Recent Heavy Ion results from ATLAS experiment Andrzej Olszewski (INP PAN Kraków) for the ATLAS Collabora@on International Nuclear Physics Conference Adelaide Convention Centre, Australia 11-16 September 2016
ATLAS Detector 44 m 25 m Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 2
ATLAS as a Heavy Ion Detector 1. Si Tracker Large coverage up to η < 2.5 Finely segmented pixel and strip detectors Tracking par6cles with p T 0.5 (0.1) GeV/c 2. High Resolu6on Calorimeters EMCal and HCal coverage up to η < 3.2 Forward Calorimeter (FCal) at 4.9 < η < 3.1 High p T probes Collision centrality Global event characteriza6on 3. Large Acceptance Muon Spectrometer Coverage up to η < 2.7 Muons from ϒ, J/ψ, Z 0 decays Heavy quarks, quarkonium suppression (J/Ψ,ϒ) Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 3
Heavy Ion Collisions at LHC The Large Hadron Collider (LHC) at CERN collides not only protons at highest energies in the world, but also lead ions and protons with lead ions This provides us with opportunity to study strongly coupled Quark Gluon Plasma (QGP) created in heavy ion collisions using so\ and hard probes Collec@ve response to ini@al condi@ons Modifica@on of energe@c parton showers in plasma Calibrate non-plasma effects in proton-lead and pp collisions Data collected by ATLAS at the LHC collider, used in Heavy Ion analyses Pb+Pb 2.76 TeV 2010 & 2011 160 μb 1 Pb+Pb 5.02 TeV 2015 680 μb 1 p+pb 5.02 TeV 2013 28 nb 1 p+p 2.76 TeV 2013 4 pb 1 p+p 5.02 TeV 2015 27 pb 1 p+p 13 TeV 2015 Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 4
Collec@ve phenomena Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 5
PbPb@2.76 TeV Harmonic flow detected first in measurements of single par@cle azimuthal angle asymmetries v n harmonics measured in the broad centrality, p T & η range Flow amplitudes having maximum in mid-central collisions and for par@cles with p T =3-4 GeV PRC 86, 014907 (2012) More general two par@cle η/φ correla@ons revealed new ridge and double-hump structures not seen before in pp collisions v n,n expansion parameters factorize to single par@cle ones, meaning they come from harmonic flow V n,n (p a T,pb T ) = v n (pa T )v n (pb T ) Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 6
2PC correla@ons: high p t jets (η/φ ~0) + low pt recoil (φ ~π) + long range ridge Study source of ridge in long range correla@ons by removal of recoil using ZYAM subtracted peripheral yield PRC 90, 044906 (2014) ppb@5 TeV Ridge distribu@on is studied by Fourier decomposi@on, v n,n parameters again factorize, agree with harmonic flow Comparing similar mul@plicity events in ppb and PbPb collisions, good agreement is found a\er rescaling to same mean p T and magnitude for v 2/4 Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 7
pp@2.76 and 13 TeV Ridge again visible, present in high mul@plicity pp collisions Large dijet correla@ons add with ridge and distort it, making ZYAM (zero at minimum) assump@on incorrect, so that it can not be used New template fit method developed Dijet recoil correla@ons accounted by scaled full Y per yields in peripheral collisions Long range ridge correla@ons measured by direct fit of sinusoidal harmonic func@on of second order PRL 116, 172301 (2016) Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 8
pp@2.76 and 13 TeV PRL 116, 172301 (2016) Like ppb, pp correla@on data are compa@ble with both near and away-side ridge, described by second order harmonic func@on The extracted Fourier coefficients, v 2;2, exhibit factoriza@on, characteris@c for a global modula@on in single-par@cle distribu@ons, as in ppb and PbPb The amplitudes, v 2, of this modula@on increase with p T for p T 3 GeV and then decrease at higher p T, similar to trend observed in ppb and PbPb They exhibit no dependence on beam energy and unlike in heavier ppb and PbPb systems, they have weak or no dependence on par@cle mul@plicity in the collision event Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 9
Flow in Small Systems Recently improved template analysis Analyzed new data for pp@5.02 TeV Reanalyzed with uniform method pp@13 TeV and ppb@5.02 TeV Confirmed pp collec@ve behavior similar to observed in heavier systems 13 TeV pp data has been described well by harmonic func@on with n=2 component only New method includes n=3 and 4 components as they are found small but not negligible ATLAS-CONF-2016-026 Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 10
System Size v n Dependence ATLAS-CONF-2016-026 v n pp very weakly depends on collision energy and mul@plicity v n pp v and v n ppb are very similar at low mul@plicity, but v n ppb increases with N ch mul@plicity a reflec@on of the increasing number of par@cipa@ng nucleons? v2 > v3 > v4 in all systems Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 11
p T Dependence of v n v 2 (p T ) shows a rise&fall, trend characteris@c for collec@ve flow observed in PbPb ATLAS-CONF-2016-026 Shape of the dependence is almost same in pp and ppb, a\er scaling pp to common max v 2 (p T ) in 5.02 and 13 TeV collisions agree v 3,4 (p T ) increase with p T both in 13 TeV pp and in ppb collisions; as for v 2 a faster increase is first observed in ppb collisions, but eventually they reach same maximum amplitudes in both systems Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 12
Forward-Backward Correlations Study long range correla@ons (LRC) between mul@plici@es in pseudorapidity bins, sensi@ve to early-@me density fluctua@ons in the longitudinal direc@on Separate short range correla@ons (SRC) coming from final state effects, by es@ma@ng their strength using ra@o of correla@ons for pairs of opposite to same charges, R(η 1,η 2 ) = C +- (η 1,η 2 )/C ±± (η 1,η 2 ), strong for SRC, weak for LRC Compare pp, p+pb and Pb+Pb collisions arxiv:1606.08170v1 [hep-ex] Full correla@on LRC pedestal SRC LRC PbPb ppb Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 13
Forward-Backward Correlations SRC contribu@on characterized by Δ SRC averaged over η space SRC dependence on N ch varying from PbPb to pp systems Power low scaling constant is smaller for smaller systems LRC are characterized by coefficients in Legendre decomposi@on, a n,n In all systems, only a 1 is important, measuring asymmetry in rapidity a 1 vary with N ch similarly in all systems, scales as predicted by independent cluster model (Berger, Nucl. Phys. B 85) Δ SRC ~ N ch -α, α~0.505, 0465, 0365 a 12 ~ N ch -α, α~0.5 arxiv:1606.08170v1 [hep-ex] Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 14
Hard probes Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 15
Heavy Quarks in QGP Produc@on of heavy flavor muons measured in PbPb@2.76 TeV collisions. Signal separated from background by consistency criterion p/pid = pid pms centered at zero pcalo(p,η,φ)/ pid Signal distribu@ons from prompt muons Background distribu@ons (non-prompt muons) shi\ed to large posi@ve values Heavy-quarks are produced at early @mes in the A+A collision High p T heavy-quarks are expected to lose energy similar to light quarks o mass dependent produc@on suppression Low p T heavy-quarks are expected to par@ally thermalize o Azimuthal anisotropy from interac@on with expanding medium Determine if heavy-quarks couple strongly or weakly to the QGP Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 16
Heavy Quarks in QGP - R AA R AA for HF muons Significant suppression observed in HF muons Suppression increasing with centrality, but smaller than in charged hadrons No p T dependence, different from charged hadrons R AA for charged hadrons ATLAS-CONF-2015-053 Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 17
Heavy Quarks in QGP v 2 v 2 for HF muons Significant v 2 is observed v 2 decreases with increasing p T, for mid-central events Smaller than inclusive hadron v 2, but similar in shape, both in p T and in centrality dependence v 2 for charged hadrons ATLAS-CONF-2015-053 Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 18
Modification of Jets in PbPb p T bins ATLAS-CONF-2015-055 High stat. 2013 pp reference used ƞ bins Enhancement observed in central collisions at low and (first @me significant) at high z/ p T. Reduc@on at mid-values. Modifica@ons decreasing for more peripheral collisions and (for high z/p T ) at higher pseudorapidity Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 19
Di-jet Asymmetry in PbPb New di-jet asymmetry measurement uses 2D unfolding to provide detector and UE free observable (truth jets) Centrality bins leading jet xj= p T2 / p T1 sub-leading In pp collisions (blue), most probable dijet configura@on is x J ~1, balanced dijets Similarly in peripheral PbPb Asymmetry increasing with centrality. In central PbPb collisions most probable configura@on for di-jets is for one jet to have HALF as much energy as the other one. Energy loss depends on path in medium ATLAS-CONF-2015-052 Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 20
Di-jet Asymmetry in PbPb Distribu@on of x J in pp becomes more narrow at high p T 1 with peak at x J ~1 Centrality 0-10%, p T bins Also in PbPb collisions asymmetry decreases for higher p T jets, restoring peak 1 at x J ~1 for p T > 200 GeV 1 Consistent with a picture in which the frac@onal energy loss decreases with jet p T ATLAS-CONF-2015-052 Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 21
Summary Azimuthal correla@ons measured in three systems: PbPb, ppb, pp o Improved 2PC analysis, abandoned ZYAM, use template fit o Results point to harmonic flow as a source of correla@ons in pp collisions with same paƒern of p T dependence (as in ppb) but insensi@ve to par@cle mul@plicity o Also long range forward-backward correla@ons found universal in these three systems Measured produc@on of heavy quarks in PbPb@2.76 TeV via muon decays o Found significant R AA suppression and flow v 2 magnitude similar to light quarks (but less varying with p T ), indica@ng a strong interac@on with maƒer Measured di-jet asymmetry and jet substructure with full bias unfolding o Proper@es of jets in PbPb@2.76 TeV are modified (wrt pp), with more par@cles seen at low and high z, as expected due to parton energy loss o Di-jet asymmetry in PbPb@2.76 TeV, increasing with centrality and disappearing for high p T jets, consistent with a picture of a parton frac@onal energy loss decreasing with its p T Adelaide, INPC 2016 Andrzej Olszewski, INP, Kraków 22