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Measurement of the hadronic photon structure function F 2 γ with L3 detector at LEP

Measurement of the hadronic photon structure function F 2 γ with L3 detector at LEP. Gy ö ngyi Baksay Florida Institute of Technology. Advisors: Dr. Marcus Hohlmann, Florida Institute of Technology Dr. Maria Kienzle-Focacci, University of Geneva (advisor at CERN).

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Measurement of the hadronic photon structure function F 2 γ with L3 detector at LEP

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  1. 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. 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. 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 Different appearances of the photon 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. (a) (b) (c) (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. (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. (b) Pointlike coupling fully calculable QED process; Large quark density at large x and logarithmic rise with Q2.  Contributions to the two-photon cross section  (b) F2/ quarks (c) gluons F2(VDM) (a) x Dissertation Defense, 05/18/2005

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

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

  8. 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 sectionusing unfolded data • Extract F2(x,Q2 )/ using analytical calculations (GALUGA) • Study x and Q2 dependenceof F2(x,Q2)/ • Compare results with theoretical predictions and previous experimental results Dissertation Defense, 05/18/2005

  9. Background:PYTHIA and DIAG 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, VDMand 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 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. 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 e- e+ HADRONS 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% tag >> 0  electron observed inside the detector antitag 0  other electron undetected e- tagged in LUMI e+ not detected *(*)interaction Dissertation Defense, 05/18/2005

  11. 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 fore+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 Selection Dissertation Defense, 05/18/2005

  12. Qvis2 well measured Qgen2 Selected events 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. After unfolding, the events are corrected for detector acceptance and efficiency: Comparison of the reconstructed xxvis and generated value xgen Probabilities that the effects measured in bin “i” are originating from the causes in bins “j”. Unfolding 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. GALUGA cross section calculated in the given Q2 and x range Radiative corrections: RADCOR [Nucl. Phys. B 253 (1985) 421; Comp. Phys. Comm. 40 (1986) 271 ] Calculates initial and final state radiation for Corrections mainly dueto initial state radiation from the electron scattered at large angle. Final state radiation  detected together with the “tagged electron” Radiation from other “electron”  negligible Target photon flux Extraction of F2 To obtainF2 Dissertation Defense, 05/18/2005

  15. 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 distributionhadron-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. 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. 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

  20. Summary and conclusions • e+e- colliders are an ideal testing ground for two-photon physics studies. • At LEP2the  cross section dominates by 2 orders of magnitude. • L3 hasexcellent resolution for photons and charged hadrons. • The hadronic final state depends on the chosenmodel, 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

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

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