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2 Lepton 2 Quark Production in 2 Photon Collisions

ECFA@Durham 1-4 september 2004. 2 Lepton 2 Quark Production in 2 Photon Collisions. W. da Silva *, F. Kapusta. LPNHE, Paris. Introduction ( computation : why, how, hypothesis ) Results (numerical, signal and background simulation) Outlook. Introduction Experimental Motivation

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2 Lepton 2 Quark Production in 2 Photon Collisions

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  1. ECFA@Durham 1-4 september 2004 2 Lepton 2 Quark Production in 2 Photon Collisions W. da Silva *, F. Kapusta LPNHE, Paris Introduction ( computation : why, how, hypothesis ) Results (numerical, signal and background simulation) Outlook

  2. Introduction • Experimental Motivation • The cross-section of 4 fermion production • has a large value at high energy • ( use leptons for luminosity measurement,......) • Study background generated by leptons • (luminosity, b and c physics • – don't forget LEP results- , • W physics .....) • Source of leptons at very low angle • (i.e. study the forward region instrumentation) • Theoretical Motivation • Study of theoretical approximations • and methods to compute the cross section • ( range of validity, test of numerical approximations,....)

  3. Introduction • Theoretical calculation • Our Computation ( and Generator) • Computation of space-like photon diagrams only ( including masses) • Use factorisation i.e. Impact factor method • ( cf Lipatov-Frolov, Serbo(1985), • Cheng-Wu) • Use semi-analytic computation • without approximation • (cf PHOTON2000/2001 proceedings) • Test with GRACE • ( for the channel) • Numerical computation including • space-like and time-like photon

  4. Numerical Results • (2 muon 2 up quark channel) • “Huge” cross section at high energy (55 pb) • Space like terms are dominant • as expected long time ago. • Good agreement between • our computation and GRACE • The Impact factor approx. • Fails slightly at low energy • (cf electron collider using • quasi real photon fluxes) • Test OK with modified GRACE • (using quasi real photon fluxes) Cross-section pb

  5. Simulation Results : • the hadronic background • Photon fluxes ( cf Telnov) • + Pythia 6.161 ( VDM + DIRect + RESolved terms) • Huge events at low Pt and low Wvis (VDM terms ... ) • Opportunity to study the total • cross-section and help to measure the luminosity ( absolute or relative) ? • Using LEP2 result ? • Example of selected events : • very loose cuts + • 2 jets with same energy back to back • Usual problem : computation time + easy rejected events

  6. Simulation Results : • The leptons + jets signal • Generator level • 2 muon 2 up quark production • Huge cross section but t-channel • ( low angle production) • Topology of back to back pairs • helps to select signal events • (loose cut + angle(muons, jets)> 90 deg.) • Other lepton sources : beauty, charm, ... • ( viewed as signal or as background ) • Luminosity measurement : • need more study • to cancel the background !

  7. Simulation Results : a photon collider LEP-like plot • Do not forget the 2 lepton 2 quark channel in inclusive lepton analyses

  8. Outlook • Theoretical point of view • more comparisons needed : space and time like terms, • with and without impact factor approximation , ..... • Experimental point of view • Simulation with huge statistics ( signal and background ) • in order to explore the high visible invariant mass • and get some numerical results for the luminosity measurement • using the 2 lepton 2 quark channel • And ....

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