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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.]

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

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

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/2005-004, February 15, 2005. L3 preprint 295, submitted to Phys. Lett. B. OPAL Phys. Lett. B. 533 (2002) 207 Dissertation Defense, 05/18/2005

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 0.006-0.556, and Q2 11-34 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

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