1. The US ATLAS Heavy Ion Program Brian A. Cole, Columbia University January 12, 2007 Phases of QCD Matter Town Meeting

2. 2 LHC Heavy Ion Program: Key Questions What is the mechanism for initial particle production at the LHC? –Production from a saturated initial state? How rapidly do produced particles thermalize or isotropize, what is the mechanism? –Faster than RHIC, slower ? How do high-energy quarks and gluons interact in the quark gluon plasma? –What is the response of the medium? What is the screening length of the QGP? What are the quasi-particles of the QGP? How does the QGP hadronize?

3. 3 LHC Heavy Ions Program: Key Questions

4. 4

5. 5 The ATLAS Central Detector Inner tracking, EM and Hadronic calorimeters, external muon spectrometers

6. 6 ATLAS from the Inside

7. 7 ATLAS Acceptance Si Tracking Muon spectrometer EM Calorimeter Hadr Calorimeter ZDC s ,  ’ ,  0, isolated  Jets Bulk observables 

8. 8 US ATLAS Heavy Ion: Primary Goals Measure dn chg /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 response to passage of quenched jet Measure Upsilon production via  +  - –Probe Debye screening in medium Study low x hard processes in p-p, p-A –Study factorization violations, BFKL, saturation

9. 9 LHC Physics Summary High p T Low x Parton Density RHIC Collectivity ??

10. 10 Jet Tomography At RHIC, studied via leading hadrons –Statistics suffer from frag. function  rates –Quenching  geometric bias –No direct measure of frag. function. At LHC: –Full jets, high p T, large rates, b jets, di-jet,  -jet  Precision jet tomography

11. 11 Jet Tomography At RHIC, studied via leading hadrons –Statistics suffer from frag. function  rates –Quenching  geometric bias –No direct measure of frag. function. At LHC: –Full jets, high p T, large rates, b jets, di-jet,  -jet  Precision jet tomography

12. 12 Parton Showers, Hard Radiation @ LHC Copious hard radiation in high Q 2 final- state parton showers,  F ~ 1/k T Both an opportunity and a challenge –Understanding jet quenching more difficult –Potentially: time-dependent probe of medium Resolving hard radiation in jets a must!

13. 13 ATLAS Calorimetery EM Long. Segmentation Hadronic Barrel Hadronic EndCap EM EndCap EM Barrel Forward

14. 14 Jet Background EM Calorimeter Long. Segmentation Jet Back ground All too wide for single photons Segmentation of first EM sampling layer so fine that heavy ion background is ~ negligible –  N chg + N   < 1,  E T  ~ 30 MeV Fine   rejection of neutral hadron decays Clean 1 st sampling  prompt  isolation  x  = 0.0028 x 0.1

15. 15 Jets in A+A Jets from PYTHIA in 0.1x0.1 (logical) towers

16. 16 Jets in A+A merged with b = 2 fm Pb+Pb event (HIJING) Jets from PYTHIA in 0.1x0.1 (logical) towers

17. 17 Jet Energy Resolution Study of different event samples embedded into central Pb+Pb HIJING (b=0-2 fm) η Results obtained from standard p-p cone algorithm w/ background subtraction. Some re-calibration still needed.

18. 18 Jet Fragmentation Observables

19. 19 ATLAS: Gamma-Jet Pythia  + jet (75 GeV) superimposed on b=4 fm HIJING Pb+Pb event, full GEANT   Jet Gamma

20. 20 ATLAS: Gamma-Jet Pythia  + jet (75 GeV) superimposed on b=4 fm HIJING Pb+Pb event, full GEANT   Background subtracted Jet Gamma

21. 21 ATLAS: Gamma-Jet, EM 1 st Layer Gamma 1 st layer unaffected by Pb+Pb background  isolation w/ 1 st layer ~ unaffected by Pb+Pb Zoom in on barrel EM calorimeter 1 st sampling layer

22. 22  ATLAS: Gamma-Jet, EM 1 st Layer Gamma EM Layer 1 E T (GeV) Isolated photon gives clean signal in EM first sampling layer Even in central Pb+Pb ! One (of 64)  rows in barrel EM calorimeter 1 st sampling layer

23. 23  ATLAS: Gamma-Jet, EM 1 st Layer EM Layer 1 E T (GeV) Zoomed Width of high-energy shower in 1 st sampling unaffected by bkgd   0,  rejection retained

24. 24 Institutional Efforts/Strengths Brookhaven (Global, flow, jets, low-x) –Expertise in global and flow measurement (PHOBOS), high-multiplicity tracking and low-x physics (BRAHMS) Columbia (Jets, Photons, low-x) –Expertise in high-pT physics, jet correlations and direct photon measurements (PHENIX) –Expertise in p-A physics Iowa State (Di-muons) –Expertise in di-muon measurements (PHENIX) SUNY Stony Brook (Chemistry) –Expertise in flow, high-pT physics, jet correlations

25. 25 Summary ATLAS-HI physics program addresses primary physics questions of interest @ LHC ATLAS provides unique capabilities with highly segmented EM calorimeters –Especially for measuring isolated direct photons –For separating jets from heavy ion background –For measuring jet shapes, hard radiation ATLAS calorimeters provide best intrinsic (i.e. in p-p) jet resolution @ LHC Can US afford NOT to participate in expt providing most precise jet measurements @ last energy frontier in heavy ion physics?

26. 26 Summary: US Effort & Impact US effort (FY09 & beyond) – ~30 physicists, 6 Ph.D students at current institutions. More desirable.  More than ½ of full ATLAS-HI program. Highly leveraged effort with big impact  Possible because of ATLAS collaboration  Synergy with high-energy groups in US US provides co-convener of ATLAS HI WG –BAC as of fall 2006 US Expertise developed @ RHIC  US will provide highly visible leadership of ATLAS-HI program.

27. 27

28. 28 ATLAS Inner Tracker 3 layers Si pixel 8 layers Si strip TR tracker Pb+Pb b=2 fm Si tracking performance

29. 29 Charged Multiplicity from Pixels 1. Truth tracks (black) 2. “B-Layer” Hits 3. Layer 1 Hits 4. Matched Tracklets HIJING Pb+Pb 5500 GeV η Φ Pixel “tracklets” Good estimate of particle densities with subset of full detector

30. 30 ATLAS Zero Degree Calorimeter Test beam @ CERN 10.06 ZDC Prototype @ CERN 10.06 Experiment Simulation p+p events, w/ precision EM module TAN region, z=140m

31. 31 Upsilon Performance

32. 32 Low-x Physics w/ ZDC 12 π 0 acceptance Log 10 (x 2 ) p T (GeV) ZDC w/ precision EM module measures semi-hard  0, , , … production at x ~ 10 -6 in p-p and p-A collisions Correlate with mid-rapidity jets

33. 33 USATLAS-HI Effort InstitutionFY07 FTEs FY08 FTEs FY09 FTEs Out year FTEs Brookhaven5.45.8 Columbia2.93.55.15.6 Iowa State1.42.43.64.1 SUNYSB Chem2.83.03.84.0 Total12.514.718.319.5 Authors w/ Ph.d19 2022 Ph.D students1256 Participation by other institutions anticipated Modest increases in above numbers would broaden scope of US ATLAS-HI program.

34. 34 ZDC  0, , , … Reconstruction In p-p and p-A, position resolution of ZDC EM module allows clean measurement of  0, , , … –Huge benefit to low-x physics program Obtained with cut on total energy, E > 200 GeV Very little background from non- vertex sources

35. 35 ATLAS Acceptance Si Tracking Muon spectrometer EM Calorimeter Hadr Calorimeter ZDC s ,  ’ ,  0, , direct  Jets, Di-jets, v 2 dn/d 2 p T d , Jet D(z) dn chg /d , v 2 chg dE T /d ,  v 2 dE T /d , v 2

36. 36 ATLAS Strengths Large Acceptance Calorimeters –Finely segmented (longitudinal and transverse) electromagnetic calorimeter  Better jet reconstruction in Pb+Pb –With highly segmented first layer  Unique feature among LHC expt’s Precision silicon tracking External Muon spectrometer Zero-degree calorimeter

37. 37 ATLAS vs CMS Jet Resolution CMS Pb+Pb Jet resolution (Nov 2005) –@ 75 GeV, CMS~16%, ATLAS~13% –@ 125 GeV, CMS~15%, ATLAS~10% –@ 175 GeV, CMS~12%, ATLAS~8% –ATLAS better than CMS even in p-p Note: ATLAS numbers from 2003