Fisica dei jets con EMCal

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Fisica dei jets con EMCal
Nicola Bianchi Bianchi_at_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
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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

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Fragmentation function modification
FF and their QCD evolution are described in the
framework of multiple parton scattering and
induced radiation
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/dynamic
  al 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

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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 C0.0060 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

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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

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Leading hadrons in HIC (RHIC)

• 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?

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Pictorial view
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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 v(Dh2Df2) lt 1, 0.7 0.3
• 80 of jet energy in R lt 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

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Why LHC

• 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
DELHC40 GeV ? need ETJet200 GeV for EgtgtDE
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Jet quenching at LHC

• MLLA parton splittingcoherence ?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

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Jet shape modification
Broadening of jet multiplicity as sensitive probe
of the matter
Gluon multiplicity distribution within RC0.3
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 ?
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Sensitivity to medium properties

• 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 pT1
  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
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EMCal in ALICE (short)

• Excellent tracking ITS, TPC
• Excellent PID TOF, RICH, TRD
• High resolution ( 3 / v E) PbWO4 Calorimetry
  for g
• PHOS but too small acceptance and PT range for
  Jet and high PT physics

EmCal Acceptance Dh 1.4 DF 110o EmCal
granularity about 12000 channels EmCal position
Back to back with the smaller PHOS
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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

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Jet rate in EMCal
Good measurement of fragmentation function 103
counts

• 104/year minbias PbPb
• inclusive jets ET200 GeV
• dijets ET170 GeV
• p0 pT75 GeV
• inclusive g pT45 GeV
• inclusive e pT25 GeV

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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,) (lt10)
• hadronic energy (25) in EMCal
• Cone algorithm Rsqrt(Dh2Df2)
• several approaches to subtract backgrounds

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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 gt pTmin
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Jet trigger

• good trigger efficiency for ETgt70 GeV in
  central PbPb
• 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

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g/p0 discrimination

• 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
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g/p0 shower shape
10 GeV
15 GeV
20 GeV
g p0
30 GeV
25 GeV
50 GeV
? same distribution at large energy ? shower
shape can be used from 10 to 30 GeV
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Direct photons

• Not an easy measurement
• g/p0 lt 0.1 for pp
• (better in central PbPb due to hadron
  suppression)
• QCD bremsstrahlung photons may dominate for
  pTlt50 GeV/c
• gjet calibration of jet energy ? precise
  measurement of modified fragmentation function

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

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Track macthing for charged
Track matching between TPC track and EMCal cluster

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

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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

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

p rejection 400 e efficiency 90
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First results from prototype
First study for energy resolution using MIPs for
calibration gt1.8 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

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Conclusion
ALICEEMCal 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 pT1 GeV/c ?
  softening of fragmentation, response of the
  medium to the jet
• excellent PID ? medium modification of jet
  hadronization