Browsing by Subject "COLLECTIVITY"

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  • The CMS collaboration; Sirunyan, A. M.; Eerola, P.; Kirschenmann, H.; Pekkanen, J.; Voutilainen, M.; Havukainen, J.; Heikkilä, J. K.; Järvinen, T.; Karimäki, V.; Kinnunen, R.; Lampen, T.; Lassila-Perini, K.; Laurila, S.; Lehti, S.; Linden, T.; Luukka, P.; Mäenpää, T.; Siikonen, H.; Tuominen, E.; Tuominiemi, J.; Tuuva, T. (2020)
    Bose-Einstein correlations of charged hadrons are measured over a broad multiplicity range, from a few particles up to about 250 reconstructed charged hadrons in proton-proton collisions at s = 13 TeV. The results are based on data collected using the CMS detector at the LHC during runs with a special low-pileup configuration. Three analysis techniques with different degrees of dependence on simulations are used to remove the non-Bose-Einstein background from the correlation functions. All three methods give consistent results. The measured lengths of homogeneity are studied as functions of particle multiplicity as well as average pair transverse momentum and mass. The results are compared with data from both CMS and ATLAS at s = 7 TeV, as well as with theoretical predictions.
  • The CMS collaboration; Eerola, P.; Kirschenmann, H.; Pekkanen, J.; Voutilainen, M.; Havukainen, J.; Heikkila, J. K.; Jarvinen, T.; Kinnunen, R.; Lampen, T.; Lassila-Perini, K.; Laurila, S.; Lehti, S.; Linden, T.; Luukka, P.; Maenpaa, T.; Siikonen, H.; Tuominen, E.; Tuominiemi, J.; Sirunyan, A. M.; Karimäki, V.; Tuuva, T. (2019)
    The almost hermetic coverage of the CMS detector is used to measure the distribution of transverse energy, E-T, over 13.2 units of pseudorapidity, eta, for pPb collisions at a center-of-mass energy per nucleon pair of root s(NN) = 5.02 TeV. The huge angular acceptance exploits the fact that the CASTOR calorimeter at -6.6 <eta <-5.2 is effectively present on both sides of the colliding system because of a switch in the proton-going and lead-going beam directions. This wide acceptance enables the study of correlations between well-separated angular regions and makes the measurement a particularly powerful test of event generators. For minimum bias pPb collisions the maximum value of dE(T)/d eta is 22 GeV, which implies an E-T per participant nucleon pair comparable to that of peripheral PbPb collisions at root s(NN) 7 = 2.76 TeV. The increase of dE(T)/d eta with centrality is much stronger for the lead-going side than for the proton-going side. The i dependence of dE(T)/d eta is sensitive to the eta range in which the centrality variable is defined. Several modern generators are compared to these results but none is able to capture all aspects of the eta and centrality dependence of the data and the correlations observed between different eta regions.
  • The CMS collaboration; Sirunyan, A. M.; Tumasyan, A.; Eerola, P.; Kirschenmann, H.; Pekkanen, Juska; Voutilainen, M.; Havukainen, J.; Heikkilä, J.K.; Järvinen, T.; Karimäki, V.; Kinnunen, R.; Lampén, T.; Lassila-Perini, K.; Laurila, S.; Lehti, S.; Lindén, T.; Luukka, P.; Mäenpää, T.; Siikonen, H.; Tuominen, E.; Tuominiemi, J.; Tuuva, T. (2021)
    Event-by-event long-range correlations of azimuthal anisotropy Fourier coefficients (v(n)) in 8.16 TeV pPb data, collected by the CMS experiment at the CERN Large Hadron Collider, are extracted using a subevent four-particle cumulant technique applied to very low multiplicity events. Each combination of four charged particles is selected from either two, three, or four distinct subevent regions of a pseudorapidity range from -2.4 to 2.4 of the CMS tracker, and with transverse momentum between 0.3 and 3.0 GeV. Using the subevent cumulant technique, correlations between v(n) of different orders are measured as functions of particle multiplicity and compared to the standard cumulant method without subevents over a wide event multiplicity range. At high multiplicities, the v(2) and v(3) coefficients exhibit an anticorrelation; this behavior is observed consistently using various methods. The v(2) and v(4) correlation strength is found to depend on the number of subevents used in the calculation. As the event multiplicity decreases, the results from different subevent methods diverge because of different contributions of noncollective or few-particle correlations. Correlations extracted with the four-subevent method exhibit a tendency to diminish monotonically toward the lowest multiplicity region (about 20 charged tracks) investigated. These findings extend previous studies to a significantly lower event multiplicity range and establish the evidence for the onset of long-range collective multiparticle correlations in small system collisions.
  • The ALICE collaboration; Acharya, S.; Brücken, E. J.; Chang, B.; Hilden, T. E.; Kim, D. J.; Litichevskyi, V.; Mieskolainen, M. M.; Orava, R.; Parkkila, J. E.; Rak, J.; Räsänen, S. S.; Saarinen, S.; Slupecki, M.; Snellman, T. W.; Trzaska, W. H.; Vargyas, M.; Viinikainen, J. (2019)
    The measurement of dielectron production is presented as a function of invariant mass and transverse momentum (p(T)) at midrapidity (vertical bar ye vertical bar < 0.8) in proton-proton (pp) collisions at a centre-of-mass energy of root s = 13 TeV. The contributions from light-hadron decays are calculated from their measured cross sections in pp collisions at root s = 7 TeV or 13 TeV. The remaining continuum stems from correlated semileptonic decays of heavy-flavour hadrons. Fitting the data with templates from two different MC event generators, PYTHIA and POWHEG, the charm and beauty cross sections at midrapidity are extracted for the first time at this collision energy: d sigma(c<(c)over bar>)/dy vertical bar(y=0) = 974 +/- 138 (stat.) +/- 140 (syst.) +/- 214(BR) mu b and d sigma(b (b) over bar)/dy vertical bar(y=0) = 79 +/- 14 (stat.) +/- 11 (syst.) +/- 5(BR) mu b using PYTHIA simulations and d sigma(c (c) over bar)/dy vertical bar(y=0) = 1417 +/- 184 (stat.) +/- 204 (syst.) +/- 312(BR) mu b and d sigma(b (b) over bar)/dy vertical bar(y=0) = 48 +/- 14 (stat.) +/- 7 (syst.) +/- 3(BR) mu b for POWHEG. These values, whose uncertainties are fully correlated between the two generators, are consistent with extrapolations from lower energies. The different results obtained with POWHEG and PYTHIA imply different kinematic correlations of the heavy-quark pairs in these two generators. Furthermore, comparisons of dielectron spectra in inelastic events and in events collected with a trigger on high charged-particle multiplicities are presented in various p(T) intervals. The differences are consistent with the already measured scaling of light-hadron and open-charm production at high charged-particle multiplicity as a function of p(T). Upper limits for the contribution of virtual direct photons are extracted at 90% confidence level and found to be in agreement with pQCD calculations. (C) 2018 The Author. Published by Elsevier B.V.
  • The ALICE collaboration; Acharya, S.; Adamova, D.; Kim, D. J.; Krizek, F.; Molander, Mikael Andreas; Novitzky, Norbert; Onnerstad, A.; Parkkila, J. E.; Rytkönen, Heidi Maria; Räsänen, Sami; Saarimäki, Oskari Antti Matti; Slupecki, M.; Trzaska, W. H. (2021)
    Two-particle Azimuthal correlations are measured with the ALICE apparatus in pp collisions at root s = 13 TeV to explore strangeness- and multiplicity-related effects in the fragmentation of jets and the transition regime between bulk and hard production, probed with the condition that a strange meson (K-S(0)) or baryon (Lambda) with transverse momentum p(T) > 3 GeV/c is produced. Azimuthal correlations between kaons or Lambda hyperons with other hadrons are presented at midrapidity for a broad range of the trigger (3 < p(T)(trigg) < 20 GeV/c) and associated particle p(T) (1 GeV/c < p(T)(assoc) < p(T)(trigg)), for minimum-bias events and as a function of the event multiplicity. The near- and away-side peak yields are compared for the case of either K-S(0) or Lambda((Lambda) over bar) being the trigger particle with that of inclusive hadrons (a sample dominated by pions). In addition, the measurements are compared with predictions from PYTHIA 8 and EPOS LHC event generators.
  • The ALICE collaboration; Acharya, S.; Adamova, D.; Kim, D. J.; Krizek, F.; Novitzky, Norbert; Onnerstad, A.; Parkkila, J. E.; Rytkönen, Heidi Maria; Räsänen, Sami; Saarimäki, Oskari Antti Matti; Slupecki, M.; Trzaska, W. H. (2021)
    Two-particle angular correlations are measured in high-multiplicity proton-proton collisions at = 13 TeV by the ALICE Collaboration. The yields of particle pairs at short-(eta similar to 0) and long-range (1.6 < || < 1.8) in pseudorapidity are extracted on the near-side ( 0). They are reported as a function of transverse momentum (p(T)) in the range 1 < p(T)< 4 GeV/c. Furthermore, the event-scale dependence is studied for the first time by requiring the presence of high-p(T) leading particles or jets for varying p(T) thresholds. The results demonstrate that the long-range "ridge" yield, possibly related to the collective behavior of the system, is present in events with high-p(T) processes as well. The magnitudes of the short- and long-range yields are found to grow with the event scale. The results are compared to EPOS LHC and PYTHIA 8 calculations, with and without string-shoving interactions. It is found that while both models describe the qualitative trends in the data, calculations from EPOS LHC show a better quantitative agreement for the p(T) dependency, while overestimating the event-scale dependency.