Study of the rare B0s and B0s decays into the π+π−μ+μ−final state

Physics Letters B 743 (2015) 46–55 Contents lists available at ScienceDirect Physics Letters B www.elsevier.com/locate/physletb Study of the rare B0 sand B0decays into the π+π−μ+μ−final state .LHCb Collaboration a r t i c l e i n f o a b s t r a c t Article history: Received 19 December 2014 Received in revised form 3 February 2015 Accepted 5 February 2015 Available online 11 February 2015 Editor: W.-D. Schlatter A search for the rare decays B0 s→π+π−μ+μ−and B0→π+π−μ+μ−is performed in a data set corresponding to an integrated luminosity of 3.0fb −1collected by the LHCb detector in proton– proton collisions at centre-of-mass energies of 7 and 8TeV. Decay candidates with pion pairs that have invariant mass in the range 0.5–1.3GeV/c2and with muon pairs that do not originate from a resonance are considered. The first observation of the decay B0 s→π+π−μ+μ−and the first evidence of the decay B0→π+π−μ+μ−are obtained and the branching fractions, restricted to the dipion-mass range considered, are measured to be B(B0 s→π+π−μ+μ−) =(8.6 ±1.5(stat) ±0.7(syst) ±0.7 (norm)) × 10−8and B(B0→π+π−μ+μ−) =(2.11 ±0.51 (stat)±0.15 (syst)±0.16 (norm)) ×10−8, where the third uncertainty is due to the branching fraction of the decay B0→J/ψ(→μ+μ−)K∗(892)0(→K+π−), used as a normalisation. ©2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. 1. Introduction Decays of the B0 sand B0mesons into a π+π−μ+μ−final state with the muons not originating from a resonance are flavour-changing neutral-current transitions,1which are expected to proceed mainly from the B0 s→f0(980)(→π+π−)μ+μ−and B0→ρ(770)0(→π+π−)μ+μ−decays, in analogy to what is observed in B0 (s)→J/ψπ+π−decays [1,2]. In the standard model (SM) these decays are governed by the b→sand b→dweak transitions and are described by loop diagrams. They are suppressed due to the Glashow–Iliopoulos–Maiani mechanism [3] and the small values of the Cabibbo–Kobayashi–Maskawa matrix elements involved [4,5]. This feature makes the B0 s→f0(980)μ+μ− and B0→ρ(770)0μ+μ−decays sensitive probes of several SM extensions, since potential non-SM amplitudes may dominate over the SM contribution [6–10]. Current SM predictions of the B0 s→ f0(980)μ+μ−branching fraction vary from 10−7to 10−9[11–13]; similar values are expected for the B0→ρ(770)0μ+μ−branching fraction [14–16]. The predictions suffer from uncertainties in the calculation of the hadronic matrix elements associated with the transitions. For the B0 s→f0(980)μ+μ−decay, the limited knowledge of the quark content of the f0(980)meson results in additional uncertainties. No experimental information exists on these decays to date. 1The inclusion of charge-conjugate processes is implied throughout. In this Letter, a search for the B0 (s)→π+π−μ+μ−decays is reported. The analysis is restricted to events with muons that do not originate from φ, J/ψ, and ψ(2S)resonances, and with pion pairs with invariant mass in the range 0.5–1.3GeV/c2. This mass range is set to include both f0(980)and ρ(770)0resonances, which overlap because of their large widths [17]. Other resonances, as well as non-resonant pions, might contribute [1,2]. However, due to the limited size of the data sample, an amplitude analysis of the π+π−mass spectrum is not attempted. The analysis is performed in a data set corresponding to an integrated luminosity of 3.0fb −1, collected by the LHCb detector in proton– proton (pp) collisions. The first 1.0fb −1of data was collected in 2011 with collisions at the centre-of-mass energy of 7TeV; the remaining 2.0fb −1in 2012 at 8TeV. The signal yields are obtained from a fit to the unbinned π+π−μ+μ−mass distribution of the decay candidates. The fit modelling and the methods for the background estimation are validated on data, by fitting the π+π−μ+μ−mass distribution of B0 (s)→J/ψ π+π−decays, while the branching fractions of B0 (s)→π+π−μ+μ−decays are normalised using B0→J/ψ K∗(892)0decays reconstructed in the same data set. 2. Detector and simulation The LHCb detector [18] is a single-arm forward spectrometer covering the pseudorapidity range 2 <η<5, designed for the study of particles containing bor cquarks. The detector includes a high-precision tracking system consisting of a silicon-strip vertex http://dx.doi.org/10.1016/j.physletb.2015.02.010 0370-2693/©2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3. LHCb Collaboration / Physics Letters B 743 (2015) 46–55 47 detector surrounding the pp interaction region [19], a large-area silicon-strip detector located upstream of a dipole magnet with a bending power of about 4Tm, and three stations of siliconstrip detectors and straw drift tubes [20] placed downstream of the magnet. The tracking system provides a measurement of momentum with a relative uncertainty that varies from 0.4% at low momentum to 0.6% at 100 GeV/c. The minimum distance of a track to a primary vertex (PV), the impact parameter (IP), is measured with a resolution of 20 μm for charged particles with high transverse momentum (pT). Different types of charged hadrons are distinguished using information from two ring-imaging Cherenkov detectors (RICH) [21]. Photon, electron and hadron candidates are identified by a calorimeter system consisting of scintillating-pad and preshower detectors, an electromagnetic calorimeter and a hadronic calorimeter. Muons are identified by a system composed of alternating layers of iron and multiwire proportional chambers [22]. Samples of simulated events are used to determine the efficiency of selecting B0 (s)→π+π−μ+μ−and B0→J/ψ K∗(892)0 decays, and to study backgrounds. In the simulation, pp collisions are generated using Pythia [23,24] with a specific LHCb configuration [25]. Decays of hadronic particles are described by EvtGen [26], in which final-state radiation is generated using Photos [27]. The model of Refs. [12,28,29] is used to describe B0 (s)→ π+π−μ+μ−decays. The interaction of the generated particles with the detector and its response are implemented using the Geant4 toolkit [30,31] as described in Ref. [32]. 3. Event selection The online event-selection (trigger) consists of a hardware stage, based on information from the calorimeter and muon systems, followed by a software stage, which applies a full event reconstruction [33]. For this analysis, the hardware trigger requires at least one muon with pT>1.48 (1.76)GeV/c, or two muons with √pT(μ1)pT(μ2)>1.3(1.6)GeV/c, in the 2011 (2012) data sample. In the software trigger, at least one of the final-state particles is required to have pT>1GeV/cand IP >100 μm with respect to all the primary pp interaction vertices in the event. Finally, the tracks of two or more final-state particles are required to form a vertex that is significantly displaced from the PVs. A multivariate algorithm is used to identify secondary vertices consistent with the decay of a bhadron [34]. In the offline selection, all charged particles are required to have pT>0.25 GeV/cand trajectories not consistent with originating from the PVs. Two oppositely charged muon candidates compatible with originating from the same displaced vertex are considered. To reject φ→μ+μ−, J/ψ →μ+μ−, and ψ(2S)→μ+μ−decays, candidates having invariant mass in the ranges 1.010–1.030, 2.796–3.216, or 3.436–3.806 GeV/c2are removed; contributions from other resonances in the μ+μ−mass spectrum such as ρ(770)0, ω(782), and ψ(4160)[35] are negligible. The muon candidates are combined with a pair of oppositely charged pions with invariant mass in the range 0.5–1.3GeV/c2 to form B0 (s)→π+π−μ+μ−candidates. For the B0→J/ψ(→ μ+μ−)K∗(892)0(→K+π−)candidates, the dimuon invariant mass is required to be in the range 2.796–3.216 GeV/c2, and the invariant mass of the pion and kaon system in the range 0.826–0.966 GeV/c2. The four tracks are required to originate from the same B0 (s)decay vertex. The B0 (s)momentum vector is required to be within 14 mrad of the vector that joins the PV with the B0 (s) decay vertex (flight distance vector). The information from the RICH, the calorimeters, and the muon systems is used for particle identification (PID), i.e., to define a likelihood for each track to be associated with a certain particle hypothesis. Requirements on the muon-identification likelihood are applied to reduce to O(10−2)the rate of misidentified muon candidates, mainly pions, whilst preserving 95% signal efficiency. In the case of B0→J/ψ K∗(892)0decays, PID requirements on kaon candidates are applied to suppress any contributions from B0 (s)→J/ψ π+π−decays with pions misidentified as kaons. In the case of B0 (s)→π+π−μ+μ−decays, a requirement on the PID of pion candidates is applied to reduce the contamination from B0→K∗(892)0(→K+π−)μ+μ−decays with kaons misidentified as pions; this background peaks around 5.25 GeV/c2 in the π+π−μ+μ−mass spectrum. A large data set of B0→ J/ψ π+π−decays is used to optimise the PID requirement of pion candidates, assuming that the proportion between misidentified B0→J/ψ K∗(892)0and B0→J/ψ π+π−decays is similar to the proportion between misidentified B0→K∗(892)0μ+μ− and B0→π+π−μ+μ−decays. The requirement retains about 55% of the signal candidates. Simulations show that additional contributions from B0 s→φ(→K+K−)μ+μ−decays with double kaon-pion misidentification are negligible. A requirement on the proton-identification likelihood of pion candidates suppresses the contamination from decays with protons misidentified as pions, with a 95% signal efficiency. After this selection, simulations show that contributions from 0 b→(→pπ−)μ+μ− and 0 b→pπ−μ+μ−decays are negligible, as are contributions from 0 b→(1520)(→pK−)μ+μ−and 0 b→pK−μ+μ−decays, where both the proton and the kaon are misidentified as pions. In addition to the above requirements, a multivariate selection based on a boosted decision tree (BDT) [36,37] is used to suppress the large background from random combinations of tracks (combinatorial background) present in the π+π−μ+μ−sample. The BDT is trained using simulated B0 s→π+π−μ+μ−events to model the signal, and data candidates with π+π−μ+μ−mass in the range 5.5–5.8GeV/c2for the background. The training is performed separately for the 2011 and 2012 data, and using simulations that reproduce the specific operational conditions of each year. The variables used in the BDT are the significance of the displacement from the PV of pion and muon tracks, the fit χ2of the B0 (s)decay vertex, the angle between the B0 (s)momentum vector and the flight distance vector, the pTof the B0 (s)candidate, the sum and the difference of the transverse momenta of pions, the difference of the transverse momenta of muons, the B0 (s)decay time, and the minimum pTof the pions. The resulting BDT output is independent of the π+π−μ+μ−mass and PID variables. Arequirement on the BDT output value is chosen to maximise the figure of merit ε/(α/2 +√Nb)[38], where εis the signal efficiency; Nbis the number of background events that pass the selection and have a mass within 30 MeV/c2of the known value of the B0 smass [17]; αrepresents the desired significance of the signal, expressed in terms of number of standard deviations. The value of αis set to 3 (5) for the 2011 (2012) data set. The resulting selection has around 85% efficiency to select signal candidates. The same BDT is used to select B0→J/ψ K∗(892)0candidates. The selected samples consist of 364 B0 (s)→π+π−μ+μ−candidates and 52 960 B0→J/ψ K∗(892)0candidates. The efficiencies of all selection requirements are estimated with simulations, except for the efficiency of the PID selection for hadrons. The latter is determined in data using large and low-background samples of D∗+ →D0(→K−π+)π+decays; the efficiencies are evaluated after reweighting the calibration samples to match simultaneously the momentum and pseudorapidity distributions of the final-state particles of B0 (s)→π+π−μ+μ− 48 LHCb Collaboration / Physics Letters B 743 (2015) 46–55 Table 1 Selection efficiencies of the 2011 and 2012 data sets; εsfor the B0 s→π+π−μ+μ− decay, εdfor the B0→π+π−μ+μ−decay, and εnfor the B0→J/ψ K∗(892)0 decay. 2011 2012 εs[%] 36.1±0.3(stat)±2.4(syst)36.9±0.3(stat)±2.3(syst) εd[%] 29.8±0.2(stat)±2.0(syst)27.5±0.2(stat)±1.7(syst) εn[%] 9.33 ±0.05 (stat)±0.35 (syst)9.74 ±0.08 (stat)±0.27 (syst) (B0→J/ψ K∗(892)0) candidates, and the distribution of the track multiplicity of the events. The final selection efficiencies for 2011 and 2012 data are reported in Table 1. The statistical uncertainties are due to the size of the calibration and simulation samples; systematic uncertainties are described in what follows. The total efficiency varies by approximately 15% in the π+π−mass range considered and it is parametrised with a second-order polynomial. The signal candidates are weighted in order to have a constant efficiency as a function of the π+π−mass spectrum. Systematic uncertainties of the efficiencies are dominated by the limited information about the signal decay-models; the main contribution comes from the unknown angular distributions of B0 (s)→π+π−μ+μ−decay products. To estimate this uncertainty, the difference in efficiencies between decays generated according to a phase-space model and to the model of Refs. [12,28,29] is considered. The resulting relative uncertainty is 5.4%. A relative uncertainty of 3.7% (2.8%) for 2011 (2012) data is estimated by considering the difference of the efficiencies evaluated in the simulation and in data for B0→J/ψ K∗(892)0decays. The same relative uncertainty is assigned to the efficiency associated with B0 (s)→π+π−μ+μ−decays, as the cancellation of this uncertainty in the ratio of the efficiencies of signal and normalisation decays may not be exact. This is due to the fact that the pTdistributions of the final-state particles are different between the decay modes. An additional 1.6% relative uncertainty is assigned to εs, due to the unknown mixture of B0 smass eigenstates in B0 s→π+π−μ+μ− decays, which results in a B0 seffective lifetime that could differ from the value used in the simulations [39]. 4. Determination of the signal yields The ratio of the branching fractions Rq≡ B(B0 (s)→π+π−μ+μ−) B(B0→J/ψ(→μ+μ−)K∗(892)0(→K+π−)) , with q =s(d) for B0 s→π+π−μ+μ−(B0→π+π−μ+μ−) decays, is the quantity being measured; it is used to express the observed yields of B0 (s)→π+π−μ+μ−decays as follows: NBq=fq fd εq εn NnRq,(1) where Nnis the B0→J/ψ K∗(892)0yield, fs/fdis the ratio of the fragmentation probabilities for B0 sand B0mesons [40], εqis the selection efficiency of B0 s→π+π−μ+μ−(B0→π+π−μ+μ−) decays, and εnthe one of B0→J/ψ K∗(892)0decays. The number of events Nnin Eq. (1) is obtained from an extended maximum likelihood fit to the unbinned μ+μ−K+π− mass distribution of the B0→J/ψ K∗(892)0candidates in the range 4.97–5.77 GeV/c2. The μ+μ−K+π−mass distribution is shown in Fig. 1 with fit projections overlaid. A sum of two Gaussian functions, with a power-law tail on either side derived from simulations, is used to describe the dominant B0→J/ψ K∗(892)0 peak and the small B0 s→J/ψ K∗(892)0contribution. All function Fig. 1. Mass distribution of B0→J/ψ K∗(892)0candidates with fit projections overlaid. The 2011 and 2012 data sets are combined. parameters are in common between the B0and B0 ssignal functions, except for the mass; the mass difference between B0 sand B0mesons is fixed to the known value [17]. An exponential function is used to model the combinatorial background. A small contamination of B+→J/ψ K+decays combined with an additional charged pion is modelled with an ARGUS function [41]. Partially reconstructed B0decays at masses lower than the B0signal are described with another ARGUS function. The fitted yields of B0→ J/ψ K∗(892)0decays are corrected by subtracting a (6.4 ±1.0)% contribution of B0→J/ψ K+π−decays [42], where the K+π− pair is in a S-wave state and does not originate from the decay of a K∗(892)0resonance. The numbers of B0→J/ψ K∗(892)0 decays are 9821 ±110 (stat) ±134 (syst) ±97 (Swave)and 23 521 ±175 (stat) ±172 (syst) ±243 (Swave)in the 2011 and 2012 data sets, respectively, where the third uncertainty is due to the S-wave subtraction. The systematic uncertainty accounts for the uncertainties in the parameters fixed in the fit to the values determined in simulations, and are calculated with the method described at the end of this section. The ratios Rsand Rdare measured from an extended maximum likelihood fit to the unbinned π+π−μ+μ−mass distribution, where the signal yields are parametrised using Eq. (1), and all other inputs are fixed. The different centre-of-mass energies result in different b¯ bproduction cross sections and selection efficiencies in the 2011 and 2012 data samples. Therefore, the two samples are fitted simultaneously with different likelihood functions, but with the parameters Rsand Rdin common. We also fit simultaneously the B0 (s)→π+π−μ+μ− and B0 (s)→J/ψ π+π−samples. The latter are selected with the B0 (s)→π+π−μ+μ−requirements, except for the dimuon mass, which is restricted to the 2.796–3.216 GeV/c2range. The B0 (s)→ J/ψ π+π−fit serves as a consistency check of the fit modelling, since the B0 (s)→π+π−μ+μ−and B0 (s)→J/ψ π+π−mass distributions are expected to be similar. In both samples, the fit range is 2GeV/c2wide and starts from 5.19 GeV/c2. This limit is set to remove partially reconstructed decays of the B0mesons with an unreconstructed π0. The stability of the fit results is checked against the extension of the fit range in the lower mass region of the B0 (s)→π+π−μ+μ−and B0 (s)→J/ψ π+π−mass distributions, where an additional component is needed in the fit to describe the partially reconstructed B0decays below 5.19 GeV/c2. LHCb Collaboration / Physics Letters B 743 (2015) 46–55 49 Fig. 2. Mass distributions of (a) the B0 (s)→J/ψ π+π−and (b) the B0 (s)→π+π−μ+μ−decay candidates in the range 5.19–5.99 GeV/c2with fit projections overlaid. The 2011 and 2012 data sets are combined. In (b), the contribution from B0 s→φμ+μ−and B+→K+μ+μ−decays are included in the fit, but they are not visible in the projection, because the corresponding yields are small. Fig. 2 shows the π+π−μ+μ−mass distributions of the B0 (s)→ J/ψ π+π−and B0 (s)→π+π−μ+μ−decay candidates in the range 5.19–5.99 GeV/c2with fit projections overlaid, where the 2011 and 2012 data sets are combined. The B0 (s)→π+π−μ+μ−and B0 (s)→J/ψ π+π−signals are described by a model similar to that used for the B0→J/ψ K∗(892)0 signal in the fit of the μ+μ−K+π−mass distribution. The B0 peak position is a common parameter for the B0 (s)→π+π−μ+μ− and B0 (s)→J/ψ π+π−fits, as well as the signal resolutions; the difference between the B0and the B0 smasses is fixed to the known value. The B0 (s)→π+π−μ+μ−signal widths are multiplied by scale factors, derived from simulations, which accounts for the different momentum spectra between non-resonant muons and muons from J/ψ meson decays. In both fits, the combinatorial background is modelled with an exponential function. Backgrounds from B0→K∗(892)0μ+μ−(B0→J/ψ K∗(892)0) decays, where kaons are misidentified as pions, are estimated using control samples of these decays reconstructed in data. They are selected as B0 (s)→π+π−μ+μ−(B0 (s)→J/ψ π+π−) candidates, except for different requirements on the PID variables of the kaon and pion candidates, as for the normalisation decay mode. To obtain the yields and the shapes of the mass distribution of the misidentified decays, the kaon candidates are assigned the pion mass, and the resulting π+π−μ+μ−mass distribution is reweighted to reproduce the PID selection of the B0 (s)→π+π−μ+μ−sample. In the final fit, the yields of the two backgrounds are constrained using Gaussian functions with means fixed to the values obtained with this method, and widths that account for a relative uncertainty in the 2011 (2012) data sample of 15% (10%) for B0→K∗(892)0μ+μ−decays, and of 2% (1%) for B0→J/ψ K∗(892)0decays. The shape of the B0→ K∗(892)0μ+μ−background is modelled with a Gaussian function with a power-law tail on the low-mass side; the shape of the B0→ J/ψ K∗(892)0background is modelled with a sum of two Gaussian functions with different means. All parameters of these functions are fixed from the values obtained in the fit to the control samples. The background from B0 s→J/ψ K∗(892)0decays is expected to be less than 0.5% [17] of the B0→J/ψ K∗(892)0yield and is neglected. Similarly, the background from B0 s→K∗(892)0μ+μ− decays is not considered. Table 2 Summary of systematic uncertainties on Rsand Rd. Source σ(Rs)[10−3]σ(Rd)[10−3] Shape of misidentified decays 0.003 0.004 Partially reconstructed decays 0.003 0.004 Combinatorial background 0.029 0.014 Signal shapes 0.020 0.014 Efficiencies 0.061 0.013 Normalisation decay yields 0.055 0.014 fs/fd0.093 – Quadratic sum 0.130 0.028 Backgrounds from decays B0 s→φ(→π+π−π0)μ+μ−with an unreconstructed π0, B0 s→η(→π+π−γ)μ+μ−with an unreconstructed γ, and B+→K+μ+μ−or B+→π+μ+μ−combined with an additional charged pion, are estimated from simulations. The mass distributions of these backgrounds are modelled with ARGUS functions with parameters fixed from fits to simulated events. Backgrounds from similar decay modes, where the muons come from the J/ψ meson, are described in the B0 (s)→J/ψ π+π−fit using the same methods. An additional contribution is given by B+ c→J/ψπ+π−π+decays, where a pion is not reconstructed. This background is modelled with a sum of two Gaussian functions, one of which has a power-law tail on the low-mass side. Backgrounds from semileptonic B0→D−(→ρ0μ−X)μ+Xdecays with ρ0→π+π−, give a negligible contribution at π+π−μ+μ−mass greater than 5.19 GeV/c2. 5. Results We measure Rs=(1.67 ±0.29 (stat) ±0.13 (syst)) ×10−3 and Rd=(0.41 ±0.10 (stat) ±0.03 (syst)) ×10−3. Systematic uncertainties are discussed below. These values correspond to 55 ± 10 (stat) ±5(syst)B0 s→π+π−μ+μ−decays and 40 ±10 (stat) ± 3(syst)B0→π+π−μ+μ−decays. The significances of the observed signals are calculated using Wilks’ theorem [43], and are 7.2σand 4.8σfor the B0 s→π+π−μ+μ−and B0→π+π−μ+μ− decays, respectively. The B0 s→π+π−μ+μ−(B0→π+π−μ+μ−) significance is obtained by considering the B0→π+π−μ+μ− (B0 s→π+π−μ+μ−) yield as a floating parameter in the fit. The systematic uncertainties are included by multiplying the signif- 50 LHCb Collaboration / Physics Letters B 743 (2015) 46–55 Fig. 3. Background-subtracted distributions of the π+π−invariant mass for (a) B0 s→π+π−μ+μ−and (b) B0→π+π−μ+μ−candidates (triangular markers). The uncertainties are statistical only. The data are compared with the background-subtracted π+π−mass distributions of (a) B0 s→J/ψ π+π−and (b) B0→J/ψ π+π−candidates (histograms). icance by the factor 1/1+(σ(syst)/σ(stat))2, where σ(stat)is the statistical uncertainty, and σ(syst)is the sum in quadrature of the contributions in Table 2, except for the uncertainty on fs/fd. Fig. 3 compares the π+π−mass spectra of B0 (s)→π+π−μ+μ− and B0 (s)→J/ψ π+π−candidates, separately for the B0 sand the B0decays. The background is subtracted using the sPlot technique [44] with the π+π−μ+μ−mass as the discriminating variable. The data show the dominance of the f0(980)resonance in the case of B0 s→J/ψ π+π−decays, and of the ρ(770)0resonance in the case of B0→J/ψ π+π−decays, as expected from previous LHCb analyses [1,2]. The B0 (s)→π+π−μ+μ−data show indications of a similar composition of the π+π−mass spectrum, although the size of the sample is not sufficient to draw a definite conclusion. Several systematic uncertainties on Rsand Rdare considered, as summarised in Table 2. The contribution due to the uncertainties on parameters that are fixed in the fit, and on the efficiencies and the yields of B0→J/ψ K∗(892)0decays that are fixed in Eq. (1), is obtained by repeating the fit, each time with the relevant parameters or inputs fixed to alternate values. These are sampled from Gaussian distributions centred at the nominal value, and whose widths correspond to the uncertainties on the fixed parameters and inputs. Known correlations between fixed parameters are taken into account. The r.m.s. spreads of the resulting Rsand Rdvalues are taken as the systematic uncertainties. The uncertainties associated with efficiencies are the sums in quadrature of their statistical and systematic uncertainties, reported in Table 1. The uncertainty on the B0→J/ψ K∗(892)0yield is the sum in quadrature of the statistical uncertainty, the systematic uncertainty, and the uncertainty due to the S-wave subtraction. Asystematic uncertainty is assigned on the estimation of the combinatorial background with the following method; pseudo experiments are generate in an extended mass range from 4.97 GeV/c2, where an additional peaking component is also added to simulate the partially reconstructed B0decays, and the pseudo data are fitted in the nominal range from 5.19 GeV/c2. The shifts between the average fitted values and the input values of Rsand Rdare taken as the systematic uncertainties. The contribution to the systematic uncertainty of Rsdue to the uncertainty on the values of fs/fdis also included. The final systematic uncertainties are the sums in quadrature of all contributions and correspond to 45% and 28% of the statistical uncertainties of Rsand Rd, respectively. 6. Conclusions The first observation of the decay B0 s→π+π−μ+μ−and the first evidence of the decay B0→π+π−μ+μ−are obtained in a data set corresponding to an integrated luminosity of 3.0fb −1collected by the LHCb detector in pp collisions at centre-of-mass energies of 7 and 8TeV. The analysis is restricted to candidates with muon pairs that do not originate from φ, J/ψ, and ψ(2S)resonances, while the pion pairs are required to have invariant mass in the range 0.5–1.3GeV/c2. About 55 B0 s→π+π−μ+μ−decays and 40 B0→π+π−μ+μ−decays are observed with significances of 7.2σand 4.8σ, respectively. Their branching fractions relative to the branching fraction of the B0→J/ψ(→μ+μ−)K∗(892)0(→K+π−)decay are measured to be B(B0 s→π+π−μ+μ−) B(B0→J/ψ(→μ+μ−)K∗(892)0(→K+π−)) =(1.67 ±0.29 (stat)±0.13 (syst)) ×10−3, B(B0→π+π−μ+μ−) B(B0→J/ψ(→μ+μ−)K∗(892)0(→K+π−)) =(0.41 ±0.10 (stat)±0.03 (syst)) ×10−3. From these ratios, the following branching fractions are obtained for the decays with the dipion-mass range considered: B(B0 s→π+π−μ+μ−) =(8.6±1.5(stat)±0.7(syst)±0.7(norm)) ×10−8and B(B0→π+π−μ+μ−) =(2.11 ±0.51 (stat)±0.15 (syst)±0.16 (norm)) ×10−8, where the third uncertainties are due to the uncertainties on the branching fraction of the normalisation decay. We use B(B0→ J/ψ K∗(892)0) =(1.30 ±0.10) ×10−3, which is the weighted average of measurements where the K+π−S-wave contribution is subtracted [45–47], B(J/ψ →μ+μ−)from Ref. [17], and B(K∗(892)0→K+π−)=2/3. LHCb Collaboration / Physics Letters B 743 (2015) 46–55 51 Assuming that the decays f0(980)→π+π−and ρ(770)0→ π+π−are the dominant transitions in the B0 s→π+π−μ+μ− and B0→π+π−μ+μ−decays, respectively, and neglecting other contributions, the B0 (s)→π+π−μ+μ−branching fractions are corrected to account for the selection efficiencies of the f0(980)and ρ(770)0resonances in the π+π−mass range considered. The following values are obtained: B(B0 s→f0(980)(→π+π−)μ+μ−) = (8.3±1.7)×10−8and B(B0→ρ(770)0μ+μ−) =(1.98 ±0.53) × 10−8, where all uncertainties are summed in quadrature. These values favour SM expectations of Refs. [12,14,15] and disfavour the B(B0 s→f0(980)μ+μ−)SM expectation of Ref. [13]. Acknowledgements We express our gratitude to our colleagues in the CERN accelerator departments for the excellent performance of the LHC. We thank the technical and administrative staff at the LHCb institutes. We acknowledge support from CERN and from the national agencies: CAPES, CNPq, FAPERJ and FINEP (Brazil); NSFC (China); CNRS/IN2P3 (France); BMBF, DFG, HGF and MPG (Germany); INFN (Italy); FOM and NWO (The Netherlands); MNiSW and NCN (Poland); MEN/IFA (Romania); MinES and FANO (Russia); MinECo (Spain); SNSF and SER (Switzerland); NASU (Ukraine); STFC (United Kingdom); NSF (USA). 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Schune 7, R. Schwemmer 38, B. Sciascia 18, A. Sciubba 25,m, A. Semennikov 31, I. Sepp 53, N. Serra 40, J. Serrano 6, L. Sestini 22, P. Seyfert 11, M. Shapkin 35, I. Shapoval 16,43,f, Y. Shcheglov 30, T. Shears 52, L. Shekhtman 34,68, V. Shevchenko 64, A. Shires 9, R. Silva Coutinho 48, G. Simi 22, M. Sirendi 47, N. Skidmore 46, I. Skillicorn 51, T. Skwarnicki 59, N.A. Smith 52, E. Smith 55,49, E. Smith 53, J. Smith 47, M. Smith 54, H. Snoek 41, M.D. Sokoloff 57, F.J.P. Soler 51, F. Soomro 39, D. Souza 46, B. Souza De Paula 2, B. Spaan 9, P. Spradlin 51, S. Sridharan 38, F. Stagni 38, M. Stahl 11, S. Stahl 11, O. Steinkamp 40, O. Stenyakin 35, F. Sterpka 59, S. Stevenson 55, S. Stoica 29, S. Stone 59, B. Storaci 40, S. Stracka 23,t, M. Straticiuc 29, U. Straumann 40, R. Stroili 22, L. Sun 57, W. Sutcliffe 53, K. Swientek 27, S. Swientek 9, V. Syropoulos 42, M. Szczekowski 28, P. Szczypka 39,38, T. Szumlak 27, S. T’Jampens 4, M. Teklishyn 7, G. Tellarini 16,f, F. Teubert 38, C. Thomas 55, E. Thomas 38, J. van Tilburg 41, V. Tisserand 4, M. Tobin 39, J. Todd 57, S. Tolk 42, L. Tomassetti 16,f, D. Tonelli 38, S. Topp-Joergensen 55, N. Torr 55, E. Tournefier 4, S. Tourneur 39, M.T. Tran 39, M. Tresch 40, A. Trisovic 38, A. Tsaregorodtsev 6, P. Tsopelas 41, N. Tuning 41, M. Ubeda Garcia 38, A. Ukleja28, A. Ustyuzhanin 64, U. Uwer 11, C. Vacca 15,e, V. Vagnoni 14, G. Valenti 14, A. Vallier 7, R. Vazquez Gomez 18, P. Vazquez Regueiro 37, C. Vázquez Sierra 37, S. Vecchi 16, J.J. Velthuis 46, M. Veltri 17,h, G. Veneziano 39, M. Vesterinen 11, J.V.V.B.Viana Barbosa 38, B. Viaud 7, D. Vieira 2, M. Vieites Diaz 37, X. Vilasis-Cardona 36,p, A. Vollhardt 40, D. Volyanskyy 10, D. Voong 46, A. Vorobyev30, V. Vorobyev 34,68, C. Voß 63, J.A. de Vries 41, R. Waldi 63, C. Wallace 48, R. Wallace 12, J. Walsh 23, S. Wandernoth 11, J. Wang 59, D.R. Ward 47, N.K. Watson 45, D. Websdale 53, M. Whitehead 48, D. Wiedner 11, G. Wilkinson 55,38, M. Wilkinson 59, M.P. Williams 45, M. 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Zhong 3 1Centro Brasileiro de Pesquisas Físicas (CBPF), Rio de Janeiro, Brazil 2Universidade Federal do Rio de Janeiro (UFRJ), Rio de Janeiro, Brazil 3Center for High Energy Physics, Tsinghua University, Beijing, China 4LAPP, Université de Savoie, CNRS/IN2P3, Annecy-Le-Vieux, France 5Clermont Université, Université Blaise Pascal, CNRS/IN2P3, LPC, Clermont-Ferrand, France 6CPPM, Aix-Marseille Université, CNRS/IN2P3, Marseille, France 7LAL, Université Paris-Sud, CNRS/IN2P3, Orsay, France 8LPNHE, Université Pierre et Marie Curie, Université Paris Diderot, CNRS/IN2P3, Paris, France 9Fakultät Physik, Technische Universität Dortmund, Dortmund, Germany 10 Max-Planck-Institut für Kernphysik (MPIK), Heidelberg, Germany 11 Physikalisches Institut, Ruprecht-Karls-Universität Heidelberg, Heidelberg, Germany 12 School of Physics, University College Dublin, Dublin, Ireland 13 Sezione INFN di Bari, Bari, Italy 14 Sezione INFN di Bologna, Bologna, Italy 15 Sezione INFN di Cagliari, Cagliari, Italy 16 Sezione INFN di Ferrara, Ferrara, Italy 17 Sezione INFN di Firenze, Firenze, Italy 18 Laboratori Nazionali dell’INFN di Frascati, Frascati, Italy 19 Sezione INFN di Genova, Genova, Italy 20 Sezione INFN di Milano Bicocca, Milano, Italy 21 Sezione INFN di Milano, Milano, Italy 22 Sezione INFN di Padova, Padova, Italy 23 Sezione INFN di Pisa, Pisa, Italy 24 Sezione INFN di Roma Tor Vergata, Roma, Italy 25 Sezione INFN di Roma La Sapienza, Roma, Italy 26 Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences, Kraków, Poland 27 AGH – University of Science and Technology, Faculty of Physics and Applied Computer Science, Kraków, Poland 28 National Center for Nuclear Research (NCBJ), Warsaw, Poland 29 Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania 30 Petersburg Nuclear Physics Institute (PNPI), Gatchina, Russia 31 Institute of Theoretical and Experimental Physics (ITEP), Moscow, Russia 32 Institute of Nuclear Physics, Moscow State University (SINP MSU), Moscow, Russia 33 Institute for Nuclear Research of the Russian Academy of Sciences (INR RAN), Moscow, Russia 34 Budker Institute of Nuclear Physics (SB RAS), Russia 35 Institute for High Energy Physics (IHEP), Protvino, Russia 36 Universitat de Barcelona, Barcelona, Spain 37 Universidad de Santiago de Compostela, Santiago de Compostela, Spain 38 European Organization for Nuclear Research (CERN), Geneva, Switzerland 39 Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland 40 Physik-Institut, Universität Zürich, Zürich, Switzerland 41 Nikhef National Institute for Subatomic Physics, Amsterdam, The Netherlands 42 Nikhef National Institute for Subatomic Physics and VU University Amsterdam, Amsterdam, The Netherlands 43 NSC Kharkiv Institute of Physics and Technology (NSC KIPT), Kharkiv, Ukraine 44 Institute for Nuclear Research of the National Academy of Sciences (KINR), Kyiv, Ukraine 45 University of Birmingham, Birmingham, United Kingdom 46 H.H. Wills Physics Laboratory, University of Bristol, Bristol, United Kingdom 47 Cavendish Laboratory, University of Cambridge, Cambridge, United Kingdom 48 Department of Physics, University of Warwick, Coventry, United Kingdom 49 STFC Rutherford Appleton Laboratory, Didcot, United Kingdom 50 School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom 51 School of Physics and Astronomy, University of Glasgow, Glasgow, United Kingdom 52 Oliver Lodge Laboratory, University of Liverpool, Liverpool, United Kingdom 53 Imperial College London, London, United Kingdom 54 School of Physics and Astronomy, University of Manchester, Manchester, United Kingdom 55 Department of Physics, University of Oxford, Oxford, United Kingdom 56 Massachusetts Institute of Technology, Cambridge, MA, United States 57 University of Cincinnati, Cincinnati, OH, United States 58 University of Maryland, College Park, MD, United States 59 Syracuse University, Syracuse, NY, United States 60 Pontifícia Universidade Católica do Rio de Janeiro (PUC-Rio), Rio de Janeiro, Brazil w 61 Institute of Particle Physics, Central China Normal University, Wuhan, Hubei, China x 62 Departamento de Fisica, Universidad Nacional de Colombia, Bogota, Colombia y 63 Institut für Physik, Universität Rostock, Rostock, Germany z 64 National Research Centre Kurchatov Institute, Moscow, Russia aa 65 Instituto de Fisica Corpuscular (IFIC), Universitat de Valencia-CSIC, Valencia, Spain ab 66 Van Swinderen Institute, University of Groningen, Groningen, The Netherlands ac 67 Celal Bayar University, Manisa, Turkey ad 68 Novosibirsk State University, Novosibirsk, Russia *Corresponding author. E-mail address: mirco.dorig[email protected] (M. Dorigo). aUniversidade Federal do Triângulo Mineiro (UFTM), Uberaba-MG, Brazil. bP.N. Lebedev Physical Institute, Russian Academy of Science (LPI RAS), Moscow, Russia. cUniversità di Bari, Bari, Italy.