Physics with the ALICE EMCal Peter Jacobs, LBNL Probing QCD matter with jets…

The Physics with the EMCal DOE Review, Dec 12-14, Partonic energy loss in a colored medium Bjorken ’82: jets lose energy in matter (calculated elastic scattering) Main mechanism: medium-induced bremsstrahlung: … Energy loss  E directly sensitive to density of the medium at the initial, hottest phase of the collision

The Physics with the EMCal DOE Review, Dec 12-14, Radiative energy loss Finite kinematics  logarithmic dependence of  E on E  need logarithmically large variation of parton energy for complete study of energy loss Medium characterized by transport coefficient:  =typical momentum transfer =gluon mean free path Total energy loss:  E~L 2 (path length), linearly dependent on color charge  E indepdendent of partonic energy E

The Physics with the EMCal DOE Review, Dec 12-14, Jet quenching at RHIC: high p T hadrons are suppressed, photons are not Hadron suppression well described by pQCD+partonic energy loss  jets lose energy in dense matter

The Physics with the EMCal DOE Review, Dec 12-14, High p T dihadron azimuthal correlations core of fireball is opaque  trigger biased towards surface recoil jet is quenched in dense matter STAR, Phys Rev Lett 91, ?

The Physics with the EMCal DOE Review, Dec 12-14, Baryon/meson enhancement in nuclear collisions Dihadron correlations exhibit jet-like features in same kinematic regime  influence of bulk medium on hadronization of jets  will hadronization of jets at LHC be similarly modified? Au+Au central collisions

The Physics with the EMCal DOE Review, Dec 12-14, Jet quenching at RHIC: summary Jets are quenched in very dense matter: unique probes of the medium But current picture is qualitative to a large extent: fragmentation and geometry bias of leading hadron trigger p T ~2-5 GeV/c: hadronization not well understood no direct evidence for radiative energy loss where is the radiation? Is it also quenched in the medium? color charge, quark mass dependence are crucial tests role of collisional energy loss? response of medium to lost energy? Future RHIC measurements: new instrumentation and larger datasets are crucial Remainder of this talk: jet studies at the LHC will complement and greatly extend the RHIC measurements

The Physics with the EMCal DOE Review, Dec 12-14, From RHIC to the LHC… Heavy ions at LHC: hard scattering at low x dominates particle production low x: calculable (saturated) initial conditions? fireball hotter and denser, lifetime longer than at RHIC dynamics dominated by partonic degrees of freedom huge increase in yield of hard probes

The Physics with the EMCal DOE Review, Dec 12-14, First jet quenching measurement at the LHC: inclusive hadron suppression Initial gluon density at LHC ~ 5-10 x RHIC: But R AA (LHC) ~ ~ R AA (RHIC): inclusive hadrons have poor sensitivity to initial conditions  need to dig deeper: full jet structure I. Vitev and M. Gyulassy, PRL 89, (2002) A. Dianese et al., Eur.Phys.J. C38, 461(2005) RHIC vs LHC

The Physics with the EMCal DOE Review, Dec 12-14, Jet measurements at the LHC High energy jets ~fully reconstructable in heavy ion collisions unbiased jet population  comprehensive study of energy loss (contrast leading particle analyses) Large kinematic reach  evolution of energy loss How high in energy? scale qhat from RHIC:  E LHC ~40 GeV  need E T Jet ~200 GeV for E>>  E Color charge, quark mass dependence over broad range  basic tests of energy loss mechanisms RHIC+LHC provide similar measurements for vastly different physical systems: comparison will provide deep insight

The Physics with the EMCal DOE Review, Dec 12-14, How to measure jet quenching? MLLA: parton splitting+coherence  angle-ordered parton cascade good description of vacuum fragmentation (PYTHIA) introduce medium effects in parton splitting Borghini and Wiedemann  =ln( E Jet / p hadron ) p T hadron ~2 GeV for E jet =100 GeV Fragmentation strongly modified at p T hadron ~1-5 GeV even for the highest energy jets

The Physics with the EMCal DOE Review, Dec 12-14, Sensitivity to medium properties 2.0 A. Morsch, ALICE Measurements at p T ~1 GeV are crucial Limitations due to (as yet unknown) background 0.7 GeVE Jet =100 GeV:

The Physics with the EMCal DOE Review, Dec 12-14, ALICE EMCal Lead-scintillator sampling calorimeter |  |<0.7,  =110 o Shashlik geometry, APD photosensor ~13K towers (  x  ~0.014x0.014)

The Physics with the EMCal DOE Review, Dec 12-14, Major physics capabilities of EMCal The EMCal significantly extends the scope of the ALICE experiment for jet quenching measurements in heavy ion collisions: 1.The EMCal provides a fast, efficient trigger for high p T jets,  (   ), electrons  recorded yields enhanced by factor ~ The EMCal markedly improves jet reconstruction through measurement of EM fraction of jet energy 3.The EMCal provides good   discrimination, augmenting ALICE direct photon capabilities at high p T 4.The EMCal provides good electron/hadron discrimination, augmenting and extending to high p T the ALICE capabilities for heavy quark jet quenching measurements

The Physics with the EMCal DOE Review, Dec 12-14, Kinematic reach of ALICE+EMCal 10 4 /year minbias Pb+Pb: inclusive jets: E T ~200 GeV dijets: E T ~170 GeV   : p T ~75 GeV inclusive  : p T ~45 GeV inclusive e: p T ~25 GeV NB: jet yields in written documentation are incorrect Good measurement of fragmentation fn: 10 3 counts

The Physics with the EMCal DOE Review, Dec 12-14, Fast (level 1) EMCal trigger L max [ p+p equivalent] (10 27 cm -2 s -1 ) interaction rate (Hz) Max rate to tape (Hz) EMCal Trigger yield enhancement  Jet (R=0.2) Pb+Pb 1.0 [4  10 4 ] 8K Ar+Ar 60 [1  10 5 ] 130K O+O 200 [5  10 4 ] 220K p+p 5  10 3 [5  10 3 ] 200K relative event rate to tape: EMCal trigger vs minbias trigger + TPC ALICE Rate to tape limited by DAQ and TPC gating (<500 Hz) Level 1 trigger (level 0 in p+p) needed to utilize luminosity EMCal enhances recorded yields of triggered hard probes by factors 10-60, depending on collision system (more discussion on trigger system in talks tomorrow)

The Physics with the EMCal DOE Review, Dec 12-14, Jet reconstruction leading hadrons Full characterization of fragmentation function and its modifications: enhancement at low p T suppression at high p T All measurements see a fraction of partonic energy  need to correct for missing fraction Control systematic uncertainties: minimize magnitude of correction minimize bias due to jet spectrum  measure as much of jet energy as possible

The Physics with the EMCal DOE Review, Dec 12-14, Jet reconstruction cont’d Fraction of jet energy outside cone R=0.3 E T ~100 GeV: R=0.3 already contains >80% of jet energy Suggests jet reconstruction strategy in heavy ions: small R to optimize S/B Large fluctuating background in heavy ion collisions  need targeted jet reconstruction algorithm Jet cone: CDF preliminary

The Physics with the EMCal DOE Review, Dec 12-14, R Energy in cone R: background and jets Central Pb+Pb Jet reconstruction in ALICE Hadronic energy: charged tracks (TPC/ITS) Electromagnetic energy: EMCal Corrections: unmeasured hadrons (neutrons, K 0 L,…) (<10%) hadronic energy in EMCal Modified UA1 cone algorithm: R=sqrt(  2 +  2 ) several approaches to subtract backgrounds S/B enhanced by: small cone radius R track p T cut Hadronic calorimetry cannot suppress backgrounds:  all LHC experiments will have same jet energy resolution

The Physics with the EMCal DOE Review, Dec 12-14, Jet energy resolution EMCal+tracking: energy resolution ~25-30% achievable with suitable R and p T cuts ultimate performance depends on actual quenching signal and background environment Resolution for E jet =100 GeV No background p T >0,1,2 GeV/c TPC + full calorimetry Central Pb+Pb, p T >1 GeV/c R Resolution contribution of unmeasured hadrons not included

The Physics with the EMCal DOE Review, Dec 12-14, largest bias for charged-only window on reconstructed energy to select most probable E generated =100 GeV PYTHIA Jet reconstruction: role of EMCal EMCal+charged recovers larger fraction of energy than charged only, with markedly better resolution  less bias on physical spectrum long tail due partly to lost neutrons+K 0 L but also to “out of cone fluctuations” from small cone radius R (aka jet splitting)  modifications to cone algorithm under study Monoenergetic: E jet =100 GeV

The Physics with the EMCal DOE Review, Dec 12-14, Jet reconstruction bias Correction factor: measured energy  parent parton energy E measured /E generated E measured or E generated mono-energetic parent physical spectrum EMCal: large improvement in bias wrt charged only  close to limit of ideal calorimetry

The Physics with the EMCal DOE Review, Dec 12-14,    discrimination  +jet: calibration of jet energy  precise measurement of modified fragmentation function X.-N. Wang et al., PRL 77, 231 (1996)   measured in EMCal (factor 8 larger acceptance than PHOS) fragmentation function from inclusive measurements of recoil in TPC ALICE kinematic reach extended to p T  ~30-40 GeV/c

The Physics with the EMCal DOE Review, Dec 12-14, Direct photons at the LHC Not an easy measurement, however:   < 0.1 for p+p (expected to be better in central Pb+Pb due to hadron suppression) QCD bremsstrahlung photons may dominate for p T <50 GeV/c  isolation cuts in heavy ion collisions? p+p Pb+Pb // CERN Yellow Report

The Physics with the EMCal DOE Review, Dec 12-14,   discrimination in EMCal single-cluster efficiency   ratio High p T : use shower shape to discriminate one shower from two merged showers Good enhancement for Pb+Pb where cross section is large (~30 GeV/c)  drives tower granularity PHOS has found effective isolation cuts  under study

The Physics with the EMCal DOE Review, Dec 12-14, Electron/hadron discrimination Significant electron yield to p T ~25 GeV/c with e/  ~0.01 EMCal provides electron trigger Dominant contribution from heavy quark jets (estimate E T jet to 50 GeV) 50 GeV: light hadron-led jets come mainly from gluons  basic test of energy loss: color- charge dependence (Wiedemann et al)

The Physics with the EMCal DOE Review, Dec 12-14, First look at electron/hadron discrimination Geant simulation with all ALICE materials Based on E/p from EMCal/tracking and shower-shape e h E/p 1/pion efficiency 10 3 electron efficiency 20 GeV First look: good hadron rejection at 20 GeV Not yet addressed: electron backgrounds

The Physics with the EMCal DOE Review, Dec 12-14, EMCal contribution to ALICE jet measurements: Trigger enhancement of high p T yields by factor Major improvement in jet reconstruction performance Extension of direct photon measurements at high p T Extension of heavy quark jet studies at high p T

The Physics with the EMCal DOE Review, Dec 12-14, ~ unbiased jet measurement over large jet energy range (~200 GeV)  evolution of energy loss excellent tracking at p T ~1 GeV/c  softening of fragmentation, response of the medium to the jet excellent PID: medium modification of jet hadronization ALICE+EMCal provides unique capabilities for jet quenching studies at the LHC

The Physics with the EMCal DOE Review, Dec 12-14, Extra slides

The Physics with the EMCal DOE Review, Dec 12-14, How does medium respond to the lost energy? 4< p T trig < 6 GeV High momentum correlation suppressed  low momentum enhanced Recoil distribution soft and broad ~ thermalized? Qualitative picture consistent  quantitative study of dynamics at low p T ? STAR, Phys Rev Lett 91, p T assoc > 2 GeV STAR nucl-ex/ cos(  ) p T assoc > 0.15 GeV STAR, Phys Rev Lett 95,

The Physics with the EMCal DOE Review, Dec 12-14, Limitations of inclusive hadron suppression Eskola et al., hep-ph/ ? Core is opaque trigger hadrons biased towards jets losing little energy R AA only provides lower bound to energy loss

The Physics with the EMCal DOE Review, Dec 12-14, Dihadrons at yet higher p T Re-emergence of recoil: dijets in central collisions Away-side yield is suppressed but finite and measurable  first differential measurement of energy loss 8 < p T (trig) < 15 GeV/c STAR preliminary 

The Physics with the EMCal DOE Review, Dec 12-14, Medium modification: longitudinal ~2 GeV E jet =100 GeV R=1

The Physics with the EMCal DOE Review, Dec 12-14, Medium-induced jet broadening Salgado and Wiedemann jet kTkT k T (tranverse to jet) in jet cone R=  C Medium-induced radiation visible at k T ~3 GeV/c  longitudinal momentum ~few GeV/c

The Physics with the EMCal DOE Review, Dec 12-14, Jets via EMCal+tracking Background suppression requires charged track cuts (next slide): hadronic calorimeter not appropriate But this approach comes at a cost: unmeasured energy (neutrons, K 0 L,…): <10% correction for hadronic energy in EMCal (~1 interaction length) Proof of principle: PHENIX and STAR M. Miller (STAR), PANIC ‘05 Inclusive jet spectrum, p+p at  s=200 GeV

The Physics with the EMCal DOE Review, Dec 12-14, Jet patch trigger in Pb+Pb good trigger efficiency for E T >~70 GeV in central Pb+Pb significant issues: background for large trigger patch sensitivity to jet quenching (softening and broadening of jet)  further discussion in Trigger talk tomorrow PYTHIA jet + HIJING background peripheral central Varying patch size (  x  )