Measurement of charm and beauty production at central rapidity versus charged-particle multiplicity in proton-proton collisions at √s = 7 TeV

This is an electronic reprint of the original article. This reprint may differ from the original in pagination and typographic detail. Author(s): Title: Year: Version: Please cite the original version: All material supplied via JYX is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user. Measurement of charm and beauty production at central rapidity versus chargedparticle multiplicity in proton-proton collisions at √s = 7 TeV ALICE Collaboration ALICE Collaboration. (2015). Measurement of charm and beauty production at central rapidity versus charged-particle multiplicity in proton-proton collisions at √s = 7 TeV. Journal of High Energy Physics, 2015(9), Article 148. https://doi.org/10.1007/JHEP09(2015)148 2015 JHEP09(2015)148 Published for SISSA by Springer Received:May 11, 2015 Revised:June 5, 2015 Accepted:August 10, 2015 Published:September 22, 2015 Measurement of charm and beauty production at central rapidity versus charged-particle multiplicity in proton-proton collisions at √s=7 TeV The ALICE collaboration E-mail: [email protected] Abstract: Prompt D meson and non-prompt J/ψ yields are studied as a function of the multiplicity of charged particles produced in inelastic proton-proton collisions at a centreof-mass energy of √s= 7 TeV. The results are reported as a ratio between yields in a given multiplicity interval normalised to the multiplicity-integrated ones (relative yields). They are shown as a function of the multiplicity of charged particles normalised to the average value for inelastic collisions (relative charged-particle multiplicity). D0, D+and D∗+mesons are measured in five pTintervals from 1 GeV/c to 20 GeV/c and for |y|<0.5 via their hadronic decays. The D-meson relative yield is found to increase with increasing charged-particle multiplicity. For events with multiplicity six times higher than the average multiplicity of inelastic collisions, a yield enhancement of a factor about 15 relative to the multiplicity-integrated yield in inelastic collisions is observed. The yield enhancement is independent of transverse momentum within the uncertainties of the measurement. The D0-meson relative yield is also measured as a function of the relative multiplicity at forward pseudo-rapidity. The non-prompt J/ψ, i.e. the B hadron, contribution to the inclusive J/ψ production is measured in the di-electron decay channel at central rapidity. It is evaluated for pT>1.3 GeV/c and |y|<0.9, and extrapolated to pT>0. The fraction of non-prompt J/ψ in the inclusive J/ψ yields shows no dependence on the charged-particle multiplicity at central rapidity. Charm and beauty hadron relative yields exhibit a similar increase with increasing charged-particle multiplicity. The measurements are compared to PYTHIA 8, EPOS 3 and percolation calculations. Keywords: Hadron-Hadron Scattering ArXiv ePrint: 1505.00664 Open Access, Copyright CERN, for the benefit of the ALICE Collaboration. Article funded by SCOAP3. doi:10.1007/JHEP09(2015)148 JHEP09(2015)148 Contents 1 Introduction 1 2 Experimental apparatus and data sample 4 3 Multiplicity definition and corrections 6 4 D-meson analysis 7 4.1 D-meson reconstruction 7 4.2 Corrections 9 4.3 Systematic uncertainties 11 4.4 Results 12 4.4.1 Studies with the charged-particle multiplicity at forward rapidity 13 5 Non-prompt J/ψ analysis 16 5.1 Non-prompt J/ψ reconstruction 16 5.2 Corrections 18 5.3 Systematic uncertainties 19 5.4 Results 21 6 Comparison of charm and beauty production 21 7 Comparison to theoretical calculations 24 7.1 PYTHIA 8 simulations 24 7.2 Comparison of data with models 26 8 Summary 29 A Tables of the results 31 The ALICE collaboration 39 1 Introduction The study of the production of hadrons containing heavy quarks, i.e. charm and beauty, in proton-proton (pp) collisions at the Large Hadron Collider (LHC) provides a way to test calculations based on perturbative Quantum Chromodynamics (pQCD) at the highest available collision energies. The inclusive production cross sections of charm mesons measured in pp collisions at the LHC at both central [1,2] and forward [3] rapidity are described by theoretical predictions based on pQCD calculations with the collinear factorisation approach at next-to-leading order (e.g. in the general-mass variable-flavour-number – 1 – JHEP09(2015)148 scheme, GM-VFNS [4]) or at fixed order with next-to-leading-log resummation (FONLL [5– 8]) within theoretical uncertainties. The comparisons suggest that charm production is under (over) estimated by the central values of the FONLL (GM-VFNS) calculations. The measured D-meson production cross sections in pp collisions at the LHC can also be described by pQCD calculations performed in the framework of kT-factorisation in the leading order (LO) approximation [9]. Beauty production cross section measurements in pp collisions at √s= 7 TeV [10–14] are well described by implementations of FONLL and GM-VFNS [7,15]. In the case of B mesons, the measured cross sections are close to the central value of the FONLL and GM-VFNS predictions. A similar situation was observed in pp collisions at √s= 1.96 TeV at the FNAL Tevatron collider [16–18]. The measurement of heavy-flavour production in pp collisions as a function of the charged-particle multiplicity produced in the collision could provide insight into the processes occurring in the collision at the partonic level and the interplay between the hard and soft mechanisms in particle production. These aspects are expected to depend on the energy and on the impact parameter (the distance between the colliding protons in the plane perpendicular to the beam direction) of the pp collision [19–21]. In the impact parameter representation of proton-proton collisions, the overlap of the nucleon wave functions in proton-proton collisions can be described by a geometrical picture with two separate transverse distance scales: the impact parameter of the collision and the transverse spatial partonic distribution [20,22–24]. In particular, pp collisions with a hard partonparton scattering are predicted to be more central (i.e. have smaller impact parameter) than minimum-bias events [20,25]. The NA27 Collaboration observed in 1988 that the average charged-particle multiplicity in events with open charm production was higher by about 20% than in events without charm production [26]. A softening of the momentum spectra of hadrons produced in association with charm was also observed. This result was interpreted as a consequence of the more central nature of collisions leading to charm production. At LHC energies, two additional contributions to charm production and its relation to multiplicity have to be considered. The first effect is the likely larger amount of gluon radiation associated to the short distance production processes at larger energies and particle transverse momenta. The second is the contribution of Multiple-Parton Interactions (MPI) [27–29], i.e. several hard partonic interactions occurring in a single pp collision. In this context, pQCD-inspired models describe the final-state particles produced in hadronic collisions with a two-component approach, namely an initial hard partonic scattering process, that gives rise to collimated clusters of hadrons (jets), and an underlying event, consisting of the final-state particles that are not associated with the initial hard scattering. While the hard scattering process can be computed with a pQCD approach, the description of the underlying event, which is thought to be dominated by particles produced in soft processes and by perturbative (mini)jets with relatively small transverse momenta (soft MPIs), is based on a phenomenological model. In particular, pQCD-based models of MPIs provide a consistent way to describe high multiplicity pp collisions, and have been implemented in recent Monte Carlo generators like PYTHIA 6 [30], PYTHIA 8 [31], and HERWIG [32]. Measurements by the CMS Collaboration of jet and underlying event prop- – 2 – JHEP09(2015)148 erties as a function of multiplicity in pp collisions at √s= 7 TeV can be better described by event generators including MPI [33,34]. The analysis of minijet production performed by the ALICE Collaboration [35] indicates that high multiplicities in pp collisions are reached through a high number of MPIs and a higher than average number of fragments per parton. Upward fluctuations of the gluon density in the colliding protons are also advocated to describe the results from high multiplicity pp collisions at the LHC [21,36,37]. Indeed, the transverse structure of the proton, as probed in hard partonic scattering processes, is predicted to play a crucial role in defining the underlying event structure and the probability of MPIs [25]. In the heavy-flavour sector, the LHCb Collaboration reported measurements of double charm production in pp collisions at the LHC (D0+X, J/ψ +X and J/ψ + J/ψ where X= D0,D+,D+ s,Λ+ c), which suggest that MPIs also play a role at the hard momentum scale relevant for cc production [38,39]. The ALICE Collaboration published the first measurement of inclusive J/ψ production as a function of charged-particle multiplicity, expressed as the pseudo-rapidity density of charged particles dNch/dηat mid-rapidity, in pp collisions at √s= 7 TeV [40]. An approximately linear increase of the yield of J/ψ with the charged-particle multiplicity was observed in a multiplicity range reaching four times the average multiplicity hdNch/dηi. The measurements at |y|<0.9 and 2.5< y < 4.0 were compatible within the uncertainties. Both the larger amount of gluon radiation and the contribution of MPI in collisions where heavy quarks are produced can induce a correlation between the yield of quarkonia and the charged-particle multiplicity produced in the collision. The measured rise of J/ψ yield with increasing multiplicity can also be described in the framework of string interaction or parton saturation models. In particular, in ref. [41] a stronger-than-linear trend in the high density domain is anticipated as a consequence of the interaction (overlap) of strings, which reduces the effective number of sources for soft-particle production. The increasing trend of J/ψ yield with multiplicity is also described in a framework in which high multiplicities are attained in pp collisions due to the contribution of higher Fock states in the proton, leading to a larger number of gluons participating in the collision [37]. It is also worth pointing out that the charged-particle densities attained in highmultiplicity pp collisions at the LHC are of the same order of magnitude as those measured in semi-peripheral heavy-ion collisions at lower centre-of-mass energies [42]. In those heavyion collisions, the measured momentum distributions of light hadrons indicate that the system undergoes a collective expansion, which can be described in terms of hydrodynamics. Recent measurements in high-multiplicity p–Pb collisions at √sNN = 5.02 TeV [43–48] and in high-multiplicity pp collisions at the LHC [49] indicate that such a collective behaviour could also be at play in these systems. If charm quarks were to follow a collective motion in high-multiplicity events, their momentum spectra would be altered, and the heavy-flavour hadron relative yields at high multiplicity would vary as a function of pT[50]. The measurements of the pT-differential prompt D meson and non-prompt J/ψ cross sections in pp collisions at √s= 7 TeV with the ALICE experiment at the LHC were published in references [1,10]. In this paper, we report the measurement of the relative open heavy-flavour production yields as a function of the charged-particle multiplicity in pp collisions at √s= 7 TeV. Open charm and beauty production is measured by recon- – 3 – JHEP09(2015)148 structing prompt D mesons and non-prompt J/ψ, i.e. J/ψ mesons coming from the decay of beauty hadrons. The experimental setup and the multiplicity estimation are described in sections 2and 3, respectively. Prompt D0, D+, D∗+mesons were measured at central rapidity, |y|<0.5, in six multiplicity intervals and five pTintervals from 1 GeV/c to 20 GeV/c (section 4). The non-prompt fraction of J/ψ production was measured in the rapidity interval |y|<0.9 in five multiplicity intervals and for pT>1.3 GeV/c and extrapolated to pT>0 (section 5). The relative yields as a function of charged-particle multiplicity are compared in section 6. Finally, model calculations are discussed and compared with data in section 7. 2 Experimental apparatus and data sample The ALICE apparatus [51] consists of a central barrel detector covering the pseudo-rapidity interval |η|<0.9, a forward muon spectrometer covering the pseudo-rapidity interval −4.0< η < −2.5, and a set of detectors at forward and backward rapidities used for triggering and event characterization. In the following, the subsystems that are relevant for the D meson and non-prompt J/ψ analyses are described. The central barrel detectors are located inside a large solenoidal magnet, which provides a magnetic field of 0.5 T along the beam direction (zaxis in the ALICE reference frame). Tracking and particle identification are performed using the information provided by the Inner Tracking System (ITS), the Time Projection Chamber (TPC) and the Time Of Flight (TOF) detectors, that have full azimuthal coverage in the pseudo-rapidity interval |η|<0.9. The detector closest to the beam axis is the ITS, which is composed of six cylindrical layers of silicon detectors, with radial distances from the beam axis ranging from 3.9 cm to 43.0 cm. The two innermost layers, with average radii of 3.9 cm and 7.6 cm, are equipped with Silicon Pixel Detectors (SPD). The two SPD layers, covering the pseudo-rapidity ranges of |η|<2.0 and |η|<1.4 respectively, have 1200 SPD readout chips. The two intermediate layers are made of Silicon Drift Detectors (SDD), while Silicon Strip Detectors (SSD) equip the two outermost layers. The high spatial resolution of the silicon sensors, together with the low material budget (on average 7.7% of a radiation length for tracks crossing the ITS perpendicularly to the detector surfaces, i.e. η= 0) and the small distance of the innermost layer from the beam vacuum tube, allow for the measurement of the track impact parameter in the transverse plane (d0), i.e. the distance of closest approach of the track to the primary vertex in the plane transverse to the beam direction, with a resolution better than 75 µm for transverse momenta pT>1 GeV/c [52]. The SPD provides also a measurement of the multiplicity of charged particles produced in the collision based on track segments (tracklets) built by associating pairs of hits in the two SPD layers. At larger radii (85 < r < 247 cm), a 510 cm long cylindrical TPC [53] provides track reconstruction with up to 159 three-dimensional space points per track, as well as particle identification via the measurement of the specific energy deposit dE/dxin the gas. The charged particle identification capability of the TPC is supplemented by the TOF [54], which is equipped with Multi-gap Resistive Plate Chambers (MRPCs) located – 4 – JHEP09(2015)148 at radial distances between 377 and 399 cm from the beam axis. The overall TOF resolution including the uncertainty on the time at which the collision took place, and the tracking and momentum resolution was about 160 ps for the data-taking period considered in these analyses. The V0 detector [55], used for triggering and for estimating the multiplicity of charged particles in the forward rapidity region, consists of two arrays of 32 scintillators each, placed around the beam vacuum tube on either side of the interaction region at z=−90 cm and z= +340 cm. The two arrays cover the pseudo-rapidity intervals −3.7< η < −1.7 and 2.8< η < 5.1, respectively. The data from proton-proton (pp) collisions at a centre-of-mass energy of √s= 7 TeV used for the analyses were recorded in 2010. The data sample consists of about 314 million minimum-bias (MB) events, corresponding to an integrated luminosity of Lint ≃5 nb−1. Minimum-bias collisions were triggered by requiring at least one hit in either of the V0 counters or in the SPD (|η|<2), in coincidence with the arrival time of proton bunches from both directions. This trigger was estimated to be sensitive to about 85% of the inelastic cross section [56]. To enrich the data sample with high multiplicity events, a High Multiplicity (HM) trigger based on the multiplicity information provided by the outer SPD layer was also used. Each readout chip of the SPD promptly asserts a digital pulse, called FastOR bit, on the presence of at least one firing pixel. A sample of about 6 million events was collected applying a selection on the minimum number of readout chips having asserted this digital pulse. The threshold was configured to select the ≈0.7% of the events with highest number of hits in the outer SPD layer. This HM-trigger sample (Lint ≃14 nb−1) provides an increase of statistics by a factor of about 2.8 relative to the MB trigger for events with more than 50 tracklets, corresponding to about six times the average multiplicity. Only events with interaction vertex reconstructed from tracks with a coordinate |z|< 10 cm along the beam line were used for the analysis. In the considered data samples, the instantaneous luminosity was limited to 0.6–1.2×1029 cm−2s−1by displacing the beams in the transverse plane by 3.8 times the RMS of their transverse profile. In this way, the interaction probability per bunch crossing was kept in the range 0.04–0.08, with a probability of collision pile-up below 4% per triggered event. An algorithm to detect multiple interaction vertices based on SPD track segments, or tracklets, was used to further reduce the pile-up contribution. An event is rejected from the analysed data sample if a second interaction vertex is found, which has at least three associated tracklets, and is separated from the first one by more than 0.8 cm along z. This removes about 48% of the pile-up events. The remaining pile-up contamination has two contributions: events with pile-up of collisions with ∆z < 0.8 cm and events in which the piled-up collisions have low-multiplicity (less than three charged particles reconstructed in the SPD). In the case of pile-up of collisions with small separation along z, the multiplicity estimation may be biased because some of the tracklets of charged particles from different interactions may be added together. According to simulations, the number of tracklets results to be biased when the piled-up vertices are separated along zby less than 0.6 cm. Combining this result with the shape of the luminous region along the beam direction and the maximum – 5 – JHEP09(2015)148 pile-up rate of 4%, the overall probability that two piled-up events induce a bias in the determination of multiplicity was found to be lower than 0.3%. The fraction of events with biased number of tracklets increases with increasing multiplicity and it was estimated to be below 2% at the highest multiplicities considered in this analysis, while the resulting bias on the measured number of tracklets was found to be negligible in all the multiplicity classes. 3 Multiplicity definition and corrections In the present analysis, the experimental estimator of the charged-particle multiplicity is the number of tracklets in the interval |η|<1.0 (Ntracklets). Tracklets are track segments defined by combining the clusters in the SPD detector with the reconstructed primary vertex position. Tracklets are required to point to the primary interaction vertex within ±1 cm in the transverse plane and ±3 cm in the zdirection [51,52]. This multiplicity estimator is the same as was used in previous studies performed for inclusive J/ψ production [40]. Monte Carlo simulations of the detector response have shown that Ntracklets is proportional to the pseudo-rapidity density of the generated charged primary particles, dNch/dη, within 2%. Primary particles are defined as prompt particles produced in the collision and all decay products, except products from weak decays of strange particles. The pseudo-rapidity coverage of the SPD detector changes with the position of the interaction vertex along the beam line, zvtx, and with time due to the variation of the number of inactive channels. The detector response over the analysed data taking period is equalised by means of a databased correction, which is applied on an event-by-event basis depending on zvtx and time. The measurements in the Ntracklets ∈[1,49] interval are performed using minimumbias triggered data, while those in the [50,80] range exploit the SPD-based HM trigger described above. The HM trigger is fully efficient for events with Ntracklets >65. The number of events and the D-meson candidate invariant mass distributions were corrected for the HM trigger inefficiency in the Ntracklets ∈[50,65] range by means of a data-driven re-weighting procedure. The Ntracklets-dependent event weights were defined from the ratio of the measured distributions of the number of tracklets in the HM and minimum-bias trigger samples. The effect of this correction on the per-event raw yield was of about 2.5%. The average dNch/dηof events in the highest Ntracklets interval was determined from the minimum-bias sample. The analysis results are presented as a function of the relative charged-particle multiplicity at central rapidity, (dNch/dη)jhdNch/dηi, where hdNch/dηi=6.01±0.01(stat.)+0.20 −0.12 (syst.) is measured in inelastic pp collisions at √s= 7 TeV with at least one charged particle in |η|<1.0 [57]. The relative quantities are used to minimise the experimental uncertainties and to facilitate the comparison with other measurements and models. The considered Ntracklets intervals and the corresponding relative charged-particle multiplicity ranges are summarised in table 1. The highest Ntracklets interval considered in the analysis extends to a multiplicity of about 9 times the hdNch/dηiof inelastic pp collisions and the average multiplicity of events in this Ntracklets interval is about six times the hdNch/dηi. The uncertainty on (dNch/dη)jhdNch/dηiis 6%; it includes the influence of (i) the deter- – 6 – JHEP09(2015)148 Ntracklets (dNch/dη)j(dNch/dη)jhdNch/dηiND0 events/106NJ/ψ events/106 [1,8] 2.7 0.45+0.03 −0.03 155.1 — [4,8] 3.8 0.63+0.04 −0.04 — 89.0 [9,13] 7.1 1.18+0.07 −0.07 46.2 50.5 [14,19] 10.7 1.78+0.10 −0.11 32.0 35.5 [20,30] 15.8 2.63+0.15 −0.17 24.7 28.0 [31,49] 24.1 4.01+0.23 −0.25 7.9 9.5 [50,80] 36.7 6.11+0.35 −0.39 1.7 — Table 1. Summary of the multiplicity intervals used for the analyses. The number of reconstructed tracklets Ntracklets, the average charged-particle multiplicity (dNch/dη)j, and the relative chargedparticle multiplicity (dNch/dη)jhdNch/dηiare detailed. The number of events analysed in the various multiplicity ranges for both the D-meson and J/ψ analyses are reported. The number of events for the Ntracklets interval [50,80] are corrected for the high multiplicity trigger efficiency, as explained in section 3. mination of the Ntracklets to dNch/dηproportionality factor, 2%, (ii) its possible deviation from linearity, 5%, (iii) and the uncertainty on the measured hdNch/dηi. The analysis of D0production is also carried out as a function of the charged-particle multiplicity in the regions −3.7< η < −1.7 and 2.8< η < 5.1, as measured with the charge collected by the V0 scintillator counters, NV0, reported in units of the minimum-ionizingparticle charge. The motivation for studying the multiplicity dependence of charmed-meson production also with this estimator is that the event multiplicity and the D-meson yields are evaluated in different pseudorapidity ranges, reducing the effects of auto-correlations. In contrast, with the Ntracklets estimator also the D-meson decay products and the charged particles produced in the fragmentation of the same charm quark are included in the multiplicity evaluation. Monte Carlo simulations demonstrate that NV0 is proportional to the charged-particle multiplicity in that pseudo-rapidity interval. In this paper we report D0relative yields as a function of the relative uncorrected multiplicity in the V0 detector, NV0hNV0i(see section 4.4.1). 4 D-meson analysis 4.1 D-meson reconstruction Charm production was studied by reconstructing D0, D+and D∗+mesons, and their antiparticles, via their hadronic decay channels D0→K−π+(with branching ratio, BR, of 3.88±0.05%), D+→K−π+π+(BR of 9.13±0.19%), and D∗+→D0π+(BR of 67.7±0.05%) with D0→K−π+[58]. D-meson candidates were selected with the same strategy as described in [1]. The selection of D0and D+decays (weak decays with mean proper decay – 7 – JHEP09(2015)148 〉 T pdy/dN 2 d〈) / T pdy/dN 2 (d 5 10 15 20 25 c<8 GeV/ T p meson, 4< 0 D c<8 GeV/ T p meson, 4< + D c<8 GeV/ T p meson, 4< + D* ALICE |<0.5y = 7 TeV, |s pp not shown〉η/dNd〈) / η/dN 6% unc. on (d± +6%/-3% normalization unc. not shown 〉η/d ch Nd〈) / η/d ch N (d 0 1 2 3 4 5 6 7 8 9 B feed-down unc. 0.4− 0.2− 0 0.2 0.4 1/2 (2) at low (high) multiplicity×B fraction hypothesis: (a) D meson with 4 < pT<8 GeV/c. 〉 T pdy/dN 2 d〈) / T pdy/dN 2 (d 5 10 15 20 25 c<12 GeV/ T p meson, 8< 0 D c<12 GeV/ T p meson, 8< + D c<12 GeV/ T p meson, 8< + D* ALICE |<0.5y = 7 TeV, |s pp not shown〉η/dNd〈) / η/dN 6% unc. on (d± +6%/-3% normalization unc. not shown 〉η/d ch Nd〈) / η/d ch N (d 0 1 2 3 4 5 6 7 8 9 B feed-down unc. 0.4− 0.2− 0 0.2 0.4 1/2 (2) at low (high) multiplicity×B fraction hypothesis: (b) D meson with 8 < pT<12 GeV/c. 〉 T pdy/dN 2 d〈) / T pdy/dN 2 (d 5 10 15 20 25 c<20 GeV/ T p meson, 12< + D* ALICE |<0.5y = 7 TeV, |s pp not shown〉η/dNd〈) / η/dN 6% unc. on (d± +6%/-3% normalization unc. not shown 〉η/d ch Nd〈) / η/d ch N (d 0 1 2 3 4 5 6 7 8 9 B feed-down unc. 0.4− 0.2− 0 0.2 0.4 1/2 (2) at low (high) multiplicity×B fraction hypothesis: (c) D meson with 12 < pT<20 GeV/c. Figure 3. D0, D+and D∗+meson relative yields for each pTinterval as a function of chargedparticle multiplicity at central rapidity. The relative yields are presented on the top panels with their statistical (vertical bars) and systematic (boxes) uncertainties, except for the feed-down fraction uncertainty that is drawn separately in the bottom panels. D0mesons are represented by red circles, D+by green squares, and D∗+by blue triangles. The position of the points on the abscissa is the average value of (dNch/dη)hdNch/dηi. For D+and D∗+mesons the points are shifted horizontally by 1.5% to improve the visibility. The diagonal (dashed) line is also shown to guide the eye. – 14 – JHEP09(2015)148 〉 T pdy/dN 2 d〈) / T pdy/dN 2 (d 5 10 15 20 25 c < 2 GeV/ T p 1 < c < 4 GeV/ T p 2 < c < 8 GeV/ T p 4 < c < 12 GeV/ T p 8 < c < 20 GeV/ T p 12 < = 7 TeVsALICE, pp |<0.5y meson, | + , D* + ,D 0 Average D not shown〉η/dNd〈) / η/dN 6% unc. on (d± +6%/-3% normalization unc. not shown 〉η/d ch Nd〈) / η/d ch N (d 0 1 2 3 4 5 6 7 8 9 B feed-down unc. 0.4− 0.2− 0 0.2 0.4 1/2 (2) at low (high) multiplicity×B fraction hypothesis: (a) pTdependence. )c<4 GeV/ T p) in 2< T pdy/dN 2 Ratio to (d 0.5 1 1.5 2 2.5 3 3.5 4 c < 2 GeV/ T p 1 < c < 8 GeV/ T p 4 < c < 12 GeV/ T p 8 < c < 20 GeV/ T p 12 < = 7 TeVsALICE, pp |<0.5y meson, | + , D* + ,D 0 Average D not shown〉η/dNd〈) / η/dN 6% unc. on (d± 〉η/d ch Nd〈) / η/d ch N (d 0 1 2 3 4 5 6 7 8 9 B feed-down unc. 0.4− 0.2− 0 0.2 0.4 1/2 (2) at low (high) multiplicity×B fraction hypothesis: (b) Ratios of pTintervals vs the 2 < pT<4 GeV/c. Figure 4. Average of D0, D+and D∗+relative yields as a function of the relative chargedparticle multiplicity at central rapidity. (a) Average of D-meson relative yields in pTintervals. (b) Ratio of the average relative yields in all pTintervals with respect to that of the 2 < pT< 4 GeV/c interval. The results are presented in the top panels with their statistical (vertical bars) and systematic (boxes) uncertainties, except for the feed-down fraction uncertainty that is drawn separately in the bottom panels. The position of the points on the abscissa is the average value of (dNch/dη)hdNch/dηi. For some pTintervals the points are shifted horizontally by 1.5% to improve the visibility. The dashed lines are also shown to guide the eye, a diagonal on (a) and a constant on (b). down fraction evolution with the charged-particle multiplicity is drawn separately in the bottom panels. The points are located on the x-axis at the average value of the relative mean multiplicity, NV0hNV0i. The uncertainty on the mean multiplicity values, NV0, was determined by comparing the mean and median values of the distributions. It was found to be below 3% for each multiplicity interval, and about 24% for the multiplicity integrated value. The uncertainty on NV0hNV0iis not displayed on this figure. These results are also summarised in tables 5and 6. The D0relative yields increase with the relative uncorrected multiplicity at forward rapidity, as measured with the V0 detector. The results in the 2 < pT<4 GeV/c and 4 < pT<8 GeV/c intervals are compatible within uncertainties. The results with the V0 multiplicity estimator indicate that the increase of the D-meson yield with the event multiplicity observed with the mid-rapidity estimator is not related to the fact that charmed mesons, originating from the fragmentation of charm quarks produced in hard partonic scattering processes, and the charged particle multiplicity are measured in the same pseudo-rapidity range. A qualitatively similar increasing trend of D-meson yield with multiplicity is indeed observed also when an ηgap is introduced between the regions where the D-mesons and the multiplicity are measured. – 15 – JHEP09(2015)148 〉 T pdy/dN 2 d〈) / T pdy/dN 2 (d 1 2 3 4 5 6 7 c < 4 GeV/ T p 2 < c < 8 GeV/ T p 4 < = 7 TeVsALICE, pp |<0.5y meson, | 0 D not shown〉 V0 N〈 / V0 N 3% unc. on ± +6%/-3% normalization unc. not shown 〉 V0 N〈 / V0 N 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 B feed-down unc. 0.4− 0.2− 0 0.2 0.4 1/2 (2) at low (high) multiplicity×B fraction hypothesis: Figure 5. D0meson relative yields at |y|<0.5 for two pTintervals as a function of the relative charged-particle multiplicity, NV0, measured at −3.7< η < −1.7 and 2.8< η < 5.1. The relative yields are presented on the top panels with their statistical (vertical bars) and systematic (boxes) uncertainties, except the uncertainty on the feed-down fraction which is drawn separately in the bottom panels. The position of the points on the abscissa is at the average value of NV0hNV0i, shifted by 1.5% to improve the visibility. The diagonal (dashed) line is also shown to guide the eye. 5 Non-prompt J/ψ analysis 5.1 Non-prompt J/ψ reconstruction The fraction of non-prompt J/ψ in the inclusive J/ψ yields, fB, was measured as a function of the charged-particle multiplicity by studying displaced J/ψ mesons that decay into electron pairs in the rapidity range |y|<0.9. This measurement, combined with the inclusive J/ψ relative yield [40], provides the multiplicity dependence of the production of beauty hadrons. J/ψ candidates were formed by combining pairs of opposite-sign electron tracks. The tracks were required to have pT>1 GeV/c, at least 70 (out of a maximum of 159) associated space points in the TPC with a χ2/ndf of the momentum fit lower than 2, and to point back to the primary interaction vertex within 1 cm in the transverse plane. The tracks were also required to have at least one associated hit in the SPD detector, with the constraint that one of the two tracks should have a hit in the first SPD layer. Electron identification was based only on the TPC information. A selection of ±3σaround the expected mean values of the specific energy deposit dE/dxfor electrons was used. To further reduce the background, a ±3.5σ(±3σ) exclusion band around the expected mean specific energy deposit for pions (protons) was also applied. In order to reduce the combinatorial background, electron candidates compatible, together with a positron candidate, with being products of γ-conversions (invariant mass below 100 MeV/c2) were removed. – 16 – JHEP09(2015)148 The measurement of fBis based on a statistical discrimination of J/ψ mesons produced at a secondary vertex displaced from the primary pp collision vertex. The signed projection of the J/ψ flight distance onto its transverse momentum vector, ~pT, was constructed as Lxy =~ L·~pT/pT, where ~ Lis the vector from the primary vertex to the J/ψ decay vertex. The pseudo-proper decay length x= (c·Lxy ·m)pTwas calculated from the observed decay length using the world-average J/ψ mass m(J/ψ) = 3096.916 ±0.011 MeV/c2[58]. The fraction of non-prompt J/ψ can be determined from a 2-dimensional un-binned loglikelihood fit to xand the unlike-sign di-electron invariant mass distributions. The fit procedure and the functions used to describe the invariant mass and the pseudo-proper decay length distributions were introduced in [10]. The fraction of non-prompt J/ψ as a function of the relative charged-particle multiplicity was determined for pT>1.3 GeV/c in five multiplicity intervals in the Ntracklets range [4,49]. The Ntracklets ∈[1,3] range was excluded from this analysis due to the poor pseudo-proper decay length resolution, R(x), and the presence of a bias in the determination of xin the case of non-prompt candidates. The resolution of the pseudo-proper decay length is determined with Monte Carlo simulations evaluating the RMS of the x distributions of reconstructed promptly produced J/ψ mesons. The event primary vertex can be computed with or without removing the decay tracks of the J/ψ candidates. The removal of the decay tracks causes a degradation of the resolution on x, especially in the low-multiplicity intervals, as a consequence of the lower precision in the determination of the primary vertex with a reduced number of tracks. For simulated events with non-prompt J/ψ, the removal of the decay tracks also results in a shift of the primary vertex position away from the secondary decay vertex of the beauty hadrons, which is reflected in a systematic shift of the mean of the xdistribution. However, one should consider that beauty quarks are always produced in pairs: the two decay tracks from the non-prompt-J/ψ, when included, pull the primary vertex towards the beauty hadron decay vertex, but the charged tracks from the decay of the second beauty quark, which enter in the barrel acceptance, pull the primary vertex in the opposite direction. The shift is larger in the lowest multiplicity bin where it reaches about 35 µm. This bias is reduced when the J/ψ decay tracks are kept in the evaluation of the primary vertex. The effect of the bias, estimated with Monte Carlo simulations, is a reduction1of the measured fBby about 20% for events with Ntracklets = 4, and it becomes negligible for Ntracklets >10. Therefore, the primary vertex was computed considering all reconstructed tracks. To correct for the remaining bias, a modification in the resolution function, R(x), used to describe the non-prompt J/ψ in the likelihood fit function was introduced, which depends on Ntracklets. In particular, the shape of the resolution function was adjusted to obtain a good matching between the function used to describe the non-prompt J/ψ in the likelihood fit (a convolution of a template of the xdistribution of J/ψ from beauty hadron decays with the resolution function [10]) and the pseudo-proper decay length distribution of reconstructed secondary J/ψ from Monte Carlo simulations. 1This shift would be greater than 50 µm in the Ntracklets interval [1,3], leading to a large bias on the extracted fBvalue (up to 35%). The correction for this bias would introduce a large systematic uncertainty. – 17 – JHEP09(2015)148 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 ) 2 cEntries/(40 MeV/ 2 4 6 8 10 12 14 16 = 7 TeVsALICE, pp -1 = 5.6 nb int L c)>1.3 GeV/ψ(J/ T p|<0.9, y| [4,8] tracklets Data Fit, all Fit, signal Fit, background 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 5 10 15 20 25 [9,13] tracklets 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 5 10 15 20 25 30 [14,19] tracklets 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 5 10 15 20 25 30 35 40 [20,30] tracklets ) 2 c) (GeV/ - e + M(e 2.4 2.6 2.8 3 3.2 3.4 3.6 3.8 4 5 10 15 20 25 30 35 [31,49] tracklets -2000 -1500 -1000 -500 0 500 1000 1500 2000 m)µEntries/(40 -1 10 1 10 2 c) < 4.0 GeV/ - e + (eM2.4 < Data Fit, all ψFit, prompt J/ ψFit, non-prompt J/ Fit, background -2000 -1500 -1000 -500 0 500 1000 1500 2000 -1 10 1 10 -2000 -1500 -1000 -500 0 500 1000 1500 2000 -1 10 1 10 -2000 -1500 -1000 -500 0 500 1000 1500 2000 -1 10 1 10 m)µPseudo-proper decay length ( -2000 -1500 -1000 -500 0 500 1000 1500 2000 1 10 Figure 6. J/ψ invariant mass and pseudo-proper decay length distributions in several multiplicity intervals with superimposed the likelihood fit results. The contributions of the signal, the background and their sum are represented with dashed, dot-dashed and full lines, respectively. In addition, the pseudo-proper decay length figures include the prompt and non-prompt contributions to the inclusive yields with dotted and long-dashed lines. Figure 6presents the invariant mass and pseudo-proper decay length distributions for pT>1.3 GeV/c for each multiplicity interval together with a projection of the result of the log-likelihood fit. 5.2 Corrections For all multiplicity intervals, the measured fraction of non-prompt J/ψ,f0 B, was corrected using the acceptance and reconstruction efficiency of prompt, hAcc ×εiprompt, and nonprompt J/ψ,hAcc ×εiB, as fB=1 + 1−f0 B f0 B·hAcc ×εiB hAcc ×εiprompt −1 .(5.1) – 18 – JHEP09(2015)148 Here all terms refer to non-prompt J/ψ with pT>1.3 GeV/c. The corrections for acceptance and efficiency were computed using Monte Carlo simulations using the GEANT3 transport code [60]. Prompt J/ψ were generated with a pTdistribution extrapolated from CDF measurements [16] and a ydistribution parameterised with the Colour Evaporation Model (CEM) [62,63]. Beauty hadrons were generated using the PYTHIA 6.4.21 event generator [30] with Perugia-0 tune [64]. The acceptance times efficiency values for prompt and non-prompt J/ψ have a minimum of 8% at pT= 2 GeV/c and a broad maximum of 12% at pT= 7 GeV/c [65]. The relative difference in efficiency between prompt and non-prompt J/ψ is only about 3%. The ratio hAcc ×εiB/hAcc ×εiprompt is assumed to be independent of multiplicity. The uncertainty related to this assumption is discussed in the next section. The measured non-prompt J/ψ fractions were extrapolated from pT>1.3 GeV/c down to pT= 0 using fextr B(pT>0) = αextr ·fB(pT>1.3 GeV/c); αextr =fmodel B(pT>0) fmodel B(pT>1.3 GeV/c),(5.2) where fmodel Brepresents a functional form modelled on existing data. It was calculated as the ratio of the differential cross section of non-prompt J/ψ, as obtained with FONLL calculations [7], to that of inclusive J/ψ, parameterised by the phenomenological function defined in [66]: fmodel B(pT) = d2σFONLL J/ψ←hB dydpT,d2σphenom J/ψ dydpT .(5.3) A combined fit to the existing results of fBin pp collisions at 7 TeV [10,13,67,68] in the rapidity bin closest to central rapidity was performed to determine the parameters of the phenomenological parameterisation. The extrapolation factor obtained is αextr = 0.99+0.01 −0.03. Its uncertainties were determined by repeating the fit by (i) excluding the LHCb data points at forward rapidities, and (ii) using for the non-prompt J/ψ cross section the upper and lower uncertainty bands of the FONLL predictions, obtained by varying the factorisation and renormalisation scales, instead of the central values. The uncertainties were determined by the maximum and minimum αextr values obtained from these fit variations. The fB fractions in all multiplicity intervals were extrapolated using the same αextr value, evaluated from the fit of the multiplicity integrated measurements. 5.3 Systematic uncertainties The systematic uncertainty introduced by the experimental resolution on the primary vertex position was evaluated by repeating the fitting procedure in two alternative ways: (i) the primary vertex was evaluated without removing the decay tracks of the J/ψ candidates. The fit was performed using the standard resolution function for non-prompt J/ψ, that does not depend on multiplicity, but the xdistribution of the non-prompt J/ψ was shifted by a multiplicity-dependent value, which was determined by the Monte Carlo simulation. (ii) The event primary vertex was computed after removing the decay tracks of the J/ψ candidates and the fit was performed using the corresponding degraded resolution – 19 – JHEP09(2015)148 function R(x) and without any shift. The resulting uncertainties decrease with increasing multiplicity, ranging from 19% in the lowest multiplicity interval to 3% at the highest multiplicities. The uncertainty related to the extrapolation of fBfrom pT>1.3 GeV/c to pT>0 was estimated with the method discussed above and it is about 3%. This uncertainty was assumed to be uncorrelated among the multiplicity intervals. The resolution function used in the fits is based on Monte Carlo simulations, which might introduce systematic effects. These were estimated by repeating the log-likelihood fits modifying the resolution function, R(x), according to (1/(1 + δ))·R(x/(1 + δ)), where δis the relative variation of the RMS of the resolution function, and it was varied from −0.1 to +0.1 to take into account the uncertainties in the Monte Carlo description. The systematic uncertainty due to the resolution function increases with multiplicity from 8% to 20%. The pTdistribution of the signal candidates (prompt and non-prompt J/ψ) could depend on the event multiplicity which could affect the shape of the resolution function which depends on the J/ψ pT. The average pTof the signal candidates was estimated from data in each multiplicity interval and found to be constant as a function of event multiplicity within statistical uncertainties about ±10%. The influence of a hpTivariation on the resolution function was determined using Monte Carlo simulations: the pTdistribution was changed, considering softer or harder pTdistributions, in order to obtain a ±10% variation of the hpTi. The corresponding variations obtained for the RMS of the resolution function are +7% and −8.5% for the softer and harder pTdistribution, respectively. The latter variations are within those quoted for the resolution function (±10%), therefore no additional uncertainty was included. The acceptance times efficiency values of prompt and non-prompt J/ψ reconstructed for pT>1.3 GeV/c are of the order of 10% and differ by 3%. The influence of the pT shape assumed in the simulation on the ratio hAcc ×εiB/hAcc ×εiprompt was evaluated by varying the average pTof the simulated J/ψ distributions within ±50%. A 1% variation in the acceptance was obtained both for prompt and non-prompt J/ψ. The corresponding variation obtained on fBthrough the eq. (5.1) is about 1%. The pseudo-proper decay length shape of the combinatorial background was determined by a fit to the xdistribution of the candidates in the sidebands of the invariant mass [10]. By varying the fit parameters within their errors an envelope of distributions was obtained, whose extremes were used in the likelihood fit to estimate the systematic uncertainty. It increases slightly with multiplicity, ranging from 1% to 5%. The uncertainty on the background invariant mass shape, which was determined by fits to the invariant mass distributions of opposite-sign candidates in each multiplicity bin, was evaluated by using like-sign distributions instead, adopting the same procedure as described in [10]. The systematic uncertainty is about 7%, independent of the chargedparticle multiplicity. The shape of the xdistribution of J/ψ from b-hadrons was evaluated using PYTHIA 6.4.21 [30]. The systematic uncertainty on its shape was computed by (i) changing the b-hadron decay kinematic, using EvtGen [61] instead of PYTHA 6.4.21 or (ii) by assuming – 20 – JHEP09(2015)148 a harder and a softer b-hadron pTdistribution, resulting in a hpTivariation of about ±15%. The resulting systematic uncertainty is about 3%, constant with multiplicity. The signal invariant mass shape was fixed from the Monte Carlo simulation which includes the detector resolution effects and the radiative decays using the EvtGen [61] package. The effect on the invariant mass signal shape due to the uncertainty on the detector material was studied with dedicated Monte Carlo simulations, where the detector material budget was varied by ±6% with respect to the nominal values [69,70]. The resulting systematic uncertainty on fBis 3% in the lowest event multiplicity interval and 5% in the highest one. The systematic uncertainties on the pseudo-proper decay length of the combinatorial background, on the pT-extrapolation uncertainty αextr and on the invariant mass shape of background are found or, in the case of αextr, assumed to be uncorrelated among multiplicity intervals. The remaining systematic uncertainties are (fully or partially) correlated in different multiplicity intervals. 5.4 Results The relative yield of J/ψ from beauty hadron decays as a function of the charged-particle multiplicity was evaluated from the inclusive J/ψ yield and the fraction of non-prompt J/ψ per multiplicity interval: dNnon−prompt J/ψ /dy DdNnon−prompt J/ψ /dyE=dNJ/ψ/dy dNJ/ψ/dy·fB hfBi.(5.4) fBis the fraction of non-prompt J/ψ in each multiplicity interval, hfBiis the fraction in the multiplicity integrated sample [10], and (dNJ/ψ/dy)hdNJ/ψ/dyiis the inclusive J/ψ relative yield measured for pT>0 in each multiplicity interval normalized to its value in inelastic pp collisions [40]. All these quantities were measured using the same data sample and the statistical correlations were taken into account. In the first charged-particle multiplicity class Ntracklets ∈[4,8], which is used for the non-prompt J/ψ analysis presented here, the relative yield of inclusive J/ψ normalized to the inelastic cross section is (dNJ/ψ/dy)hdNJ/ψ/dyi= 0.41±0.07 (stat)±0.01 (syst). The values of fBextrapolated to pT>0 were used in eq. (5.4), providing the non-prompt J/ψ relative yields for pT>0. The relative yields of inclusive J/ψ were also recomputed for pT>1.3 GeV/c and no difference was observed with respect to those for pT>0 within the uncertainties. The results for the fraction of non-prompt J/ψ for both pT>0 and pT>1.3 GeV/c, the relative yields of prompt and non-prompt J/ψ in each multiplicity bin for pT>0 are summarized in tables 7and 8and shown in figure 7. 6 Comparison of charm and beauty production Figure 8(a) presents prompt D meson and inclusive J/ψ results to compare open and hidden charm production. The average prompt D-meson results are shown in the 2 < – 21 – JHEP09(2015)148 〉 η /d ch Nd〈) / η /d ch N(d 0 1 2 3 4 5 from b hadrons (%)ψFraction of J/ 0 10 20 30 40 50 = 7 TeVsALICE, pp c) > 1.3 GeV/ψ(J/ T p| < 0.9, y| Figure 7. Non-prompt J/ψ fraction as a function of the relative charged-particle multiplicity at central rapidity for pT>1.3 GeV/c. The vertical bars represent the statistical uncertainties, while the empty boxes stand for the systematic uncertainties. The width and the height of these empty boxes indicate the measurement uncertainty on the horizontal and vertical axis respectively. The dashed line shows the value of fBmeasured in the same pTrange and integrated over multiplicity [10]. The shaded area represents the statistical and systematic uncertainties on the multiplicity-integrated result added in quadrature. pT<4 GeV/c interval with the pT-integrated inclusive J/ψ measurement2at central and forward-rapidity by the ALICE experiment [40]. The results for prompt J/ψ at central rapidity from this paper (pT>0) and for prompt D mesons (2 < pT<4 GeV/c) are compared in figure 8(b). A similar increase of the relative yield with the charged-particle multiplicity is observed for open and hidden charm production both at central and forward rapidities. Figure 8(c) superimposes the open charm and beauty production measurements reported in this paper showing the average prompt D-meson results in the 2 < pT<4 GeV/c interval and the pT-integrated non-prompt J/ψ measurement at central rapidity. The results are compatible within the measurement uncertainties. Open charm, open beauty and hidden charm hadron relative yields present a similar increase with charged-particle multiplicity. The comparison of open and hidden heavy flavour production suggests that this behaviour is most likely related to the c¯c and b¯ b production processes, and is not significantly influenced by hadronisation. The enhancement of the heavy-flavour relative yields with the charged-particle multiplicity is qualitatively consistent with the calculations of the contribution from MPIs to particle production at LHC 2After the inclusive J/ψ measurement was published in reference [40], there was an improvement of the ALICE measurement of the inelastic cross section in pp collisions at √s= 7 TeV. The improved evaluation of the inelastic cross section does not rely on Monte Carlo, hence the systematic uncertainty is larger [56]. To allow a proper comparison with the results reported here, we updated the published inclusive J/ψ measurement by the corresponding change of the trigger efficiency for inelastic collisions 0.864/0.85. The normalisation uncertainties were also changed from 1.5% to +6 −3%. – 22 – JHEP09(2015)148 〉 T pdy/dN 2 d〈) / T pdy/dN 2 (d 5 10 15 20 25 c<4 GeV/ T p|<0.5, 2<y meson | + , D* + , D 0 Average D >0 T p|<0.9, y, | - e + e→ ψJ/ >0 T p<4.0, y, 2.5< - µ + µ → ψJ/ = 7 TeVsALICE, pp not shown〉η/dNd〈) / η/dN 6% unc. on (d± +6%/-3% normalization unc. not shown 〉η/d ch Nd〈) / η/d ch N (d 0 1 2 3 4 5 6 7 8 9 B feed-down unc. 0.4− 0.2− 0 0.2 0.4 1/2 (2) at low (high) multiplicity×B fraction hypothesis: (a) D meson with 2 < pT<4 GeV/c and inclusive J/ψ. 〉 T pdy/dN 2 d〈) / T pdy/dN 2 (d 5 10 15 20 25 c<4 GeV/ T p|<0.5, 2<y meson | + , D* + , D 0 Average D >0 T p|<0.9, y, | - e + e→ ψPrompt J/ = 7 TeVsALICE, pp not shown〉η/dNd〈) / η/dN 6% unc. on (d± +6%/-3% normalization unc. not shown 〉η/d ch Nd〈) / η/d ch N (d 0 1 2 3 4 5 6 7 8 9 B feed-down unc. 0.4− 0.2− 0 0.2 0.4 1/2 (2) at low (high) multiplicity×B fraction hypothesis: (b) D meson with 2 < pT<4 GeV/c and prompt J/ψ. 〉 T pdy/dN 2 d〈) / T pdy/dN 2 (d 5 10 15 20 25 c<4 GeV/ T p|<0.5, 2<y meson | + , D* + , D 0 Average D >0 T p|<0.9, y, | - e + e→ ψNon-prompt J/ = 7 TeVsALICE, pp not shown〉η/dNd〈) / η/dN 6% unc. on (d± +6%/-3% normalization unc. not shown 〉η/d ch Nd〈) / η/d ch N (d 0 1 2 3 4 5 6 7 8 9 B feed-down unc. 0.4− 0.2− 0 0.2 0.4 1/2 (2) at low (high) multiplicity×B fraction hypothesis: (c) D meson with 2 < pT<4 GeV/c and non-prompt J/ψ. Figure 8. Average D meson and J/ψ relative yields as a function of the relative charged-particle multiplicity at central rapidity. D-meson yields are shown for 2 < pT<4 GeV/c, while J/ψ yields are for pT>0. (a) Inclusive J/ψ results for |y|<0.9 are represented by empty black circles [40], inclusive J/ψ results for 2.5< y < 4.0 by black filled symbols [40], and prompt D mesons by red filled circles. (b) Prompt J/ψ results for |y|<0.9 are represented by green filled crosses, and prompt D mesons by red filled circles. (c) Non-prompt J/ψ results for |y|<0.9 are represented by blue filled squares, and prompt D mesons by red filled circles. The relative yields are presented on the top panels with their statistical (vertical bars) and systematic (boxes) uncertainties except the uncertainty on the feed-down fraction for D mesons, which is drawn separately on the bottom panels. The points are located on the x-axis at the average value of (dNch/dη)hdNch/dηi. The diagonal (dashed) line is drawn to guide the eye. – 23 – JHEP09(2015)148 ever, the PYTHIA 8.157 [31] event generator seems to under-estimate the increase of heavy flavour yields with the charged-particle multiplicity at high multiplicities. Acknowledgments We would like to thank P. Skands, co-author of PYTHIA 8, and K. Werner and B. Guiot, coauthors of EPOS 3, for fruitful discussions and for providing their theoretical calculations. The ALICE Collaboration would like to thank all its engineers and technicians for their invaluable contributions to the construction of the experiment and the CERN accelerator teams for the outstanding performance of the LHC complex. The ALICE Collaboration gratefully acknowledges the resources and support provided by all Grid centres and the Worldwide LHC Computing Grid (WLCG) collaboration. The ALICE Collaboration acknowledges the following funding agencies for their support in building and running the ALICE detector: State Committee of Science, World Federation of Scientists (WFS) and Swiss Fonds Kidagan, Armenia, Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnol´ogico (CNPq), Financiadora de Estudos e Projetos (FINEP), Funda¸c˜ao de Amparo `a Pesquisa do Estado de S˜ao Paulo (FAPESP); National Natural Science Foundation of China (NSFC), the Chinese Ministry of Education (CMOE) and the Ministry of Science and Technology of China (MSTC); Ministry of Education and Youth of the Czech Republic; Danish Natural Science Research Council, the Carlsberg Foundation and the Danish National Research Foundation; The European Research Council under the European Community’s Seventh Framework Programme; Helsinki Institute of Physics and the Academy of Finland; French CNRS-IN2P3, the ‘Region Pays de Loire’, ‘Region Alsace’, ‘Region Auvergne’ and CEA, France; German Bundesministerium fur Bildung, Wissenschaft, Forschung und Technologie (BMBF) and the Helmholtz Association; General Secretariat for Research and Technology, Ministry of Development, Greece; Hungarian Orszagos Tudomanyos Kutatasi Alappgrammok (OTKA) and National Office for Research and Technology (NKTH); Department of Atomic Energy and Department of Science and Technology of the Government of India; Istituto Nazionale di Fisica Nucleare (INFN) and Centro Fermi — Museo Storico della Fisica e Centro Studi e Ricerche “Enrico Fermi”, Italy; MEXT Grant-in-Aid for Specially Promoted Research, Japan; Joint Institute for Nuclear Research, Dubna; National Research Foundation of Korea (NRF); Consejo Nacional de Cienca y Tecnologia (CONACYT), Direccion General de Asuntos del Personal Academico(DGAPA), M´exico, Amerique Latine Formation academique — European Commission (ALFA-EC) and the EPLANET Program (European Particle Physics Latin American Network); Stichting voor Fundamenteel Onderzoek der Materie (FOM) and the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; Research Council of Norway (NFR); National Science Centre, Poland; Ministry of National Education/Institute for Atomic Physics and National Council of Scientific Research in Higher Education (CNCSI-UEFISCDI), Romania; Ministry of Education and Science of Russian Federation, Russian Academy of Sciences, Russian Federal Agency of Atomic Energy, Russian Federal Agency for Science and Innovations and The Russian Foundation for Basic Research; Ministry of Education of Slovakia; Department of Science and Technology, South Africa; Centro de Investiga- – 30 – JHEP09(2015)148 ciones Energeticas, Medioambientales y Tecnologicas (CIEMAT), E-Infrastructure shared between Europe and Latin America (EELA), Ministerio de Econom´ıa y Competitividad (MINECO) of Spain, Xunta de Galicia (Conseller´ıa de Educaci´on), Centro de Aplicaciones Tecnol´ogicas y Desarrollo Nuclear (CEADEN), Cubaenerg´ıa, Cuba, and IAEA (International Atomic Energy Agency); Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW); Ukraine Ministry of Education and Science; United Kingdom Science and Technology Facilities Council (STFC); The United States Department of Energy, the United States National Science Foundation, the State of Texas, and the State of Ohio; Ministry of Science, Education and Sports of Croatia and Unity through Knowledge Fund, Croatia. Council of Scientific and Industrial Research (CSIR), New Delhi, India. A Tables of the results Table 4reports the results of the relative average D-meson yields per inelastic collision as a function of the relative charged-particle multiplicity at mid-rapidity in several D-meson transverse momentum intervals, see figure 4. The corresponding relative average D-meson yields normalised to the visible cross section instead of the inelastic one are presented in table 3. Table 6summarises the relative D0yields per inelastic collision as a function of the relative raw multiplicity measured with the V0 detector at forward rapidity, see figure 5. These relative D0yields are also presented in table 5normalised to the visible cross section. Table 7reports the fraction of non-prompt J/ψ to the inclusive J/ψ yields as a function of the relative charged-particle multiplicity at mid-rapidity, see figure 7. The relative prompt and non-prompt J/ψ yields per inelastic collision are reported in table 8, while table 7presents these yields normalised to the visible cross section. – 31 – JHEP09(2015)148 Ntracklets interval [1,8] [9,13] [14,19] [20,30] [31,49] [50,80] pT(GeV/c) (d2N/dydpT)hd2N/dydpTi×trigger 1–2 0.39 ±0.09 ±0.05+0.07 −00.94 ±0.21 ±0.11+0.14 −0.07 1.45 ±0.36 ±0.15+0.16 −0.22 3.60 ±0.66 ±0.40+0.27 −0.82 5.33 ±1.59 ±0.62+0 −2.01 — 2–4 0.18 ±0.01 ±0.01+0.02 −00.91 ±0.05 ±0.06+0.07 −0.03 1.94 ±0.10 ±0.11+0.10 −0.14 3.64 ±0.16 ±0.15+0.13 −0.40 7.67 ±0.48 ±0.40+0 −1.42 14.04 ±2.21 ±1.79+0 −3.28 4–8 0.15 ±0.01 ±0.01+0.01 −01.01 ±0.04 ±0.06+0.06 −0.03 1.95 ±0.08 ±0.10+0.09 −0.12 4.13 ±0.12 ±0.22+0.12 −0.35 9.36 ±0.39 ±0.45+0 −1.49 16.94 ±1.60 ±1.45+0 −3.15 8–12 0.14 ±0.02 ±0.01+0.01 −00.69 ±0.07 ±0.05+0.04 −0.02 2.41 ±0.15 ±0.15+0.11 −0.15 4.27 ±0.25 ±0.26+0.14 −0.42 8.74 ±0.72 ±0.57+0 −1.48 — 12–20 — — 2.14 ±0.30 ±0.26+0.07 −0.10 4.15 ±0.51 ±0.43+0.09 −0.28 11.60 ±1.59 ±1.18+0 −1.31 — Table 3. Average of D0, D+and D∗+mesons relative yields for the sum of particle and antiparticle in several multiplicity and pTintervals for pp collisions at √s= 7 TeV as a function of the relative charged-particle multiplicity at central rapidity. The values are reported together with their uncertainties, which are quoted in the the order: statistical, systematic and feed-down contribution uncertainties. The yields reported here are not corrected by the trigger selection efficiency, they are normalised to the visible cross section. (dNch/dη)hdNch/dηi 0.45+0.03 −0.03 1.18+0.07 −0.07 1.78+0.10 −0.11 2.63+0.15 −0.17 4.01+0.23 −0.25 6.11+0.35 −0.39 pT(GeV/c) (d2N/dydpT)hd2N/dydpTi 1–2 0.45 ±0.11 ±0.05+0.09 −01.11 ±0.25 ±0.13+0.17 −0.08 1.70 ±0.43 ±0.18+0.19 −0.26 4.24 ±0.78 ±0.47+0.32 −0.96 6.27 ±1.87 ±0.73+0 −2.37 — 2–4 0.21 ±0.02 ±0.01+0.02 −01.08 ±0.06 ±0.06+0.08 −0.04 2.28 ±0.12 ±0.13+0.12 −0.16 4.28 ±0.19 ±0.17+0.16 −0.48 9.02 ±0.57 ±0.47+0 −1.67 16.51 ±2.60 ±2.11+0 −3.86 4–8 0.18 ±0.01 ±0.01+0.01 −01.18 ±0.05 ±0.07+0.07 −0.04 2.30 ±0.09 ±0.12+0.11 −0.14 4.85 ±0.15 ±0.26+0.14 −0.42 11.02 ±0.46 ±0.53+0 −1.75 19.92 ±1.89 ±1.71+0 −3.71 8–12 0.16 ±0.02 ±0.01+0.01 −0.00 0.81 ±0.08 ±0.06+0.05 −0.03 2.84 ±0.17 ±0.18+0.13 −0.17 5.02 ±0.29 ±0.31+0.17 −0.50 10.28 ±0.85 ±0.67+0 −1.74 — 12–20 — — 2.52 ±0.35 ±0.30+0.09 −0.11 4.88 ±0.61 ±0.51+0.11 −0.33 13.65 ±1.87 ±1.38+0 −1.54 — Table 4. Average of D0, D+and D∗+mesons relative yields for the sum of particle and antiparticle in several multiplicity and pTintervals for pp collisions at √s= 7 TeV as a function of the relative charged-particle multiplicity at central rapidity. The values are reported together with their uncertainties, which are quoted in the the order: statistical, systematic and feed-down contribution uncertainties. The yields reported here are per inelastic event. – 32 – JHEP09(2015)148 NV0hNV0i 0.43 1.0 1.5 2.2 3.3 pT(GeV/c) (d2N/dydpT)hd2N/dydpTi×trigger 2–4 0.15 ±0.02 ±0.01+0.02 −00.65 ±0.07 ±0.03+0.06 −0.03 1.28 ±0.11 ±0.07+0.09 −0.12 2.81 ±0.15 ±0.11+0.13 −0.39 4.22 ±0.33 ±0.24+0 −0.99 4–8 0.17 ±0.02 ±0.01+0.02 −00.80 ±0.07 ±0.04+0.07 −0.04 1.50 ±0.12 ±0.08+0.10 −0.13 2.57 ±0.15 ±0.14+0.11 −0.34 4.53 ±0.34 ±0.24+0 −1.00 Table 5. D0meson relative yields for the sum of particle and antiparticle in several multiplicity and pTintervals for pp collisions at √s= 7 TeV as a function of the relative average multiplicity in the V0 detector, NV0hNV0i. The yields reported here are not corrected by the trigger selection efficiency, they are normalised to the visible cross section. NV0hNV0i 0.43 1.0 1.5 2.2 3.3 pT(GeV/c) (d2N/dydpT)hd2N/dydpTi 2–4 0.18 ±0.02 ±0.01+0.02 −00.76 ±0.08 ±0.04+0.07 −0.04 1.50 ±0.13 ±0.08+0.11 −0.14 3.31 ±0.18 ±0.13+0.15 −0.46 4.96 ±0.38 ±0.28+0 −1.16 4–8 0.20 ±0.02 ±0.01+0.02 −00.94 ±0.09 ±0.05+0.08 −0.04 1.76 ±0.14 ±0.09+0.12 −0.16 3.02 ±0.18 ±0.16+0.13 −0.40 5.33 ±0.40 ±0.29+0 −1.18 Table 6. D0meson relative yields for the sum of particle and antiparticle in several multiplicity and pTintervals for pp collisions at √s= 7 TeV as a function of the relative average multiplicity in the V0 detector, NV0hNV0i. The yields reported here are normalised to the inelastic cross section. – 33 – JHEP09(2015)148 Ntracklets fB(%) fextr B(%) (dNprompt J/ψ /dy)hdNprompt J/ψ /dyi×trigger (dNnon−prompt J/ψ /dy)hdNnon−prompt J/ψ /dyi×trigger [4,8] 10.1±7.8±2.5 10.2±7.9±2.5 0.37 ±0.07 ±0.01 0.24 ±0.20+0.05 −0.04 [9,13] 20.8±6.9±2.7 20.9±6.9±2.7 0.80 ±0.14 ±0.04 1.20 ±0.39+0.19 −0.14 [14,19] 15.6±6.3±2.0 15.7±6.3±2.0 1.95 ±0.31 ±0.24 2.06 ±0.84+0.37 −0.35 [20,30] 16.7±6.7±3.3 16.8±6.7±3.3 2.61 ±0.46 ±0.27 2.99 ±1.23+0.51 −0.47 [31,49] 19.0±11.9±4.2 19.0±12.0±4.2 6.50 ±1.50 ±0.31 8.70 ±5.75+1.41 −1.24 Table 7. Fraction of non-prompt J/ψ measured for pT>1.3 GeV/c,fB(%), and extrapolated down to pT>0, fextr B(%) in the various Ntracklets intervals. Prompt and non-prompt J/ψ relative yields for pT>0 are also reported in the different multiplicity intervals. The first and second uncertainties correspond to the statistical and systematic uncertainties, respectively. The yields reported here are not corrected by the trigger selection efficiency, they are normalised to the visible cross section. (dNch/dη)hdNch/dηi(dNprompt J/ψ /dy)hdNprompt J/ψ /dyi(dNnon−prompt J/ψ /dy)hdNnon−prompt J/ψ /dyi 0.63+0.4 −0.40.44 ±0.08 ±0.01 0.28 ±0.23+0.06 −0.05 1.18+0.07 −0.07 0.94 ±0.17 ±0.05 1.41 ±0.46+0.22 −0.17 1.78+0.10 −0.11 2.29 ±0.36 ±0.28 2.42 ±0.99+0.44 −0.41 2.63+0.15 −0.17 3.07 ±0.54 ±0.32 3.52 ±1.45+0.60 −0.55 4.01+0.23 −0.25 7.65 ±1.76 ±0.36 10.24 ±6.76+1.66 −1.46 Table 8. Prompt and non-prompt J/ψ relative yields for pT>0 in different multiplicity bins. The first and second uncertainties correspond to the statistical and systematic uncertainties respectively. The yields reported here are normalised to the inelastic cross section. Open Access. 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Borel15 , A. Borissov96 , M. Borri82 , F. Boss´u65 , M. Botje81 , E. Botta27 , S. B¨ottger52 , P. Braun-Munzinger97 , M. Bregant119 , T. Breitner52 , T.A. Broker53 , T.A. Browning95 , M. Broz40 , E.J. Brucken46 , E. Bruna111 , G.E. Bruno33 , D. Budnikov99 , H. Buesching53 , S. Bufalino36 ,111 , P. Buncic36 , O. Busch93 , Z. Buthelezi65 , J.T. Buxton20 , D. Caffarri36 ,30 , X. Cai7, H. Caines136 , L. Calero Diaz72 , A. Caliva57 , E. Calvo Villar103 , P. Camerini26 , F. Carena36 , W. Carena36 , J. Castillo Castellanos15 , A.J. Castro124 , E.A.R. Casula25 , C. Cavicchioli36 , C. Ceballos Sanchez9, J. Cepila40 , P. Cerello111 , B. Chang122 , S. Chapeland36 , M. Chartier123 , J.L. Charvet15 , S. Chattopadhyay131 , S. Chattopadhyay101 , V. Chelnokov3, M. Cherney86 , C. Cheshkov129 , B. Cheynis129 , V. Chibante Barroso36 , D.D. Chinellato120 , P. Chochula36 , K. Choi96 , M. Chojnacki80 , S. Choudhury131 , P. Christakoglou81 , C.H. Christensen80 , P. Christiansen34 , T. Chujo127 , S.U. Chung96 , C. Cicalo106 , L. Cifarelli12 ,28 , F. Cindolo105 , J. Cleymans89 , F. Colamaria33 , D. Colella33 , A. Collu25 , M. Colocci28 , G. Conesa Balbastre71 , Z. Conesa del Valle51 , M.E. Connors136 , J.G. Contreras11 ,40 , T.M. Cormier84 , Y. Corrales Morales27 , I. Cort´es Maldonado2, P. Cortese32 , M.R. Cosentino119 , F. Costa36 , P. Crochet70 , R. Cruz Albino11 , E. Cuautle63 , L. Cunqueiro36 , T. Dahms92 ,37 , A. Dainese108 , A. Danu62 , D. Das101 , I. Das51 ,101 , S. Das4, A. Dash120 , S. Dash48 , S. De131 ,119 , A. De Caro31 ,12 , G. de Cataldo104 , J. de Cuveland43 , A. De Falco25 , D. De Gruttola12 ,31 , N. De Marco111 , S. De Pasquale31 , A. Deloff77 , E. D´enes135 , G. D’Erasmo33 , D. Di Bari33 , A. Di Mauro36 , P. Di Nezza72 , M.A. Diaz Corchero10 , T. Dietel89 , P. Dillenseger53 , R. Divi`a36 , Ø. Djuvsland18 , A. Dobrin57 ,81 , T. Dobrowolski77 ,i, D. Domenicis Gimenez119 , B. D¨onigus53 , O. Dordic22 , A.K. Dubey131 , A. Dubla57 , L. Ducroux129 , P. Dupieux70 , R.J. Ehlers136 , D. Elia104 , H. Engel52 , B. Erazmus113 ,36 , D. Eschweiler43 , B. Espagnon51 , M. Estienne113 , S. Esumi127 , D. Evans102 , S. Evdokimov112 , G. Eyyubova40 , L. Fabbietti37 ,92 , D. Fabris108 , J. Faivre71 , A. Fantoni72 , M. Fasel74 , L. Feldkamp54 , D. Felea62 , A. Feliciello111 , G. Feofilov130 , J. Ferencei83 , A. Fern´andez T´ellez2, E.G. Ferreiro17 , A. Ferretti27 , A. Festanti30 , J. Figiel116 , M.A.S. Figueredo123 , S. Filchagin99 , D. Finogeev56 , F.M. Fionda104 , E.M. Fiore33 , M.G. Fleck93 , M. Floris36 , S. Foertsch65 , P. Foka97 , S. Fokin100 , E. Fragiacomo110 , – 39 –