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Hard probes capabilities of ALICE: Jets and Direct Photons

Hard probes capabilities of ALICE: Jets and Direct Photons. Andreas Morsch CERN, Geneva. Hard Probes 2006, Asilomar, June 9-15, 2006. Outline. Jet Physics Jets at LHC: New perspectives and challenges High- p T di-hadron correlations Reconstructed Jets g -Jet Correlations Summary. ^.

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Hard probes capabilities of ALICE: Jets and Direct Photons

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  1. Hard probes capabilities of ALICE:Jets and Direct Photons Andreas Morsch CERN, Geneva Hard Probes 2006, Asilomar, June 9-15, 2006

  2. Outline • Jet Physics • Jets at LHC: New perspectives and challenges • High-pT di-hadron correlations • Reconstructed Jets • g-Jet Correlations • Summary

  3. ^ Nuclear mod. Factor RAA vs <q> A. Dainese, C. Loizides, G. Paic s = 5500 GeV Hump-backed plateau from toy model x = ln(Ejet/phadron) Jet physics at LHC • As for RHIC energies, RAA at LHC will only give lower limit on transport parameter. • Reason: Surface and trigger bias • We can reduce the trigger and surface bias by studying reconstructed jets and increase sensitivity to medium parameters. • Using reconstructed jets we can study directly • Modification of the leading hadron • Additional hadrons from gluon radiation • Transverse heating.

  4. Jet physics at LHC: New perspectives #Jets ET>ETmin • At LHC rates are high at energies at which jets can be reconstructed over the large background from the underlying event. • Reach to about 200 GeV • Provides lever arm to measure the energy dependence of the medium induced energy loss • 104 jets needed to study fragmentation function in the z > 0.8 region. • To make use of the high rate we need trigger ! Pb-Pb 1 month of running |h| < 0.5 More than 1 jet > 20 GeV per central collision

  5. Jet physics at LHC: New challenges • The high production rates also represent a challenge • More than one particle pT > 7 GeV per event • 1.5 TeV background energy in cone of R = Dh2+Df2 < 1 ! • Challenge for jet reconstruction algorithms ! • We want to measure modification of leading hadron and the hadrons from the radiated energy. Small S/B where the effect of the radiated energy should be visible: • Low z • Low jT • Large distance from the jet axis • Low S/B in this region is a challenge !

  6. central Pb–Pb pp New challenges: Apparatus • Also preparing ALICE for jet physics represents a challenge. • Existing: Tracking system • Momentum resolution < 6% up to pT = 100 GeV • For jet structure analysis • Tracking down to 100 MeV • Excellent Particle ID • New: For improved energy resolution and trigger: EMCAL • Pb-scintillator • Energy resolution ~15%/√E DpT/pT

  7. Azimuthal correlation baseline PYTHIA 6.2 Di-hadron correlations:from RHIC to LHC • Di-hadron correlations will be studied at LHC in an energy region where full jet reconstruction is not possible (E < 30 GeV). • What will be different at LHC ? • Number of hadrons/event large • Decreases S/B at LHC but increases also overall statistics • The width of the away-side peak increases to higher order processes • Wider h-correlation (loss of acceptance for fixed h-widow) due to smaller xB • Power-law behavior of x-section (ds/dpT ~ 1/pTn) changes from n = 8 at RHIC and n = 4 at LHC • Changes the trigger bias on parton energy See also, K. Filimonov, J.Phys.G31:S513-S520 (2005)

  8. RHIC/STAR-like central Au-Au (1.8 107 events) LHC/ALICE central Pb-Pb (107 events), no-quenching Scaling from RHIC to LHC From STAR pTtrig = 8 GeV/c pTtrig > 8 GeV • S/B and significance for away-side correlations can be estimated by scaling rates between RHIC and LHC • Ratio of inclusive hadron cross-section • Replace N(pT) ~1/pT8 by ~1/pT4 50 1/25

  9. Di-hadron correlations with ALICE STAR LHC, ALICE acceptance HIJING Simulation 4 105 events M. Ploskon, ALICE INT-2005-49 O(1)/2p “Peak Inversion”

  10. Under study • For pT < 7 GeV many particles per event • Look for other possibilities to quantify jet-like correlations • Example: Averaged Power-spectra (auto-correlations)

  11. pTtrig > 8 GeV hep-ph/0606098 The biased trigger bias <pTpart> is a function of pTtrig but also pTassoc, s, near-side/away-side, DE See also, K. Filimonov, J.Phys.G31:S513-S520,2005

  12. From di-hadron correlations to jets • Strong bias on fragmentation function • … which we want to measure • Very low efficiency, example: • 1.1 106 Jets produced in central Pb-Pb collisions (|h| < 0.5) • ~1500 Jets selected using leading particles pT > 60 GeV

  13. Reduction of the trigger biasby collecting more energy from jet fragmentation… Unbiased parton energy fraction - production spectrum induced bias

  14. Jets reconstructed from charged particles: Energy contained in sub-cone R Need reduced cone sizes and transverse momentum cut ! How to reconstruct jets in HI environment:Optimal cone size 1.5 TeV in cone of R = 1 Background: E ~ R2 85% of jet energy Jets can be reconstructed using reduced cone size, but what is the energy resolution ?

  15. What determines the energy resolution ? • There exist different kind of energy fluctuations that contribute to the intrinsic energy resolution in HIC • Fluctuations caused by event-by-event variations of the impact parameter for a given centrality class. • Strong correlation between different regions in h-f plane • ~R2 • Can be eliminated using impact parameter dependent background subtraction • Poissonian fluctuations of uncorrelated particles • DE = N[<pT>2 +DpT2] • ~R • Correlated particles from common source (low-ET jets) • ~R • Out-of-cone Fluctuations Ejet = 100 GeV Resolution limited by out-of-cone fluctuations common to all experiments ! pT > 0 GeV 1 GeV 2 GeV

  16. Reconstructed energy for monochromatic jets Tail towards higher energies = Trigger bias ET = 100 GeV DE/E ~ 50% DE/E ~ 30%

  17. Expected resolution including EMCAL Jet reconstruction using charged particles measured by TPC + ITS and neutral energy from EMCAL. Sarah Blyth, QM2004

  18. Trigger performance Background rejection set to factor of 10 =>HLT Centrality dependent thresholds on patch energy A. Mischke and P. Jacobs, ALICE INT-2005-50

  19. ALICE performance studiesWhat has been achieved so far ? • Full detector simulation and reconstruction of HIJING events with embedded Pythia Jets • Implementation of a core analysis frame work • Reconstruction and analysis of charged jets.

  20. Energy spectrum from charged jets Cone-Algorithm: R = 0.4, pT > 2 GeV Selection efficiency ~30% as compared to 6% with leading particle ! No deconvolution, but GaussE-n ~ E-n

  21. Jet structure observables Bump from background Background subtraction under study.

  22. Hump-back plateau Erec > 100 GeV Bias due to incomplete reconstruction. Statistical error 104 events 2 GeV High z (low x): Needs improved resolution (EMCAL). Low z (high x): Systematic error is a challenge, needs reliable tracking. Also good statistics (trigger is needed)

  23. jT Q jT-Spectra Statistical error 104 events Background small where transverse heating is expected.

  24. Q tform = 1/(QkT) tsep = 1/Q More to come … • Dijet correlations • “Sub-jet” Suppression ? • Look for “hot spots” at large distance to jet axis • Small formation time • Can we observe ~10 GeV parton suppression within 100 GeV jets ? R0 = 1fm Q

  25. Dominant processes: g + q → γ+ q (Compton) q + q → γ + g (Annihilation) pT > 10 GeV/c max min EMCal TPC g g IP PHOS Photon-tagged jets g-jet correlation • Eg = Ejet • Opposite direction • Direct photons are not perturbed by the medium • No surface bias • Parton in-medium-modification through the fragmentation function D(z), z = phadron/Eg

  26. Prompt g are likely to be produced isolated Two parameters define g isolation: Cone size R pT threshold candidate isolated if: no particle in cone with pT > pTthres pT sum in cone, SpT < SpTthres R PHOS • pp collisions R = 0.2, pTthres = 0.7 GeV/c • Identification Probability100 % • Misidentification4.5 % • Signal/Background 13 • Pb-Pb collisions R = 0.2, pTthres = 2 GeV/c • Identification Probability 50 % • Misidentification 7 % • Signal/Background 4.2 Promp photon identification:Isolation cut method G. Conesa, ALICE-INT-2005-014, HCP 2005 proceedings

  27. signal x5 Identifying prompt g in ALICE Prompt g reach ~ 100 GeV Statistics for on months of running: 2000 g with Eg > 20 GeV Eg reach increases to 40 GeV with EMCAL

  28. Background non-quenched HIC background Signal quenched jet Fragmentation function Pb-Pb collisions Sensitivity ~ 5% for z < 0.4

  29. Summary • Copious production of jets in PbPb collisions at the LHC • < 20 GeV many overlapping jets/event • Di-hadron correlations • Background conditions require jet identification and reconstruction in reduced cone R < 0.3-0.5 • ALICE will measure jet structure observables (jT, fragmentation function, jet-shape) for reconstructed jets. • High-pT capabilities (calorimetry) needed to reconstruct parton energy • Good low-pT capabilities are needed to measure particles from medium induced radiation. • In this sense ALICE is now optimized for jet studies in HIC • ALICE can measure photon tagged jets with • Eg > 20 GeV (PHOS + TPC) • Eg > 40 GeV (EMCAL+TPC) • Sensitivity to medium modifications ~5%

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