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Diffraction at HERA

Diffraction at HERA. Anna Mastroberardino Calabria University On behalf of the H1 & ZEUS Collaborations. HSQCD 2004 St. Petersburg, Russia 18 – 22 May 2004. Outline. Introduction to diffraction Diffractive structure function of the proton QCD fits of diffractive data

leah-morrison
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Diffraction at HERA

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  1. Diffraction at HERA Anna Mastroberardino Calabria University On behalf of the H1 & ZEUS Collaborations HSQCD 2004 St. Petersburg, Russia 18 – 22 May 2004

  2. Outline • Introduction to diffraction • Diffractive structure function of the proton • QCD fits of diffractive data • Test of QCD factorization with jets and charm • Exclusive vector mesons • Summary

  3. Q2 W e’(k’) Q2 e (k) *(q)  X W xIP IP P (p) P’(p’) t What is diffraction? Standard DIS in a frame in which the proton is very fast (Breit frame): The struck quark carries fraction x  Q2/W2 of the proton momentum W = photon-proton centre of mass energy Diffraction: exchange of colour singlet (IP) producing a rapidity GAP in the particle flow The pomeron carries fraction xIP of the initial proton momentum The struck parton carries fraction βof the Pomeron momentum

  4. Why diffraction ? DIS probes the partonic structure of the proton Diffractive structure function Diffractive cross section Diffractive DIS probes the partonic structure of colour singlet exchange HERA has opened up the small x domain 920 GeV proton 27.5 GeV electron ~ 10% of low-x DIS diffractive at HERA What role does it play?

  5. Selection methods Two systems X and Y well separated in phase space with low masses MX ,MY << W Y : proton or p-dissociation carries most of the hadronic energy X : vector meson, photon or photon-dissociation Diffractive events are characterized by: scattered proton almost intact no forward energy deposition flat vs ln MX2 distribution ln MX2 -2 0 2 4 6 8 Diffractive peak Large Rapidity Gap MX – Method Proton Tagging

  6. Factorization in Diffractive DIS • QCD factorization for diffractive DIS holds (Collins, Bereira & Soper, Trentadue & Veneziano) universal partonic cross section (same as in inclusive DIS) diffractive parton distribution function – evolve according to DGLAP universal for diffractive ep DIS (inclusive, dijets, charm) • If in addition postulate Regge factorization (Ingelman & Schlein) M.Kapishin, F2D measurements 6

  7. New results from ZEUS Proton tagging method MX method MN < 2.3 GeV Transition from very low Q2 to DIS (0.03 <Q2<100 GeV2)

  8. Recent results from H1 Integrate over t  • high precision measurement of • b and Q2 dependences • QCD fit • (DGLAP evolution of diffractive pdfs) • (coming later)

  9. Measurement of b & Q2 dependences • Regge factorization holds for xIP< 0.01 • Weak b dependence:looks like a photonmore than a proton • Scaling violations positive up to large b:large gluon contribution • DGLAPevolution based fit describes the data

  10. H1 NLO QCD fit – diffractive PDFs Integrated fraction of exchanged momentum carried by gluons (75  15)% Diffractive data fitted in similar way to proton F2 data • Parametrize Flavour Singlet (quarks + antiquarks) and gluons at Q2 = 3 GeV2 • Evolve according to NLO DGLAP and fit • Determine quark sea and gluon distribution Diffractive interactions gluon dominated

  11. ZEUS NLO QCD fit to F2D and charm (LPS) • xIP <0.01 • QCDNUM • Regge factorisation assumption possible for this small data set • DL flux • initial scale Q2=2 GeV2 • zf(z)=(a1+a2z+a3z2)(1-x)a4 • other PDFs parametrisation tried • Thorne-Robert variable-flavour-number-scheme • QCD fit describes data • fractional gluon momentum • shape of pdfs not well constrained [F2D(3)cc from DESY-03-094]

  12. Factorization in Diffractive DIS – experimental test universal partonic cross section (same as in inclusive DIS) diffractive parton distribution function – evolve according to DGLAP universal for diffractive ep DIS (inclusive, dijets, charm) If QCD factorization holds diffractive parton densities are universal - Test: use diffractive pdfs obtained so far from inclusive data to predict other final state cross sections • diffractive DIS ? • hadron – hadron scattering?

  13. A test of QCD factorization: jets and charm (H1) Use results of NLO QCD fit to predict the rate of diffractive production of dijets and charm in DIS • NLO calculations based on H1 pdfs describe data well • QCD factorization in DDIS holds

  14. Factorization in Diffractive DIS – experimental test • diffractive DIS ? Diffractive structure function of antiproton It holds • hadron-hadron scattering ? Factorization not expected to work - Indeed it does not: diffractive dijets at the Tevatron: suppression by a factor of 10 factorization breaking • understood in terms of (soft) rescattering corrections of the spectator partons (Kaidalov, Khoze, Martin, Ryskin) But several other approaches … • also a suppression of resolved g processes, supposed to be similar to pp ?

  15. Diffractive dijets in photoproduction Real photon (Q2~ 0) can develop hadronic structure photoproduction similar to hadron-hadron interaction • Xg=partonic momentum for dijet production • photon remnant energy1 - xg • LO comparison: no evidence for a suppression of resolved with respect to direct • NLO comparison ?

  16. Diffractive dijets in photoproduction NLO calculations compared to preliminary H1 data (Klasen and Kramer, DESY 04-011) • NLO comparison: agreement between data and MC found if resolved contribution suppressed by a factor of 0.34 • rate of suppression expected from theoretical models

  17. V IP V p p p p Vector Meson production (JPC=1--): r, f, J/y,U,... W • Exclusive VM production calculable in pQCD • NLO calculation available forJ/Y photoproduction • Sensitivity to gluons in proton 2-gluon exchange probability of finding 2 gluons in the proton cross section: rise with W: increasing with hard scale

  18. s (gp Vp), Q2=0 MV W 1/  x gp centre-of-mass energy Vector Meson production Small MV (MV2  1 GeV2): Incoming dipole behaves like a normal-size hadron. Flat s vs W reflects flat gluon distribution for Q2  0 Soft regime Large MV: Fast growth of s with W reflects growth of gluon distribution with decreasing x Hard regime

  19. Exclusive J/Y Meson production sL/sT vs Q2 0<Q2<100 GeV2 Pomeron trajectory • -pQCD models describe data • strong sensitivity to (generalized) gluon • need NLO to constrain gluon density aIP(t) not consistent with soft diffractive measurement

  20. Summary New high precision HERA data have improved our understanding of diffraction: • Diffractive processes are dominated by gluons pQCD • Regge factorization works to a good approximation Non perturbative phenomenology • Diffractive pdfs are universal within HERA • - QCD factorization holds in diffractive DIS - On the way to understanding the large breaking of factorization at Tevatron – soft re-scattering • Vector mesons: steep W dependence • - Pure pQCD approach successful Need to discriminate models HERA–II: a lot of more data coming

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