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Jets and High-pt Physics with ALICE at the LHC

Jets and High-pt Physics with ALICE at the LHC. Andreas Morsch CERN. Outline. Introduction Jets at RHIC and LHC: New perspectives and challenges High- p T di-hadron correlations Reconstructed Jets Jet Structure Observables g -Jet Correlations. Jets in nucleus-nucleus collisions.

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Jets and High-pt Physics with ALICE at the LHC

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  1. Jets and High-pt Physics with ALICE at the LHC Andreas Morsch CERN

  2. Outline • Introduction • Jets at RHIC and LHC: New perspectives and challenges • High-pT di-hadron correlations • Reconstructed Jets • Jet Structure Observables • g-Jet Correlations

  3. Jets in nucleus-nucleus collisions • Jets are the manifestation of high-pT partons produced in a hard collisions in the initial state of the nucleus-nucleus collision. • These partons undergo multiple interaction inside the collision region prior to fragmentation and hadronisation. • In particular they loose energy through medium induced gluon radiation and this so called “jet quenching” has been suggested to behave very differently in cold nuclear matter and in QGP. • The properties of the QGP can be studied through modification of the fragmentation behavior • Hadron suppression • Jet structure.

  4. Jet Physics at RHIC p+p @ s = 200 GeV STAR Au+Au @ sNN = 200 GeV In central Au-Au collisions standard jet reconstruction algorithms fail due to the large energy from the underlying event (125 GeV in R< 0.7) and the relatively low accessible jet energies (< 20 GeV). Use leading particles as a probe.

  5. Quantities studied Hadron Suppression Similar RCP: Ratio central to peripheral “away side” pT(assoc) Hadron Correlations: pT(trig) – pT(assoc) Df(trig, assoc) … “same side” pT (trig)

  6. STAR Phys. Rev. Lett. 91, 072304 (2003). Pedestal&flow subtracted Evidence for Jet Quenching • In central Au+Au • Strong suppression of inclusive hadron yield in Au-Au collisions • Disappearance of away-side jet • No suppression in d+Au • Hence suppression is final state effect.

  7. Eskola et al., hep-ph/0406319 RAA~0.2-0.3 for broad range of q Surface emission bias • RHIC measurements are consistent with pQCD-based energy loss simulations. However, they provide only a lower bound to the initial color charge density.

  8. A. Dainese, C. Loizides, G. Paic s = 5500 GeV From toy model Reconstructed Jet Jet Physics at LHC: Motivation • Study of reconstructed jets increases sensitivity to medium parameters by reducing • Trigger bias • Surface bias • Using reconstructed jets to study • Modification of the leading hadron • Additional hadrons from gluon radiation • Transverse heating. x = ln(Ejet/phadron)

  9. Jet Physics at LHC: New perspectives • Jet rates are high at energies at which they 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. Pb-Pb O(103) un-triggered (ALICE) => Need Trigger

  10. Jet Physics at LHC: New challenges • More than one jet ET> 20 GeV per event • More than one particle pT > 7 GeV per event • 1.5 TeV in cone of R = Dh2+Df2 < 1 ! • 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 !

  11. central Pb–Pb pp New Challenges for ALICE • Existing TPC+ITS+PID • |h| < 0.9 • Excellent momentum resolution up to 100 GeV • Tracking down to 100 MeV • Excellent Particle ID • New: EMCAL • Pb-scintillator • Energy resolution ~15%/√E • Energy from neutral particles • Trigger capabilities

  12. ALICE Set-up Size: 16 x 26 meters Weight: 10,000 tons TOF TRD HMPID TPC PMD ITS Muon Arm PHOS

  13. 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 (P) large • Leads to increased signal and background at LHC • Background dominates, significance independent of multiplicity • Increased width of the away-side peak (NLO) • Wider h-correlation (loss of acceptance for fixed h-widow) • Power law behavior ds/dpT ~ 1/pTn with 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)

  14. 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 • Scale rates between RHIC and LHC • Ratio of inclusive hadron cross-section • N(pT) ~ pT4

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

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

  17. From di-hadron correlations to jets • Strong bias on fragmentation function • … which we want to measure • Low selectivity of the parton energy • Very low efficiency, example: • ~6% for ET > 100 GeV • 1.1 106 Jets produced in central Pb-Pb collisions (|h| < 0.5) • No trigger: ~2.6 104 Jets on tape • ~1500 Jets selected using leading particles

  18. Reduction of the trigger biasby collecting more energy from jet fragmentation… Unbiased parton energy fractionproduction spectrum induced bias

  19. Reconstructed Jets: Objectives • Reduce the trigger bias as much as possible by collecting of maximum of jet energy • Maximum cone-radius allowed by background level • Minimum pT allowed by background level • Study jet structure inclusively • Down to lowest possible pT (z, jT) • Collect maximum statistics using trigger.

  20. Jet Finder in HI Environment:Principle Rc Loop1: Background estimation from cells outside jet cones Loop2: UA1 cone algorithm to find centroid using cells after background subtraction

  21. Jet Finder based on cone algorithms • Input: List of cells in an h-f grid sorted in decreasing cell energy Ei • Estimate the average background energy Ebg per cell from all cells. • For at least 2 iterations and until the change in Ebg between 2 successive iterations is smaller than a set threshold: • Clear the jet list • Flag cells outside a jet. • Execute the jet-finding loop for each cell, starting with the highest cell energy. If Ei – Ebg > Eseed and if the cell is not already flagged as being inside a jet: • Set the jet-cone centroid to be the center of the jet seed cell (hc, fc) = (hi, fi) • Using all cells with (hi-h)2+(fi-f)2 < Rc of the initial centroid, calculate the new energy weighted centroid to be the new initial centroid. • Repeat until difference between iterations shifts less than one cell. • Store centroid as jet candidate. • Recalculate background energy using information from cells outside jets.

  22. Jets reconstructed from charged particles: Energy contained in sub-cone R Need reduced cone sizes and transverse momentum cut ! Optimal Cone Size Jet Finders for AA do not work with the standard cone size used for pp (R = 0.7-1). R and pT cut have to be optimized according to the background conditions. E ~ R2

  23. Background Fluctuations • Background fluctuations limit the energy resolution. • 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 subtrcation. • Poissonian fluctuations of uncorrelated particles • DE = N[<pT>2 +DpT2] • ~R • Correlated particles from common source (low-ET jets) • ~R • Out-of-cone Fluctuations

  24. Background Fluctuations Evt-by-evt background energy estimation

  25. Signal fluctuationsResponse function for mono-chromatic jets ET = 100 GeV DE/E ~ 50% DE/E ~ 30%

  26. Putting things together:Intrinsic resolution limit Ejet = 100 GeV Background included pT > 0 GeV 1 GeV 2 GeV Resolution limited by out-of-cone fluctuations common to all experiments !

  27. Expected resolution including EMCAL Jet reconstruction using charged particles measured by TPC + ITS And neutral energy from EMCAL.

  28. Trigger performance Trigger on energy in patch Dh x Df Background rejection set to factor of 10 =>HLT Centrality dependent thresholds

  29. Jet trigger Reference systems • Compare central Pb+Pb to reference measurements • Pb+Pb peripheral: vary system size and shape • p+A: cold nuclear matter effects • p+p (14 TeV): no nuclear effects, but different energy • p+p (5.5 TeV): ideal reference, but limited statistics All reference systems are required for a complete systematic study Includes acceptance, efficiency, dead time, energy resolution

  30. Jet yield in 20 GeV bin Jet yields: one LHC year Large gains due to jet trigger Large variation in statistical reach for different reference systems

  31. Resolution buys statistics

  32. ALICE performanceWhat 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. • Quenching Studies on fragmentation function.

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

  34. Jet structure observables Low z (high x): Systematics is a challenge, needs reliable tracking. Also good statistics (trigger is needed)

  35. Hump-back plateau Bias due to incomplete reconstruction. Statistical error 104 events Erec > 100 GeV 2 GeV

  36. Systematics of background subtraction Background energy is systematically underestimated (O(1 GeV)) Corrections under study (thesis work of R. Dias Valdez)

  37. jT Q jT-Spectra Bias due to incomplete reconstruction. Statistical error 104 events Erec > 100 GeV

  38. Estimate quenching at LHC: Pythia-based simulation with quenching Large R, no pT cut Dashed: quenched jet (central Pb+Pb) Solid: unquenched (p+p) Quenching Studies Compare distributions with and without quenching The measurement: ratio of dashed over solid = Pb+Pb(central)/p+p

  39. Afterburner A Pythia hard scattering Initial and Final State Radiation Afterburner B Pythia Hadronization Afterburner C . . . Toy Models Nuclear Geometry (Glauber) Jet (E) → Jet (E-DE) + n gluons (“Mini Jets”) • Two extreme approaches • Quenching of the final jet system and radiation of 1-5 gluons. (AliPythia::Quench using Salgado/Wiedemann - Quenching weights) • Quenching of all final state partons and radiation of many (~40) gluons (I. Lokhtin: Pyquen)* )*I.P. Lokhtin et al., Eur. Phys. J C16 (2000) 527-536 I.P.Lokhtin et al., e-print hep-ph/0406038 http://lokhtin.home.cern.ch/lokhtin/pyquen/

  40. ratio ALICE+EMCal in one LHC year

  41. Benchmark measurement:p+Pb reference With EMCal: jet trigger+ improved jet reconstruction provides much greater ET reach

  42. Benchmark measurement:Peripheral Pb+Pb reference Without EMCal, significant quenching measurements beyond ~100 GeV are not possible

  43. Summary of statistical reach Large : ~10% error requires several hundred signal events (Pb central) and normalization events (pp,pA). Large z>0.5 requires several thousand events • The EMCAL • extends kinematic range by 40–125 GeV • improves resolution (important at high z) • Some measurements impossible w/o EMCAL

  44. 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 • ~10 GeV parton suppression within 100 GeV jets ? R0 = 1fm Q

  45. Dominant processes: g + q → γ+ q (Compton) q + q → γ + g (Annihilation) pT > 10 GeV/c max min EMCal TPC g IP PHOS Photon-tagged jets g-jet correlation • Eg = Ejet • Opposite direction • Direct photons are not perturbed by the medium • Parton in-medium-modification through the fragmentation function g

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

  47. Background non-quenched HIC background Signal quenched jet Fragmentation function Pb-Pb collisions

  48. Summary • Copious production of jets in Pb-Pb collisions at the LHC • < 20 GeV many overlapping jets/event • Inclusive leading particle correlation • Background conditions require jet identification and reconstruction in reduced cone R < 0.3-0.5 • At LHC we 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. • EMCAL will provide trigger capabilities which are in particular needed to perform reference measurements (pA, pp, ..) • 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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