1. Fisica dei jets con EMCal
Nicola Bianchi
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

2. 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

3. 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

4. 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= 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

5. 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

6. Leading hadrons in HIC (RHIC)
STAR, Phys Rev Lett 91, ?
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?

7. 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:

8. 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

9. 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

10. 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 …

11. 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 

12. 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

13. 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 channels
EmCal position :
Back to back with the smaller PHOS

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

23. 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%

24. 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

25. 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