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Dissertation Defense, 05/18/20051 Measurement of the hadronic photon structure function F 2 γ with L3 detector at LEP Gyöngyi Baksay Florida Institute.

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Presentation on theme: "Dissertation Defense, 05/18/20051 Measurement of the hadronic photon structure function F 2 γ with L3 detector at LEP Gyöngyi Baksay Florida Institute."— Presentation transcript:

1 Dissertation Defense, 05/18/20051 Measurement of the hadronic photon structure function F 2 γ 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

2 Dissertation Defense, 05/18/20052 Topics of Discussion Theoretical considerations Theoretical considerations Kinematics Kinematics The L3 detector The L3 detector Analysis method Analysis method Results Results Summary and conclusions Summary and conclusions Dissertation defense: April 18, 2005 Gyöngyi Baksay

3 Dissertation Defense, 05/18/20053 “ direct/bare” photon QED: photon mediator =>structureless: “ direct/bare” photon Free photon: Free photon: zero rest mass m=0 Virtual photon: 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 Photon Structure Function One possible description: Photon Structure Function Different appearances of the photon Virtual photon cloud: Vacuum polarization:

4 Dissertation Defense, 05/18/20054 Kinematics (e + e -  *  ( * )  e + e - hadrons) 4-momentum transfer: 4-momentum fraction: “Virtuality” Virtual photon 4-momenta: Center-of-mass energy; h=particle measured in the detector Four momentum fraction:  - process:

5 Dissertation Defense, 05/18/20055 Contributions to the two-photon cross section x F 2  (VDM) gluons F2/F2/F2/F2/ quarks  (a) (c) (b)  Pointlike coupling (b) Pointlike coupling  fully calculable QED process; Large quark density at large x and logarithmic rise with Q 2. QCD corrections (hard interaction) (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 Q 2. (c) (a)(b) non-perturbative VDM (soft interactions) (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 Q 2 : more momentum goes into radiated gluons; shift to lower x.

6 Dissertation Defense, 05/18/20056 CERN, LEP, and the L3 detector highest cm. energy reached: 209 GeV L3 CERN LEP

7 Dissertation Defense, 05/18/20057 Data sets and cross-sections Present data analysis   700 pb -1 LEP2

8 Dissertation Defense, 05/18/20058 Analysis Method Monte Carlo Programs: Monte Carlo Programs: PHOJET, PYTHIA, TWOGAM Triggers and Selection: Triggers and Selection: select single-tag  two-photon events Unfolding: Unfolding: x vis distorted, hadrons partially detected, obtain x true distribution using Bayes Theorem Determine measured cross section Determine measured cross section using unfolded data Extract F 2  (x,Q 2 )/  Extract F 2  (x,Q 2 )/  using analytical calculations (GALUGA) Study x and Q 2 dependence Study x and Q 2 dependence of F 2  (x,Q 2 )/  Compare results Compare results with theoretical predictions and previous experimental results

9 Dissertation Defense, 05/18/20059 Monte Carlo Programs PHOJET(1.05c): PHOJET(1.05c): hadron-hadron,photon-hadron,photon-photon collisions soft and hard processesQCD  Dual Parton Model (soft and hard processes) with QCD improved improved parton model.  Two-photon luminosity calculated from the flux of transversely polarized photons. PYTHIA(6.203): PYTHIA(6.203): general purpose MC, (LO) hard-scattering processes, elastic, diffractive and low p t events. direct, resolved, VDM hard scales  Classification according to photon interactions: direct, resolved, VDM and hard scales: photon virtualities and parton p t. TWOGAM(1.71): direct, resolved, VDM direct, resolved, VDM processes separately generated  3 cross sections adjusted to fit x distribution of the data.  Photon flux: exact (LO) formula Detector simulation: Detector simulation: GEANT and GEISHA Background: Background: PYTHIA and DIAG stat. MC >5 x stat. data; MC’s reconstructed the same way as data

10 Dissertation Defense, 05/18/ How do we see it in the detector?  tag >> 0  electron observed inside the detector  antitag  0  other electron undetected e+e+e+e+ e-e-e-e-  *  (*) interaction “Single-tag” process HADRONS Triggers and selection e + not detected 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 60 O, p t >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 E tag /E beam >0.7  LUMI polar angle Anti-tag condition To ensure low virtuality of the target photon E max /E beam <0.2 e - tagged in LUMI

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

12 Dissertation Defense, 05/18/ Selected events well measured Q gen 2 Q vis 2 Unfolding Energy of the target photon is not known (second electron undetected) e tag final state hadrons Reconstruct events using information from e tag and final state hadrons hadrons partially detected Boost of  system  hadrons partially detected x vis distribution is distorted compared to the x true Observed x vis distribution is distorted compared to the x true distribution Multidimensional method based on Bayes Theorem Correction with MC’s: Pythia,Twogam (compare x-shapes)

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

14 Dissertation Defense, 05/18/ Extraction of F 2  To obtain F 2  GALUGA cross section calculated in the given Q 2 and x range Target photon flux 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

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

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

17 Dissertation Defense, 05/18/ Q 2 evolution of F 2  44% CL 71% CL fit:

18 Dissertation Defense, 05/18/ Comparison with other LEP experiments and GRV-set1 Each experiment uses different methods.  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  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.]

19 Dissertation Defense, 05/18/ Kinematical range:LEP2 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 LUMI Q 2 range

20 Dissertation Defense, 05/18/ 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 GeV 2. 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. Summary and conclusions

21 Dissertation Defense, 05/18/ Thank you!


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