Heavy Ion Physics with ATLAS Helio Takai Brookhaven National Laboratory US-ATLAS Software Meeting August 28, 2003

Introduction Heavy Ion Collisions at the LHC will allow us to study a unique QCD system at the limit of extreme energy densities. This system will contain tens of thousands of gluons, quarks and anti-quarks in a relatively small volume. RHIC results suggest that a 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. ATLAS provides an excellent experimental tool for accessing this physics.

R AA vs. R dA for Identified  0 d-Au results rule out CGC as the explanation for Jet Suppression at Central Rapidity and high p T d+Au Au+Au PHENIX

Azimuthal distributions pedestal and flow subtracted Near-side: p+p, d+Au, Au+Au similar Back-to-back: Au+Au strongly suppressed relative to p+p and d+Au Suppression of the back-to-back correlation in central Au+Au is a final-state effect

Jet Quenching

Jet Quenching at High P T at LHC

The ATLAS detector Hadronic Calorimeter Electromagnetic Calorimeter Inner Detectors Silicon Pixels Silicon Strips Transition Radiation Tracker Superconducting Solenoid Muon chambers Superconducting Coils for Toroidal Field for Muon System

The World Class Calorimeter System ATLAS Calorimeters provide for optimal jet measurements. 1) Full coverage: |  | < 4.9 and  = 2  This is crucial probing low x gluons at a pQCD scale. When x 1 >> x 2 then forward coverage is required. Back-to-back jet studies require large coverage. Z 0 -jet and  -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. 3)State of the Art Excellent segmentation, energy and timing resolution. High rate capability for critical p-p and p-A comparisons.

ATLAS Heavy Ion Physics Program The initial goal is to determine the global Nucleus-Nucleus collision properties by the measure of total E T, N ch Elliptic Flow. There is interest in studying jet physics in heavy ion collisions as a means to probe the early stages of the system formed. This translates into 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 also provide a solid baseline for the understanding of A-A system. Ultra-Peripheral collisions will be studied.

ATLAS Simulations dd How well does ATLAS perform for Heavy Ion Physics? Results we show today were obtained from full GEANT-3 simulations of the ATLAS detector HIJING (dN ch /dy ~ 8000) has been used as the event generator, which is conservative because of the high particle multiplicities.

ATLAS Simulation Chain Standard ATLAS Reconstruction Tools Underlying Pb-Pb Event Physics of Interest

The problem with background Exact dN/dy at LHC energies is unknown! HIJING does not have proper treatment of quenching and we don’t use it. We use all default parameters. Event Generator workshop was held few weeks ago.

Full Simulation Details Full GEANT-3 Simulation of ATLAS detector Heavy Ion Simulation effort coupled to recent ATLAS Data Challenge 1 activity for proton-proton simulations. Central Pb-Pb event takes ~ 6-8 CPU-hours in US-ATLAS computing facility. Restricted tracks to -3.2 <  < +3.2 for initial studies. Forward calorimeter studies is the next step. Tracking detector threshold identical to full proton-proton studies and includes low energy delta-rays.

ATLAS Visualizer for HIJING (b=0) Event

Event Visualizer

Global Measurements x z y Quantities such as energy flow, particle multiplicity, anisotropic 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. Excellent event-by-event fluctuation capability. Full ATLAS Simulation

Impact Parameter Determination Events in ATLAS can be characterized by the measurement of the 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. Full ATLAS Simulation

Tracking with Inner Detectors  Occupancies Pixel Detector Silicon Tracker Silicon Pixel occupancy is quite low ~ 1% even for b=0 HIJING events and including standard ATLAS simulation thresholds! Silicon Tracker (SCT) occupancy is ~10-15% for b=0 HIJING events and provides significant tracking assistance. Full ATLAS Simulation

Tracking Reconstruction Tracking uses the Pixel and SCT detectors. They provide a maximum of 11 independent points for track reconstruction. The Tracking code used in the reconstruction is XKALMAN and is used for proton-proton events. Vertex constraint can be imposed in nucleus- nucleus collisions due to low luminosity. The magnetic field used is 2 T. At this magnetic field, there are a large number of “loopers”. Many loopers were not included in the reconstruction to reduce processing time but increase fake rate. Further vertex selections may reduce fakes. At larger momentum (p T >15 GeV) tracking will require help from the calorimeter system. Hadronic tile calorimeter is fairly quiet and thus should provide excellent additional rejection via E threshold cut. Electron identification via E/p match will greatly enhance the Z  e + e -.

Tracking Reconstruction -2.5<  <2.5 Tile Calorimeter threshold cut will have excellent rejection. Full simulation results 1-2 weeks away. Detailed reconstruction with p T thr = 300 MeV/c Track requires all planes. Reconstruction threshold = 1 GeV. Most fakes in forward directions. Full ATLAS Simulation

Tracking Resolution -2.5<  <2.5 Plot shows the average reconstruction resolution. Note that in proton-proton in the central barrel we expect ~2.5% ! Full ATLAS Simulation

Jet Physics Modifications of the jet properties by the “hot medium” created by the nucleus-nucleus collision will provide a unique way to study the medium itself. Hard scattered partons in the medium radiate gluons and in the process “lose” energy. This could manifest itself as an increase in the jet cone size or an effective suppression of the jet cross section with a fixed cone size. The measurement of the fragmentation function distribution is the most direct way to observe any changes.

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

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 due to soft particles ranging out. Hadronic calorimeter is relatively quiet even in b=0 HIJING events! Energy in 0.1x0.1 tower as function of . Full ATLAS Simulation

55 GeV Jet PYTHIA only The Result ! PYTHIA + HIJING overlayed event. After average background subtraction Full ATLAS Simulation

280 GeV event 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”. Remember we are using b=0 HIJING events as our test case. Full ATLAS Simulation

Jet Reconstruction Efficiency A “good” jet is defined as the one that finds a match in the generated event within a cone radius of 0.2. Fakes are the ones that do not fulfill the requirement. Fakes include HIJING jets. Track matching may reduce the number of “real” fakes. Full ATLAS Simulation Very promising results with high jet reconstruction efficiency!

Jet Reconstruction

Jet Energy Resolution Excellent jet energy resolution. Energy resolution is close to a high luminosity L~10 34 proton-proton run. This fact also means that large contingent of high energy ATLAS participants are interested in working on these issues. Full ATLAS Simulation

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

The problem with jets Present work assumes that background is uniform. The number one problem that we need to address is the azimuthal asymmetry in the background, namely v 2. Difficulty is to generate a semi-decent background when so much is unknown. v 2 is p T dependent. When generating asymmetric background base on HIJING events this needs to be taken into account, e.g. mini-jets. Simulation based on Phobos v 2 generator is in progress.

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.

b-tagging Preliminary study of the b-jet tagging using the Pixel and SCT detectors with the algorithm used for high p T proton-proton environment is shown here: Left, the rejection of light quark jet as function of tagging efficiency for p-p, and right for heavy ions. In both cases Higgs events were used. A combination of displaced vertex and muon tagging may improve the overall rejection factor.

Fragmentation Function ATLAS can measure 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 “truth” given by the event generator. Full ATLAS Simulation

Jet Fragmentation (2) Evaluate sensitivity by studying core E t for different jet energies. In Pythia but ~ no change in HI bkgd. For large jet energies see peak that shifts with increasing jet energy. Qualitatively: core E t sensitive to ~ 10% quark/gluon energy  E. Quantitative analysis underway. “core” E t = energy in 3x3 EM cell x 0.1 hadronic cell

Jet Fragmentation Variation of average E T core with fragmenting parton (jet) energy for Pythia simulated jets and for these same jets embedded in central Pb+Pb events

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.

Upsilon Reconstruction  distribution for the reconstructed Y and muon p T distribution from upsilon decay.

Upsilon Reconstruction HIJING muon  and p T distributions.

Probes of deconfinement Initial figures on mass resolutions using Upsilon events alone indicate a system resolution of 200 MeV without Silicon and 130 MeV with Silicon. 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!

Upsilon mass resolution Problems: Reconstruction of upsilon in the heavy ion environment is under way. HIJING seem not to have correct physics for background. Therefore we are considering mixing PYTHIA events. J/  seems doable in the forward direction.

Proton-Nucleus Physics Study of p-A collisions is LHC To provide baseline for heavy ion measurements. Physics intrinsically compelling Nuclear Structure function Mini-jet production, multiple semi-hard scattering Gluon saturation – probe high gluon density Color Glass Condensate In the pA environment we can fully benefit from ATLAS detector capabilities, e.g. tracking, because particle multiplicities are lower than in proton-proton collisions at design luminosity. Because of this “low” particle multiplicity we can run at high luminosity, e.g Astrophysics interest ultra-high energy cosmic rays (> GZK)

Proton-Nucleus In proton-nucleus run the detector occupancy will be low enough so that all the detectors are available in the ATLAS full rapidity. Preliminary work on proton-nucleus simulation has started using the ATLAS fast simulator. ATLFAST works very well for general survey studies in low luminosity (10 33 ) proton-proton runs. An example of the jet inclusive spectra is shown. The threshold is 10 GeV. Jets are produced by HIJING. p+Pb dN/dE T

Trigger DAQ For Pb-Pb collisions the interaction rate is 8 kHz, 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

3 Jet Events ! 3 jet events result from the radiation of a hard gluon. Since the gluon couples to two color charges it is expected to lose twice as much energy from induced gluon radiation. Suppression of 3 jet events would be significant because it indicates energy loss coupled to the parton color charge !

Conclusions and Outlook ATLAS is being constructed as a world class high energy experiment, and is available for heavy ion 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. Full ATLAS simulations indicate excellent jet reconstruction and the ability to measure fragmentation functions. 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 (including for example displaced vertex B tagging). We are preparing a Letter of Intent to LHCC. It should be ready by fall 2003.

Looking Forward to the Highest Energy Reactions!

Extras

Low-x 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~ Note that xg(x) is enhanced by A 1/3 ~ 6 in Pb over the proton. RHICLHC

Tracking Efficiency vs  Tracking efficiency is higher at the endcaps, but fake rate is also much higher over there…. p T >1 GeV Full ATLAS Simulation

Alternative Jet Finding Algorithms We are exploring different techniques to reconstruct jets. This will aid in the jet finding and reconstruction. Tracks do not know about neutrals. The plot below uses tracking for jet reconstruction. Tracks with low p T could be suppressed for jet finding. Events from WH events were over- layed on top of HIJING events and jets reconstructed using the cone algorithm, with a radius of 0.4. Dashed line is without background More work is required to prove its usefulness.

DOE Charge for Progress Report 1) Results from work during the past year that effectively address the panel’s concerns. 2) New developments within the collaborations and updated proposed costs and schedules. 3) A well articulated plan that prioritizes R&D work that may be needed, establishes the timescales needed to accomplish it, and carefully considers the risks of inaction on each item. 4) Computing resources that may be needed.

EM Calorimeter Segmentation

ATLAS Full Coverage Recent results from STAR and PHENIX at RHIC have shown the crucial additional information provided by back-to-back coverage. Near full coverage by ATLAS calorimetry.