Fisica dei jets con EMCal

Nicola BianchiBianchi@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 20062nd 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

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

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.

FF and their QCD evolution are described in the framework of multiple parton scattering and induced radiation

Leading hadrons in SIDIS

• 1 free parameter Cquark-gluon correlation strength in nuclei• From 14N data C=0.0060 GeV2:• HERMES : cold but static nuclei Esta 0RA

2 ; 0 gluon density and RA6 fm• RHIC : hot but expanding exp Esta (20/RA); 0 initial medium formation time • Gluon density at RHIC ~ 30 times higher than in cold matter

22ANsg RmCznE

Parton energy loss : Landau-Migdal-Pomeranchuk interference patternH-T term in the QCD evolution equation of FFs

ddpdT

ddpNdpR

TNN

AA

TAA

TAA /

/)(

2

2

fmGeVq /0ˆ 2

fmGeVq 21ˆ

fmGeVq 2155ˆ

Medium charact. by gluon transport coeff.: 2

ˆ q= typical momentum transfer= gluon mean free path

•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?

Leading hadrons in HIC (RHIC)

Where does thisassociated radiationgo to ?

How does this partonthermalize ?

What is the dependence on parton identity ?

Egluon Equark, m0 Equark, m0

RHIC has not succeeded in significantly improve the following picture:

Pictorial view

• 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 = √(2+2) < 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 jets

LHC

RHIC

SPS

(h+

+h-)/20

17 GeV

200 GeV

5500 GeV=√s

LO p+p y=0Heavy 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: ELHC~40 GeV

need ETJet~200 GeV for E>>E

Why LHC

=ln(EJet/phadron)

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

• hadron enhancement at low relative pT

• hadron suppression at large relative pT

… like in DIS at low and high z …

Jet quenching at LHC

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 E C C, is the effective cut-off of radiated spectrumBroadening is expected to be

Jet shape modification

c ˆ q L2 2Lq̂

2.0 0.7 GeVEJet=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

Sensitivity to medium properties

TPC

PHOS

TRD

RICH

EMCal

Support Structure

TOF

EmCal Acceptance = 1.4 = 110o

EmCal granularity:about 12000 channels

EmCal position :Back to back with the smaller PHOS

•Excellent tracking : ITS, TPC•Excellent PID : TOF, RICH, TRD

•High resolution (~ 3% / √ E) PbWO4 Calorimetry for : PHOS but too small acceptance and PT range for Jet and high PT physics

EMCal in ALICE (short)

The EMCal extends the scope of the ALICE experiment for jet quenching :

• The EMCal provides a fast, efficient trigger for high pT jets, (), 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 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

Major physics capabilities of EMCal

Good measurement of fragmentation function: 103

counts

104/year minbias Pb+Pb:

• inclusive jets: ET~200 GeV

• dijets: ET~170 GeV

• : pT~75 GeV

• inclusive : pT~45 GeV

• inclusive e: pT~25 GeV

Jet rate in EMCal

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(2+2)

• several approaches to subtract backgrounds

Jet reconstruction Typical for jet reconstruction : combination of e.m and hadronic calorimeters, but no hadronic calorimeter in ALICE

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 carried by particle with pT > pT

min

Energy (charged) contained in sub-cone R

Jet trigger

PYTHIA jet + HIJING background

peripheral

central Varying patc

h size

(x)

• 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

discrimination

0 pt (GeV/c)

• low pt: invariant mass analysis• medium pt : evt by evt shower shape• high pt : isolation cut• neutrons : up to 2-3 GeV from TOF• , f0(?)

Invariant mass (up to 10 GeV)

10 GeV

10 GeV 15 GeV

30 GeV 50 GeV

20 GeV

0

→ same distribution at large energy→ shower shape can be used from ~10 to 30 GeV

25 GeV

/0 shower shape

Direct photons

p+p

Pb+Pb/

CERN Yellow Report

Not an easy measurement:

• < 0.1 for p+p(better in central Pb+Pb due to hadron suppression)• QCD bremsstrahlung photons may dominate for pT<50 GeV/c• +jet: calibration of jet energy precise measurement of modified fragmentation function

• measured in EMCal • fragmentation function from measurements of recoil in TPC

Track macthing for charged

TPC track – EMCal hit (cm)

Track matching between TPC track and EMCal cluster

TPC

TRD+TOF

EMCal

electron photon

• electron identification and reconstruction• removal of charge hadronic energy deposition in EMCal

e/h discrimination

e

h

E/p

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

rejection 400e efficiency 90%

Study of semi-leptonic decay of massive quarks :•Sensitivity to mass due to suppression of gluon radiation in dead-cone C < mQ/E•Sensitivity to color charge

First results from prototype

X, Y [cm]Y

ield

First study for energy resolution:using MIPs for calibration :=>~1.8% + 9.5%/ E

First study for position resolution(large beam size)

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