Exploring Photon-Photon Collisions at the LC: Insights into Particle Physics

1. gg physics at the LC Giulia Pancheri - INFN-Frascati AdA: where e+e- collisions were born Where e+e- collisions were born G. Pancheri - gamma gamma at LC

2. For same cm energy  xsections are larger G. Pancheri - gamma gamma at LC

3. Photon-photon processes • Formation of neutral resonances with other quantum numbers than in e+e- , with C=+ and J=0,2 • Higher mass reach since particles can be produced individually (not necessarily in pairs) • The ggH loop is sensitive to all charged fundamental particles of the theory • Direct pair production of new fermions, charged scalars, charged vectors • Pair production of neutral scalars, vectors (in loop, also gg gg • Measurement of parton densities of real real photons (in eg mode) G. Pancheri - gamma gamma at LC

4. Normal mode of operation ECM approx up to 1/2 ee Non-monochromatic Can access JP=0± QCD and some top Total cross-sections Photon Collider ECM ~ 0.8 EeeCM Lum ~ 0.2 ee Higgs and top physics EWSB, SUSY are accessible and interesting photon-photon G. Pancheri - gamma gamma at LC

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6. Photon Collider Energy is transferred to the photon up to almost 80% of initial electron’s • Compton Back Scattering of laser light G. Pancheri - gamma gamma at LC

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10. QCD tests • Pomeron? • Jets • Photon densities • Models for total cross-section G. Pancheri - gamma gamma at LC

11. Models for total x-sections • Interest lies in QCD role • What is the Pomeron? The Reggeon? • Are these concepts universal? • Or do they just phenomenologically describe our ignorance? • How can ILC help ? A.de Roeck, R. Godbole, A. Grau, G. Pancheri, JHEP 2003 G. Pancheri - gamma gamma at LC

12. s=Bs-h + Ase+ Cse1 • Fit3 C≠ 0 e=0.093 e1=0.418 • Fit 1 C=0 e=0.250 • Fit2 C=0 e=0.093 as in pp G. Pancheri - gamma gamma at LC

13. se : e controls the rise • Could e be the same for all hadronic cross-sections? • The total gg x-section is extracted from data which do not include all the phase space • MC simulations are based on minijets models which indeed are the best in simulating L3 data G. Pancheri - gamma gamma at LC

14. se : e controls the rise • Should e be the same for all hadronic cross-sections? • Yes if the model • is based on Regge poles and a universal Pomeron pole exchange or • On Gribov factorization alone • Not necessarily if • The model has some connection with QCD and photon densities play a role G. Pancheri - gamma gamma at LC

15. A realistic QCD model should relate the fit to QCD phenomenological inputs quantities like densities etc. G. Pancheri - gamma gamma at LC

16. The BN Eikonal Minijet model includes kt resummation R.Godbole, A. Grau, G.Pancheri, Y.Srivastava PRD 2005 A. Corsetti, A. Grau, G.Pancheri, Y. Srivastava PLB 1996 • Multiple parton interactions : optical theorem and eikonal representation for Tel(s,t) • Hard scattering to drive the rise due to 1/x • Soft gluons down to zero momentum to tame the rise G. Pancheri - gamma gamma at LC

17. The hard cross-section • Mini-jet cross-section S∫ densities ∫dpt ds/dpt G. Pancheri - gamma gamma at LC

18. The hard scattering part • qq,qg and mostly Minijet cross-section depends upon • parton densities • GRV, MRST, CTEQ for protons • GRS, CJK for photons • pt cutoff ptmin=1~ 2 GeV g g g g G. Pancheri - gamma gamma at LC

19. Soft resummation Probablity of total KT from infinite # of soft gluons ∫ d2b eiKTb exp{-∫d3n(k)[1-e-iktb]} depends upon single gluon energy • maximum : use Kinematics • minimum : 0 if Bloch-Nordsieck states G. Pancheri - gamma gamma at LC

20. Role of resummation An infinite number of soft quanta • down to zero momentum but how? next slides • Up to an energy dependent limit qmax • Higher hadron energy possibility of more small x partons with “high energy” (≈1-2 GeV) higher qmax G. Pancheri - gamma gamma at LC

21. Maximum soft gluon energy • q1 and q2 : any two partons • X : the 2-jet final state • Q2≥4 p2tmin • qmax depends on x1,x2 • We average it over densities G. Pancheri - gamma gamma at LC

22. Zero momentum quanta • Soft gluons need to be resummed if they are indeed soft ≈1/k • Resummation implies integration over dkt • What matters will be ∫as(kt )dkt f(kt) and not as(0) G. Pancheri - gamma gamma at LC

23. Soft gluons give b-distributions In eikonal representation stot≈2∫d2b [1-e-n(b,s)/2] • n(b,s)=average # of collisions at distance b, at energy √s • b-distribution is needed Our ansatz: b-distribution = Fourier transform of soft gluon Kt distribution G. Pancheri - gamma gamma at LC

24. How the model works • Choose ptmin = 1÷2 GeV for mini-jets • Choose parton densities • Calculate minijet x-section • Calculate qmax for soft gluons • Calculate A(b,s) for given qmax • Calculate nhard (b,s)=A(b,s) sjet(ptmin,s) • Parametrize nsoft • Evaluate n(b,s)= nsoft + nhard • Eikonalize stot≈2∫d2b [1-e-n(b,s)/2] G. Pancheri - gamma gamma at LC

25. qmax for ptmin=1.15 geV G. Pancheri - gamma gamma at LC

26. sjet for ptmin=1.15 GeV green band MRST G. Pancheri - gamma gamma at LC

27. b-distribution for hard collisions from soft gluon resummation for ptmin=1.15 GeV for singular as G. Pancheri - gamma gamma at LC

28. Example of Eikonalized proton-antiproton total cross-section for ptmin=1.15 G. Pancheri - gamma gamma at LC

29. Comparison with proton data R.Godbole, Grau R. Hedge G. Pancheri Y. Srivastava Les Houches 2005 GGPS PRD 2005 G. Pancheri - gamma gamma at LC

30. photon-photon : eikonal minijet and BN model G. Pancheri - gamma gamma at LC

31. Conclusions gg in Photon Collider Mode would be very interesting for Higgs, WW, unknown resonances, etc. In regular mode, QCD and total cross-sections measurements can give insight on details of low x physics complementary to LHC G. Pancheri - gamma gamma at LC