Studying collective phenomena in pp and A-A collisions with the ALICE experiment at the LHC

Transcription

1 Studying collective phenomena in pp and A-A collisions with the ALICE experiment at the LHC Ivan Ravasenga Politecnico di Torino and I.N.F.N. 56 th course, Erice (Sicily, Italy) Ivan Ravasenga

2 Collectivity & Two-particle angular correlations Pb-Pb p-pb pp Pb-Pb collisions Bulk-dominated regime, where hydrodynamic modelling gives a good description of the data p-pb collisions Excess structure in the correlation forms two ridges. Consistent with Colour Glass Condensate based predictions and hydro model calculations pp collisions (high mult.) Ridge-like structure visible at Δφ 0 At higher p T trig (5-6 GeV/c) the ridge almost disappears. Qualitatively similar to what is observed in central Pb-Pb collisions at LHC energies. Completely unexpected in minimum bias pp collisions and pp MC models 2

3 Outline ❶ALICE experimental apparatus ❷Transverse-momentum spectra ❸Proton-to-pion ratio ❹Blast-wave model ❺Comparison to theoretical models 3

4 A Large Ion Collider Experiment Particle ID. HMPID Cherenkov angle: σ ~ 3 mrad ITS PID via de/dx, tracking and vertexing σ de/dx ~ % TOF Time-of-flight: σ TOF ~ 80 ps V0 Trigger and Trigger + centrality centrality determination by slicing the measured signal determination amplitude distribution. Centrality/multiplicity classes defined as the percentile of the hadronic cross section. TPC Tracking, PID through de/dx (σ de/dx ~ 5%) 4

5 Transverse momentum spectra Pb-Pb Pb-Pb Spectra in Pb-Pb: the spectra become harder as the multiplicity increases and the change is most pronounced for heavier particles Effect of radial flow 5

6 Transverse momentum spectra Pb-Pb Pb-Pb Xe-Xe Spectra in Pb-Pb: the spectra become harder as the multiplicity increases and the change is most pronounced for heavier particles Effect of radial flow Spectra in Xe-Xe: lower p T reach but, at low p T, similar behaviour as in Pb-Pb 6

7 Transverse momentum spectra Pb-Pb Pb-Pb pp Spectra in Pb-Pb: the spectra become harder as the multiplicity increases and the change is most pronounced for heavier particles Effect of radial flow Spectra in pp: softer compared to Pb-Pb. A shape dependence across multiplicities is observed. 7

8 Proton-to-pion ratios Pb-Pb, Xe-Xe Pb-Pb 5.02 TeV vs Xe-Xe 5.44 TeV Typical flow bump at around p T = 3 GeV/c, more evident in central collisions Compatible structure in the two colliding systems 8

9 Proton-to-pion ratios Pb-Pb, Xe-Xe pp Pb-Pb 5.02 TeV vs Xe-Xe 5.44 TeV Typical flow bump at around p T = 3 GeV/c, more evident in central collisions Compatible structure in the two colliding systems pp 13 TeV Similar flow-like feature, the peak is more suppressed compared to A-A Multiplicity dependence is observed as in A-A 9

10 Blast-wave fit to spectra Boltzmann-Gibbs blast-wave model: a three parameters simplified hydrodynamical model* * Phys. Rev. C 48 (1993) 2462 E d3 N dp 3 න p T sinh ρ m T cosh ρ Rm T I 0 K 0 T 1 r dr kin β T m T = m p T ρ = tanh 1 r β T β T (r) = β s R n The resulting spectrum is a superposition of individual thermal sources, each boosted with the boost angle ρ n: exp. of velocity profile profile T kin : kinetic freeze-out temperature β T r : transverse velocity distribution β s : surface velocity ρ: boost angle Xe-Xe Simultaneous Boltzmann-Gibbs fit to π, K and p using Pb-Pb 2.76 TeV fit ranges Good description of data Clear manifestation of strong radial flow in central heavy-ion collisions 10

11 Blast-wave fit to spectra in pp, p-a, A-A Large systems Larger β T for central Pb-Pb collisions Comparable T kin and β T in Pb-Pb and Xe-Xe collisions at a similar dn ch /dη 11

12 Blast-wave fit to spectra in pp, p-a, A-A p-pb Stronger radial gradients Large systems Larger β T for central Pb-Pb collisions Comparable T kin and β T in Pb-Pb and Xe-Xe collisions at a similar dn ch /dη Small systems p-pb & pp vs A-A p-pb and Pb-Pb show a similar trend consistent with the presence of radial flow in p-pb collisions. At similar dn ch /dη, o o comparable T kin for p-pb and Pb-Pb, whereas β T is significantly higher in p- Pb (color reconnection effects under study) pp and p-pb shows a similar trend and values are comparable Larger T kin in pp (~ MeV) 12

13 [Pb-Pb 0-5 %] Spectra compared to models iebe-vishnu (arxiv: v1; Phys.Rev. C92, (2015) & (R) (2015)) Viscous hydrodynamics (QGP expansion) + Hadron cascade model (UrQMD) to simulate the evolution of the hadron resonance gas Trento initial conditions: effective model where entropy is deposited proportional to the generalized mean of nuclear overlap density AMPT initial conditions: initial state includes fluctuations at the nucleonic and subnucleonic levels and considers pre-equilibrium dynamics of partonic matter. Good agreement at low p T 13

14 [Pb-Pb 0-5 %] Spectra compared to models EPOS-LHC (Phys.Rev. C 92, (2015)) Non uniform fireball divided in the core (high density) and corona (lower density). Describes better particle ratios in central Pb-Pb collisions McGill (Phys. Rev. C 95, (2017)) IP-Glasma initial condition matched to hydrodynamic variables and evolved using viscous hydrodynamic model (MUSIC). Good agreement at low p T 14

15 [p-pb 5-10 % and pp] Comparison to models p-pb 5-10% pp 1 Bozek, PRC 85, (2012) 2 Roesler et al., arxiv:hep-ph/ arxiv: v1 4 arxiv: v3; 5 arxiv: v1 p-pb collisions (Phys. Lett. B 728 (2014) 25-38) Kraków 1 : event-by-event (3+1)-D perfect fluid hydrodynamic Reproduces particle spectra reasonably well DPMJET 2 : QCD-inspired model based on Glauber- Gribov formalism Fails to reproduce particle spectra EPOS-LHC: pi, K and p reasonably well reproduced especially at low p T pp collisions PYTHIA 8 3 (with CR) describes better integrated particle ratios. EPOS-LHC agrees better in low p T ranges DIPSY 4 with color ropes correctly reproduces the p/π shape at low p T better agreement at higher p T and low dn ch /dη HERWIG7 5 is an event generator that performs simulations at next-to-leading order in QCD Fails to describe data 15

16 Summary Particle transverse-momentum spectra and angular correlations have been measured by ALICE in pp, p-pb, Xe-Xe and Pb-Pb at different collision energies Hardening of particle spectra with increasing event multiplicity Radial flow Radial flow in p-pb from Blast-wave model Flow in p-pb Ridge-like structure observed in 2-particle angular correlation in pp collisions Collectivity in pp? Color reconnection effects are under investigation Mimic radial flow Blast-wave model Similar kinetic freeze-out temperature in Pb-Pb and p-pb at similar dn ch /dη, but larger β T in small systems Larger β T in p-pb, pp Low-p T particle production is described better by hydrodynamical models or by QCD-inspired models which go beyond an incoherent superposition of parton-parton scatterings (e.g. via color reconnection, color ropes or corecorona) 16

17 Backup slides 17

18 Event centrality & multiplicity in ALICE pp, p-pb, Pb-Pb Centrality/multiplicity defined as the percentile of the hadronic cross section corresponding to a particle multiplicity above a given threshold. Event multiplicity classes defined from the amplitude of the signal in the VZERO detectors. Pb-Pb The centrality of the collision is directly related to the impact parameter (b). Pb b Pb Transverse plane view Pb-Pb p-pb p p pp 18

19 Blast-wave fit to spectra Boltzmann-Gibbs blast-wave model: a three parameters simplified hydrodynamical model* * Phys. Rev. C 48 (1993) 2462 E d3 N dp 3 න p T sinh ρ m T cosh ρ Rm T I 0 K 0 T 1 r dr kin β T m T = m p T ρ = tanh 1 r β T β T (r) = β s R n The resulting spectrum is a superposition of individual thermal sources, each boosted with the boost angle ρ n: exp. of velocity profile profile T kin : kinetic freeze-out temperature β T r : transverse velocity distribution β s : surface velocity ρ: boost angle pp Simultaneous Boltzmann-Gibbs fit to π, K and p using Pb-Pb 2.76 TeV fit ranges Good description of data in the fit range 19

20 Particle ratios in Pb-Pb (2.76 vs 5.02 TeV) p/π K/π p/π: indication of a slightly higher radial flow in central collisions compared to lower energy. K/π: no significant change observed in the comparison of the two energies 20

21 Particle ratios pp Pb-Pb no energy dep. energy dep. No energy dependence pp at 2.76, 5.02, 7 and 13 TeV K/π: no significant change with s p/π: shift of the maximum towards higher p T with increasing s in the intermediate p T region. Pb-Pb at 2.76 vs TeV Indication of a slightly higher radial flow in central collisions compared to lower energies. pp (approx. baseline), p-pb and Pb-Pb* K/π: same within sys and stat uncertainties. p/π: similar flow-like features for p Pb and Pb Pb systems. *Phys. Lett. B 760 (2016) 720 Above pp All Same for all systems 21

22 Particle ratios in pp vs Pythia 8 generator K/π: no significant change with s. p/π: in the intermediate p T region the peak shifts towards higher p T with increasing s Comparison to Pythia8 Monash 2013 (arxiv: v1) In general Pythia 8 event generator overestimates p/π and underestimates K/π in a percentage depending on p T. 22

23 Yield ratio in pp vs s They include new preliminary points in pp at s = 5.02 TeV For s > 0.9 TeV no dependence on the centre-of-mass energy is seen within the uncertainties 23

24 p T -integrated yield ratios p/π p/π Eur. Phys. J. C75 (2015) no. 5, 226 K/π No significant evolution is observed with respect to lower energy data. K/π and p/π measured in peripheral Pb-Pb collisions are compatible with the ones in pp and p-pb collisions. No significant evolution of ratios with multiplicity is observed. In Pb-Pb collisions they are centrality independent 1 system-size independence. [1] Phys. Rev. C 93(3) (2016)

25 Comparison with models pp p-pb 5-10% p-pb collisions (Phys. Lett. B 728 (2014) 25-38) Krakow 1 : event-by-event (3+1)-D perfect fluid hydrodynamic Reproduces particle spectra reasonably well DPMJET 2 : QCD-inspired model based on Glauber-Gribov formalism Fails to reproduce particle spectra EPOS-LHC: pi, K and p reasonably well reproduced especially at low p T pp collisions Pythia 8 3 generator overestimates p/π and underestimates K/π 1 Bozek, PRC 85, (2012) 2 Roesler et al., arxiv:hep-ph/ arxiv: v1 25

26 Comparison with models Pb-Pb 60-80% Pb-Pb 70-80% EPOS-LHC (Phys.Rev. C 92, (2015)) Non uniform fireball divided in the core (high density) and corona (lower density). Describes better π, K, K/π in peripheral Pb-Pb collisions iebe-vishnu with TRENTo and AMPT initial conditions Good agreement at low p T 26

27 [Pb-Pb 2.76 TeV] Particle ratios vs models Kraków: good description of particle ratios Fries et al. 1 includes recombination processes and pqcd at high p T Good description of p/π between 4 an 7 GeV/c EPOS: describes better p/π. For K/π, good agreement only at low p T Tends to overestimates the peaks 27 1 Phys. Rev. C 68, (2003)

28 Particle ratios Pb-Pb pp p-pb Pb-Pb Low mult. High mult. Pb-Pb at 2.76 vs 5.02 TeV Indication of a slightly higher radial flow in central collisions compared to lower energies. pp, p-pb and Pb-Pb p/π: similar flow-like features for pp, p-pb and Pb-Pb systems 28

29 p T -integrated yield ratios vs multiplicity p/π K/π [Pb-Pb] No significant evolution is observed with respect to lower energy data. K/π and p/π measured in peripheral Pb-Pb collisions are compatible with the ones in pp and p-pb collisions The chemical composition is independent of collision system at same dn ch /dη 29