Fisica dei jets con EMCal Nicola Bianchi Bianchi@lnf.infn.it Hadron suppression in DIS Hadron suppression in HIC at RHIC Hadron and jet quenching at LHC The case for an ElectroMagnetic Calorimeter for ALICE Physics performances of EMCal 2nd Convegno Nazionale su fisica di ALICE. Vietri sul mare, May 30 - June 1 2006

Deep Inelastic Scattering DIS and SIDIS are powerful tools to study parton distribution and fragmentation functions in the vacuum Underlying effects in the nuclear medium are better tested due to the static and known density of the system Input for HIC in modification of partonic distribution functions (EMC valence quark at large x, shadowing effects, gluon saturation at low x ..) Input for HIC in modification of partonic fragmentation functions (parton energy loss, pre-hadronic formation and interaction, hadron formation time ..) Virtuality (Q2) is exactly measured in DIS/SIDIS

Fragmentation function modification FF and their QCD evolution are described in the framework of multiple parton scattering and induced radiation Rescattering without gluon-radiation: pt-broadening. Rescattering with another q : mix of quark and gluon FF. Gluon-rescattering including gluon-radiation: dominant contribution in QCD evolution of FF. Importance to measure the full kinematical/dynamical dependence : transverse broadening : high energy mixing of hadron species : good PID longitudinal effect (hadron suppression at large z/ enhancement at low z) : full momentum acceptance

Leading hadrons in SIDIS Parton energy loss : Landau-Migdal-Pomeranchuk interference pattern H-T term in the QCD evolution equation of FFs 1 free parameter Cquark-gluon correlation strength in nuclei From 14N data C=0.0060 GeV2: HERMES : cold but static nuclei DEsta  r0RA2 ; r0 gluon density and RA6 fm RHIC : hot but expanding DEexp  DEsta (2t0/RA); t0 initial medium formation time Gluon density at RHIC ~ 30 times higher than in cold matter

Leading hadrons in HIC (RHIC) m= typical momentum transfer l= gluon mean free path Medium charact. by gluon transport coeff.: Photons are not suppressed High pT hadrons are suppressed according to pQCD + partonic energy loss Hadron suppression supplies only a lower limit on the energy loss Need to go to higher pT to study QCD evolution Need to study full jet quenching

Leading hadrons in HIC (RHIC) STAR, Phys Rev Lett 91, 072304 ? core of fireball is opaque  trigger biased towards surface recoil jet is quenched in dense matter But current picture is qualitative to a large extent: pT ~2-5 GeV/c: hadronization not well understood (quark recombination?) 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?

Pictorial view Where does this associated radiation go to ? How does this parton thermalize ? What is the dependence on parton identity ? RHIC has not succeeded in significantly improve the following picture:

Why jets Jets are characterized by the fact that transverse momenta of associated particles transverse to jet axis (jT) are small compared to jet momentum (collimation). Collimation increases with energy Jet cone is (simply) defined as: R = √(Dh2+Df2) < 1, 0.7 … 0.3 80% of jet energy in R < 0.3 ! Leading particle has only approximately the direction and energy of the original parton Jet as an entity (parton hadron duality ) stays unchanged Map out observables as a function of parton energy Partons traveling through a dense color medium are expected to loose energy via medium induced gluon radiation, “jet quenching”, and the magnitude of the energy loss depends on the gluon density of the medium gluon radiation

Why LHC LHC RHIC LO p+p y=0 Heavy ions at LHC: SPS (h++h-)/2 p0 17 GeV 200 GeV 5500 GeV = √s LO p+p y=0 Heavy ions at LHC: hard scattering at low x dominates particle production fireball hotter and denser, lifetime longer than at RHIC weakly (?) interacting QGP initial gluon density at LHC 5-10 x RHIC dynamics dominated by partonic degrees of freedom huge increase in yield of hard probes Large kinematic range  evolution of energy loss How high in energy? scale qhat from RHIC: DELHC~40 GeV  need ETJet~200 GeV for E>>DE

Jet quenching at LHC pThadron~2 GeV for Ejet=100 GeV MLLA: parton splitting+coherence angle-ordered parton cascade good description of vacuum fragmentation (PYTHIA) introduce medium effects in parton splitting =ln(EJet/phadron) pThadron~2 GeV for Ejet=100 GeV hadron enhancement at low relative pT hadron suppression at large relative pT … like in DIS at low and high z …

Jet shape modification Broadening of jet multiplicity as sensitive probe of the matter Gluon multiplicity distribution within RC=0.3 : Broadening ( kt to jet direction) is expected for large energy loss DE aC wC, is the effective cut-off of radiated spectrum Broadening is expected to be 

Sensitivity to medium properties 2.0 0.7 GeV EJet=100 GeV: Experimental requirements: Trigger on jet Measurement of total jet energy Full hadron distribution inside the jet cone (charged and neutral) Measurements the full distribution down to pT~1 GeV PID for the study of the jet composition Need to add to the ALICE excellent charged particle ID and momentum reconstruction a Large Electromagnetic Calorimeter

EMCal in ALICE (short) Dh = 1.4 DF = 110o Excellent tracking : ITS, TPC Excellent PID : TOF, RICH, TRD High resolution (~ 3% / √ E) PbWO4 Calorimetry for g : PHOS but too small acceptance and PT range for Jet and high PT physics TPC PHOS TRD RICH EMCal Support Structure TOF EmCal Acceptance Dh = 1.4 DF = 110o EmCal granularity: about 12000 channels EmCal position : Back to back with the smaller PHOS

Major physics capabilities of EMCal The EMCal extends the scope of the ALICE experiment for jet quenching : The EMCal provides a fast, efficient trigger for high pT jets, g(p0), electrons  recorded yields enhanced by factor ~10-60 The EMCal markedly improves jet reconstruction through measurement of EM fraction of jet energy with less bias The EMCal provides good g/p0 discrimination, augmenting ALICE direct photon capabilities at high pT The EMCal provides good electron/hadron discrimination, augmenting and extending to high pT the ALICE capabilities for heavy quark jet quenching measurements

Jet rate in EMCal p0: pT~75 GeV Good measurement of fragmentation function: 103 counts 104/year minbias Pb+Pb: inclusive jets: ET~200 GeV dijets: ET~170 GeV p0: pT~75 GeV inclusive g: pT~45 GeV inclusive e: pT~25 GeV

Jet reconstruction Typical for jet reconstruction : combination of e.m and hadronic calorimeters, but no hadronic calorimeter in ALICE Charged Charged + neutral RMS [GeV] 21 15 Econe/ET 0.50 0.77 Efficiency 67% 80% Hadronic energy: charged tracks (TPC/ITS) Electromagnetic energy: EMCal Corrections: unmeasured hadrons (neutrons, K0L,…) (<10%) hadronic energy (25%) in EMCal Cone algorithm: R=sqrt(Dh2+Df2) several approaches to subtract backgrounds

Jet signal/background R and pt cut should be optimized: maximize signal energy minimize signal fluctuations minimize background contribution (R2) and fluctuation (R) background mostly at low pt (98% below 2 GeV) Energy (charged) contained in sub-cone R Energy carried by particle with pT > pTmin

Jet trigger peripheral Varying patch size (DhxDf) central PYTHIA jet + HIJING background peripheral central Varying patch size (DhxDf) good trigger efficiency for ET>~70 GeV in central Pb+Pb background for large trigger patch centrality dependent threshold required (need input from a centrality-multiplicity detector) 10 % sensitivity to jet quenching (softening and broadening of jet) below 70 GeV

g/p0 discrimination p0 pt (GeV/c) ~6 ~50 10 GeV low pt: invariant mass analysis medium pt : evt by evt shower shape high pt : isolation cut neutrons : up to 2-3 GeV from TOF h, f0(?) Invariant mass (up to 10 GeV) 10 GeV

g/p0 shower shape 10 GeV 15 GeV 20 GeV g p0 25 GeV 30 GeV 50 GeV → same distribution at large energy → shower shape can be used from ~10 to 30 GeV

Direct photons g/p0 g Not an easy measurement: g/p0 < 0.1 for p+p Pb+Pb g/p0 CERN Yellow Report Not an easy measurement: g/p0 < 0.1 for p+p (better in central Pb+Pb due to hadron suppression) QCD bremsstrahlung photons may dominate for pT<50 GeV/c g+jet: calibration of jet energy  precise measurement of modified fragmentation function g g measured in EMCal fragmentation function from measurements of recoil in TPC

Track macthing for charged Track matching between TPC track and EMCal cluster TPC TRD+TOF EMCal electron photon TPC track – EMCal hit (cm) electron identification and reconstruction removal of charge hadronic energy deposition in EMCal

e/h discrimination e h E/p p rejection 400 e efficiency 90% Electron/hadron discrimination : Geant simulation with all ALICE materials Based on E/p from EMCal/tracking Good hadron rejection at 20 GeV Energy resolution better than 10 %/ E (GeV) Prototype beam test data under analysis Study of semi-leptonic decay of massive quarks : Sensitivity to mass due to suppression of gluon radiation in dead-cone qC < mQ/E Sensitivity to color charge p rejection 400 e efficiency 90%

First results from prototype First study for energy resolution: using MIPs for calibration : =>~1.8% + 9.5%/ E First study for position resolution (large beam size) X, Y [cm] Yield Final test at FNAL in November: Energy and position resolution Timing Stability (GMS, T, V) Hadron response

Conclusion ALICE+EMCal provides unique capabilities for jet quenching studies at the LHC challenge with respect to leading hadron physics at RHIC  larger pt, hard regime ~ unbiased jet measurement over large jet energy range (~200 GeV)  evolution of energy loss excellent tracking down to pT~1 GeV/c  softening of fragmentation, response of the medium to the jet excellent PID  medium modification of jet hadronization