Modification of charged-particle jets in event-shape engineered Pb–Pb collisions at √sNN = 5.02 TeV

This is a self-archived version of an original article. This version may differ from the original in pagination and typographic details. Author(s): Title: Year: Version: Copyright: Rights: Rights url: Please cite the original version: CC BY 4.0 https://creativecommons.org/licenses/by/4.0/ Modification of charged-particle jets in event-shape engineered Pb–Pb collisions at √sNN = 5.02 TeV © 2024 The Author(s). Published by Elsevier B.V. Funded by SCOAP³ Published version ALICE Collaboration ALICE Collaboration. (2024). Modification of charged-particle jets in event-shape engineered Pb–Pb collisions at √sNN = 5.02 TeV. Physics Letters B, 851, Article 138584. https://doi.org/10.1016/j.physletb.2024.138584 2024 Phys. Lett. B 851 (2024) 138584 Available online 18 March 2024 0370-2693/© 2024 The Author(s). Published by Elsevier B.V. Funded by SCOAP³. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Contents lists available at ScienceDirect Physics Letters B journal homepage: www.elsevier.com/locate/physletb Letter Modification of charged-particle jets in event-shape engineered Pb–Pb collisions at √𝑠NN =5.02 TeV .ALICE Collaboration⋆ A R T I C L E I N F O A B S T R A C T Editor: M. Doser Dataset link: https:// www .hepdata .net /record /ins2681682 Charged-particle jet yields have been measured in semicentral Pb–Pb collisions at center-of-mass energy per nucleon–nucleon collision √𝑠NN =5.02 TeV with the ALICE detector at the LHC. These yields are reported as a function of the jet transverse momentum, and further classified by their angle with respect to the event plane and the event shape, characterized by ellipticity, in an effort to study the path-length dependence of jet quenching. Jets were reconstructed at midrapidity from charged-particle tracks using the anti-𝑘Talgorithm with resolution parameters 𝑅= 0.2 and 0.4, with event-plane angle and event-shape values determined using information from forward scintillating detectors. The results presented in this letter show that, in semicentral Pb–Pb collisions, there is no significant difference between jet yields in predominantly isotropic and elliptical events. However, out-of-plane jets are observed to be more suppressed than in-plane jets. Further, this relative suppression is greater for low transverse momentum (<50 GeV/c) 𝑅= 0.2 jets produced in elliptical events, with out-of-plane to in-plane jet-yield ratios varying up to 5.2𝜎between different event-shape classes. These results agree with previous studies indicating that jets experience azimuthally anisotropic suppression when traversing the QGP medium, and can provide additional constraints on the path-length dependence of jet energy loss. 1. Introduction At very high energy densities, ordinary hadronic matter undergoes a transition to become a strongly interacting state of deconfined quarks and gluons. This new state of matter is referred to as the quark–gluon plasma (QGP) [1]. Calculations using quantum chromodynamics (QCD) on the lattice predict a crossover transition between these phases at a temperature of about 150 MeV that can be reached in the laboratory via ultrarelativistic collisions of heavy ions [2,3]. Experimental studies of heavy-ion collisions thus offer a compelling opportunity to explore the properties of the strongly interacting medium, and form the main physics program of the ALICE experiment at the LHC [4]. Jets, sprays of hadrons resulting from high-transverse-momentum (𝑝T) partons produced in hard-scattering processes, are sensitive to a variety of QGP properties [5–7]. Because jets are produced early in a collision, indeed much earlier than the formation of the QGP at 𝜏QGP ∼0.5 fm/c, they experience its whole evolution. Jets interact with and are modified by this medium as they traverse it, resulting in a collection of effects known as jet quenching. The observation of jet quenching at both RHIC and LHC energies is therefore considered to be a main signature of QGP formation [8–11], and the microscopic mechanism by which jet quenching occurs has been the subject of significant theoretical and experimental investigation. Models predict that partons can lose energy ⋆E-mail address: alice -publications @cern .ch. collisionally and/or radiatively in the weakly-coupled limit, with radiative contributions expected to dominate in the high-𝑝Tregime [12,13]. Moreover, it is predicted that there is a direct relationship between the path-length dependence of parton energy loss and the relative contributions of the different mechanisms. In a static medium, collisional and radiative energy loss are expected to have a linear and quadratic dependence on the length of plasma traversed, respectively [14,15]. Measuring this dependence would therefore offer a direct way to probe the underlying mechanisms of jet–medium interactions, but doing so has so far proven to be challenging. Past measurements, e.g. the dijet asymmetry [16–18], are heavily influenced by fluctuations in jet–medium interactions, making it difficult to extract an underlying path-length dependence [19]. Another such measurement, the jet 𝑣2(the second Fourier coefficient in the azimuthal distribution of jet momenta in the transverse plane), shows a significant azimuthal anisotropy in jet yields in Pb–Pb collisions [20–22]. However, medium fluctuations limit the ability to constrain the underlying physics mechanisms that drive this behavior. Event-shape engineering (ESE) [23], a technique that classifies events according to their anisotropies using the magnitude of the reduced flow vector, offers a new experimental approach to overcome these difficulties and constrain the path-length dependence of jet energy loss [24]. This approach is advantageous in that it allows for the https://doi.org/10.1016/j.physletb.2024.138584 Received 8 September 2023; Received in revised form 27 February 2024; Accepted 13 March 2024 Physics Letters B 851 (2024) 138584 2 ALICE Collaboration selection of events for which the thermodynamic properties are similar, but for which the spatial anisotropies vary significantly. This is done by isolating events within a centrality class that have particularly round or elliptical geometries. Previous measurements have shown that the elliptic flow coefficient 𝑣2of charged particles varies significantly at a fixed collision centrality [25–27]. In addition, the mean 𝑝Tof the particle yields is larger for elliptical events than for isotropic events. Results using ESE in the heavy-flavor sector show similar indications [28,29]. These measurements reveal the sizeable potential that ESE has to connect observables from the soft and hard momentum scales. Combining the precision afforded by jet measurements with the control that ESE provides to constrain the collision geometry, it is possible to learn about this interplay of physics phenomena from high to low 𝑝T[30]. In the analysis presented in this letter, this interplay is studied by considering an event shape in conjunction with the jet angle with respect to the event-plane Ψ2, which is defined by the beam axis and the vector of the collision impact parameter. The distance the jet traverses through the medium when traveling parallel to the event plane (inplane) is, on average, shorter than when it travels perpendicular to the event plane (out-of-plane). As such, the azimuthal anisotropy of the jet spectra provides initial information about the path-length dependence of parton energy loss. By then applying ESE, the relative difference between inand out-of-plane jet path-lengths can be increased or decreased. This is especially true in the case of semicentral collisions, where the system is usually (but not necessarily) elliptical [30]. In semicentral Pb–Pb collisions at √𝑠NN =2.76 TeV, the charged-particle 𝑣2ratio for the 30% most elliptical events compared to the 30% most isotropic events is ∼1.3 [26]. This ratio was approximated by considering the average of the charged-particle 𝑣2values reported in differentiated centrality and ellipticity windows. With this in consideration, comparing inand out-of-plane jet spectra for events with different ellipticities can reduce the contribution of medium shape fluctuations and increase understanding of the path-length dependence of jet energy loss. In this letter, results of event-shape engineered jet yields in 30–50% Pb–Pb collisions at √𝑠NN =5.02 TeV are presented. Jets were reconstructed from charged-particle tracks for resolution parameters 𝑅 =0.2 and 𝑅 =0.4, within a jet transverse-momentum range of 35 <𝑝 T,ch jet < 120 GeV/cand 40 <𝑝 T,ch jet <120 GeV/c, respectively. The in-plane and out-of-plane jet yields are presented according to the ellipticity of the collision system quantified event-by-event, which allows for the exploitation of average differences in jet path length. 2. Experimental setup The ALICE experiment is a general-purpose detector located at the LHC. It is optimized to provide high momentum resolution and excellent particle identification over a broad momentum range, up to the highest multiplicities [31]. The primary ALICE sub-detectors used in this analysis are the Inner Tracking System (ITS), Time Projection Chamber (TPC), and V0 detectors. For more information on the ALICE apparatus and its performance, see Refs. [32,33]. The ITS is a silicon-based tracking detector used for reconstruction of charged tracks and primary vertex identification [34]. It consists of six layers having increasing radii around the nominal collision point. The first two layers are Silicon Pixel Detectors (SPD), followed by two layers of Silicon Drift Detectors (SDD), and two layers of Silicon Strip Detectors (SSD). The TPC is a large cylindrical gaseous detector, covering a pseudorapidity range of |𝜂| <0.9over the full azimuthal angle [35]and providing excellent tracking performance up to high particle multiplicities and momenta. The tracks used for jet reconstruction in this analysis were measured by both the ITS and the TPC, and were accepted for 𝑝T>0.15 GeV/cand pseudorapitidies of |𝜂| <0.9. The tracks have a momentum resolution of 𝜎𝑝T∕𝑝T∼0.8% at 𝑝T=1 GeV/c, which increases to 𝜎𝑝T∕𝑝T∼2%at 𝑝T=10GeV/c[33]. In central Pb–Pb collisions, the tracking efficiency ranges from approximately 65% to 82% for increasing 𝑝T[8]. The V0A and V0C, scintillation detectors located at pseudorapidities 2.8 <𝜂<5.1and −3.7 <𝜂<−1.7, respectively, were used to select the Pb–Pb minimum-bias and semicentral events according to their summed amplitudes [36,37]. In this analysis, the V0C was used for calculating the reduced flow vector 𝑞2, defined in Eq. (1), as it is closer to midrapidity than the V0A. It can therefore produce a wider 𝑞2distribution, thus accessing the best separation between different event shapes. The V0A was used for calculating event-plane angles. Using these forward detectors to measure the event shape and event-plane angle minimizes the autocorrelations between these quantities and the charged-particle jets at midrapidity. Details of the 𝑞2and event-plane angle measurements are given in the next section. 3. Data analysis The results presented in this letter are derived from a sample of Pb–Pb collisions collected by the ALICE experiment during the 2018 LHC heavy-ion run. The data sample considered in this work was recorded with a semicentral trigger based on the V0 signal amplitude, which allowed for the collection of a large sample of Pb–Pb collisions in the 30–50% centrality class [31]. Only events having a primary vertex within ±10 cm of the nominal interaction point along the beam line (𝑧 direction) were accepted. An additional selection criterion was applied to remove pile-up, utilizing the correlation between the number of hits in the ITS and TPC detectors. After applying these criteria, a total of approximately 54 million events were selected for this study. Jets were reconstructed from charged-particle tracks [38]with the FastJet anti-𝑘Talgorithm [39,40]. The 𝑝T-scheme recombination strategy was chosen to combine tracks using their transverse momenta [39,41]. The resolution parameters 𝑅 =0.2and 𝑅 =0.4were studied, where each jet was required to contain a leading track with 5 <𝑝 T<100 GeV/c. The leading track requirement was chosen to reduce contamination from combinatorial jets. The jet axis was required to be within |𝜂jet| <0.9 −𝑅, where 𝜂jet is the pseudorapidity of the jet axis. Furthermore, for each jet the quantity Δ𝜑 =𝜑jet −Ψ 2was calculated. This is the difference in azimuthal angle between the jet axis and the event-plane angle Ψ2, where Ψ2is determined from the V0A signals. The average combinatorial background was subtracted using an area-based technique [38,42,43]. With this method, the background transverse-momentum density per unit area, 𝜌, was determined eventby-event after removing the two leading 𝑘Tjets [44]. The jet energies were corrected for the underlying-event contribution by subtracting the event-averaged density multiplied by the jet area. The residual background fluctuations, together with detector effects, were then corrected on a statistical basis using a 2D Bayesian unfolding procedure [45,46]. The choice to use a 2D procedure was made so as to correct for the differences in background arising from the jet angle with respect to Ψ2, as well as to account for any correlated bin migration in Δ𝜑and 𝑝T,ch jet . This was done using a 4D response matrix constructed from PYTHIA 8 (Monash tune) [47,48]jets transported through the ALICE detector by a GEANT3-based simulation [49]and embedded into real Pb–Pb events. The data was binned in 𝑝T,ch jet and | cos(Δ𝜑)|for both truthand reconstructed-level jets. Before filling the response matrix, 2% of simulated tracks were randomly rejected before jet-finding to account for the worsened tracking efficiency in the high track-density environment of Pb–Pb collisions. This level of degradation was estimated using HIJING simulations of 0–10% central Pb–Pb collisions [50]. The 2D jet distribution was then unfolded using six iterations of the Bayesian procedure, with the PYTHIA 8 distribution used as the prior. After unfolding, corrections were applied to the jet yields for the kinematic and reconstruction efficiencies. Here, the kinematic efficiency refers to the inefficiency introduced by truth-level jets that were reconstructed outside of the measured 𝑝T,ch jet range, thus not entering the unfolding procedure. This was computed for each bin by taking the ratio of the truth-level spectrum reconstructed in the measured range to the truth level-spectrum reconstructed within 10 <𝑝 T,ch jet <200 GeV/c. Physics Letters B 851 (2024) 138584 3 ALICE Collaboration Fig. 1. Distribution of 𝑞2values as a function of centrality in Pb–Pb collisions at √𝑠NN =5.02 TeV. Pink lines demarcate the 30th and 70th 𝑞2percentiles, as calculated within 1%-wide centrality intervals. The reconstruction efficiency accounts for truth-level jets that were not found at detector-level. Corrections were also applied to account for the event-plane resolution when the event-plane angle was considered. The reported 𝑝T,ch jet ranges are 35 − 120 GeV/cand 40 − 120 GeV/c for 𝑅 =0.2and 𝑅 =0.4jets, respectively. These ranges were chosen to satisfy the requirement of having a kinematic efficiency above 75% for each generator-level 𝑝T,ch jet bin, as well as to ensure stability when varying the lower limit of the 𝑝T,ch jet range considered in the unfolding procedure. To study the event-shape dependence, events were classified according to the magnitude of the reduced flow vector 𝑞2[51]as measured with the V0C, defined as 𝑞2=|Q2|∕√𝑀, (1) where M represents the charged-particle multiplicity and Q2represents the second harmonic flow vector, defined as Q2=(∑ 𝑖 𝑤𝑖cos(2𝜑𝑖),∑ 𝑖 𝑤𝑖sin(2𝜑𝑖)).(2) Here, 𝜑𝑖and 𝑤𝑖are the azimuthal angle and signal weight, respectively, of the 𝑖-th segment of the V0C detector [33,52]. Samples of events with the 30% smallest and largest 𝑞2were selected for this study and will be henceforth referred to as 𝑞2-small and 𝑞2-large. These designations represent isotropic and elliptical event topologies, respectively. Fig. 1 shows the distribution of 𝑞2values as a function of collision centrality in Pb–Pb collisions at √𝑠NN =5.02 TeV. The pink lines demarcate the 30th and 70th percentiles in 𝑞2. Note that the 𝑞2-small sample contains a significant fraction of events with non-zero 𝑣2, so, while this sample is characterized as comparatively isotropic, there still exists some significant anisotropy within the sample [26]. The slope of the distribution indicates that the average values of 𝑞2are slightly larger for more central collisions, which can introduce a centrality bias in the event class selection. To avoid this correlation bias, the 𝑞2classification was done within 1%-wide centrality intervals. The event-plane angle Ψ2, given by the direction of Q2, was measured with the V0A detector. The inand out-of-plane axes were defined as parallel and perpendicular to Ψ2, respectively. Jets were considered inand out-of-plane when they were reconstructed within 30◦in azimuth of these axes. This restriction from the traditional ±45◦definition was made to enforce larger differences between inand out-of-plane path lengths and to increase the potential differences in jet yields [30]. The use of opposed detectors for 𝑞2and Ψ2is advantageous for avoiding autocorrelations between these observables and for reducing detectorresolution corrections. To account for the smearing of the reaction-plane angle due to the event-plane resolution, the ratios of inand out-of-plane jet yields were corrected using a procedure analogous to that used for 𝑣2measurements [53]. First, the 𝑣2was calculated using 𝑣2=𝜋 3√3 1 𝑅2 𝑁in −𝑁out 𝑁in +𝑁out ,(3) where 𝑅2is the second harmonic event-plane resolution, and 𝑁in and 𝑁out are the inand out-of-plane jet yields, respectively. Note that the coefficient 𝜋∕(3√3) in this formula is specific to this analysis, in which the inand out-of-plane definitions are at ±30◦around Ψ2and the vector perpendicular to it, as described above. The event-plane resolution 𝑅2was calculated using the three-sub-event method [53], where the particles measured by the V0A, V0C, and TPC detectors were used to construct the three separate sub-events. For 𝑞2-small samples, 𝑅2is 0.55, whereas for 𝑞2-large samples it is 0.68. After calculating the corrected 𝑣2, the corrected ratio  =𝑁out ∕𝑁in was obtained by inverting Eq. (3)and assuming a perfect resolution 𝑅2=1. To correct the individual spectra for the event-plane resolution, conservation of jet yields within the fiducial volume (𝑁measured in +𝑁measured out = 𝑁corrected in +𝑁corrected out ) was additionally considered, such that 𝑁corrected in =𝑁measured in +𝑁measured out 1+,𝑁 corrected out =𝑁measured in +𝑁measured out 1+1∕. (4) For the ratio of out-of-plane to in-plane jet yields, the magnitude of this correction varies from 5 to 25%. Note that 𝑁mid remains unmodified, where 𝑁mid is the jet yield reconstructed between ±30◦−60 ◦of the event plane. This correction procedure is exact when assuming a negligible contribution from higher order harmonics. Additionally, the contribution of non-flow to the measured yield ratios was estimated using PYTHIA 8. Here, non-flow refers to the 𝑣2contribution from forward multi-jets that result in a biased determination of Ψ2. It was found that, for cases where an intermediate 𝑝T,ch jet jet is produced at midrapidity, a recoiling jet strikes the V0A in <4% of instances. The relative contribution from these events to the jet 𝑣2is estimated to be less than 20%. The presented results are not corrected for this possible effect. The systematic uncertainties of the charged-particle jet yields and their ratios are summarized in Tables 1and 2, respectively. The ranges of systematic uncertainties are listed for the measured 𝑝T,ch jet range. The systematic uncertainty on the tracking efficiency was calculated by randomly rejecting an additional 4% of PYTHIA 8 tracks used in the embedding procedure, representing the uncertainty in the single-track efficiency in the Pb–Pb environment. The jet finding was then repeated and the response matrix recalculated, resulting in the largest source of uncertainty for the measured spectra. The uncertainty in the unfolding procedure was quantified by varying the number of iterations of unfolding, the shape of the prior 𝑝T,ch jet and Δ𝜑spectra, and the lower limit of the measured range (referred to as the truncation). The shape variation was done by reweighting the unfolding prior according to the ratio between the PYTHIA 8 and data spectra in both 𝑝T,ch jet and eventplane angle. The number of unfolding iterations was varied by ±1. The lower 𝑝T,ch jet limit for the jets that entered into the unfolding procedure was varied by ±5 GeV∕𝑐. Finally, the systematic uncertainty of the event-plane resolution was obtained by varying 𝑅2by 2%. This 2% variation accounts for the difference in event-plane resolution observed when it is calculated using the 𝜒-ratio method as opposed to the threesub-event method [26,53]. Note that this uncertainty is only considered for the measurements that are differentiated in Δ𝜑. For the ratios of the spectra, the systematic uncertainties in the numerator and denominator were treated as correlated, and the resulting systematic uncertainty was Physics Letters B 851 (2024) 138584 4 ALICE Collaboration Table 1 Relative systematic uncertainties for the charged-particle jet yields as measured in 30–50% Pb–Pb collisions at √𝑠NN =5.02 TeV. Values are reported as percentages. Reported ranges reflect the minimum and maximum values of the uncertainties over the measured 𝑝T,ch jet range. Here, <1 indicates an uncertainty with a decimal value greater than zero but less than one. R=0.2 R=0.4 𝑞2-small 𝑞2-large 𝑞2-small 𝑞2-large in-plane out-of-plane in-plane out-of-plane in-plane out-of-plane in-plane out-of-plane Tracking efficiency 6–15 6–12 8–10 6–16 4–15 6–15 6–20 <1–17 Unfolding iterations <1<1<1<1<1–2 <1–4 1–3 <1–3 Unfolding prior <1–3 <1–2 <1–2 <1–2 2–6 <1–5 <1–8 2–5 Unfolding truncation <1<1<1<1<1–10 <1–7 1–13 <1–9 Event-plane determination <1<1<1<1<1<1<1<1 Total 6–15 6–12 8–10 6–16 8–16 6–16 12–21 9–17 Table 2 Relative systematic uncertainties for the ratios of charged-particle jet yields as measured in 30–50% Pb–Pb collisions at √𝑠NN =5.02 TeV. Values are reported as percentages. Reported ranges reflect the minimum and maximum values of the uncertainties over the measured 𝑝T,ch jet range. Here, <1 indicates an uncertainty with a decimal value greater than zero but less than one. R=0.2 R=0.4 𝑞2-large/𝑞2-small 𝑞2-small 𝑞2-large 𝑞2-large/𝑞2-small 𝑞2-small 𝑞2-large out-/in-plane out-/in-plane out-/in-plane out-/in-plane Tracking efficiency 1–3 <1–2 <1–5 1–3 4–9 1–9 Unfolding iterations <1<1<1<1<1–6 2–6 Unfolding prior <1–3 <1–2 <1–3 <1–3 1–5 <1–12 Unfolding truncation <1<1<1<1–3 1–18 1–21 Event-plane determination N/A <1<1N/A <1<1 Total 1–4 2–3 1–5 1–5 5–21 4–25 obtained by making the above-described variations and calculating the deviations on the ratio itself. The total systematic uncertainties were calculated as quadratic sums of the different sources by assuming the independence of all contributions. 4. Results The 𝑝T,ch jet -differential charged-particle jet yields for resolution parameters 𝑅 =0.2and 𝑅 =0.4are shown in Fig. 2. Included are the results for the event classes 𝑞2-small and 𝑞2-large, differentiated for inplane and out-of-plane jets. The systematic uncertainties are indicated by the boxes and are highly correlated among the different measurements. The ratios of charged-particle jet yields for the 𝑞2-large to 𝑞2-small event classes are shown in Fig. 3, for both 𝑅 =0.2and 𝑅 =0.4. Considering these results as ratios allowed for a reduction in the systematic uncertainties due to correlations of the uncertainties between the spectra, thus improving the sensitivity of this measurement. The results are consistent with unity, indicating that azimuthally-integrated jet yields are not sensitive to collision ellipticity. This stands in contrast to the yield enhancement seen in elliptical collisions at low 𝑝T[25], where particle spectra are not governed by quenching, but rather by the hydrodynamic expansion of the medium. Fig. 4shows the ratio of out-of-plane to in-plane jet yields for the 𝑞2small and 𝑞2-large event classes, for jets with 𝑅 =0.2(left) and 𝑅 =0.4 (right). These results were corrected for the event-plane resolution, as described in the previous section. The measured ratios are significantly below unity, indicating that jets lose more energy on average when traveling out-of-plane than when traveling in-plane. This is consistent with the idea that the magnitude of jet energy loss is driven, at least in part, by the path length traversed in the medium. For 𝑅 =0.4jets, further conclusions regarding event-shape dependent azimuthal anisotropy are limited by the large experimental uncertainties. For 𝑅 =0.2jets, the ratios for 𝑞2-small and 𝑞2-large are similar at high 𝑝T,ch jet . For 𝑝T,ch jet <50 GeV/c, however, there is an indication that out-of-plane jets in the 𝑞2large class are more suppressed relative to in-plane jets than those in the 𝑞2-small class. The significance of this separation from 35 <𝑝 T,ch jet <50 GeV/cis 5.2𝜎. This result is qualitatively in agreement with observations of D mesons [29]. This effect is expected due to the increased path-length differences between inand out-of-plane directions for highly elliptical collision geometries, which is supported by Trajectum calculations [30,54]. In these calculations, probes were generated in the initial state at the location of nucleon–nucleon collisions, and propagated through the hydrodynamically evolving medium while remaining unmodified. The average path lengths of these probes were calculated for events with varying 𝑞2, and differentially for inand out-of-plane emission angles. While the results of this study show that the average traversed path length of the probes does not vary significantly with event 𝑞2, it does change as a function of the probe angle with respect to Ψ2. This variation with Ψ2can be further augmented when considering 𝑞2-large events, and suppressed when considering 𝑞2-small events. The outcome of these Trajectum calculations shows that by using ESE, the ratio of out-of-plane to in-plane path lengths can be increased in semicentral collisions by ∼10% with respect to the inclusive case. The results presented in this letter are consistent with these calculations, assuming that the traversed path length of jets is an important factor for determining their energy loss. These Trajectum studies do not, however, allow one to conclude anything about the 𝑝T,ch jet -dependence of this energy loss or explain the apparent convergence of ratios at high 𝑝T,ch jet . Despite the absence of phenomenological descriptions, a possible understanding of the experimental results at high 𝑝T,ch jet can be obtained by considering that the charged-particle jet 𝑅AA increases and the charged-particle jet 𝑣2decreases with increasing 𝑝T,ch jet [20,55]. It is therefore expected that any path-length-dependent signal accessible to ESE measurements would decrease at high 𝑝T,ch jet . Moreover, the precision of the measurement presented here is statistically limited at high 𝑝T,ch jet . It is therefore difficult to establish if the convergence of out-of-plane to in-plane ratios for elliptical and isotropic events is a true physics phenomenon, or is rather a consequence of the limited experimental precision accessible at high 𝑝T,ch jet . Physics Letters B 851 (2024) 138584 5 ALICE Collaboration Fig. 2. Charged-particle jet yields for 𝑅 =0.2(left) and 𝑅 =0.4(right) jets in Pb–Pb collisions at √𝑠NN =5.02 TeV. Results are shown for the 𝑞2-small and 𝑞2-large event classes, for inand out-of-plane jets. The bars (boxes) represent statistical (systematic) point-by-point uncertainties. Fig. 3. Ratio of the charged-particle jet yields of the 𝑞2-large to the 𝑞2-small event classes, in Pb–Pb collisions at √𝑠NN =5.02 TeV. The results are reported for 𝑅 =0.2and 𝑅 =0.4jets. This measurement demonstrates the potential of the ESE technique and paves the way for future studies with larger data samples. However, a full interpretation of these results requires detailed comparisons to model calculations, which will allow for more quantitative conclusions on the path-length dependence of energy loss. 5. Conclusions In this letter, the measured event-shape engineered jet yields are reported for resolution parameters 𝑅 =0.2and 𝑅 =0.4in semicentral Pb–Pb collisions at √𝑠NN =5.02 TeV. The magnitude of the reduced second harmonic flow vector 𝑞2was used to select event classes that are particularly isotropic (𝑞2-small) and elliptical (𝑞2-large). The jet spectra from these two event classes are consistent within their uncertainties. However, a significant deviation between jet spectra is observed when these jets are classified according to their azimuthal angle with respect to the event plane. It is indicated that jets lose more energy out-of-plane compared to in-plane, consistent with the measurement of a non-zero 𝑣2of jets at the LHC. Furthermore, for 𝑅 =0.2jets in highly elliptical events, the differences between the modification of out-of-plane and inplane jets at low 𝑝T,ch jet are found to be more significant than in more isotropic events. Model calculations employing a realistic parton shower in event-by-event hydrodynamical simulations, such as LBT [56,57], JETSCAPE [58], or JEWEL on a (2+1)D background [59,60], are needed to further interpret these results and gain insight into the path-length dependence of jet quenching. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability This manuscript has associated data in a HEPData repository at: https://www .hepdata .net /record /ins2681682. Acknowledgements 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: A. I. Alikhanyan National Science Laboratory (Yerevan Physics Institute) Foundation (ANSL), State Committee of Science and World Federation of Scientists (WFS), Armenia; Austrian Academy of Sciences, Austrian Science Fund Physics Letters B 851 (2024) 138584 6 ALICE Collaboration Fig. 4. Ratios of out-of-plane to in-plane charged-particle jet yields for the 𝑞2-large and 𝑞2-small event classes in Pb–Pb collisions at √𝑠NN =5.02 TeV. The results are reported for 𝑅 =0.2(left) and 𝑅 =0.4(right) jets, and are corrected for the event-plane resolution. (FWF): [M 2467-N36] and Nationalstiftung für Forschung, Technologie und Entwicklung, Austria; Ministry of Communications and High Technologies, National Nuclear Research Center, Azerbaijan; Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Financiadora de Estudos e Projetos (Finep), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) and Universidade Federal do Rio Grande do Sul (UFRGS), Brazil; Bulgarian Ministry of Education and Science, within the National Roadmap for Research Infrastructures 20202027 (object CERN), Bulgaria; Ministry of Education of China (MOEC), Ministry of Science & Technology of China (MSTC) and National Natural Science Foundation of China (NSFC), China; Ministry of Science and Education and Croatian Science Foundation, Croatia; Centro de Aplicaciones Tecnológicas y Desarrollo Nuclear (CEADEN), Cubaenergía, Cuba; Ministry of Education, Youth and Sports of the Czech Republic, Czech Republic; The Danish Council for Independent Research—Natural Sciences, the Villum Fonden and Danish National Research Foundation (DNRF), Denmark; Helsinki Institute of Physics (HIP), Finland; Commissariat à l’Énergie Atomique (CEA) and Institut National de Physique Nucléaire et de Physique des Particules (IN2P3) and Centre National de la Recherche Scientifique (CNRS), France; Bundesministerium für Bildung und Forschung (BMBF) and GSI Helmholtzzentrum für Schwerionenforschung GmbH, Germany; General Secretariat for Research and Technology, Ministry of Education, Research and Religions, Greece; National Research, Development and Innovation Office, Hungary; Department of Atomic Energy, Government of India (DAE), Department of Science and Technology, Government of India (DST), University Grants Commission, Government of India (UGC) and Council of Scientific and Industrial Research (CSIR), India; National Research and Innovation Agency - BRIN, Indonesia; Istituto Nazionale di Fisica Nucleare (INFN), Italy; Japanese Ministry of Education, Culture, Sports, Science and Technology (MEXT) and Japan Society for the Promotion of Science (JSPS) KAKENHI, Japan; Consejo Nacional de Ciencia y Tecnología (CONACYT), through Fondo de Cooperación Internacional en Ciencia y Tecnología (FONCICYT) and Dirección General de Asuntos del Personal Academico (DGAPA), Mexico; Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO), Netherlands; The Research Council of Norway, Norway; Commission on Science and Technology for Sustainable Development in the South (COMSATS), Pakistan; Pontificia Universidad Católica del Perú, Peru; Ministry of Education and Science, National Science Centre and WUT ID-UB, Poland; Korea Institute of Science and Technology Information and National Research Foundation of Korea (NRF), Republic of Korea; Ministry of Education and Scientific Research, Institute of Atomic Physics, Ministry of Research and Innovation and Institute of Atomic Physics and Universitatea Nationala de Stiinta si Tehnologie Politehnica Bucuresti, Romania; Ministry of Education, Science, Research and Sport of the Slovak Republic, Slovakia; National Research Foundation of South Africa, South Africa; Swedish Research Council (VR) and Knut & Alice Wallenberg Foundation (KAW), Sweden; European Organization for Nuclear Research, Switzerland; Suranaree University of Technology (SUT), National Science and Technology Development Agency (NSTDA) and National Science, Research and Innovation Fund (NSRF via PMU-B B05F650021), Thailand; Turkish Energy, Nuclear and Mineral Research Agency (TENMAK), Turkey; National Academy of Sciences of Ukraine, Ukraine; Science and Technology Facilities Council (STFC), United Kingdom; National Science Foundation of the United States of America (NSF) and United States Department of Energy, Office of Nuclear Physics (DOE NP), United States of America. 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