Transcript Fisica dei jets con EMCal

Fisica dei jets con EMCal
Nicola Bianchi
[email protected]
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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: ptbroadening.
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
DE  n  Dz g  C s2 mN RA2
• 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)
R AA ( p T ) 
d 2 N AA / dp T d 
T AA d 2 NN / dp T d 
qˆ  0 GeV 2 / fm
qˆ  1GeV 2 fm
qˆ  5  15 GeV 2 fm
 2 = typical momentum transfer
Medium charact. by gluon transport coeff.: qˆ 
 = 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?
Pictorial view
RHIC has not succeeded in significantly improve the following picture:
How does this parton
thermalize ?
What is the dependence on
parton identity ?
Where does this
associated radiation
go to ?
DE gluon  DE quark, m 0  DE quark, m0
Why jets
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gluon radiation
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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 = √(D2+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
Why LHC
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
(h++h-)/2
p0
=
5500
GeV
200 GeV
17 GeV
LHC
RHIC
SPS
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
√
s
Jet quenching at LHC
• MLLA: parton splitting+coherence angle-ordered parton cascade
• good description of vacuum fragmentation (PYTHIA)
• introduce medium effects in parton splitting
pThadron~2 GeV for
Ejet=100 GeV
=ln(EJet/phadron)
• 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 C wC,
w c  qˆ L2 2 is the effective cut-off of radiated spectrum
Broadening is expected to be  q̂L
Sensitivity to medium properties
EJet=100 GeV: 2.0 0.7 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)
•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
EmCal Acceptance
D = 1.4
DF = 110o
EmCal granularity:
about 12000 channels
EMCal
Support
Structure
TPC
RICH
TRD
TOF
EmCal position :
Back to back with the
smaller PHOS
PHOS
Major physics capabilities of EMCal
The EMCal extends the scope of the ALICE experiment for jet quenching :
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The EMCal provides a fast, efficient trigger for high pT jets, g(p0),
electrons  recorded yields enhanced by factor ~10-60
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The EMCal markedly improves jet reconstruction through measurement
of EM fraction of jet energy with less bias
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The EMCal provides good g/p0 discrimination, augmenting ALICE direct
photon capabilities at high pT
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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
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(D2+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
PYTHIA jet +
HIJING background
• 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
~6
~50
p0 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
g/p0 shower shape
10 GeV
15 GeV
20 GeV
30 GeV
50 GeV
g
p0
25 GeV
→ same distribution at large energy
→ shower shape can be used from ~10 to 30 GeV
Direct photons
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/p0
Pb+Pb
p+p
CERN Yellow Report
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
electron photon
TPC
TRD+TOF
EMCal
TPC track – EMCal hit (cm)
• electron identification and reconstruction
• removal of charge hadronic energy deposition in EMCal
e/h discrimination
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
e
h
E/p
p rejection 400
e efficiency 90%
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
First results from prototype
First study for position resolution
(large beam size)
Yield
First study for energy resolution:
using MIPs for calibration :
=>~1.8% + 9.5%/ E
X, Y [cm]
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