FAS, 68th Annual Meeting Orlando, March 12-13, 2004

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Measurement of the photon structure function Measurement of the photon structure function

FF22γγ(x,Q(x,Q22) with the LUMI detector at L3 ) with the LUMI detector at L3

Gyongyi BaksayGyongyi Baksay

Florida Institute of Technology

Melbourne, Florida, USA

Advisor: Advisor:

Dr. Marcus HohlmannDr. Marcus Hohlmann

FAS, 68th Annual Meeting Orlando, March 12-13, 2004

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Topics of DiscussionTopics of Discussion

Introduction:CERN, L3, LUMIIntroduction:CERN, L3, LUMI

Theoretical considerationsTheoretical considerations

Data analysis and resultsData analysis and results

SummarySummary

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IntroductionIntroduction

highest centre-of-mass energy : 207 GeV (Giga-electron Volts)

LEP, CERN, Switzerland, France (future LHC)

Two-photon reactions dominantTwo-photon reactions dominant

FAS, 68th Annual Meeting Orlando, March 12-13, 2004

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The L3 experimentThe L3 experiment

MAIN SUBSYSTEMS: central tracker (SMD, TEC) electromagnetic (ECAL), hadronic (HCAL) calorimeters, and muon chambers.

Tagging: Luminosity Monitor (LUMI), Very Small Angle Tagger (VSAT), Active Lead Rings (ALR), Electromagnetic Calorimeter endcaps

e+ e-

FAS, 68th Annual Meeting Orlando, March 12-13, 2004

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The photonThe photon QED: Photon mediator.

Photon structureless: direct/bare photon Heisenberg uncertainty principle:

Photon violates conservation of energy:

f or interacts => parton content resolved, photon reveals its structure.

Photon extended object=> charged fermions+gluons Dual nature of photon: direct or resolved One possible description: Photon Structure Function

1tΔE ff

f

FAS, 68th Annual Meeting Orlando, March 12-13, 2004

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The different appearances of the photonThe different appearances of the photon

“bare photon”

Lepton pair production => process can be calculated in QED

Quark pair production => QCD corrections

The QED structure functions can only be used for the analysis of leptonic final states.

For hadronic final states the leading order QED diagrams are not sufficient and QCD corrections are important.

Does not reveal a structure

photon fluctuates into a hadronic state which subsequently interacts

Photon: QED-photon couples to fermions (quarks & leptons)

Photon interactions receive several contributions:

FAS, 68th Annual Meeting Orlando, March 12-13, 2004

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LUMILUMI LUMILUMI

ELECTROMAGNETIC CALORIMETERELECTROMAGNETIC CALORIMETER

HADRON CALORIMETERHADRON CALORIMETER

HADRON CALORIMETERHADRON CALORIMETER

ELECTROMAGNETIC CALORIMETERELECTROMAGNETIC CALORIMETER

BEAM BEAM PIPEPIPE

ee++ee--

tag

(*)

*

eetagtag--

eeantitagantitag++

tag >> 0 electron observed inside the detector

antitag 0 other electron undetected “single-tag”

ee++ee-- e e++ee- - ** ** e e++ee-- + hadrons deep-inelastic scattering reaction + hadrons deep-inelastic scattering reaction

antitag 0LUMILUMI LUMILUMI

FAS, 68th Annual Meeting Orlando, March 12-13, 2004

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)cos1(222tagbeamtagEEqQ

)(222222 qpQPWQQx

For single tagged events: P P 00

22

2

WQ

Qx

22221

2 )()()( * pqEEqqW

),(),,( 21 * pEqqEq

Single-tag variables:

21

2

21

21

2121

212

2

22

22

221

21

222

22

02)(k

:fermion ginteractin outgoing theof squared mass

0

0

)2,1(),,(

**

**

qq

Q

qq

qx

qxqqqxq

Qq

QQq

pEq

ipEq

ii

ii

i

i

Photon Structure FunctionPhoton Structure Function FF22

(x,Q(x,Q22)) ~ ~ probabilityprobability that the probe photon with virtuality Q2 sees a parton (quark or gluon)

with momentum fraction x inside the target quasi-real photon.

The Bjorken variable x tells us what fraction of the photon four momentum was carried by the particle which participated to the interaction: the target photon itself or a parton (quark or gluon) inside the photon.

)]Q(x,Fy)Q(x,)Fy)(1[(1xQ

2π

dxdQ

)Q(x,2γ

L22γ

22

4

2

2

2

)X(ke(q)e(k)γdσ 'tag

*

0y),(θcosEE1K)q)/(p(py tag2

beamtag

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Analysis MethodAnalysis Method1) Selection2) Split x and Q2 in several bins3) Unfolding

energy of the target photon is not known Correction with MC

(Pythia, Phojet, Twogam)4) Calculate measured cross section:

5)5) FF22(x,Q(x,Q22)) obtained using analytically calculated

differential cross section (program Galuga)

efficiencytrigger acceptanceL

NN backgroundunfolded

unfolding

Q2 “well” measured

Correlations between the generated and measured Q2, x, W; MC: Phojet

Example: selection 1998

FAS, 68th Annual Meeting Orlando, March 12-13, 2004

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Evolution of FEvolution of F22 with xwith x

FF22(x,Q(x,Q22) vs x with the different contributions:) vs x with the different contributions:

VDM, QCD, QPMVDM, QCD, QPM

x

F2(VDM)

quarks

gluons

FF22//

Preliminary results:

FAS, 68th Annual Meeting Orlando, March 12-13, 2004

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Q2 evolution of F2

Expected LUMI-L3 results

add data points to the low x region!

High statistics! Test of QCD and QED.

FAS, 68th Annual Meeting Orlando, March 12-13, 2004

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Summary

Photon is not just a simple structureless object. It’s more than that! It can fluctuate into other states (resolved photon, QCD corrections). The photon can be regarded as an object with an internal structure consisting of charged fermions and gluons.

Photon structure function analyzed for ee++ee-- e e++ee- - ** ** e e++ee-- + hadrons + hadrons Results obtained at LEP/ L3 (using LUMI for tagging

the scattered electron) provides the highest statistics ever obtained (highest c.m. energy).

The Grand Daddy prominence

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Thank you!