The Heavy-Ion Physics Programme with the ATLAS Detector & new results from WA98 Laurent Rosselet Cartigny, September 14 th 2007

At  s = 200 GeV/A, QGP is unexpectedly a perfect fluid with no viscosity Pb+Pb at LHC:  s = 5.5 TeV/A  1148 TeV for central collisions T~500 MeV, energy density~30 GeV/fm 3 cf LQCD prediction for the transition: 1 GeV/fm 3 Letter of Intent in 2004 Physics Performance Report for 2007 Idea: study of the Quark-Gluon Plasma

ATLAS Heavy-ion physics programme  Global variable measurement dN/dη dE T /dη elliptic flow azimuthal distributions  Jet measurement and jet quenching  Quarkonia suppression  J/Ψ  c  p-A physics  Ultra-Peripheral Collisions (UPC) Idea: take full advantage of the large calorimeter and μ-spectrometer A Zero Degree Calorimeter is being added for trigger and UPC tagging Direct information from QGP x z y TAN region, z=140m, on each side 2x4 modules of tungsten/quartz sandwich

4 Central Pb-Pb collisions  Simulation: HIJING+GEANT dN ch /dη| max ~ 3200 in central Pb-Pb c.f from RHIC extrapolation  Large bulk of low p T particles is stopped in the first layer of the EM calorimeter (60% of energy)  μ-spectrometer occupancy in Pb-Pb < high-L p-p  η    0.1 Tower (ΔΦxΔη) 0.1  0.1 Tower

—Only Pixel and SCT detectors —At least 10 hits out of 11 per track —At most 1 shared hits —For p T : GeV/c: efficiency 60-70% fake rate < 1% p T -resolution ~3% 2000 reconstructed tracks from HIJING (b=0) events with p T > 1 GeV and |  | < 2.5 Fake rate at high p T can be reduced by matching with calorimeter data TRT not considered for this study. Expected to be partially (fully) usable in central (peripheral) Pb collisions => electron identification Track reconstruction |  |<1

Original idea: color screening prevents various ψ, , χ states to be formed, when T→T trans to QGP (color screening length < size of resonance) Heavy quarkonia suppression Modification of the potential can be studied by a systematic measurement of heavy quarkonia states characterized by different binding energies and dissociation temperatures ~thermometer for the plasma In fact: complex interplay between suppression and regeneration

For |η| < 2 (12.5% acc+eff) we expect 15K   +  - /month of 10 6 s For |  | 1.5 GeV we expect 100K J/  +  - /month 4 different strategies have been investigated: A low p T di-muon trigger is under study (with a muon p T >1.5 GeV) Studies of J/  e + e - and  e + e -, of  c decaying into J/ , of open heavy flavors are under way

Jet quenching  Suppression of high-z hadrons and increase of soft hadrons in jets.  Induced gluon radiation results in the modification of jet properties like a broader angular distribution.  Conical structure which may result from Cherenkov radiation or shock- waves from partons traversing the medium (Mach cone).  Effective suppression of the jet cross section within a fixed cone size. Measuring jet profile is the most direct way to observe any change.  Advantage of LHC over RHIC: full jets with large rates, di-jets,  -jets, Z 0 - jets, b-jets. Energy loss of fast partons by excitation and gluon radiation, larger in QGP

PYTHIA jets embedded with central Pb+Pb HIJING events Main task: separation of jets from backgr. Several jet algorithms and methods of subtraction are tested (average and local) Jet studies Cone algorithm Fast k T jet finder Jet reconstruction & fitting algorithm with first radial moment Fragmentation functions using ID tracks Di-jets b-tagged jets  +jet Z 0 +jet

Jet studies (II) Jet position resolution (R=0.4)  Jet energy resolution Standard ATLAS solution - cone algorithm - is intensively studied with different samples Jet finding & energy measurement work for E T > 40 GeV (15 GeV in pp)

11 Summary  Global observables, including elliptic flow, should be accessible from day-one, even with a very low luminosity (early scheme)  Jet physics (jet quenching) is very promising, jet reconstruction is possible despite the additional background study of di-jet,  -jet, Z 0 -jet correlations possibility to study separately light and heavy q-jets  Heavy-quarkonia physics (suppression in dense matter) well accessible, capability to measure and separate  and  ’, to measure the J/  using a specially developed  tagging method  A study of , J/   e + e - and of open heavy flavor prod. is under way  Low-x physics and UPC will also be accessible Laurent Rosselet, HEP 2007, Manchester, July 20 th 2007

12 ATLAS HI Physics Group Brookhaven National Laboratory, USA Charles University, Prague, Czech Republic Columbia University, Nevis Laboratories, USA University of Geneva, Switzerland IHEP, Protvino, Russia IFJ PAN, Krakow, Poland Iowa State University, USA PUC, Santiago, Chile JINR, Dubna, Russia MePHI, Moscow, Russia Chemistry Department, Stony Brook University, USA Yale University, USA

New result from WA98 First observation of a large p T particle (  0 ) suppression at SPS energy in central heavy- ion collisions (related to jet quenching) Submitted to Physical Review Letters this month