1. Gyöngyi Baksay Florida Institute of Technology
Measurement of the hadronic photon structure function F2γ with L3 detector at LEP
Gyöngyi Baksay
Florida Institute of Technology
Advisors:
Dr. Marcus Hohlmann, Florida Institute of Technology
Dr. Maria Kienzle-Focacci, University of Geneva (advisor at CERN)
Dissertation defense: April 18, 2005
Dissertation Defense, 05/18/2005

2. Dissertation Defense, 05/18/2005
Topics of Discussion
Theoretical considerations
Kinematics
The L3 detector
Analysis method
Results
Summary and conclusions
                                     
                      
Dissertation Defense, 05/18/2005
Gyöngyi Baksay
Dissertation defense: April 18, 2005

3. Dissertation Defense, 05/18/2005
Different appearances of the photon
QED: photon mediator =>structureless: “direct/bare” photon
Free photon: zero rest mass m=0
Virtual photon: “off-mass shell” m0
* emitted and reabsorbed:  t ħ/ E
* violates conservation of energy, * ff
fermion or anti-fermion further interacts=> parton content resolved
photon extended object (charged fermions+gluons):
”resolved” photon
Another dual nature of photon: direct or resolved
One possible description: Photon Structure Function
Virtual photon cloud:
Vacuum polarization:
Dissertation Defense, 05/18/2005

4. Kinematics (e+e-*(*)  e+e- hadrons)
Virtual photon 4-momenta:
 - process:
Four momentum fraction:
4-momentum transfer:
“Virtuality”
Center-of-mass energy;
h=particle measured in the detector
4-momentum fraction:
Dissertation Defense, 05/18/2005

5. Dissertation Defense, 05/18/2005
Contributions to the two-photon cross section 
(a)
(b)
(c)
(b)
F2/
quarks
(c)
gluons
F2(VDM)
(a) x
(a) non-perturbative VDM (soft interactions) : superposition of ρ, ω, and φ. Ignoring gluon emission the VDM structure function (electron-nucleon scattering) shows Bjorken scaling. Increasing Q2: more momentum goes into radiated gluons; shift to lower x. 
(b) Pointlike coupling  fully calculable QED process; Large quark density at large x and logarithmic rise with Q2.
(c) QCD corrections (hard interaction)  DGLAP evolution equation; presence of quark & gluon density. Large corrections in NLO  PDF’s do not converge; must be measured at a certain value of Q2.
Dissertation Defense, 05/18/2005

6. CERN, LEP, and the L3 detector
highest cm. energy reached: 209 GeV
LEP L3
Dissertation Defense, 05/18/2005

7. Data sets and cross-sections
LEP2
Present data analysis 700 pb-1
Dissertation Defense, 05/18/2005

8. Dissertation Defense, 05/18/2005
Analysis Method
Monte Carlo Programs: PHOJET, PYTHIA, TWOGAM
Triggers and Selection: select single-tagtwo-photon events
Unfolding: xvis distorted, hadrons partially detected, obtain xtrue distribution using Bayes Theorem
Determine measured cross section using unfolded data
Extract F2(x,Q2 )/ using analytical calculations (GALUGA)
Study x and Q2 dependence of F2(x,Q2)/
Compare results with theoretical predictions and previous experimental results
Dissertation Defense, 05/18/2005

9. Dissertation Defense, 05/18/2005
Monte Carlo Programs
PHOJET(1.05c): hadron-hadron,photon-hadron,photon-photon collisions
Dual Parton Model (soft and hard processes) with QCD
improved parton model.
Two-photon luminosity calculated from the flux of transversely
polarized photons.
PYTHIA(6.203): general purpose MC, (LO) hard-scattering processes, elastic, diffractive and low pt events.
Classification according to photon interactions: direct, resolved, VDM and hard scales: photon virtualities and parton pt.
TWOGAM(1.71):
direct, resolved, VDM processes separately generated
3 cross sections adjusted to fit x distribution of the data.
Photon flux: exact (LO) formula
Background: PYTHIA and DIAG
Detector simulation: GEANT and GEISHA
stat. MC >5 x stat. data; MC’s reconstructed the same way as data
Dissertation Defense, 05/18/2005

10. Dissertation Defense, 05/18/2005
How do we see it in the detector?
Triggers and selection
“Single-tag” process
Triggers:
Single-tag trigger
70% Ebeam deposited in ECAL or LUMI, in coincidence with 1 track in the central tracking chambers
TEC trigger
Outer TEC trigger: 2 tracks back-to-back in the transverse plane within 60O, pt>150 MeV
Inner TEC trigger: complementary; at least two tracks in the internal chambers with any configuration of tracks.
trig  97%
LUMI-tag condition
Highest energy cluster; shape e.m. shower
Etag/Ebeam>0.7
LUMI polar angle
Anti-tag condition
To ensure low virtuality of the target photon
Emax/Ebeam<0.2
tag >> 0  electron observed inside the detector
antitag  0  other electron undetected
e- tagged in LUMI e-
HADRONS
e+ not detected e+
*(*) interaction
Dissertation Defense, 05/18/2005

11. Dissertation Defense, 05/18/2005
Selection
Hadronic channel (e+e- e+e- hadrons):
Hadrons in the final state contain several:
At least 4 additional particles
Ntracks + N  4
Track (chambers): pt>100 MeV, <10 mm
Photon (BGO): E>100 MeV, not assoc. with charged track
Ntracks=2: e+e- e+e-l+l- (l=leptons) excluded
Background rejection for e+e- Zqq
  events: low energy in the central detectors
EECAL+HCAL<0.4
 misidentified as the tagged electron.
Exclude low Wvis
Exclude resonances and low efficiency region
Wvis>4 GeV
Dissertation Defense, 05/18/2005

12. Dissertation Defense, 05/18/2005
Selected events
well measured
Qgen2
Qvis2
Unfolding
Energy of the target photon is not known (second electron undetected)
Reconstruct events using information from etag and final state hadrons
Boost of  system  hadrons partially detected
Observed xvis distribution is distorted compared to the xtrue distribution
Multidimensional method based on Bayes Theorem
Correction with MC’s: Pythia,Twogam (compare x-shapes)
Dissertation Defense, 05/18/2005

13. Dissertation Defense, 05/18/2005
Unfolding
Probabilities that the effects measured in bin “i” are originating from the causes in bins “j”.
After unfolding, the events are corrected for detector acceptance and efficiency:
Comparison of the reconstructed xxvis and generated value xgen
Causes: xgen,j
Effects: xvis,i
Number of unfolded events assignable to each of the causes:
Unfolded events
Experimentaly observed events
Correlation between generated and measured MC events
Correlation, i.e. “Smearing matrix”:
For Sji=1 each observed event xvis must come from one of the causes xgen.
Dissertation Defense, 05/18/2005

14. Dissertation Defense, 05/18/2005
Extraction of F2
To obtain F2
Radiative corrections:
RADCOR [Nucl. Phys. B 253 (1985) 421; Comp. Phys. Comm. 40 (1986) 271 ]
Calculates initial and final state radiation for
Corrections mainly due to initial state radiation from the electron scattered at large angle.
Final state radiation  detected together with the “tagged electron”
Radiation from other “electron”  negligible
GALUGA cross section calculated in the given Q2 and x range
Target photon flux
Dissertation Defense, 05/18/2005

15. Dissertation Defense, 05/18/2005
Comparison: PYTHIA & TWOGAM
Dissertation Defense, 05/18/2005

16. Evolution of F2 with x GRV*
fPL perturbatively calculable. For fhad use approximate similarity of the vector meson and the pion is used.
Starting distribution hadron-like (based on VDM)
Galuga calculation: GVDM to a  form factor comparison: 2%.
Estimation of the radiative corrections 2%
Dissertation Defense, 05/18/2005
*GRV[M. Glück, E. Reya, and A. Vogt, Photonic Parton Distributions, Phys. Rev. D 46, (1992) 1973.]

17. Dissertation Defense, 05/18/2005
Q2 evolution of F2
fit:
44% CL
Dissertation Defense, 05/18/2005
71% CL

18. Comparison with other LEP experiments and GRV-set1
MC’s predict different shapes for x
Differences between MC’s larger than differences between different experiments [LEP  working group: Eur. Phys. J. C 23 (2002) 201.]
Comparison has its limits ! Each experiment uses different methods.
Other experiments: expectations of a MC generated with a well defined PDF
Present L3 measurements: analytical calculations (GALUGA)
Radiative corrections: present L3 measurements and OPAL
Dissertation Defense, 05/18/2005

19. Dissertation Defense, 05/18/2005
Kinematical range:LEP2
LUMI Q2 range
ALEPH Eur. Phys. J. C. 30 (2003) 145
DELPHI Bejing Conference (preliminary) 2004
L3 Phys. Lett. B 447 (1999) 147 and
This analysis: L3 preprint, CERN-PH-EP/ ,
February 15, L3 preprint 295, submitted to Phys. Lett. B.
OPAL Phys. Lett. B. 533 (2002) 207
Dissertation Defense, 05/18/2005

20. Dissertation Defense, 05/18/2005
Summary and conclusions
e+e- colliders are an ideal testing ground for two-photon physics studies.
At LEP2 the  cross section dominates by 2 orders of magnitude.
L3 has excellent resolution for photons and charged hadrons.
The hadronic final state depends on the chosen model, which needs to be tuned to mach the data distribution.
High energy data with high statistics allowed precision measurements of the photon structure function testing QCD and QED predictions in the kinematical range: x , and Q GeV2.
The data are best reproduced by the higher-order parton density function GRV. Due to the high energy obtained with the LEP accelerator, it was possible to measure in addition to the 3 light quarks the effect of the heavier charm quark.
Dissertation Defense, 05/18/2005

21. Dissertation Defense, 05/18/2005
Thank you! 
Dissertation Defense, 05/18/2005