Measurement of the photon structure function F2? with single tag events for the Q2 range 6-43 GeV2 in two-photon collisions at LEP2

1
Measurement of the photon structure function F2?
with single tag events for the Q2 range 6-43 GeV2
in two-photon collisions at LEP2

• Gyongyi Baksay
• Marcus Hohlmann (Advisor)
• Florida Institute of Technology
• Melbourne, Florida, USA

2
Topics of Discussion

• Introduction LEP, L3, LUMI, ?? physics

• Theoretical considerations

• Measurement of F2?

• Data analysis and results

• Future plans and conclusion

3
Introduction
Two-photon reactions dominant
LEP, CERN, Switzerland, France (future LHC)
collides e e-, highest centre-of-mass energy
208GeV
4
The L3 experiment
MAIN SUBSYSTEMS central tracker (SMD, TEC)
electromagnetic (ECAL), hadronic (HCAL)
calorimeters, and muon chambers.
e
e-
Tagging Luminosity Monitor (LUMI), Very Small
Angle Tagger (VSAT), Active Lead Rings (ALR),
Electromagnetic Calorimeter endcaps
5
Subdetectors of LUMI

• A calorimeter made of 304 Bismuth Germanium Oxide
  (BGO) crystals, arranged in 16 sectors of 19
  crystals (of varying size) each.
• A silicon detector placed in front of each
  monitor measures the polar angle ? and the
  azimuthal angle ? with very good accuracy.

6
The different appearances of the photon, (QED and
QCD)
Photon
quarks
Mediator of the electromagnetic force, couples to
charged objects (QED) Lepton pair production gt
process can be calculated in QED Quark pair
production gt QCD corrections
leptons (e, ?, ?)
photon fluctuates into a hadronic state which
subsequently interacts
Photon interactions receive several contributions
bare photon
Does not reveal a structure
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.
7
Single tag events
large scattering angle gt electron observed
inside the detector gt tag one scattered
electrons detected gt single-tag event
ee- ? ee- ? ?? ee- hadrons deep-inelastic
scattering reaction
e
e-
?
8
Photon Structure Function

• Why do we measure the photon structure function
  of photon ?
• understand complex behavior of interacting
  photons (test QED, QCD)
• creation of matter out of two photons

F2?(x,Q2) probability 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.
9
Goal measure the cross section for the
single-tagged ?? process to extract the photon
structure function F2? (x, Q2)
, k
X (hadrons)
Main Processes contributing to the ee- ? ee-
? ? ? ee- hadrons cross section
Single-tag variables
Direct process (QPM)
VDM

For single tagged events P ?0
Single Resolved
10
Analysis Method
unfolding

• Selection
• Split x and Q2 in several bins
• Unfolding
• (energy of the target photon is not known
• Correction with MC
• 4) Calculate measured cross section
• Compare measured cross section
• to analytical calculations (program Galuga)

Q2 well measured
11
Evolution of F2? with x
F2?(x,Q2) vs x with the different
contributionsVDM, QCD, QPM


F2?/?
quarks
F2?(VDM)
gluons
after unfolding with PHOJET
x
12
Q2 evolution of F2?
Expected LUMI-L3 results add data points to the
low x region! Test of QCD

• Comparison
• LEP (ALEPH, DELPHI, L3, OPAL)
• TPC detector at PEP/SLAC, Stanford
• PLUTO, JADE, and TASSO detectors at
• DESY, Hamburg, Germany
• TOPAZ, AMY detectors at KEK-TRISTAN,
• Japan.

Present measurements data collected in 1998,
1999, 2000 at center-of-mass energies between
13
Work done

• Selections, high enough statistics
• Binning for (x, Q2), comparable to other
  experiments assure high enough statistics for
  each bin
• Unfolding for x
• Calculate measured differential cross section
• Extract F2? versus x

Work to be done Determine F2? versus Q2
What improves compared to previous measurements?

• Higher statistics, better precision
• The physics advantage of this high energy
  reached at LEP2 comes from the increased
  center-of-mass energy between the virtual photon
  probe and the real photon target. This means that
  the photon structure can be measured at lower x
  values than was done before.
• Better detector conditionsgtLUMI Q2 (6-43 GeV2)
  with a low W, means low xgt test of QCD

14
(No Transcript)