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Measurements of two-particle correlations in e+e− collisions at 91 GeV with ALEPH archived data
Available online at www.sciencedirect.com
Nuclear Physics A 982 (2019) 483–486 www.elsevier.com/locate/nuclphysa
XXVIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2018)
Measurements of two-particle correlations in e+e− collisions at 91 GeV with ALEPH archived data Yen-Jie Leea , Anthony Badeaa , Austin Batya , Christopher McGinna , Gian Michele Innocentia , Jesse Thalera , Michael Petersa , Tzu-An Shengb , Paoti Changb , Marcello Maggic a Massachusetts b National
Institute of Technology, Cambridge, USA Taiwan University, Taipei, Taiwan Sezione di Bari, Bari, Italy
c INFN
Abstract The first measurements of two-particle angular correlations for charged particles emitted in e+ e− collisions at a center-ofmass energy of 91 GeV are presented. The archived data are collected with the ALEPH detector at LEP. The correlation functions are measured over a broad range of pseudorapidity and azimuthal angle of the charged particles. Those results are compared to predictions from the PYTHIA event generator. In contrast to the results from high charged particle multiplicity nucleon-nucleon, nucleon-nucleus and nucleus-nucleus collisions, where long-range correlations with a large pseudorapidity gap are observed, no significant enhancement of long-range correlations is observed with respect to PYTHIA predictions, which do not include additional final state interactions of the outgoing partons. Keywords: correlation function, electron-position annihilation, LEP, ALEPH, QCD
Two-particle correlations in high-energy collisions provide valuable information for characterizing Quantum Chromodynamics and have been studied previously for a broad range of collision energies in protonproton (pp) [1, 2], proton-nucleus (pA) [3, 4, 5], deutron-nucleus (dA) [6], and nucleus-nucleus (AA) [7, 8] collisions. Such measurements can elucidate the underlying mechanism of particle production and reveal possible collective effects resulting from the high particle densities accessible in these collisions. Studies of two-particle angular correlations are typically performed the using the two-dimensional Δη − Δφ correlation functions, where Δφ is the difference in the azimuthal angle φ between the two particles and Δη is the difference in pseudorapidity η = − ln(tan(θ/2)). The polar angle θ is defined relative to the reference axis, which is in the counterclockwise hadron beam direction. Of particular interest in studies of collective effects is the long-range (large |Δη|) structure of the two-particle correlation functions. In this region, the function is less susceptible to other known sources of correlations such as resonance decays and the fragmentation of energetic partons. Measurements in high-energy AA collisions have shown significant modifications of the long-range structure with respect to minimum-bias pp collisions, over a very wide range of collision energies [9, 10, 11, 12]. The long-range correlations are interpreted as a consequence of the hydrodynamical https://doi.org/10.1016/j.nuclphysa.2018.09.018 0375-9474/© 2018 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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Y.-J. Lee et al. / Nuclear Physics A 982 (2019) 483–486
flow of the produced strongly interacting medium [13] and are closely related to initial collision geometry and its fluctuations [14]. Those measurements allow for the extraction of the fundamental transport properties of the medium using hydrodynamic models. Recently, measurements in pp, pPb collisions and dAu collisions have revealed the emergence of long-range, near-side (Δφ ∼ 0) correlations in the selection of collisions with very high number of final state particles. This long-range correlation has inspired a large variety of theoretical models [15, 16, 17]. The physical origin of the phenomenon is not yet fully understood. Due to the complexity of the hadron-hadron collisions, possible initial state correlations of the partons, such as those arise from color-glass condensate [18, 19], could complicate the interpretation of the pp, pA and dA data. Studies of high charged particle multiplicity e+ e− collisions, where the initial kinematics of the collisions are well-defined, could bring significant insights [20]. In these proceedings, the first measurements of the two-particle correlation function charged particles emitted in e+ e− collisions at a center-of-mass energy of 91 GeV is presented. Archived data collected with the ALEPH detector [21] at LEP is used and converted to an MIT Open Data format [22]. Details of the selection of hadronic events are described in [23]. Events are accepted with at least five good tracks and with a total charged energy in excess of 15 GeV. The sphericity axis and its polar angle (Θsph ) reconstructed with both tracker and calorimeters is required to be | cos Θsph | < 0.9 to ensure that the event is well contained within the detector. Charged particles with transverse momentum pT > 0.2 GeV/c and |η| < 1.74 are selected for the correlation function analysis. High multiplicity events are sampled using charged particle multiplicity N in each event. The first step in extracting the correlation function was to divide the sample into N bins. For each multiplicity class, “trigger” particles are defined as charged particles in the selected transverse momentum range. Particle pairs are then formed by associating every trigger particle with the remaining charged particles in the same pT interval as the trigger particle. The per-trigger-particle associated yield is defined as S (Δη,Δφ) 1 d2 Npair Ntrig dΔηdΔφ = B(0, 0) × B(Δη,Δφ) , where Ntrig is the number of trigger particles in the event, Δη and Δφ are the differences in η and φ of the pair. The signal distribution, S (Δη, Δφ), is the per-trigger-particle yield of d2 Nsame particle pairs in the same event which is defined as S (Δη, Δφ) = N1trig dΔηdΔφ . The mixed-event background 2
mix
d N distribution, used to account for random combinatorial background, is defined as B(Δη, Δφ) = N1trig dΔηdΔφ and is constructed by pairing the trigger particles from two random events in the same event multiplicity interval. The symbol N mix denotes the number of pairs taken from the mixed event, while B(0, 0) represents the mixed-event associated yield for both particles of the pair going in the same direction and thus having full pair acceptance. Therefore, the ratio B(0, 0)/B(Δη, Δφ) represents the pair-acceptance correction factor used to derive the corrected per-trigger-particle associated yield distribution. The signal and background distributions are first calculated for each event and then averaged over all the events within the track multiplicity class. Simulated events with the PYTHIA event generator (version 6.1 [24]) are used to derive efficiency correction factors for charged particles. In Fig. 1, the two-particle correlation functions from high (N>35) multiplicity events are presented using the beam axis as the reference axis for the calculation of η and φ of the charged particles. The dominant features of the correlation function are the jet peak near (Δη, Δφ) = (0, 0) for pairs of particles originating from the same jet and the away-side structure at Δφ ∼ π for pairs of particles from back-to-back jets. Correlated yields are obtained from an zero-yield-at-minimum (ZYAM) method [25] as a function of |Δφ| averaged over 1.6 < |Δη| < 3.0, which are shown in the right panel of Fig. 1. Different from the results from pp and pA collisions, no “ridge”-like structure is found at Δφ ∼ 0. To investigate the collective behavior transverse to the direction of the color string of the outgoing quark and anti-quark pairs from Z decay, the event thrust axis is used to replace the role of the electron beam axis. The charged particle pseudorapidity and polar angles are recalculated with respect to the thrust axis. The obtained correlation functions are shown in Fig. 2. In the away-side at Δφ ∼ π, the elongated structure is more similar to the ones obtained in pp and pA collisions. The correlated yields as a function of Δφ are consistent with archived simulated PYTHIA events, where no additional final state interactions of the partons are implemented. To suppress the contribution of jet correlations and increase the sensitivity to soft gluon emissions perpendicular to the thrust axis, a pseudorapidity selection of the charged particle |η| < 1.6 is applied. The resulting correlation function and the correlated yields with this selection are shown in Fig. 3. The observed correlated yields are also consistent with results from simulated PYTHIA events.
Y.-J. Lee et al. / Nuclear Physics A 982 (2019) 483–486
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Fig. 1. (Left panel) Two-particle correlation functions in e+ e− collisions at 91 GeV for events with particle multiplicity N > 35. The sharp near-side peaks from jet correlations have been truncated to better illustrate the structure outside that region. (Right panel) Correlated yield obtained from ZYAM procedure as a function of |Δφ| averaged over 1.6 < |Δη| < 3.0. The error bars correspond to statistical uncertainties, while the shade areas denote the systematic uncertainties.
Fig. 2. (Left panel) Two-particle correlation functions with thrust axis in e+ e− collisions at 91 GeV for events with particle multiplicity N > 35. The sharp near-side peaks from jet correlations have been truncated to better illustrate the structure outside that region. (Right panel) Correlated yield obtained from ZYAM procedure as a function of |Δφ| averaged over 1.6 < |Δη| < 3.0. The error bars correspond to statistical uncertainties, while the shade areas denote the systematic uncertainties.
Fig. 3. (Left panel) Two-particle correlation functions with thrust axis in e+ e− collisions at 91 GeV for events with particle multiplicity N > 35. Charged particles with |η| < 1.6 are used in the correlation function calculations. The sharp near-side peaks from jet correlations have been truncated. (Right panel) Correlated yield obtained from ZYAM procedure as a function of |Δφ| averaged over 1.6 < |Δη| < 3.0. The error bars correspond to statistical uncertainties, while the shade areas denote the systematic uncertainties.
In summary, the first measurements of two-particle angular correlations for charged particles emitted in e+ e− collisions at a center-of-mass energy of 91 GeV are presented using archived data collected with the ALEPH detector at LEP. The correlation functions are measured over a broad range of pseudorapidity
486
Y.-J. Lee et al. / Nuclear Physics A 982 (2019) 483–486
and azimuthal angle of the charged particles. Those results using either the electron beam axis or the event thrust axis are compared to predictions from the PYTHIA event generator. In contrast to the results from high charged particle multiplicity pp, pA and AA collisions, where long-range correlations with large pseudorapidity gap are observed, no significant enhancement of long-range correlations is observed with respect to PYTHIA predictions, which do not include additional final state interactions of the outgoing partons. Those results serve as an important reference to the pp, pA and AA collisions. References [1] V. Khachatryan, et al., Observation of Long-Range Near-Side Angular Correlations in Proton-Proton Collisions at the LHC, JHEP 09 (2010) 091. arXiv:1009.4122, doi:10.1007/JHEP09(2010)091. √ [2] G. Aad, et al., Observation of Long-Range Elliptic Azimuthal Anisotropies in s =13 and 2.76 TeV pp Collisions with the ATLAS Detector, Phys. Rev. Lett. 116 (17) (2016) 172301. arXiv:1509.04776, doi:10.1103/PhysRevLett.116.172301. [3] S. Chatrchyan, et al., Observation of long-range near-side angular correlations in proton-lead collisions at the LHC, Phys. Lett. B718 (2013) 795–814. arXiv:1210.5482, doi:10.1016/j.physletb.2012.11.025. √ [4] B. Abelev, et al., Long-range angular correlations on the near and away side in p-Pb collisions at sNN = 5.02 TeV, Phys. Lett. B719 (2013) 29–41. arXiv:1212.2001, doi:10.1016/j.physletb.2013.01.012. √ [5] G. Aad, et al., Observation of Associated Near-Side and Away-Side Long-Range Correlations in sNN =5.02??TeV Proton-Lead Collisions with the ATLAS Detector, Phys. Rev. Lett. 110 (18) (2013) 182302. arXiv:1212.5198, doi:10.1103/PhysRevLett.110.182302. √ [6] A. Adare, et al., Quadrupole Anisotropy in Dihadron Azimuthal Correlations in Central d+Au Collisions at sNN =200 GeV, Phys. Rev. Lett. 111 (21) (2013) 212301. arXiv:1303.1794, doi:10.1103/PhysRevLett.111.212301. [7] K. Aamodt, et al., Elliptic flow of charged particles in Pb-Pb collisions at 2.76 TeV, Phys. Rev. Lett. 105 (2010) 252302. arXiv:1011.3914, doi:10.1103/PhysRevLett.105.252302. [8] S. Chatrchyan, et al., Centrality dependence of dihadron correlations and azimuthal anisotropy harmonics in PbPb collisions at √ sNN = 2.76 TeV, Eur. Phys. J. C72 (2012) 2012. arXiv:1201.3158, doi:10.1140/epjc/s10052-012-2012-3. [9] B. B. Back, et al., The PHOBOS perspective on discoveries at RHIC, Nucl. Phys. A757 (2005) 28–101. arXiv:nucl-ex/0410022, doi:10.1016/j.nuclphysa.2005.03.084. [10] I. Arsene, et al., Quark gluon plasma and color glass condensate at RHIC? The Perspective from the BRAHMS experiment, Nucl. Phys. A757 (2005) 1–27. arXiv:nucl-ex/0410020, doi:10.1016/j.nuclphysa.2005.02.130. [11] K. Adcox, et al., Formation of dense partonic matter in relativistic nucleus-nucleus collisions at RHIC: Experimental evaluation by the PHENIX collaboration, Nucl. Phys. A757 (2005) 184–283. arXiv:nucl-ex/0410003, doi:10.1016/j.nuclphysa.2005.03.086. [12] J. Adams, et al., Experimental and theoretical challenges in the search for the quark gluon plasma: The STAR Collaboration’s critical assessment of the evidence from RHIC collisions, Nucl. Phys. A757 (2005) 102–183. arXiv:nucl-ex/0501009, doi:10.1016/j.nuclphysa.2005.03.085. [13] J.-Y. Ollitrault, Anisotropy as a signature of transverse collective flow, Phys. Rev. D46 (1992) 229–245. doi:10.1103/PhysRevD.46.229. [14] B. Alver, G. Roland, Collision geometry fluctuations and triangular flow in heavy-ion collisions, Phys. Rev. C81 (2010) 054905, [Erratum: Phys. Rev.C82,039903(2010)]. arXiv:1003.0194, doi:10.1103/PhysRevC.82.039903, 10.1103/PhysRevC.81.054905. [15] A. Bzdak, B. Schenke, P. Tribedy, R. Venugopalan, Initial state geometry and the role of hydrodynamics in proton-proton, proton-nucleus and deuteron-nucleus collisions, Phys. Rev. C87 (6) (2013) 064906. arXiv:1304.3403, doi:10.1103/PhysRevC.87.064906. [16] K. Dusling, W. Li, B. Schenke, Novel collective phenomena in high-energy proton?proton and proton?nucleus collisions, Int. J. Mod. Phys. E25 (01) (2016) 1630002. arXiv:1509.07939, doi:10.1142/S0218301316300022. [17] L. He, T. Edmonds, Z.-W. Lin, F. Liu, D. Molnar, F. Wang, Anisotropic parton escape is the dominant source of azimuthal anisotropy in transport models, Phys. Lett. B753 (2016) 506–510. arXiv:1502.05572, doi:10.1016/j.physletb.2015.12.051. [18] F. Gelis, E. Iancu, J. Jalilian-Marian, R. Venugopalan, The Color Glass Condensate, Ann. Rev. Nucl. Part. Sci. 60 (2010) 463–489. arXiv:1002.0333, doi:10.1146/annurev.nucl.010909.083629. [19] K. Dusling, R. Venugopalan, Comparison of the color glass condensate to dihadron correlations in proton-proton and protonnucleus collisions, Phys. Rev. D87 (9) (2013) 094034. arXiv:1302.7018, doi:10.1103/PhysRevD.87.094034. [20] J. L. Nagle, R. Belmont, K. Hill, J. Orjuela Koop, D. V. Perepelitsa, P. Yin, Z.-W. Lin, D. McGlinchey, Minimal conditions for collectivity in e+ e− and p + p collisions, Phys. Rev. C97 (2) (2018) 024909. arXiv:1707.02307, doi:10.1103/PhysRevC.97.024909. [21] D. Decamp, et al., ALEPH: A detector for electron-positron annnihilations at LEP, Nucl. Instrum. Meth. A294 (1990) 121–178, [Erratum: Nucl. Instrum. Meth.A303,393(1991)]. doi:10.1016/0168-9002(90)91831-U. [22] A. Tripathee, W. Xue, A. Larkoski, S. Marzani, J. Thaler, Jet Substructure Studies with CMS Open Data, Phys. Rev. D96 (7) (2017) 074003. arXiv:1704.05842, doi:10.1103/PhysRevD.96.074003. [23] R. Barate, et al., Studies of quantum chromodynamics with the ALEPH detector, Phys. Rept. 294 (1998) 1–165. doi:10.1016/S0370-1573(97)00045-8. [24] T. Sjostrand, P. Eden, C. Friberg, L. Lonnblad, G. Miu, S. Mrenna, E. Norrbin, High-energy physics event generation with PYTHIA 6.1, Comput. Phys. Commun. 135 (2001) 238–259. arXiv:hep-ph/0010017, doi:10.1016/S0010-4655(00)00236-8. [25] N. N. Ajitanand, J. M. Alexander, P. Chung, W. G. Holzmann, M. Issah, R. A. Lacey, A. Shevel, A. Taranenko, P. Danielewicz, Decomposition of harmonic and jet contributions to particle-pair correlations at ultra-relativistic energies, Phys. Rev. C72 (2005) 011902. arXiv:nucl-ex/0501025, doi:10.1103/PhysRevC.72.011902.
Nuclear Physics A 982 (2019) 483–486 www.elsevier.com/locate/nuclphysa
XXVIIth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 2018)
Measurements of two-particle correlations in e+e− collisions at 91 GeV with ALEPH archived data Yen-Jie Leea , Anthony Badeaa , Austin Batya , Christopher McGinna , Gian Michele Innocentia , Jesse Thalera , Michael Petersa , Tzu-An Shengb , Paoti Changb , Marcello Maggic a Massachusetts b National
Institute of Technology, Cambridge, USA Taiwan University, Taipei, Taiwan Sezione di Bari, Bari, Italy
c INFN
Abstract The first measurements of two-particle angular correlations for charged particles emitted in e+ e− collisions at a center-ofmass energy of 91 GeV are presented. The archived data are collected with the ALEPH detector at LEP. The correlation functions are measured over a broad range of pseudorapidity and azimuthal angle of the charged particles. Those results are compared to predictions from the PYTHIA event generator. In contrast to the results from high charged particle multiplicity nucleon-nucleon, nucleon-nucleus and nucleus-nucleus collisions, where long-range correlations with a large pseudorapidity gap are observed, no significant enhancement of long-range correlations is observed with respect to PYTHIA predictions, which do not include additional final state interactions of the outgoing partons. Keywords: correlation function, electron-position annihilation, LEP, ALEPH, QCD
Two-particle correlations in high-energy collisions provide valuable information for characterizing Quantum Chromodynamics and have been studied previously for a broad range of collision energies in protonproton (pp) [1, 2], proton-nucleus (pA) [3, 4, 5], deutron-nucleus (dA) [6], and nucleus-nucleus (AA) [7, 8] collisions. Such measurements can elucidate the underlying mechanism of particle production and reveal possible collective effects resulting from the high particle densities accessible in these collisions. Studies of two-particle angular correlations are typically performed the using the two-dimensional Δη − Δφ correlation functions, where Δφ is the difference in the azimuthal angle φ between the two particles and Δη is the difference in pseudorapidity η = − ln(tan(θ/2)). The polar angle θ is defined relative to the reference axis, which is in the counterclockwise hadron beam direction. Of particular interest in studies of collective effects is the long-range (large |Δη|) structure of the two-particle correlation functions. In this region, the function is less susceptible to other known sources of correlations such as resonance decays and the fragmentation of energetic partons. Measurements in high-energy AA collisions have shown significant modifications of the long-range structure with respect to minimum-bias pp collisions, over a very wide range of collision energies [9, 10, 11, 12]. The long-range correlations are interpreted as a consequence of the hydrodynamical https://doi.org/10.1016/j.nuclphysa.2018.09.018 0375-9474/© 2018 Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
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Y.-J. Lee et al. / Nuclear Physics A 982 (2019) 483–486
flow of the produced strongly interacting medium [13] and are closely related to initial collision geometry and its fluctuations [14]. Those measurements allow for the extraction of the fundamental transport properties of the medium using hydrodynamic models. Recently, measurements in pp, pPb collisions and dAu collisions have revealed the emergence of long-range, near-side (Δφ ∼ 0) correlations in the selection of collisions with very high number of final state particles. This long-range correlation has inspired a large variety of theoretical models [15, 16, 17]. The physical origin of the phenomenon is not yet fully understood. Due to the complexity of the hadron-hadron collisions, possible initial state correlations of the partons, such as those arise from color-glass condensate [18, 19], could complicate the interpretation of the pp, pA and dA data. Studies of high charged particle multiplicity e+ e− collisions, where the initial kinematics of the collisions are well-defined, could bring significant insights [20]. In these proceedings, the first measurements of the two-particle correlation function charged particles emitted in e+ e− collisions at a center-of-mass energy of 91 GeV is presented. Archived data collected with the ALEPH detector [21] at LEP is used and converted to an MIT Open Data format [22]. Details of the selection of hadronic events are described in [23]. Events are accepted with at least five good tracks and with a total charged energy in excess of 15 GeV. The sphericity axis and its polar angle (Θsph ) reconstructed with both tracker and calorimeters is required to be | cos Θsph | < 0.9 to ensure that the event is well contained within the detector. Charged particles with transverse momentum pT > 0.2 GeV/c and |η| < 1.74 are selected for the correlation function analysis. High multiplicity events are sampled using charged particle multiplicity N in each event. The first step in extracting the correlation function was to divide the sample into N bins. For each multiplicity class, “trigger” particles are defined as charged particles in the selected transverse momentum range. Particle pairs are then formed by associating every trigger particle with the remaining charged particles in the same pT interval as the trigger particle. The per-trigger-particle associated yield is defined as S (Δη,Δφ) 1 d2 Npair Ntrig dΔηdΔφ = B(0, 0) × B(Δη,Δφ) , where Ntrig is the number of trigger particles in the event, Δη and Δφ are the differences in η and φ of the pair. The signal distribution, S (Δη, Δφ), is the per-trigger-particle yield of d2 Nsame particle pairs in the same event which is defined as S (Δη, Δφ) = N1trig dΔηdΔφ . The mixed-event background 2
mix
d N distribution, used to account for random combinatorial background, is defined as B(Δη, Δφ) = N1trig dΔηdΔφ and is constructed by pairing the trigger particles from two random events in the same event multiplicity interval. The symbol N mix denotes the number of pairs taken from the mixed event, while B(0, 0) represents the mixed-event associated yield for both particles of the pair going in the same direction and thus having full pair acceptance. Therefore, the ratio B(0, 0)/B(Δη, Δφ) represents the pair-acceptance correction factor used to derive the corrected per-trigger-particle associated yield distribution. The signal and background distributions are first calculated for each event and then averaged over all the events within the track multiplicity class. Simulated events with the PYTHIA event generator (version 6.1 [24]) are used to derive efficiency correction factors for charged particles. In Fig. 1, the two-particle correlation functions from high (N>35) multiplicity events are presented using the beam axis as the reference axis for the calculation of η and φ of the charged particles. The dominant features of the correlation function are the jet peak near (Δη, Δφ) = (0, 0) for pairs of particles originating from the same jet and the away-side structure at Δφ ∼ π for pairs of particles from back-to-back jets. Correlated yields are obtained from an zero-yield-at-minimum (ZYAM) method [25] as a function of |Δφ| averaged over 1.6 < |Δη| < 3.0, which are shown in the right panel of Fig. 1. Different from the results from pp and pA collisions, no “ridge”-like structure is found at Δφ ∼ 0. To investigate the collective behavior transverse to the direction of the color string of the outgoing quark and anti-quark pairs from Z decay, the event thrust axis is used to replace the role of the electron beam axis. The charged particle pseudorapidity and polar angles are recalculated with respect to the thrust axis. The obtained correlation functions are shown in Fig. 2. In the away-side at Δφ ∼ π, the elongated structure is more similar to the ones obtained in pp and pA collisions. The correlated yields as a function of Δφ are consistent with archived simulated PYTHIA events, where no additional final state interactions of the partons are implemented. To suppress the contribution of jet correlations and increase the sensitivity to soft gluon emissions perpendicular to the thrust axis, a pseudorapidity selection of the charged particle |η| < 1.6 is applied. The resulting correlation function and the correlated yields with this selection are shown in Fig. 3. The observed correlated yields are also consistent with results from simulated PYTHIA events.
Y.-J. Lee et al. / Nuclear Physics A 982 (2019) 483–486
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Fig. 1. (Left panel) Two-particle correlation functions in e+ e− collisions at 91 GeV for events with particle multiplicity N > 35. The sharp near-side peaks from jet correlations have been truncated to better illustrate the structure outside that region. (Right panel) Correlated yield obtained from ZYAM procedure as a function of |Δφ| averaged over 1.6 < |Δη| < 3.0. The error bars correspond to statistical uncertainties, while the shade areas denote the systematic uncertainties.
Fig. 2. (Left panel) Two-particle correlation functions with thrust axis in e+ e− collisions at 91 GeV for events with particle multiplicity N > 35. The sharp near-side peaks from jet correlations have been truncated to better illustrate the structure outside that region. (Right panel) Correlated yield obtained from ZYAM procedure as a function of |Δφ| averaged over 1.6 < |Δη| < 3.0. The error bars correspond to statistical uncertainties, while the shade areas denote the systematic uncertainties.
Fig. 3. (Left panel) Two-particle correlation functions with thrust axis in e+ e− collisions at 91 GeV for events with particle multiplicity N > 35. Charged particles with |η| < 1.6 are used in the correlation function calculations. The sharp near-side peaks from jet correlations have been truncated. (Right panel) Correlated yield obtained from ZYAM procedure as a function of |Δφ| averaged over 1.6 < |Δη| < 3.0. The error bars correspond to statistical uncertainties, while the shade areas denote the systematic uncertainties.
In summary, the first measurements of two-particle angular correlations for charged particles emitted in e+ e− collisions at a center-of-mass energy of 91 GeV are presented using archived data collected with the ALEPH detector at LEP. The correlation functions are measured over a broad range of pseudorapidity
486
Y.-J. Lee et al. / Nuclear Physics A 982 (2019) 483–486
and azimuthal angle of the charged particles. Those results using either the electron beam axis or the event thrust axis are compared to predictions from the PYTHIA event generator. In contrast to the results from high charged particle multiplicity pp, pA and AA collisions, where long-range correlations with large pseudorapidity gap are observed, no significant enhancement of long-range correlations is observed with respect to PYTHIA predictions, which do not include additional final state interactions of the outgoing partons. Those results serve as an important reference to the pp, pA and AA collisions. References [1] V. Khachatryan, et al., Observation of Long-Range Near-Side Angular Correlations in Proton-Proton Collisions at the LHC, JHEP 09 (2010) 091. arXiv:1009.4122, doi:10.1007/JHEP09(2010)091. √ [2] G. Aad, et al., Observation of Long-Range Elliptic Azimuthal Anisotropies in s =13 and 2.76 TeV pp Collisions with the ATLAS Detector, Phys. Rev. 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