ATLAS Heavy Ion Physics Andrzej Olszewski (INP PAN Kraków) for the ATLAS Collaboration

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków2 Heavy Ion Studies with the ATLAS Detector Building of ATLAS detector is progressing well First pp collisions in 2008 LHC physics program includes four weeks of heavy ion physics running per year The primary collision system is Pb+Pb at 5.5. TeV/n, considered also p+Pb and p+p at 5.5 TeV Heavy Ion beams expected to be commissioned in 2009, first significant heavy ion running in 2009/2010

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków3 Heavy Ion Collisions at LHC RHIC: Energy 200 GeV/n observations Strong quenching of high transverse momentum particles Near-Perfect liquid behavior of a collective motion in medium Collective motion of hadrons generated at parton level Parton saturation manifested in low multiplicity of final hadrons LHC: Energy 5.5 TeV/n opportunities Initial energy density ~5 times higher Lifetime of a quark-gluon plasma much longer Large rates of hard probes over a broad kinematical range Discovery of two new forms of QCD matter sQGP – strongly coupled Quark-Gluon Plasma CGC – a saturated gluon initial state (QGP-source) Era of quantitative experimental exploration of thermal QCD

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków4 Unprecedented acceptance for A+A physics both in p T and rapidity, with full azimuthal coverage ATLAS Acceptance High p T probes Muons from , J/ , Z 0 decays Tracking particles with p T  1.0 GeV/c Global event characterization Heavy quarks, quarkonia

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków5 ATLAS Heavy Ion Program Measure dN ch /d , dE T /d  (total+EM) –Characterize gross properties of initial state –Test saturation predictions Measure charged, inclusive ,  0 elliptic flow –Probe early collective motion of (s/t/w)QGP Measure jets, jet fragmentation,  -Jet, di-jet, … –Precision tomography of QGP & its properties –Medium effects in jet quenching Measure quarkonia production rates via  +  - decays –Probe Debye screening in medium Study low x hard processes in p-p, p-A –Study factorization violations, saturation

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków6 Histogram – true N ch Points – reconstructed N ch N ch (|  | < 3) 10% <3% Global Event Characterization Day-one measurements: N ch, dN ch /d ,  E T, dE T /d , b dN ch /d  |  =0  3200 (HIJING, no quenching) dN ch /d  |  =0  6000 (HIJING, with quenching) Reconstruction errors ~5% Single Pb+Pb event, b =0-1fm Tracklets in pixel detector

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków7 Fourier expansion of the azimuthal distribution of particles: Isotropic radial flow Anisotropic flow v 1 : directed v 2 : elliptic View of a A+A collision with impact parameter b  0 Elliptic Flow Distribution of azimuthal angle  (v 2 ) vs true reaction plane position,  R y’ x’ x y RR b V 2 =0.042 -R-R Correlation of signals with flow Generated v 2 = EM EndCap Calo 0.036EM FCAL Calo 0.029EM Barrel Calo 0.032Hit clusters, Pixel layer Hit clusters, Pixel layer Hit clusters, Pixel layer 2  V 2 (  R )  Data Type 0.025HAD FCAL Calo

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków8 Flow v 2 vs   Flow v 2 vs N ch N ch Event Plane Method Method: v 2   cos[2(  -  R )]  Reaction plane estimated from flow Reconstructed flow is close to the input 5% flow Reconstructed v 2 is flat against , N ch Remaining difference (~10%) is due to non - flow correlations and will be accounted for by MC corrections =0=0

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków9 Lee-Yang-Zeroes Method

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków10 Jet Studies Goal is to determine medium properties. Jet quenching Need to measure jet shapes: - Fragmentation function using tracking - Core ET and jet profile using calorimeters - Neutral leading hadrons using EM calorimeters LHC: Copious hard radiation in high Q 2 final-state parton showers Both an opportunity and a challenge –Understanding jet quenching more difficult –Potentially: time-dependent probe of medium Resolving hard radiation in jets a must! Fine segmentation of first EM sampling layer helps Jet Background  x  = x 0.1 All too wide for single photons

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków11 Jet Energy Resolution Study of different event samples embedded into central Pb+Pb HIJING (b=0-2 fm) Results obtained using a standard pp cone algorithm Another possibility is studied − Fast K T Jet Finder

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków12 Color screening prevents various ψ, , χ states to be formed when T→T c to QGP (color screening length < size of resonance) Quarkonia suppression Measurements of suppression patterns in production of heavy quarkonia states are an ideal thermometer for the plasma Important to separate  (1s) and  (2s) states!

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków13  +  - reconstruction |  |  2 For |η| < 2 (12.5% acc+eff) we expect 15K  /month of 10 6 s at L=4  cm -2 s -1 p T  >3 GeV |  |  1|  |  2|  |  2.5 Acceptance + efficiency 4.7%12.5%17.5% Resolution123 MeV145 MeV159 MeV S/B S/√ S+B Rate/month5,70015,00021,200 Low momentum muons measured by tagged ID tracks Identified by coincidence with track segment in  -spectrometer

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków14 J/  +  - reconstruction A trigger with muon p T >1.5 GeV is more efficient if torroidal field is reduced for HI runs. The mass resolution is 15% worse but we gain a factor 2-3 in statistics |  |  2.5, p T   1.5 GeV We expect 8K to 100K J/  +  - per month of 10 6 s at L=4  cm -2 s -1 |  |  2.5 p T  > 3 GeVp T  > 1.5 GeV Acceptance + efficiency 0.055%0.530% Resolution68 MeV S/B S/√ S+B Rate/month 11, ,000

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków15 Low-x Physics with ZDC π 0 acceptance ZDC will be used for centrality and Ultraperipheral  (  -  and  -nucleon) Pb+Pb collisions ZDC reconstructs also neutral particles at very high rapidities: physics processes at very low x, e.g. Color Glass Condensate

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków16 Summary  ATLAS Heavy Ion physics program addresses primary physics questions of interest at the LHC  Global observables, including elliptic flow, should be accessible from day-one, even with a very low luminosity (early scheme)  Jet physics is very promising with Atlas unique capabilities of measuring isolated direct photons, separating jets from heavy ion background, measuring jet shapes, hard radiation components  Z+jet, γ+jet, jet-jet correlations  Heavy-quarkonia physics with capability to measure and separate  and  ’, J/  using a specially developed  tagging method  Low-x physics & Ultra-peripheral collisions  Heavy quarks (esp. b physics)

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków17 BACKUPS

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków18 Summary of Ions CollisionR (fm) Luminosity (cm -2 s -1 ) dN ch /dy (maximum) Interaction rate p+p~11x10 34 <2501 GHz 208 Pb+ 208 Pb7.11x10 27 <80008 kHz 40 Ar+ 40 Ar4.16x10 28 < kHz p+ 208 Pb1x10 30 <1502 MHz p+ 40 Ar1x10 31 <1206 MHz The desired species for a systematic HI study are as follow In addition different colliding energies would provide for the study of different energy densities.

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków19 IonMassdN/dyR (fm)Luminosity Pb208<8, Sn x10 28 Kr84< x10 28 Ar40< O p+Pb,Ar< d+Pb,Ar10 31 p+p Beam Energy: 2.75 and 1.00 Tev/nucleon

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków20 Calorimeters Granularity ATLAS calorimeters covers a large pseudo-rapidity range |  |<5.0 Both EM and Hadronic calorimeters are segmented longitudinally in several compartments. The first section of the EM calorimeter is finely segmented in eta strips. Coverage |  |<3.2 Segmentation Long.3 Segmentation  x01 Segmentation  x0.025 Segmentation  x0.25 Coverage |  |<1.7 Segmentation Long.3 Segmentation  1 0.1x0.1 Segmentation  2 0.1x0.1 Segmentation  3 0.2x0.1 Coverage 3.1<  <4.9 Segmentation Long.4 Segmentation  (all) 0.2x0.2 EM Barrel and EndcapHadronic Tile Hadronic LAr Forward Calorimeter Coverage 1.5<  <3.2 Segmentation Long.4 Segmentation  1 0.1x0.1 Segmentation  2 0.2x0.2

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków21 Examples of Calorimeter Performance Electromagnetic Energy Resolution EM Angular Resolution EM Timing Resolution Hadronic Calorimeter Energy Resolution The above performance was achieved with test beam modules

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków22 Detector Occupancies b = 0 – 1fm Si detectors: Pixels < 2% SCT < 20% TRT: too high, unusable (limited usage for PbPb collisions is under investigation) Muon Chambers: 0.3 – 0.9 hits/chamber (<< pp at cm -2 s -1) Calorimeters ( |η|< 3.2 ) On average: ~ 2 GeV/Tower ~.3 GeV/Tower

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków23 Track Reconstruction -Pixel and SCT detectors -Threshold p T  1 GeV -Tracking cuts: -At least 10 hits out of 11(13) available in the barrel (end-caps) -All three pixel hits -At most 1 shared hits -  2 /dof > 4 For p T : GeV/c: efficiency ~ 70 % fake rate < 1% Momentum resolution ~ 3% (2% - barrel, 4-5% end-caps) Efficiency Ghosts Fakes

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków24 Jet Studies with Tracks -Jets with E T = 100 GeV -Track p T > 3 GeV Fragmentation function Momentum component perpendicular to jet axis PbPb  HIJING-unquenched  pp dN/dj T broader in PbPb than in pp (background fluctuations)

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków25 How to measure  ?  Global method (A): use tracks fully traversing the  -spectrometer, which allows momentum measurement in the standalone  -spectrometer, then associate with ID tracks through a global fit.  Tagging method (B): select ID tracks whose extrapolation coincide with a track segment in the  -spectrometer.  Advantage of A over B: better p measurement (true for Z 0,not J/ ,  ), better purity.  Advantage of B over A: lower p threshold => better acceptance (3 instead of 4 GeV).  For this study, A+B are used, with a priority to method A when possible. Selection of pairs with at least one  from method A. Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków25

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków26  +  - reconstruction |  |  2 For |η| < 2 (12.5% acc+eff) we expect 15K  /month of 10 6 s at L=4  cm -2 s -1 A di-muon trigger study is under way Global fit Global+tag p T  >3 GeV |  |  1||  2||  2|  |  2.5 Acceptance + efficiency 2.6% 4.7% 8.1% 12.5% 12.0% 17.5% Resolution123 MeV145 MeV159 MeV S/B S/√ S+B Rate/month 10,000 15,000

Isola d'Elba, HCP 2007Andrzej Olszewski, INP, Kraków27 J/  +  - reconstruction If a trigger is possible forward with a muon p T >1.5 GeV, we gain a factor 4 in statistics…A solution might be to reduce the toroidal field for HI runs |  |  2.5, p T   1.5 GeV We expect 8K to 100K J/  +  - per month of 10 6 s at L=4  cm -2 s -1 Global fit Global+tag |  |  2.5 p T  > 3 GeVp T  > 1.5 GeV Acceptance + efficiency 0.039% 0.055% 0.151% 0.530% Resolution68 MeV S/B S/√ S+B Rate/month 8,000 11,000 30, ,000