-hadron correlation measurement to probe fragmentation function

within ALICE@LHC

Presented by: Renzhuo Wan

Institute of Particle Physics Central China Normal University, Wuhan, China

and Yaxian Mao, Gustavo Conesa, Yves Schutz, Chuncheng Xu,

Zhongbao Yin and Daicui Zhou

For the ALICE Collaboration and CCNU group

The international (7th national) workshop for QCD phase transition and heavy ion collisions July 10~13, 2008, Hefei, Anhui

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Motivation ALICE experiment γ-hadron correlations measurement Summary

Outline

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• The medium is almost transparent to photons• Balancing the jet and the photon provides a measurement

of the medium modification experienced by the jet• Study of fragmentation function

Motivation

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• Hard scattered partons interact with the color dense medium

• Probe the energy loss• From pp 900GeV, 10TeV and 14TeV to PbPb 5.5TeV

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LHC/ALICE experiment

• Explore the properties of nuclear matter at extreme high energy density• Explore the dynamics of the transition from deconfined matter toward hadronic matter

Charged hadrons: ITS, TPC, TRD, TOF =360º || < 0.9 p/p < 5% at pT < 100GeV

Photons: PHOS =100º || < 0.12 E/E = 3%/E

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• Direct photons

－ Prompt pQCD photons (E > 20 GeV)

• g compton scattering• qqbar 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

Photon sources

q

qg

g

γ

q

qg

γ

• Decay photons– Hadrons, mainly π0, ~90%

q

qg

γ q

q g

γ

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1. Identify prompt photons with ALICE PHOS detector (PID + Isolation Cut)

2. Construct -charged hadrons correlation from detected events (detector response)

3. Compare the imbalance distribution (CF) to the fragmentation function (FF)

4. Do the same study in -jet events (signal) and jet-jet events (background).

5. Estimate the contribution of hadrons from underlying events

6. Start with pp, base line measurement in AA

Strategy

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+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: 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]

MC data production

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• We can discriminate , e and 0 from anything else, based on:– CPVCPV : Charged particle identification

– TOFTOF : Identification of massive low pT particles

– PHOSPHOS : Hadron rejection via shower shape analysis

Shower from: • single photon/e : e1/e2 = 1

• 0 (p0 > 30 GeV/c) : e1/e2 > 1

Photon identification (PID)

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• Prompt are 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 %

22R

R

PHOS

TPC

candidate

IP

Isolation Cut (1)

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• S/B: – ~ 0.1 at pT =25 GeV/c (generated events)

– ~ 0.3 at pT =25 GeV/c (reconstructed events + SSA)

– > 10 at pT = 25 GeV/c (after IC selection)

Isolation Cut (2)

×3

×10

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(0, 0)

R

IP

Fragmentation Function

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• Momentum imbalance variable– z-h= -pTh

· pT / |pT|2

• In leading-order kinematics (s)

– z-h pTh / pT

• According to momentum conservation,– pT = k = Eparton

• So,

– (exp.) z-h z (th.)

γ-hadron correlation

pTh

pT

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• Photon and hadron momenta cuts must be very asymmetric:

pTcut >> pTh

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

Kinematics condition

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Azimuthal correlation for γ-jet

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• Estimated counting statistics in one pp run for 2 PHOS modules

• Systematic errors from misidentified 0

Photon spectrum after one year of data taking

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• Hadrons spatial distribution from underlying events (UE) is isotropic: UE (|-hadron |<0.5 ) ≅ UE (0.5 <|-hadron |<1.5 )

• Calculate UE contribution on the same side as photon where there is no jet contribution

Underlying Event (UE)

For pt h > 2GeV/c,

contribution from UE represents less than 15%

99%

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Imbalance distribution is equivalent to fragmentation function for z = 0.12 – 0.8.

Comparison CF with FF

pT cut

Fragmentation & decay

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• 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.

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Summary

• The modification is best measured in the jet fragmentation function

• A valuable approach to access the medium properties

• Currently the only way to identify low energy jets (< 50 GeV) in AA

• Verified the feasibility of such a measurement with the ALICE experiment in pp at 14 TeV

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Thank you !