The Heavy-Ion Physics Programme with the ATLAS Detector

& new results from WA98

Laurent Rosselet

Cartigny, September 14th 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/fm3 cf LQCD prediction for the transition: 1 GeV/fm3

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 dET/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

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Central Pb-Pb collisions

Simulation: HIJING+GEANT

dNch/dη|max~ 3200 in central Pb-Pb

c.f. 1200 from RHIC extrapolation

Large bulk of low pT 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 0.5

0.10.1 Tower(ΔΦxΔη)

0.10.1 Tower

—Only Pixel and SCT detectors—At least 10 hits out of 11 per track—At most 1 shared hits

—For pT: 1 - 10 GeV/c:

efficiency 60-70%

fake rate < 1%

pT-resolution ~3%

• 2000 reconstructed tracks from HIJING (b=0) events with pT > 1 GeV and || < 2.5• Fake rate at high pT 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→Ttrans to QGP (color screening length < size of resonance)

Heavy quarkonia suppression

state J/ c

' (1s) b

(2s) b' (3s)

Mass [GeV} 3.096 3.415 3.686 9.46 9.859 10.023 10.232 10.355B.E. [GeV] 0.64 0.2 0.05 1.1 0.67 0.54 0.31 0.2

Td/Tc --- 0.74 0.15 --- --- 0.93 0.83 0.74

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

1.10 0.74 0.15 2.31 1.13 0.93 0.83 0.74

For |η| < 2 (12.5% acc+eff) we expect

15K +-/month of 106s For || <2.5, pT

>1.5 GeV we expect 100K J/+-/month

4 different strategies have been investigated: http://dpnc.unige.ch/users/ros/quarkoniav3.ps

A low pT di-muon trigger is under study (with a muon pT>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, Z0-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 kT jet finder

Jet reconstruction & fitting algorithm with first radial moment

Fragmentation functions using ID tracks

Di-jets

b-tagged jets

+jet

Z0+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 ET > 40 GeV (15 GeV in pp)

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SummaryGlobal 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, Z0-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 accessibleLaurent Rosselet, HEP 2007, Manchester, July 20th 2007

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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 pT

particle (0) suppression at SPS energy in central heavy-ion collisions

(related to jet quenching)

Submitted to Physical Review Letters this month