1. Heavy Ion Physics with the ATLAS detector Helio Takai Brookhaven National Laboratory takai@bnl.gov IV INTERNATIONAL SYMPOSIUM ON LHC PHYSICS AND DETECTORS Fermilab May 1-3,2003

2. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 2 Introduction Heavy Ion Collisions at the LHC will allow us to study an unique QCD system at the limit of extreme energy densities. This system will contain tens of thousands of gluons, quarks and antiquarks in a relatively small volume. RHIC results suggest that hard scattering embedded in the system is one of the best ways to probe the early stages of the matter formed in these collision. The complexity of heavy ion collision will require the understanding of proton-nucleus and proton-proton collisions preferably in the same detector acceptance.

3. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 3 Nucleus-Nucleus Collision A central nucleus-nucleus collision creates the highest energy density system. Because of the size of the colliding system not all nucleons interact and they will depend on the impact parameter, b. b

4. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 4 RHIC (Phenix) Results  0 spectra for central and peripheral collisions. Results show the suppression of  0 in central collisions.

5. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 5 ATLAS and Heavy Ions Interest in Heavy Ion Physics renewed after RHIC results showed strong evidences for jet quenching. ATLAS is a detector primarily designed for high p T physics at the LHC design luminosity and well suited for the study of jet physics. The calorimeter has a large coverage (|  |<4.9), excellent resolution and fine granularity. Heavy Ion collisions produce a HUGE number of particles. Event generators such as HIJING predicts about dN/dy~6,000. How well does ATLAS perform for Heavy Ion Physics? Results shown here are obtained from full G3 simulations of the ATLAS detector Hijing has been used, which is conservative because of the high particle multiplicities All results are VERY preliminary

6. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 6 The ATLAS detector Tile Calorimeter Hadronic Endcap Forward Calorimeter Electromagnetic LAr Inner Detector

7. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 7 Simulations HIJING b=0 event in the ATLAS detector. Events for b=0-15fm were generated in the range of |  |<3.2.

8. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 8 Global Measurements x z y Quantities such as energy flow, particle multiplicity, anisotropy in energy flow, permit us to characterize the event. Any particular measured quantity should be studied in function of these variables. dN/d  from the pixel detector compared to what is given by the event generator.

9. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 9 Impact Parameter Determination Events in ATLAS can be characterized by the measurement of the event charged particle Multiplicity or Total Transverse Energy. Correlation between impact parameter and number of hits in the pixel detector. Comparison of the impact parameter resolution obtained by three distinct techniques.

10. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 10 Tracking with ID - Occupancies Pixel Detector Silicon Tracker Impact parameter b=0-1 fm, HIJING event generator. Track reconstruction with the Inner Detectors is challenging due to the high occupancy. From the three systems, the TRT has very high occupancy that make it unusable for heavy ion physics.

11. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 11 Tracking Reconstruction Track reconstruction performed with ATLAS tracking code using the Pixel and SC systems. Simulation include  -rays. Efficiencies are slightly lower for two layer pixel configuration. Momentum resolution is approximately 2.4%. Efficiency has a flat dependency with , except in the transition regions between barrel and endcap.

12. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 12 Jet reconstruction Modifications of the jet properties by the “hot media” created by the nucleus-nucleus collision will provide a unique way to study it. Hard scattered partons in the media radiate gluons and in the process “lose” energy (the overall energy is conserved). This could manifest as an increase in the jet cone size. The measurement of the fragmentation function distribution is the most direct way to observe any changes.

13. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 13 Jet Reconstruction We have performed initial work on reconstructing jets in the heavy ion environment. Pythia jets were overlapped to HIJING produced “background” as means to produce jet events. Heavy Ion “background” (HIJING) has a very soft particle spectra. Remind you of the ATLAS calorimeter system:

14. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 14 Jet Reconstruction Energy in 0.1x0.1 tower in the EM and HAD calorimeter for |  |<3.2. Most of the energy is in the EM calorimeter Energy in 0.1x0.1 tower as function of . The pronounced dips are the transiton region between barrel and endcap calorimeters. PRELIMINARY

15. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 15 55 GeV Jet Pythia only The Result! Pythia + Hijing mixed event. After average background subtraction For lower jet energies we found that it is easier to find jets if the first EM compartment is not included. Later the energy from the 1st compartment must be recovered.

16. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 16 280 GeV event Plots for 280 GeV jets. Preliminary efficiency numbers show that jet reconstruction efficiency is larger than 90% above 50 GeV. Below 50 GeV the efficiency lowers to approximately 75% with an increase in the number of “ghosts”. Resolution and Efficiencies are now being worked out!!! (well, they didn’t make it for this talk…)

17. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 17 Beauty quark Radiative quark energy loss is qualitatively different for heavy and light quarks. Finite velocity of heavy quarks at finite transverse momentum leads to suppression of co-linear gluon emission. Tagging of the b jets is possible via the high p T muon in the spectrometer or via displaced vertex. We are currently investigating both possibilities.

18. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 18 B-tagging Preliminary study of the b-jet tagging using the Pixel and SCT detectors and the algorithm used for high p T proton-proton environment. Left, the rejection of light quark jet as function of tagging efficiency for p-p, and right for heavy ions. In both cases W+H events were used. A combination of displaced vertex and muon tagging may improve the overall rejection factor. PRELIMINARY!

19. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 19 Fragmentation Function We are considering measuring the fragmentation function via the EM cluster in the jet or charged particle momentum in the jet. The study below was performed for 75 GeV Pythia generated jets. Reconstructed EM cluster in the jet compared to the sum of  0 energies. Reconstructed charged particle track in the jet compared to the given by the event generator. The efficiency is 85%.

20. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 20 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. The detectors used for the Upsilon mass reconstruction are the Muon Spectrometer, Silicon Tracker and the Pixel Detector.

21. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 21 Probes of deconfinement Initial figures on mass resolutions using Upsilon events alone indicate a system resolution of 130 MeV, with an estimated S/B~0.6. Muons with p T >3GeV are tracked backwards to the ID and the mass calculated from the overall fit. Overlay with HIJING Event is now under way!

22. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 22 Proton-Nucleus physics Study of p-A collisions is important @ LHC To provide baseline for heavy ion measurements. Physics intrinsically compelling Nuclear Structure function Mini-jet production, multiple semi-hard scattering. Gluon saturation – probe QCD @ high gluon density Etc... In the pA environment we can fully benefit from ATLAS detector capabilities, e.g. tracking, because particle multiplicities are lower than in pp collisions at design luminosity. Because of this “low” particle multiplicity we can run at high luminosity, e.g. 10 31. Astroparticle Physics interest.

23. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 23 Trigger DAQ For Pb+Pb collisions the interaction rate is 8kHz, a factor of 10 smaller than LVL 1 bandwidth. We expect further reduction to 1kHz by requiring central collisions and pre-scaled minimum bias events (or high p T jets or muons). The event size for a central collision is ~ 5 Mbytes. Similar bandwidth to storage as pp at design L implies that we can afford ~ 50 Hz data recording. ~200 Hz

24. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 24 Physics Program The initial goal is to determine the global Nucleus-Nucleus collision properties by the measure of total E T, N ch and Flow. There is interest in studying jet physics in heavy ion collision as a mean to probe the early stages of the system formed. This comprises in measuring jet (di-jet) cross sections, and measure jet properties in jet+jet,  +jet, Z 0 +jet and  *+jet channels. Heavy quarks should have gluon radiation suppressed due to their mass. The B-jet should not be quenched. A direct probe of deconfinement is the suppression of the  or  states. These can be reconstructed using the muon spectrometer and the inner detector. Proton-nucleus collisions are interesting on its own and will provide a solid baseline for the understanding of AA system. Ultra Peripheral collisions will be studied.

25. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 25 Conclusions and Outlook ATLAS is general purpose detector with excellent performance for high p T physics. Features like calorimeter coverage, granularity and resolution give us good potential for high p T probes in heavy ion collisions, e.g. jet quenching. ATLAS is complementary to ALICE and CMS. The physics topics outlined in this presentation can be likely addressed with the present detector layout (except for zero degree calorimetry to study UPC). For pA and light AA collisions the experimental environment is quieter than pp collisions at design L and therefore we can benefit from the full detector performance capabilities. Heavy ion physics studies in ATLAS are progressing with the aim of preparing a Letter of Intent to LHCC by mid 2003.

26. Helio Takai, BNL, IV International Symposium on LHC Physics and Detectors, Fermilab, May 2003 26 EM calorimeter segmentation