Light flavor hadron spectra at low p T and search for collective phenomena in high multiplicity pp, p-Pb and Pb-Pb collisions measured with the ALICE experiment
NNuclear Physics A 00 (2018) 1–4
NuclearPhysics A
Light flavor hadron spectra at low p T and search for collectivephenomena in high multiplicity pp, p–Pb and Pb–Pb collisionsmeasured with the ALICE experiment C. Andrei (for the ALICE Collaboration)
National Institute for Physics and Nuclear Engineering, Bucharest, Romania
Abstract
Comprehensive results on transverse momentum distributions and their ratios for identified light flavor hadrons ( π , K, p) at low p T and mid-rapidity as a function of charged particle multiplicity are reported for pp collisions at 7 TeV. Particle mass dependenthardening of the spectral shapes in Pb–Pb collisions at 2.76 TeV were attributed to hydrodynamical flow and quantitatively param-eterized with Boltzmann-Gibbs Blast Wave fits. In this contribution, we investigate the existence of collective phenomena in smallsystems: pp, p–Pb and peripheral Pb–Pb where similar patterns are observed in multiplicity dependent studies. Keywords:
LHC, ALICE, light-flavor hadrons, multiplicity, transverse momentum spectra, collective phenomena
Identified charged particle transverse momentum spectra in p–pbar collisions were extensively studied as a func-tion of incident energy below 900 GeV at CERN SppS [1] and up to 1800 GeV at Fermilab Tevatron [2]. Starting from200 GeV, a deviation of the (cid:104) p T (cid:105) of kaons as a function of √ s relative to the expectations based on the extrapolation ofISR data was reported by the UA5 Collaboration [3]. The E735 Collaboration saw evidence for a mass dependence ofthe (cid:104) p T (cid:105) as a function of c.m. energy from 300 to 1800 GeV and of the (cid:104) p T (cid:105) as a function of d N ch / d η at 1800 GeV [2].The origin of these experimental observations is still under debate. QCD inspired models like PYTHIA [4] and EPOS[5] reproduce the experimental trends observed at Tevatron considering multiple partonic interactions and rescatteringor a hydrodynamic type evolution with flux tube initial conditions, respectively. At about four times larger incidentenergies, the case of the present study, such processes become more important and contribute to a large energy trans-fer, well beyond the deconfinement energy density. Therefore, it is quite probable that at such energies a piece ofmatter of proton size explodes hydrodynamically once the energy transfer becomes significantly large [6].Transverse momentum distributions and their relative ratios for identified positive charged hadrons π + , K + , p atmid-rapidity in pp collisions at √ s = p T range goes from 0.2 GeV / c , 0.3 GeV / c and 0.5 GeV / c to 2.6 GeV / c ,1.4 GeV / c and 2.6 GeV / c for π + , K + and p respectively, significantly larger than previously reported results [8]. 60million inelastic pp collisions collected by the ALICE experiment during the 2010 run at LHC, using minimum bias(MB) and high multiplicity (HM) triggers [9], at √ s = | η | < ±
10 cm from the center of the TimeProjection Chamber (TPC) and the Time-Of-Flight array (TOF) subdetectors. A p T dependent Distance of Closest1 a r X i v : . [ h e p - e x ] A ug . Andrei et al. / Nuclear Physics A 00 (2018) 1–4
Figure 1: a) Upper row - charged particle multiplicity dependence of the transverse momentum distributions for π + ,K + and p in pp collisions at 7 TeV; z raw = (cid:68) N rawch (cid:69) mult bin / (cid:68) N rawch (cid:69) mult > . Bottom row - ratio of transverse momentumdistributions in a given multiplicity bin (z) relative to mult >
0; b) Upper row - p T dependence of the particle ratiosK + / p, p / π + and p / K + in pp collisions at 7 TeV for two multiplicity bins. Bottom row - the ratio of the upper distributionsrelative to the one for mult > + p − . T ) cm in the transverse plane and 2 cm in the longitudinal direction wereused in order to reduce the contamination from weak decays of strange particles, conversion or secondary hadronicinteractions in the detector material. The present analysis has been done in | y | < a priori probabilities were obtained from the experimental data. The contaminations from weak decays productsand from particles produced by the interactions with the detector material were estimated based on a data drivenmethod. All the correction factors were estimated using simulations based on the PYTHIA event generator [10] (tunePerugia0 [11]) and the GEANT3 [12] transport code. The corrections determined for the MB case were applied forall the multiplicity bins and their variations as a function of multiplicity, proved to be very small, were included in thesystematic errors.The fully corrected p T spectra for π + , K + and p were obtained by selecting events in eight multiplicity bins inthe raw charged particle multiplicity distribution, 7-12, 13-19, 20-28, 29-39, 40-49, 50-59, 60-71 and 72-82 or, usingthe scaled multiplicity z raw = (cid:68) N rawch (cid:69) mult bin / (cid:68) N rawch (cid:69) mult > , [0.7-1.4), [1.4-2.1), [2.1-3.0), [3.0-4.2), [4.2-5.2), [5.2-6.3),[6.3-7.5) and [7.5-8.6]. The results are presented in Fig. 1a - upper row. In the bottom row of Fig. 1a the ratios of p T distributions at di ff erent multiplicities relative to the mult > p T region which shows a tendency to level o ff at larger p T values. The amount of depletion clearly depends on the mass of the species and on the multiplicity, i.e. it isenhanced going from pions to protons and with increasing multiplicity for a given mass. The K + / π + , p / π + and p / K + ratios as a function of p T are plotted in Fig. 1b - upper row for the second and the sixth multiplicity bins. The generaltrends observed in Fig. 1a can be seen in a more quantitative way in these representations, especially in the bottomrow of Fig. 1b, where the ratios of relative yields for the two multiplicities to the ones corresponding to N rawch > / π + and p / K + at low p T , increasing with the multiplicity and decreasingtowards 1.4-2.0 GeV / c looks similar with the trends observed in A–A collisions, the heavier particles being pushedtowards larger transverse momenta. Such behavior, observed in Pb–Pb collisions at 2.76 TeV [13], was attributed to2 . Andrei et al. / Nuclear Physics A 00 (2018) 1–4
Figure 2: a) p T dependence of p / π + ratio for the second and highest multiplicity bins in pp collisions; b) p T dependenceof (p + p) / ( π + + π − ) ratio in p–Pb for 60-80% and 0-5% multiplicity classes and Pb–Pb for 80-90% and 0-5% centrality[14]; c) The ratios of the ratios presented in a) and b).the existence of collective transverse flow. The evolution of the p T spectra shape with charged particle multiplicity inpp collisions at 7 TeV and p–Pb at 5.02 TeV [14] and with centrality in Pb–Pb at 2.76 TeV [13] is rather similar.The p T dependence of p / π + for the second and highest multiplicity bins and of (p + p) / ( π + + π − ) ratio for p–Pbin 60-80% and 0-5% multiplicity classes and for Pb–Pb at 80-90% and 0-5% centrality [14] are presented in Fig. 2aand Fig. 2b, respectively. The push of protons towards larger p T values relative to pions with increasing centralityor multiplicity is present for all three systems. Quantitatively, this can be followed in Fig. 2c where the ratios of theratios shown in Fig. 2a and Fig. 2b are presented. The ratio for pp follows closely the p–Pb trend as a function of p T .Based on these similarities one could investigate to what extent the quality and parameters of simultaneous fits oftransverse momentum distributions for di ff erent multiplicities or centralities using expressions inspired by hydrody-namic models are also similar. The quality of fits for π + , K + and p, based on the Boltzman-Gibbs Blast Wave (BGBW)expression [15]: E d Nd p ∼ f ( p T ) = (cid:90) R m T K ( m T cosh ρ/ T kin ) I ( p T sinh ρ/ T kin ) rdr (1)where m T = (cid:113) m + p T ; β r ( r ) = β s ( rR ) n ; ρ = tanh − β r , is similar within the error bars for all three systems. The p T range on which the p T spectra follows the hydro shape increases going towards higher multiplicity in pp and p–Pb orhigher centrality in Pb–Pb.In Fig. 3a the results of the fits for pp collisions at 7 TeV in terms of kinetic freeze-out temperature ( T kin ) versustransverse expansion velocity ( (cid:104) β T (cid:105) ) are presented for di ff erent multiplicity bins and compared with the results ob-tained for p–Pb and Pb–Pb as a function of multiplicity classes and centrality, respectively [14]. While the T kin - (cid:104) β T (cid:105) correlation as a function of multiplicity in pp overlaps with the one corresponding to p–Pb as a function of multiplicityclasses, for Pb–Pb the decrease of T kin and increase of (cid:104) β T (cid:105) with centrality is more enhanced. The observed trendsare not reproduced by PYTHIA in absolute values although a somewhat similar trend seems to be present if ColorReconnection is included.Further information accessed from these fits is the expansion profile, given by the parameter n. The n- (cid:104) β T (cid:105) cor-relation as a function of multiplicity or centrality for the three systems is presented in Fig. 3b. Towards high chargedparticle multiplicity, the transverse expansion velocity approaches a linear dependence as a function of position in thefireball for pp, the trend overlapping with the one corresponding to p–Pb and Pb–Pb.In conclusion, transverse momentum spectra of positive identified charged hadrons as a function of charged par-ticle multiplicity in pp collisions at √ s = / π + and p / K + at low p T , increasing with the multiplicity and decreasing towards larger p T , similarwith the trends observed in p–Pb and Pb–Pb collisions, is seen. The kinetic freeze-out temperature ( T kin ), expansion3 . Andrei et al. / Nuclear Physics A 00 (2018) 1–4
Figure 3: a) Freeze-out T kin versus transverse expansion velocity (cid:104) β T (cid:105) for di ff erent multiplicity bins for pp collisionsat 7 TeV (black triangles) comparison with the results obtained for p–Pb (dark blue dots) for di ff erent multiplicityclasses and Pb–Pb (red circles) as a function of centrality [14]; b) Expansion profile n versus transverse expansionvelocity (cid:104) β T (cid:105) for di ff erent multiplicity bins for pp collisions at 7 TeV (green dots) compared with the results obtainedfor p–Pb (dark blue triangles) dots for di ff erent multiplicity classes and Pb–Pb (red squares) as a function of centrality[14]. The (cid:104) β T (cid:105) values increase with multiplicity (pp and p–Pb) or centrality (Pb–Pb).velocity ( (cid:104) β T (cid:105) ) and its profile extracted from the simultaneous fits of the π + , K + and p spectra with BGBW, show amultiplicity dependence trend similar with the ones obtained for p–Pb and Pb–Pb collisions as a function of multiplic-ity classes or centrality, respectively. However, a conclusion about similar mechanisms for the three systems has tobe taken with caution. Detailed investigations based on theoretical approaches such as hydrodynamic models, partonbased Gribov-Regge theory, Color Glass Condensate, Color Reconnection, will give insight to the underlying physicsof this similar behavior observed at LHC energies. References [1] R.E.Ansorge et al., Phys.Lett.B, 167, (1986), p. 476.[2] T.Alexopoulos et al., Phys.Rev.D, 48, (1993), p. 984.[3] G.J.Alner et al., Nucl.Phys.B, 258, (1985), p. 505.[4] T.Sj¨ostrand and P.Z.Skands, arXiv:hep-ph / / o901.2852.[5] K.Werner et al., Phys.Rev.C, 74, (2006), p. 044902.[6] S.Z.Belenkji, L.D. Landau, Del Nuov.Cim., Suppl. Vol.III, Serie X, (1956), p. 15; G.A.Malekhin, Zh.Eksp.Teor.Fiz., 35, (1958), p. 1185;L.Van Hove, Phys.Lett.B, 118, (1982), p. 138; B.Andersson et al., Physica Scripta, 20, (1979), p. 10.[7] K.Aamodt et al., Eur. Phys. J. C, 68, (2010), p. 345.[8] CMS Collaboration, Eur. Phys. J. C, 72, (2012), p. 2164.[9] ALICE Collaboration, arXiv:nucl-ex / / /0610012.[12] R.Brun, F.Carminati, S.Giani, CERN-W5013.[13] B. Abelev et al., Phys.Rev.C, 88, (2013), p. 044910.[14] B. Abelev et al., Phys.Lett.B, 728, (2014), p. 25.[15] E.Schnedermann et al., Phys.Rev.C, 48, (1993), p. 2462.