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Heavy Ion Collisions at LHC with the ATLAS Detector

Heavy Ion Collisions at LHC with the ATLAS Detector. Helio Takai takai@bnl.gov Brookhaven National Laboratory. (J. Nagle, B. Cole and S. White). Hadron 2002 Bento Gonçalves, April 2002. Before we start. First of all my sincere thanks to the organizers !.

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Heavy Ion Collisions at LHC with the ATLAS Detector

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  1. Heavy Ion Collisions at LHC with the ATLAS Detector Helio Takai takai@bnl.gov Brookhaven National Laboratory (J. Nagle, B. Cole and S. White) Hadron 2002 Bento Gonçalves, April 2002

  2. Before we start • First of all my sincere thanks to the organizers ! • After I accepted the invitation I got worried ! What can I talk about? I decided to talk about heavy ion physics with the ATLAS detector that it is still far in the future. However it has been our experience in ATLAS that help from our theoretical colleagues in planning the experiments has been a big plus. So this is the spirit of this presentation. Helio Takai, Brookhaven National Laboratory

  3. Before we start Another reason why I got worried is because I have been away from for a long time - almost 8 years! So I will behave myself, and make no jokes about Our Lady of Perpetual Motion, the Church of Christ Geologist or the Jojoba Witnesses. However one can’t pass the opportunity to mention that everybody missed one good heavy ion experiment: Helio Takai, Brookhaven National Laboratory

  4. Plan for this presentation Flirting with heavy ions The ATLAS detector Heavy Ion Physics at LHC energies A nuclear physics program for ATLAS What is going to happen next Summary and conclusions Helio Takai, Brookhaven National Laboratory

  5. ATLAS and Heavy Ions • ATLAS is an experiment designed to study the origins of the electroweak symmetry breaking in proton-proton collisions, search for SUSY, and exotica - e.g., extra dimensions. • ATLAS is designed to acquire data at full LHC design luminosity of L=1034 cm-2s-1, or 40 MHz collision rate. • The detector is designed to cover a range of |h|<5.0 in rapidity with inner tracking, calorimetric coverage and muon spectrometer. • ATLAS is also a large collaboration with over 2,000 physicists from 157 institutions worldwide, including Brazil. Helio Takai, Brookhaven National Laboratory

  6. ATLAS and Heavy Ions • Recent RHIC data suggests that jets may be quenched. • CERN theorists have been sponsoring a one year long workshop on hard probes in heavy ion physics. • This has sparked renewed interest by a group of physicists within ATLAS to revisit the heavy ion physics program with the detector. RAA 2 1 0 2 4 pT(GeV/c) Helio Takai, Brookhaven National Laboratory

  7. ATLAS and Heavy Ions • Large calorimetric coverage detector are well suited for jet physics • Jets at LHC heavy ion energies are similar to Tevatron jets. • For about one year, since the workshop in BNL, we have been developing a nuclear physics program for the ATLAS detector. • However there are many hurdles to overcome … people, funding, etc… Helio Takai, Brookhaven National Laboratory

  8. The people • The institutions involved in ATLAS heavy ions working group are: • Brookhaven National Laboratory • CERN • Columbia University • Prague • Rio de Janeiro • University of Geneva • Helsinki Helio Takai, Brookhaven National Laboratory

  9. LHC and heavy ions • LHC, the Large Hadron Collider, is now under construction and will accelerate heavy ions at an energy of 2.75 TeV/nucleon • The energy available at the center of mass of a Lead-Lead collision is well over a 1,000 TeV. • The energy density is expected to be about 30 times what is currently observed at RHIC • New and exciting physics could be observed by probing the hot QCD matter with hard probes. • But what is ATLAS exactly? Where is the experiment? Helio Takai, Brookhaven National Laboratory

  10. CERN CERN BNL Where is CERN? It is in Switzerland, not Swaziland… and about 6,000 km from New York. Helio Takai, Brookhaven National Laboratory

  11. CERN Mt. Blanc CMS Mt Blanc des Americaine ATLAS Helio Takai, Brookhaven National Laboratory

  12. The ATLAS detector Helio Takai, Brookhaven National Laboratory

  13. Inner Detectors The Semiconductor Tracker are also silicon devices. The ATLAS inner detector is composed of three systems: Pixel, SCT and TRT The transition radiation tracker are gas detectors and used to identify electrons. The pixel detectors are silicon devices and are located near to the collision point. Helio Takai, Brookhaven National Laboratory

  14. Inner Detectors The same event, in full and low luminosity running conditions. Helio Takai, Brookhaven National Laboratory

  15. Calorimeters • Calorimeters in ATLAS cover a wide range of pseudo-rapidity, |h|<5. • The electromagnetic calorimeter is realized in liquid argon technology • The hadronic calorimeter is implemented as a iron-scintillator device, except in the forward direction. • The very forward region is covered by axial drift liquid argon calorimeter. Helio Takai, Brookhaven National Laboratory

  16. Electromagnetic Calorimeter Used by the Inner tracker Helio Takai, Brookhaven National Laboratory

  17. g g p0 Electromagnetic Calorimeter Segmentation The Electromagnetic Calorimeter is segmented longitudinally and transversely. Will be used for p0 identification. Segmentation helps the identification of g in the background of p0s. Helio Takai, Brookhaven National Laboratory

  18. Hadronic Tile Calorimeter The hadronic tile calorimeter ‘hugs’ the liquid argon electromagnetic calorimeter. It is built in iron-scintillator technology and readout by WLS optical fibers. Helio Takai, Brookhaven National Laboratory

  19. Hadronic Tile Calorimeter Helio Takai, Brookhaven National Laboratory

  20. Calorimeter Characteristics Helio Takai, Brookhaven National Laboratory

  21. Jet Energy Resolution hadronic Large acceptance allows for detection of back-to-back jets. Jet Energy Resolution : Electromagnetic Ha! But this is for pp collisions!!! Di-jet event in ATLAS Helio Takai, Brookhaven National Laboratory

  22. Toroid! Toroid! Toroid! Muon Spectrometer Muon detectors Air Core Toroidal Magnet System Helio Takai, Brookhaven National Laboratory

  23. Muon Spectrometer Helio Takai, Brookhaven National Laboratory

  24. Muon spectrometer It takes 3 GeV to go through. The rest stops! Helio Takai, Brookhaven National Laboratory

  25. Muon Spectrometer Performance Spectrometer only Spectrometer + ID Helio Takai, Brookhaven National Laboratory

  26. The case for Heavy Ions • ATLAS is a detector appropriate for high pT physics. • It has a finely segmented calorimeter which is appropriate for jet physics. The energy resolution for jets is superb. • It has a stand alone muon spectrometer with good momentum resolution. • It is designed for high data rates for proton-proton collisions • But, is there interesting physics to be done in the high Q2 regime? Helio Takai, Brookhaven National Laboratory

  27. Heavy Ion collisions …the picture says it all !!!! Helio Takai, Brookhaven National Laboratory

  28. 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 - 10-5. Note that xg(x) is enhanced by A1/3 ~ 6 in Pb over the proton. LHC RHIC Helio Takai, Brookhaven National Laboratory

  29. 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 possible gluon plasma. Helio Takai, Brookhaven National Laboratory

  30. Freeing the Gluons (the QLF) 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, quark 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. Helio Takai, Brookhaven National Laboratory

  31. 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 to DE a L2 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….. Helio Takai, Brookhaven National Laboratory

  32. 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 ! Helio Takai, Brookhaven National Laboratory

  33. ATLAS Jet Measurements ATLAS can 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 Helio Takai, Brookhaven National Laboratory

  34. Jet Profile Analysis The induced gluon radiation may be measurable due to the broader angular energy distribution than from the jet. proton-proton jet cone Possible observation of reduced “jet” cross section from this effect. U.A. Wiedemann, hep-ph/0008241. BDMS, hep-ph/0105062. Helio Takai, Brookhaven National Laboratory

  35. 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. Helio Takai, Brookhaven National Laboratory

  36. g-Jet Physics Wang and Huang, hep-ph/9701227 ATLAS g-jet rate is very large, thus allowing for detailed studies. 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 Helio Takai, Brookhaven National Laboratory

  37. 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 Helio Takai, Brookhaven National Laboratory

  38. g*-Jet Physics One can also study virtual photon-jet events, whereg* m+ m-. Rate is down two orders of magnitude fromg-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. Helio Takai, Brookhaven National Laboratory

  39. 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. Helio Takai, Brookhaven National Laboratory

  40. 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 highpTp0andh Coverage over a broad range of pseudorapidity Helio Takai, Brookhaven National Laboratory

  41. Unique Opportunity at LHC A parton propagating through the hot QCD media will radiate gluons loosing energy. The overall energy is, however, conserved therefore the measured jet properties will be modified. The modification of the jet fragmentation function is the most sensitive physical quantity. Other expected modifications are the cone radius, etc. Whatever measurement is done, it is worthwhile balancing the jet energy by the opposite g, g* or Z0. Therefore large coverage is beneficial. Helio Takai, Brookhaven National Laboratory

  42. Heavy Quarks Dokshitzer and Kharzeev have suggested that heavy quarks would not radiate as much as light quarks when propagating through the media. Therefore b-jets should not be quenched as much as light quark jets. b-jets can be identified by the dislocated vertex or by tagging jets with the associated muon. t-quarks although interesting do not live long enough to ‘feel’ the media. Helio Takai, Brookhaven National Laboratory

  43. p-A Physics in ATLAS • Study of p-A collisions is essential @ LHC • To provide baseline for heavy ion measurements. • Physics intrinsically compelling • Mini-jet production, multiple semi-hard scattering. • Shadowing – test of “Eikonal” QCD. • Gluon saturation – probe QCD @ high gluon density. • Test factorization. • Multiple hard scattering – Measure parton correlations in nucleon (and nucleus ?) • ATLAS is ideal detector for p-A studies •  coverage, calorimeter performance, b tagging, lepton identification, inner tracking. Helio Takai, Brookhaven National Laboratory

  44. RHIC and LHC, g+hadron colliders gbeamRhic=100 S=2mA’ =>Nucleus at rest,effective Lorentzgeff=2*gbeam2-1 Heavy Ions e-Hadron collider Helio Takai, Brookhaven National Laboratory

  45. Ultra Peripheral Collisions Heavy Ion Physics= Opportunities with a tool that we are just learning to exploit (c.f. e+e- physics) LHC energy scale Equivalentgflux Up to 100’s of TeV Helio Takai, Brookhaven National Laboratory

  46. Summary of the physics program The ATLAS Nuclear Physics program includes Global variable measurements: measurement of ET, dET/dh, charged particle multiplicity N, dN/dh, and elliptical flow. Jet Physics :g+jet, g*+jet, Z0+jet Heavy Quarks : beauty jets Quarkonia : suppression p+A physics Ultra Peripheral Collisions Helio Takai, Brookhaven National Laboratory

  47. Conclusions and Outlook ATLAS is a world class detector designed for high rate proton-proton collisions and will provide great opportunity to study high Q2 physics in nucleus nucleus collisions. The ATLAS heavy ion working group is being formed and simulation is under way. Input from Theorist and Experimentalists are very welcome Helio Takai, Brookhaven National Laboratory

  48. Conclusions and Outlook Strong input from phenomenologists is the ticket for a successful program. We have experience in the proton-proton program and has made a huge difference. In early april the proposed program was presented to DOE as a letter of intent and will formalize a proposal to LHCC late fall (in the northern hemisphere) - so join in!!! Helio Takai, Brookhaven National Laboratory

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