Report

Diffraction at HERA Anna Mastroberardino Calabria University On behalf of the H1 & ZEUS Collaborations HSQCD 2004 St. Petersburg, Russia 18 – 22 May 2004 1 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 2 What is diffraction? Standard DIS in a frame in which the proton is very fast (Breit frame): Q2 The struck quark carries fraction x Q2/W2 of the proton momentum W W = photon-proton centre of mass energy Diffraction: exchange of colour singlet (IP) producing a rapidity GAP in the particle flow Q2 *(q) W X xIP IP t The pomeron carries fraction xIP of the initial proton momentum The struck parton carries fraction β of the Pomeron momentum 3 Why diffraction ? d 2 4 2 y2 2 1 y F2 ( x, Q ) 2 4 2 dxdQ xQ 2[1 R( x, Q )] F2 ei2 xfi x,Q2 DIS probes the partonic structure of the proton i Diffractive structure function Diffractive cross section d 4 4 2 2 ddQ dxIP dt Q 4 D ( 4) y2 2 1 y F ( , Q , xIP , t ) 2 D ( 4) 2(1 R ) 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 W 300 GeV Q 2 105 GeV 2 x 105 What role does it play? 4 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 -2 Large Rapidity Gap 0 2 4 6 8 MX – Method ln MX2 Diffractive peak Proton Tagging 5 Factorization in Diffractive DIS QCD factorization for diffractive DIS holds (Collins, Bereira & Soper, Trentadue & Veneziano) F2D ~ fi/pD ˆi 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) F2D( 4) f IP / p ( xIP , t )F2IP (Q2 , ) F2IP (Q2 , ) evolves followingDGLAP equations 6 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) 7 Recent results from H1 r D(4) F2 D(4) y2 2(1 y 2 y ) 2 FL D(4) r D F2 D at low y r D F2 D if FL D 0 Integrate over t rD ( 3) 1.5 Q2 1600GeV2 high precision measurement of and Q2 dependences QCD fit (DGLAP evolution of diffractive pdfs) (coming later) 8 Measurement of & Q2 dependences Regge factorization holds for xIP< 0.01 Weak dependence: looks like a photon more than a proton Scaling violations positive up to large : large gluon contribution DGLAP evolution based fit describes the data 9 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 10 ZEUS NLO QCD fit to F2D and charm • xIP <0.01 (LPS) • 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-flavournumber-scheme QCD fit describes data ( 2 / ndf 37.9 / 36) fractional gluon momentum (82 8( stat ) 9( sys ))% shape of pdfs not well constrained [F2D(3)cc from DESY-03-094] 11 Factorization in Diffractive DIS – experimental test F2D ~ fi/pD ˆi 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? 12 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 13 Factorization in Diffractive DIS – experimental test diffractive DIS ? It holds Diffractive structure function of antiproton 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 processes, supposed to be similar to pp ? 14 Diffractive dijets in photoproduction Real photon (Q2~ 0) can develop hadronic structure photoproduction similar to hadron-hadron interaction X= partonic momentum for dijet production photon remnant energy 1 - x LO comparison: no evidence for a suppression of resolved with respect to direct NLO comparison ? 15 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 16 Vector Meson production V V (JPC=1--): r, f, J/y,U,... W p IP p p Exclusive VM production calculable in pQCD p 2-gluon exchange NLO calculation available for J/ photoproduction probability of finding 2 gluons in the proton Sensitivity to gluons in proton cross section: ~ S2 Q6 2 xG( x, Qeff ) 2 Q 2 M V2 x~ W2 rise with W: increasing with hard scale 2 for J / xG( x, Qeff ) ~ x 0.2 ~ W ~ 0.8 17 Vector Meson production (p Vp), Q2=0 Soft regime Small MV (MV2 1 GeV2): Incoming dipole behaves like a normal-size hadron. Flat vs W reflects flat gluon distribution for Q2 0 MV Hard regime Large MV : Fast growth of with W reflects growth of gluon distribution with decreasing x W 1/ x p centre-of-mass energy 18 Exclusive J/ Meson production 0<Q2<100 GeV2 L/T vs Q2 Pomeron trajectory - pQCD models describe data - strong sensitivity to (generalized) gluon - need NLO to constrain gluon density IP(t) not consistent with soft diffractive measurement 19 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 Diffractive pdfs are universal within HERA - QCD factorization holds in diffractive DIS Non perturbative phenomenology - 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 20