Measurements of multi-particle correlations and collective flow with the ATLAS detector

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1 Measurements of multi-particle correlations and collective flow with the detector omasz Bold AGH University of Science and enology, Krakow, Poland he measurement of flow harmonics of arged particles from v to v 7 in Pb+Pb collisions in the wide range of transverse momentum and pseudorapidity provides not only a way to study the initial state of the nuclear collisions and soft particle collective dynamics, but also provides insight into jet quening via the measurement of flow harmonics at high transverse momenta. he longitudinal fluctuations of and event-plane angles Ψ n are also presented. he longitudinal flow decorrelations have contributions from -magnitude fluctuations and event-plane twist. A four-particle correlator is used to separate these two effects. Results show that both effects have a linear dependence on pseudorapidity separation from v to v 5, and show a small but measurable variation with collision energy. While collectivity is well established in collisions involving heavy nuclei, its evidence in pp collisions is less clear. In order to assess the collective nature of multiparticle production, the correlation measurements are extended to include azimuthal correlations measured using multi-particle cumulants. he measurements of multi-particle cumulants c {- 8} confirm the evidence for collective phenomena in p+pb and low-multiplicity Pb+Pb collisions. For pp collisions, the measurements of cumulants do not yet provide clear evidence for collectivity as they are susceptible to event-by-event multiplicity fluctuations. A new modified cumulant method, whi suppresses both the contribution of multiplicity fluctuations and non-flow effects, is used to address this issue. PoS(EPS-HEP7)56 he European Physical Society Conference on High Energy Physics 5 July, 7 Venice Speaker. On behalf of the Collaboration c Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives 4. International License (CC BY-NC-ND 4.).

2 omasz Bold. Introduction he correlation studies in [] concentrate on detailed analysis of the QGP dynamics in Pb+Pb collisions. Smaller systems of p+pb or pp collisions are also in the area of interest where an unquestioned QGP signature has still to be found.. Correlation studies in Pb+Pb he arged-hadrons azimuthal flow coefficients of up to v 7 are measured in Pb+Pb data using the scalar product (SP) and two-particle correlations (PC) methods []. he measurement is performed in a wide range of transverse momenta, p for -8% most central events with the emphasise on the most central collisions Non-zero Preliminary values Pb+Pb, 5 µb are found p [GeV].3 for up Preliminary to vpb+pb, 7 in5 µb the intermediate p [GeV] ( 3)% s = 5. ev (3 4)% s = 5. ev (4 5)%.5 <.5.5 <.5.5 <.5 p range as shown in Figure. Results obtained with PC and SP methods are consistent. he inte-... grated harmonics vary little with centrality except for the v whi maximises at mid-centralities due to the sensitivity to the initial-state. eccentricities... ) (p.3.. Preliminary Pb+Pb v v 3 v 4 s =5. ev, µb v 5 v 6 pb [GeV] a.5<p <5 GeV < <5 (-5)%.3 Preliminary Pb+Pb, 5 µb.3 Preliminary Pb+Pb, 5 µb.3 Preliminary Pb+Pb, 5 µb ( 5)% s = 5. ev (5)% s = 5. ev ( )% s = 5. ev.5 <.5.5 <.5.5 < PoS(EPS-HEP7) Preliminary Pb+Pb, 5 µb p [GeV].3 Preliminary Pb+Pb, Preliminary 5 µb ( 5)% (5)%.3 Preliminary Pb+Pb, 5 µb ( )% s = 5. ev Pb+Pb, (5 6)% s 5 = µb 5. ev.5 (6 7)% s = 5. ev.5.5 <.5.5 (7 8)% s <.5.5 = 5. ev < Preliminary Pb+Pb, 5 µb.3 Preliminary Pb+Pb, 5 µb.3 centrality Preliminary [%] Pb+Pb, 5 µb p [GeV] ( 3)% s = 5. ev (3 4)% s = 5. ev (4 5)% s = 5. ev.5 <.5.5 <.5.5 < Preliminary Pb+Pb, 5 µb p [GeV].3 Preliminary Pb+Pb, 5 µb.3 In the Pb+Pb collisions the flow longitudinal decorrelation s is studied [3]. (5 6)% = 5. ev s he correlators Preliminary Pb+Pb, 5 µb (6 7)% = 5. ev.5 <.5.5 (7 8)% s = 5. ev <.5.5 <.5 sensitive the flow magnitude r n n;k and event-plane twist R. n n; are measured as a function of.. pseudorapidity,. he decorrelation of the magnitude shown in Figure for =6.5.5 and v 3 increases.5 with indicating breaking of PC factorisation. to -particle correlation. with. he effect. has different dynamics as a function of centrality for v and v 3. he r n n;k remains centrality independent for the latter. he effect is weaker at higher.5 center-of-mass energies..5 he R n n; correlators, shown < Preliminary Pb+Pb, 5 µb p [GeV] s = 5. ev.5.5 < p < 5. GeV.5 Figure : he as a function of p measured with PC (left), SP methods (center) and integrated over the p range of.5-5 GeV and pseudorapidity <.5 (left) as a function of collision centrality in Pb+Pb... collisions at s = 5. ev []. in Figure 3, show a similar magnitude and behaviour with and energy indicating that the eventplane twist has an equally significant contribution to the longitudinal dynamics Small system measurements 3. Sub-event cumulants he observation of the ridge structure in pp collisions at the LHC [4] opened the discussion on the presence of collectivity in small collision systems or a different origin of the observed phenomena. It was found that a method robust against local correlations due to jets or resonances has to be established to resolve the puzzle. For instance in the cumulants measurement not only

3 r r r r.95 Pb+Pb %, 7µb 5. ev,µb Pb+Pb -5% 5. ev Pb+Pb 5% 5. ev Pb+Pb -% 5. ev omasz Bold r ; r ; r ; r ; Pb+Pb -3% %, 7µb.9 Pb+Pb 5.% ev,µb, 7µb ev,µb r ; Pb+Pb -3%.9 Pb+Pb 5.-3% % ev, 7µb ev,µb Pb+Pb 5. ev,µb -3% 5. ev.5.5 r ; Pb+Pb 3-4% -5% Pb+Pb 5.-5% ev ev Pb+Pb 3-4% Pb+Pb % -5% ev ev.5.5 Pb+Pb 3-4% 5. ev.5.5 r ; Pb+Pb 4-5% 5% Pb+Pb 5.5% ev ev Pb+Pb 4-5% Pb+Pb % 5% ev ev.5.5 Pb+Pb 4-5%.5.5 r ; Pb+Pb -% 5-6% Pb+Pb 5. -% ev ev Pb+Pb 5-6% Pb+Pb 5. -% 5-6% ev ev.5.5 Figure.95 : he r n n;k correlator as a function of at two energies for n = (top) and n = 3 (bottom) in bins.95 of centrality Pb+Pb [3]. -5% Pb+Pb 5% Pb+Pb -%.95, 7µb Pb+Pb -3% Pb+Pb -5%, 7µb 5. ev,µb ev Pb+Pb 3-4% Pb+Pb 5% 5. ev ev 5. ev Pb+Pb 4-5% Pb+Pb -% Pb+Pb.9 -%, 7µb Pb+Pb -% 5. ev,µb 5..8 ev, 7µb ev,µb.5 5. ev.9 5. ev 5. ev.5.5 Pb+Pb 5-6% 5. ev ev the quantitative but also qualitative results may depend on an arbitrary oice of kinematics of Pb+Pb -4% Pb+Pb -4% 5. ev Figure 3: he R n n; correlator as a function of at two energies for n = (left) and n = 3 (right) in bins of.95 Pb+Pb -3% Pb+Pb 3-4% Pb+Pb 4-5% centrality [3]. reference particles [5]. A novel method of sub-event cumulants is devised to reduce the non-flow background on measured correlations [6]. In this method the event is partitioned into sub-events (two or three in case of pp measurements) and particles from disparate sub-events are used. In practice, the formulation of traditional cumulants is modified as follows: PoS(EPS-HEP7)56 {} n = e in(φ φ ) {} n a b = e in(φ a φ b ) (3.) {4} n = e in(φ +φ φ 3 φ 4 ) {4} n a b,c = e in(φ a +φ a φ b 3 φ c 4 ) (3.) c n {4} = {4} n {} n c a b,c n {4} = {4} n a b,c {} n a b {} n a c (3.3) where the particles are taken from three ranges of pseudorapidity into whi the full detector cover-

4 omasz Bold age <.5 is split. he angle brackets denote an event average while double angel brackets denote averaging over the event and then over many events. he flow coefficient, {4} = 4 cn {4}, is then only defined when the cumulant c n {4} assumes positive values while positive cumulant is an indicator of lack of collectivity. In the measurement [7], a cross-eck of this method on the p+pb collision system is performed as shown in the left panel of Figure 4 where the results obtained from the standard cumulant, two sub-events or three sub-events methods are consistent except for very peripheral collisions where only three sub-events method indicates collectivity (c {4} < ). he results of the method application to the pp system is shown in the right panel of Figure 4 where the three subevents method indicates presence of non-local correlation. he comparison of the measurements obtained with the PC method and three sub-event is shown in Figure 5 for the pp collisions at two energies as well as for p+pb. Flow at the level of 4% independent of the multiplicity is observed in all conditions. In comparison to the PC measurements deploying various teniques of subtracting non-flow contributions estimated from peripheral collisions the three sub-events results yield smaller v. he v 3 in pp system is found to be consistent with zero. c {4} p+pb 5. ev 8 nb 3 Standard method -subevent method 3-subevent method.3<p <3 GeV N for.3<p <3 GeV c {4} pp 5. ev.7 pb 5 Standard method -subevent method 3-subevent method.3<p <3 GeV N for.3<p <3 GeV Figure 4: he c {4} measured with the standard, two sub-events and three sub-event methods in p+pb collisions (left) and pp collisions (right) [7]. v..5 pp, s = 5. ev,.7 pb.3<p <3 GeV N for.3<p <3 GeV pp, s = 3 ev,.9 pb.3<p <3 GeV N for.3<p <3 GeV p+pb, s = 5. ev, 8 nb.3<p <3 GeV N for.3<p <3 GeV v {} template fit v {} peripheral subtraction v {4} three-subevent method PoS(EPS-HEP7) Figure 5: he v measured in pp at s = 5. ev (left), at 3 ev (centre) and p+pb at s = 5. ev (right) with a sub-event cummulant method as well as wiht PC methods with two non-flow removal procedures based on peripheral collisions [7]. 4. Summary In these proceedings correlation measurements in Pb+Pb, p+pb and pp collision systems are presented. measures flow harmonics up to v 7 in a broad range of p. he lon- 3

5 omasz Bold gitudinal decorrelations are studied in Pb+Pb at.76 and 5. ev. It is found that the slope of this decorrelation is slightly bigger at lower energies. Both, the flow magnitude ange and the event-plane twist influence the decorrelation at a similar level. For small collision systems a novel method of sub-event cumulants is proposed in order to robustly eliminate non-flow effects. he v coefficient is found to be about 4% in a wide range of event activity quantified by arged particle multiplicity in pp as well as in p+pb collisions. Acknowledgements his work was supported in part by the National Science Center grant UMO-5/9/B/S/96 and by PL-Grid Infrastructure. References [] Collaboration, JINS 3 (8) S83. [] Collaboration, -CONF-65, [3] Collaboration, arxiv:79.3 [nucl-ex]. [4] CMS Collaboration, JHEP 9 () 9 [arxiv:9.4 [hep-ex]]. [5] Collaboration, Eur. Phys. J. C 77 (7) no.6, 48 [arxiv: [hep-ex]]. [6] J. Jia, M. Zhou and A. rzupek, arxiv:7.383 [nucl-th]. [7] Collaboration, arxiv: [hep-ex]. PoS(EPS-HEP7)56 4