1 Jets in diffraction and factorization at HERA Alice Valkárová Charles University, Prague On behalf of H1 and ZEUS collaborations

Low x workshop, Kolimpari2 HERA collider experiments 27.5 GeV electrons/positrons on 920 GeV protons → √s=318 GeV two experiments: H1 and ZEUS HERA I: 16 pb -1 e-p, 120 pb –1 e+p HERA II: ∼ 500 pb -1, ∼ 40% polarisation of e+,e- closed July 2007, still lot of excellent data to analyse…… e DIS: Probe structure of proton → F 2 Diffractive DIS: Probe structure of color singlet exchange → F 2 D

Low x workshop, Kolimpari3 W ´ HERA: ~ 10% of low-x DIS events are diffractive Diffraction and diffraction kinematics momentum fraction of color singlet exchange fraction of exchange momentum, coupling to γ* t 4-momentum transfer squared MyMy Two classes of diffractive events: Q 2 ~0 → photoproduction Q 2 >>0 → deep inelastic scattering (DIS)

Low x workshop, Kolimpari4 Diffractive Event Selection 1) Proton Spectrometers: ZEUS: LPS ( ) H1: FPS ( ),VFPS ( ) t measurement access to high x IP range free of p-dissociation background at low x IP small acceptance  low statistics ☠ 2) Large Rapidity Gap, H1, ZEUS: Require no activity beyond η max t not measured, some p-diss background ☠ 3) M x method, ZEUS: Diffractive vs non-diffractive: exponential fall off vs constant distribution in ln M x 2 Some p-diss contribution ☠ diff. non-diff e p

Low x workshop, Kolimpari5 Dijets in diffractive ep scattering photon directly involved in hard scattering x γ =1 (at parton level), due to hadronization and resolution not exactly true for measured x γ photon fluctuates into hadronic system, which takes part in hadronic scattering,dominant at Q 2 ≃ 0 x γ <1 (at parton level) x γ - fraction of photon’s momentum in hard subprocess LO DIS, direct PHPresolved PHP Pointlike photon Resolved photon

Low x workshop, Kolimpari6 QCD factorization Get PDF from inclusive diffraction predict cross sections for exclusive diffraction Hard scattering QCD matrix element,perturbatively calculated, process dependent Universal diffractive parton densities, identical for all processes → DPDFs – obey DGLAP, universal for diff. ep DIS (inclusive,dijet,charm) → universal hard scattering cross section (same as in inclusive DIS) proven for DIS (J.Collins (1998)) not proven for photoproduction!

Low x workshop, Kolimpari7 Dijets in diffractive DIS, H1 z IP is the most sensitive variable to test gluonic part of DPDFs – difference between fits A and B at high z IP. Data agree with NLO prediction, NLO with fit B is more close to data 4< Q 2 < 80 GeV < y < 0.7 x IIP < 0.03 P* tjet1 > 5.5 GeV P* tjet2 > 4 GeV -3. < η* jets < 0. Data 99/00 published, JHEP,0710:042,(2007)

Low x workshop, Kolimpari8 Dijets in diffractive DIS, H1 For z IP < 0.4 NLO predictions using fits 2006 A and B agree with data very well Combined QCD fit for inclusive and dijet DIS data….. A B jets Largest difference

Low x workshop, Kolimpari9 Data 99/00 Eur.Phys.J.C52: 83 (2007) Dijets in diffractive DIS, ZEUS E* tjet1 > 5 GeV 5 < Q 2 < 100 GeV 2 Conclusions: the best agreement of data and NLO for H fit B and MRW 2006 H1, ZEUS: for DIS dijets factorization holds….. R=data/NLO(ZEUS LPS)

Low x workshop, Kolimpari10 Photoproduction as hadronic process resolved contribution expected to be suppressed by factor 0.34 (Kaidalov,Khoze,Martin,Ryskin:Phys.Lett.B567 (2003),61) Rescattering leads to factorization breaking and rapidity gap fill up suppression of cross section ~ 1 – (rapidity survival probability) HERA resolved photoproduction Secondary interactions between spectators Jets in photoproduction thought to be ideal testing ground for rescattering Factorization broken by β dependent factor ~ 10

Low x workshop, Kolimpari11 H1: E tjet1 > 5 GeV suppression of factor ~0.5 ZEUS: E tjet1 > 7.5 GeV weak (if any) suppression ( ) Neither collaboration sees difference between the resolved and direct regions, in contrast to theory! Possible explanation of differences between H1 and ZEUS (DIS 2007) Different phase space of both analyses ……..? Dijets in photoproduction, one year ago….

Low x workshop, Kolimpari12 E t dependence of suppression? H1 and ZEUS observe the data have harder E t slope than NLO From the DIS 2008 talk of W.Slomiński, ZEUS results Double ratio NLO/data for photoproduction and DIS Very useful !!!! Cancelations of several uncertainties (DPDFs for example) Figure extracted from the published results of H1

Low x workshop, Kolimpari13 New H1 analysis – data 99/00 Tagged photoproduction, luminosity 3x larger than for 97 diffractive events found by Large Rapidity Gap method (LRG)

Low x workshop, Kolimpari14 Two cut scenarios To crosscheck previous H1 results To approach closest to ZEUS cuts

Low x workshop, Kolimpari15 No difference in survival probabilities for resolved and direct regions of x γ, like in previous H1 and ZEUS analyses Lower E t cut scenario Hadronization corrections δ hadr =MC(hadr)/MC(parton) Frixione/Ridolfi → H Fit A H Fit B H Fit Jets Kramer/Klasen → H Fit B (thanks to Michael K.) 2 programs for NLO calculations, 3 sets of DPDFs:

Low x workshop, Kolimpari16 Lower E t cut scenario Another hint of E t harder slope for data than NLO Integrated survival probabilities (ISP) Within errors no difference in ISP using different DPDFs

Low x workshop, Kolimpari17 Higher E t cut scenario Now much more „direct-like“ events than in low E t analysis, peak at higher x γ Integrated survival probabilities (ISP) Larger ISP than for lower E t cut scenario → more close to ZEUS results!!!

Low x workshop, Kolimpari18 Higher E t cut scenario E t dependence not excluded but cannot be independently verified ZEUS – W.Slomiński

Low x workshop, Kolimpari19 Do we need global suppression? M.Klasen,G.Kramer: Review of factorization breaking in diffractive photoproduction of dijets, DESY , Modern Phys.Lett.A,2008, an attempt to interprete ZEUS and H1 results with NLO resolved only suppression R=0.31 (0.29) for H1 and R=0.53 (0.45) for ZEUS publish analyses H1, 2007ZEUS, 2007

Low x workshop, Kolimpari20 Global and resolved only suppression… Global suppression 0.53 Data H1 preliminary, suppression of NLO resolved component 0.3 (see KK paper) H1 lower Et cut scenario Much worse agreement than for global suppression

Low x workshop, Kolimpari21 Global and resolved only suppression… Global suppression 0.53 Data H1 preliminary, suppression of NLO resolved component 0.3, (see KK paper) H1 lower Et cut scenario No E t dependence but prize is worse agreement data&NLO for x γ and also for other distributions!!!

Low x workshop, Kolimpari22 Summary new H1 dijet photoproduction data – older H1 results confirmed – within errors is gap survival probability (GSP) ~ 0.5. in higher E t cut scenario (similar to ZEUS) GSP is ~ 0.6, more close to ZEUS results. hint that GSP is dependent on E t of the leading jet, for low E t jets seems to be suppression more significant. the evidence that GSP is not different for direct and resolved events remains (originally not expected ) the problem are rather the theoretical uncertainties than the lack of experimental results! (see huge scale uncertainties). important for LHC predictions….(depend on DPDFs from HERA, and GSP)

Low x workshop, Kolimpari23 What DPDF is the „best“? The most sensitive variable → z IP Fits 2006 A,B valid only to z IP = 0.8 Fit 2007 jets to z IP = 0.9 Poorly constraint DPDFs give large uncertainties at large z IP. Fits 2006 A and 2007 Jets represent extremes. Fit 2006 B is in the middle…. Lower E t cut scenario Higher E t cut scenario