Two- and Mul,-par,cle Azimuthal Correla,ons in Small Collision Systems with the ATLAS Detector

Transcription

1 Two- and Mul,-par,cle Azimuthal Correla,ons in Small Collision Systems with the ATLAS Detector Adam Trzupek on behalf of the ATLAS experiment Ins$tute of Nuclear Physics PAS XXVI Interna$onal Conference on Ultrarela$vis$c Heavy-Ion Collisions (Quark MaAer 2017) Chicago, USA, 5 th - 11 th February 2017

2 Ridge in High Mul,plicity p+pb and pp Collisions 2 ATLAS, PRC 90, (2014) p+pb ridge ATLAS, PRL 116, (2016) pp ridge Strongly interac$ng QGP in small systems? arxiv: v n in 5 and 13 TeV pp and 5 TeV p+pb from 2PC ATLAS-CONF mul$-par$cle correla$ons in 5 and 13 TeV pp, 5 TeV p+pb and low-mul$plicity 2.76 TeV Pb+Pb

3 ATLAS Detector Three main subsystems: Inner Detector (ID) tracking η <2.5 Calorimetry η <4.9 FCal 3.1< η <4.9 Muon Spectrometer - η <2.7 Fast trigger systems: Level 1 (L1) High Level Trigger (HLT) 3 FCal

4 Two-par,cle Correla,ons (2PC) in 5 and 13 TeV pp and 5 TeV p+pb (arxiv: ) Two-par$cle Correla$on func$on: Per-trigger par$cle yield 4 S and B are the same and mixed event pairs distribu$ons The average number of associated par$cles per trigger par$cle in Δϕ bin Data samples: 13 TeV pp (64 nb 1 ), for 5 TeV pp (170 nb 1 ) and 5 TeV p+pb (28 nb 1 ) Δη >2 arxiv:

5 Template-fiVng Method To separate the ridge from other correla$ons (e.g. dijets), ATLAS developed a template fiing procedure (PRL 116, (2016)) Template fit func$on (with 2 free parameters v n,n, F): Y templ (Δϕ) = Y ridge (Δϕ) + F Y periph (Δϕ) azimuthal modula$on N ch rec <20 5 = + FY(Δϕ) low Δϕ no ZYAM G (1 + 2Σv n,n cos(nδϕ)) no suppressed zero Y templ successfully describes Y distribu$ons The factoriza$on works well in different N ch rec and p T -ranges v n,n (p T a,p T b )= v n (p T a )v n (p T b ) Δϕ φ Δϕ

6 System Size, Energy and N c Dependence of v n 6 6 v n pp has only a weak dependence on energy and mul$plicity v n pp and v n p+pb are similar at low mul$plicity, but v n p+pb increases with N ch rec v 2 >> v 3 >> v 4 for all systems arxiv:

7 p T Dependence of v n 7 v 2 (p T a )= v 2,2 (p T a,p T b )/ v 2,2 (p T b,p T b ) v 2 (p T ) in 5.02 and 13 TeV pp collisions agree v 2 (p T ) shows a rise & fall, trend characteris$c for collec$ve flow observed in PbPb v 3,4 (p T ) in 5 TeV p+pb and 13 TeV pp collisions rise with p T but a faster increase is observed for p+pb system arxiv:

8 v 4 /v 2 2 Ra,o in 13 TeV pp and 5.02 TeV p+pb Collisions 8 arxiv: The ra$o is constant for both systems but is higher in pp than in p+pb collisions stronger non-linear coupling in pp

9 Mul,-par,cle correla,ons (ATLAS-CONF ) 9 To directly explore collec$vity in small systems, mul$-par$cle correla$ons are measured in 5 and 13 TeV pp, 5 TeV p+pb and lowmul$plicity 2.76 TeV Pb+Pb collisions (k=1,2,3,4): corr n {2k} e in [ (ϕ 1 Φ n )+.. (ϕ k+1 Φ n )...] = cos[n(ϕ ϕ k+1...)] = v n 2k Using cumulants suppresses non-flow correla$ons, e.g. c n {4} = corr n {4} 2 corr n {2} Inner/outer brackets denote averaging over tracks/events Cumulants are measured for reference par$cles in two p T ranges: GeV and GeV Averaging over events is performed in unit bins of the number of reference par$cles, M ref (main selec$on, EvSel_M ref ) Results are compared in <N ch (p T >0.4GeV)> (corrected for efficiency) 2 v n {4} = 4 c n {4} nega$ve

10 Event Selec,on for Cumulant Calcula,ons 10 ATLAS-CONF c 2 {4} cumulant for 13 TeV pp is posi$ve except for the <N ch >range of: , where c 2 {4}<0 within 1 2 σ Collec$ve effects could be hidden by non-flow correla$ons For performance study, different event selec$on in unit bins of the number of par$cles with p T >0.4 GeV was considered (EvSel_N ch ) c 2 {4} is nega$ve for <N ch > 60 non-flow correla$ons associated with mul$plicity fluctua$ons may give nega$ve contribu$ons to measured c 2 {4} in EvSel_N ch

11 The Ellip,c Flow Cumulant c 2 {4} ATLAS-CONF pp cumulants are posi$ve Larger c 2 {4} for higher p T -> indica$ve for non-flow correla$ons In 3 sub-event cumulants non-flow is significantly reduced ATLAS-CONF ( ) cos φ 1 a +φ 2 a φ 3 b φ 4 c 3 sub-event c 2 {4} cumulants are nega$ve for 5 and 13 TeV pp collisions See M. Zhou talk on Wednesday pp 13 TeV

12 The Ellip,c Flow Cumulant c 2 {4} 12 ATLAS-CONF For p+pb and Pb+Pb collisions, for c 2 {4} < 0 c 2 {4} is larger at higher p T -> relevant for collec$ve flow

13 System Size Dependence of c 2 {4} 13 ATLAS-CONF c 2 {4} pp < c 2 {4} ppb < c 2 {4} PbPb

14 Ellip,c Flow Harmonics v 2 14 v 2 {2k} of mul$ par$cle cumulants (k=1,2,3,4) provide important informa$on on ini$alstate anisotropy ATLAS-CONF v 2 harmonics are larger for Pb+Pb than for p+pb v 2 {4}, v 2 {6} v 2 {8} are similar for p+pb as well as for Pb+Pb

15 Ra,os: v 2 {4}/v 2 {2, Δη >2}, v 2 {6}/v 2 {4}, v 2 {8}/v 2 {6} 15 ATLAS-CONF For Pb+Pb collisions all three ra$os are independent of <N ch > for N ch >120 v 2 {6}/v 2 {4}, v 2 {8}/v 2 {6} are close to 1 but v 2 {6}/v 2 {4} < v 2 {8}/v 2 {6} v 2 {4}/v 2 {2, Δη >2} 0.85 (0.7) for Pb+Pb (p+pb) collisions Significant fluctua$ons in ini$al geometry in both systems (Li Yan and Jean-Yves Ollitrault, PRL 112 (2014) )

16 v 3 and v 4 Harmonics 16 ATLAS-CONF v 3 {2, Δη >2} is similar for p+pb and Pb+Pb collisions v 3 {2, Δη >2} pp <v 3 {2, Δη >2} for p+pb and Pb+Pb collisions v 4 {2, Δη >2} pp is comparable to that for p+pb and Pb+Pb collisions, for <N ch > < 120 For <N ch > > 120: v 4 {2, Δη >2} ppb < v 4 {2, Δη >2} PbPb

17 Comparison to Other Results 17 ATLAS-CONF For 13 TeV pp collisions CMS c 2 {4} cumulant is roughly consistent with ATLAS result for EvSel_N ch selec$on method but it has a larger magnitude than c 2 {4} for the selec$on EvSel_M ref For p+pb and Pb+Pb collisions, ATLAS and CMS v 2 {2, Δη >2}, v 2 {4}, v 2 {6}, v 2 {8} agree very well

18 Summary 18 Using a template fit method ATLAS has observed v 2, v 3 and v 4 harmonics in 5 and 13 TeV pp collisions v n pp are constant with mul$plicity and energy v n pp and v n p+pb are similar at low mul$plicity, but v n p+pb increase with mul$plicity v 2 pp and v 2 p+pb have similar p T dependence c 2 {4} cummulants measured in 5 & 13TeV pp are posi$ve -> collec$ve effects could be obscured by non-flow correla$ons Mul$plicity fluctua$ons tend to shi cumulants to nega$ve values For 5 TeV p+pb and 2.76 TeV Pb+Pb collisions collec$vity is supported by v 2,3,4 {2, Δη >2}, v 2 {4}, v 2 {6}, v 2 {8} measurements v 2 {4}, v 2 {6} v 2 {8} are similar The data provide useful constraints on the theore,cal modeling of the underlying mechanism

19 Template-fitting Method Fit func$on successfully describes Y distribu$ons in all N ch rec intervals If v n,n cos(n φ) modula$on arises from modula$on of the singlepar$cle φ distribu$ons, then v n,n should factorize: v n,n (p T a,p T b )= v n (p T a )v n (p T b ) The factoriza$on works well in different N ch rec and p T -ranges arxiv: