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Yaxian Mao, Daicui Zhou, Yves Schutz In ALICE Physics Workgroup: High p T and photons ( for ALICE collaboration -- Wuhan). Gamma- Hadron Correlation MeasureMENT WITH ALICE at LHC. Fragmentation g. p 0. Jet. Why -jet/hadron correlation ?. - PowerPoint PPT Presentation
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28 April 2008MYX@Nanjing 1
Yaxian Mao, Daicui Zhou, Yves Schutz
In ALICE Physics Workgroup: High pT and photons
( for ALICE collaboration -- Wuhan)
Why -jet/hadron correlation?
The photon 4-momentum remains unchanged by the medium and sets the reference of the hard process
Balancing the jet and the photon provides a measurement of the medium modification experienced by the jet
Allows to measure jets in an energy domain (E < 50 GeV) where The jet looses a large fraction of its energy (E ≈ 20 GeV) The jet cannot be reconstructed in the AA environment
28 April 2008MYX@Nanjing 2
Fragmentation Jet
Prompt
0
q
qg
g
γ
Photon sources
Direct photons (the signal) Prompt pQCD photons (E > 10
GeV) g Compton scattering qq annihilation Fragmentation
Photons produced by the medium (Eγ < 10 GeV) Bremsstrahlung Jet conversion Thermal
28 April 2008MYX@Nanjing 3
q
qg
γ
q
q g
γ
q
g q
γ
q
qg
γ
Photon sources
Decay photons (the background) Hadrons, mainly
π0
But suppressed by the medium
28 April 2008MYX@Nanjing 4
E
Rat
e
Hadron Decay
QGP Thermal
Jet conversion
Bremsstrahlung
pQCD prompt
10GeV
High pT hadrons suppression
• Hard scattered partons interact with the color dense medium
• The energy loss is imprinted in the fragmentation hadrons
• The medium is transparent to photons28 April 2008MYX@Nanjing 5
dydpd
dydpd
AAAA
T
pp
T
AA
2
2
N
1R
From RHIC to LHC
The medium is formed with energy densities larger by a factor 2-3: a different QGP ?
The lifetime of the QGP is increased by a factor 2-3: more favorable for observation
28 April 2008MYX@Nanjing 6
From RHIC to LHC
Cross section of hard probes increased by large factors 105 for very high pT jets
Differential measurements become possible Jet fragmentation
function Photon tagging
28 April 2008MYX@Nanjing 7
28 April 2008MYX@Nanjing 8
=100º || < 0.12E/E = 3%/E
PHOS
TPC
Charged hadrons:
Photons:
=360º || < 0.9p/p < 5% at E < 100GeV
h
Measurement with ALICE@LHC
Strategy for a feasibility study Identify prompt photons with ALICE PHOS
detector (PID + Isolation Cut) Construct -charged hadrons correlation from
detected events (detector response) Compare the imbalance distribution (CF) and
fragmentation function (FF) Do the same study in -jet events (signal) and
jet-jet events (background). Estimate the contribution of hadrons from
underlying events. Start with pp, base line measurement
for AA measurement
28 April 2008MYX@Nanjing 9
Monte Carlo Data production +jet in final state – jet @ √s = 14 TeV
Prompt is the signal under study: 6×105 events (5 GeV < E < 100 GeV)
2 jets in final state jet – jet @ √s = 14TeV These events constitute the background: high-pT 0
[O(S)] and fragmentation [O(2S)]: 24×105 events
(5 GeV < E jet < 200 GeV)
ALICE offline framework AliRoot Generator: PYTHIA 6.214; PDF: CTEQ4L Luminosity: Lint = 10 pb -1
Acceptance: two PHOS modules [-0.13, 0.13]; = [259, 301]
28 April 2008MYX@Nanjing 10
Generated prompt photon from -jet events
Cross section of generated photons from -jet events in pp@14TeV
Dashed lines are the simulation bins28 April 2008MYX@Nanjing 11
-jet in pp@14TeV
Generated decay photon from jet-jet events
Cross section of generated photons from jet-jet events in pp@14TeV
Dashed lines are the simulation bins28 April 2008MYX@Nanjing 12
jet-jet in pp@14TeV(π0→γ+γ)
Photon identification (PID) We can discriminate , e and 0 from
anything else : based on: CPVCPV : Charged particle identification TOFTOF : Identification of massive low pT
particles EMCAEMCA : Hadron rejection via shower
topology (SSA)
28 April 2008MYX@Nanjing 13
Photon PID Efficiency discriminate and 0 (SSA) 30 GeV < E < 100
GeV High identification efficiency, ~ 70%, Misidentification efficiency decreasing from 70% to 20%
Not good enough 28 April 2008MYX@Nanjing 14
true 70%
false 20%
Isolation cut (IC) Prompt are likely to be
produced isolated Cone size
pT threshold candidate isolated if:
no particle in cone with pT > pT thres
pT sum in cone, ΣpT < ΣpTthres
pp collisions; R = 0.3, ΣpTthres = 2.0 GeV/c
Identification probability 98 %
Misidentification 3 %
Signal/Background >10 28 April 2008MYX@Nanjing 15
22R
R
PHOS
TPC
candidate
IP
Isolation cut (IC)
S/B: ~ 0.07 at pT =20 GeV/c (generated events)
~ 0.1 at pT =20 GeV/c (detected events)
> 10 at pT = 20 GeV/c (after IC selection)28 April 2008MYX@Nanjing 16
Photon spectrum after one year data taking
Estimated counting statistics in one pp run for 2 PHOS modules
Systematic errors from misidentified 0
28 April 2008MYX@Nanjing 17
Fragmentation function
Construct jets within jet finder in PYTHIA; Calculate fragmentation function of these
jets: the distribution of charged hadrons as a function of the fraction of jet momentum z = pT/ET
jet
Requirement: reconstruction of jet energies28 April 2008MYX@Nanjing 18
R = (-0)2 + (-0)2 =1
(0, 0)
R
IP
Jet
Jet
Tagging jet with photon
Search identified prompt photon (PHOS) with largest pT (E > 20 GeV)
Search leading particle : -leading180º
Eleading > 0.1 E
Reconstruct the jet: Particles around the leading inside a
cone A more simpler method is …
28 April 2008MYX@Nanjing 1919
min max
leading
R
IP
PHOS
EMCal
TPC
-hadron correlation Momentum imbalance variable
z-h= -pTh · pT / |pT|2
In leading-order kinematics (s)
z-h pTh / pT
According momentum conservation, pT = k = Eparton
Therefore, (exp.) z-h z (th.)
28 April 2008MYX@Nanjing 20
Kinematics conditions on CF Photon and hadron momenta cuts must
be very asymmetric:
pTcut >> pT h
cut Photon must be produced directly from
the partonic process and not from a jet fragmentation:
isolated and pT> 20GeV/c
Photon – hadrons are back to back: /2 < < 3/2
28 April 2008MYX@Nanjing 21
Experimental imbalance distribution
Statistical errors correspond to one standard year of data taking with 2 PHOS modules.
Systematic errors is contributed by decay photon contamination and hadrons from underlying events.
Imbalance distribution is equivalent to fragmentation function for z = 0.1 – 0.8
28 April 2008MYX@Nanjing 22
Summary Tagging jets with direct prompt photons is
the unique approach to identify low energy jets (Ejet < 50 GeV) in AA
The medium modification on jets is best measured in the jet fragmentation function
The fragmentation function can be measured in photon – charged hadrons correlations
The feasibility of such a measurement with the ALICE experiment has been evaluated in pp at 14 TeV
Next time , different kinematics cuts and AA …
28 April 2008MYX@Nanjing 23
Thanks for your attention!
28 April 2008MYX@Nanjing 24
Back up
Hard Probes: an essential tool Hard probes involve high momentum
(pT) or high mass transfer: pT, M >> QCD : pertubative regime of QCD
thus calculable 1/(pT, M) << QGP
formation: produced in early phase of the collision
pT, M >> T medium : they decouple from the medium
Observe how the medium (AA) modifies the hard probes as compared to its vacuum (pp) properties
28 April 2008MYX@Nanjing 26
Di-hadron correlation
• No back-to-back high pT correlation in central Au+Au collisions compared to dAu or pp collisions: the hard scattered parton looses energy via gluon radiation when traversing the color dense medium
28 April 2008MYX@Nanjing 27
Jet
Jet
Di-hadron correlation
• Back-to-back low pT correlation reflects the radiated energy through the fragmentation of low pT gluons
28 April 2008MYX@Nanjing 28
Jet
Jet
Performance
• Momentum resolution in central barrel better than 4%• Energy resolution in PHOS better than 1.5% for E > 10
GeV28 April 2008MYX@Nanjing 29
PHOSCentral Barrel, pp
Imbalance distribution from NLO (by F. Arleo)
Within higher kinematics condition, the medium effects can be measured by imbalance distribution (CF) instead of fragmentation function (FF).
28 April 2008MYX@Nanjing 30
Underlying events estimation
Based on: Hadrons spatial distribution from underlying
events (ue) is isotropic:
ue (|-hadron |<0.5 ) ≅ ue (0.5 <|-hadron
|<1.5 )
Strategy: Calculate ue contribution on the same side as
photon where there is no jet contribution
28 April 2008MYX@Nanjing 31