gamma- hadron correlation measurement with alice at lhc

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


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


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


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


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



=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


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]


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)


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


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 …


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.)


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


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


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 …


Thanks for your attention!


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


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


Jet

Jet

Di-hadron correlation

• Back-to-back low pT correlation reflects the radiated energy through the fragmentation of low pT gluons


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).


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


gamma- hadron correlation measurement with alice at lhc

Documents

jetjet events background

final state jet jet

jet events signal

gevthe jet

gev e jet

gev wherethe jet

medium e

prompt photons