Exploring Gluon Structure and Jet Physics in LHC Heavy Ion Program

1. ATLAS Heavy Ion Program 1) LHC heavy ion physics is very interesting 2) LHC heavy ion physics is complementary to RHIC and EIC 3) ATLAS has strong heavy ion physics capabilities and interests

2. Unprecedented Gluon Densities HERA experiments have observed a dramatic increase in the gluon density at low x. This increase must end at some point when the gluon density saturates. Large Hadron Collider Pb-Pb collisions probe the gluon structure below x~10-3 - 10-6. Note that xg(x) is enhanced by A1/3 ~ 6 in Pb over the proton. LHC RHIC

3. LHC and RHIC The saturation scale is much larger at the LHC than at RHIC. Thus, the initial partonic state may be dominated by the saturation region (described as a color glass condensate). Also, the cross section for high pT processes is much larger, thus yielding better pQCD calibrated probes of the created gluon plasma.

4. Freeing the Gluons In a future Electron-Ion Collider (EIC) one can probe the low-x gluon structure one gluon at a time. At the LHC, tens of thousands of gluons, quarks and antiquarks are made physical in the laboratory in every collision ! Very complementary physics. Then we can study the nature of this very hot bath of partons (QGP) ! The plasma should be hotter and live longer than at RHIC.

5. Jet Probes of the Plasma Partons are expected to lose energy via induced gluon radiation in traversing a dense partonic medium. Coherence among these radiated gluons leads toDEaL2 q q We want to measure the modification of jet properties as we change the gluon density and path length. Baier, Dokshitzer, Mueller, Schiff, hep-ph/9907267 Gyulassy, Levai, Vitev, hep-pl/9907461 Wang, nucl-th/9812021 and many more…..

6. ATLAS Calorimeters ATLAS Calorimeters provide for optimal jet measurements. 1) Full coverage: |h| < 4.9 and Df = 2p This is crucial probing low x gluons at a pQCD scale. When x1 >> x2 then forward coverage is required. Back-to-back jet studies require large coverage. Z0-jet and g-jet need high statistics in p-p, p-A, and A-A. 2) Complete Hadronic and Electromagnetic Calorimeters Important to be insensitive to jet composition. Complementary to ALICE jet energy measure method. 3) State of the Art Excellent segmentation, energy and timing resolution. High rate capability for critical p-p and p-A comparisons.

7. ATLAS Jet Rates In one month of Pb-Pb running with three experiments at LHC, ATLAS will measure an enormous number of jets. Vitev - extrapolated to Pb-Pb ATLAS accepted jets for central Pb-Pb Jet pT > 50 GeV 30 million ! Jet pT > 100 GeV 1.5 million Jet pT > 150 GeV 190,000 Jet pT > 200 GeV 44,000 Note that every accepted jet event is really an accepted jet-jet event since ATLAS has nearly complete phase space coverage !

8. ATLAS Jet Measurements ATLAS should measure jets with E > 70 GeV with reasonable resolution and efficiency in the highest multiplicity central Pb-Pb events. More detailed studies are currently underway. Substantial background reduction can be achieved by simultaneously finding back-to-back jets. 200 GeV jet overlay on central Pb-Pb event with ATLAS segmentation h f

9. Jet Profile Analysis The induced gluon radiation may be measurable due to the broader angular energy distribution than from the jet. U.A. Wiedemann, hep-ph/0008241. BDMS, hep-ph/0105062. q<200 - 80% of jet energy contained 5% loss of energy outside q<120 - 70% of jet energy contained 8% loss of energy outside Possible observation of reduced “jet” cross section from this effect.

10. Fragmentation Functions ATLAS can measure identified p0 and h mesons via photons. Excellent energy and timing resolution will help to limit background. 2nd sample 3x5 cluster invariant mass calculation for identification. 2nd sample cluster two g shower shape identification. Dh x Df = 0.025 x 0.025 Photon opening angle in degrees 1st sample cluster two g separation and total energy measure in 2nd sample. Dh x Df = 0.003 x 0.1 pT (GeV) Background and resolution studies are underway.

11. g-Jet Physics Wang and Huang, hep-ph/9701227 ATLAS g-jet rate is very large, thus allowing for detailed studies of the opposite side jet. In one month, over 1000 events with g energy = 60 GeV in a 1 GeV bin ! Above a certain pT~30 GeV, ATLAS can no longer cleanly separate single g from two g resulting from a p0 decay. We are investigating whether with isolation cuts on a single high energy shower and an opposite side jet, which process dominates (1) g-jet events (2) jet-jet events with one jet with a high z fragmentation

12. Beauty Jets m Radiative quark energy loss is qualitatively different for heavy and light quarks. Finite velocity of heavy quarks at finite pT leads to suppression of co-linear gluon emission (“dead-cone” effect). ATLAS can tag B jets via a high pT muon in the muon detectors. D n B b b Y.L.Dokshitzer and D.E. Kharzeev, hep-ph/0106202

13. ATLAS Muon Coverage ATLAS muon acceptance matches that of the calorimeter. In one month, ATLAS can measure over 200,000 muon tagged Beauty jets. Lin and Vogt, hep-ph/9808214 The standalone muon momentum resolution is ~ 2% for muons with pT = 10-100 GeV and a low pT cutoff ~ 4 GeV.

14. g*-Jet Physics One can also study virtual photon-jet events, where g* m+ m-. Rate is down two orders of magnitude from g-jet. Good muon coverage makes this possible. In one month in central Pb-Pb, ATLAS would accept ~ 10,000 events with pT > 40 GeV. Z0-jet reconstruction is possible, but less than 500 total Z0 events per month.

15. Probes of Deconfinement Upsilon states (1s,2s,3s) span a large range in binding energy and thus their suppression pattern may allow for a mapping on the onset in the screening on the long range color confining potential. ATLAS is currently investigating the mass resolution in the muon system with alternate track matching algorithms.

16. z y x Correlated Global Measures ATLAS will measure many global observables and have high statistics for correlating them with high pT probes. 1) Transverse energy 2) Charged particle multiplicity 3) Zero degree energy 4) Reaction plane Jet observables as a function of reaction plane Azimuthal distribution of high pTp0 and h Coverage over a broad range of pseudorapidity

17. ATLAS Heavy Ion Physics • ATLAS is just at the beginning of what promises to be a strong program in relativistic heavy ion physics at the Large Hadron Collider. • We are leading the effort to characterize the performance capabilities and explore new physics signatures in the unique LHC environment. • A state of the art detector is available to us to extend our studies at RHIC to much higher gluon densities and with much higher pT probes. • An exciting future awaits.

18. Extras To reach the very low x-Bjorken one should measure at forward rapidities.

19. p0 Invariant Yield per Event Central data Scaled pp Shadowing + Cronin Energy Loss Transverse Momentum (GeV/c) Results at RHIC At RHIC, we have measured jet fragments averaged over the distribution of jet energies. PHENIX and STAR have an exciting observation of suppressed hadron yields.

20. We Need the Full Picture