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Understanding Diffraction in Particle Physics: HERA Discoveries and QCD Factorization Experiments

Explore the fascinating world of particle diffraction through the lens of HERA collaborations. Learn about QCD fits, exclusive vector mesons, and the role of diffractive structure function in probing protonic structure.

leah-morrison
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Understanding Diffraction in Particle Physics: HERA Discoveries and QCD Factorization Experiments

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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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