FAS, 68th Annual Meeting Orlando, March 12-13, 20041 Measurement of the photon structure function F...
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Transcript of FAS, 68th Annual Meeting Orlando, March 12-13, 20041 Measurement of the photon structure function F...
FAS, 68th Annual Meeting Orlando, March 12-13, 2004
1
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:
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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(),,(
**
**
Q
qx
qxqqqxq
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
FAS, 68th Annual Meeting Orlando, March 12-13, 2004
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Thank you!