Diffraction at HERA Alice Valkárová Charles University, Prague Meeting Yerevan 2010 10.1.2019 AFP meeting, Yerevan 2010

Content factorisation and DPDFs factorisation tests HERA and diffraction factorisation and DPDFs factorisation tests longitudinal part of DPDF forward jets in diffraction 10.1.2019 AFP meeting, Yerevan 2010

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: ∼ 550 pb-1 closed July 2007, still lot of excellent data to analyse…… DIS: Probe structure of proton → F2 10.1.2019 AFP meeting, Yerevan 2010

H1 data Lower energies of the proton, special runs to establish longitudinal part of PDF and DPDF. 10.1.2019 AFP meeting, Yerevan 2010

H1 detector 10.1.2019 AFP meeting, Yerevan 2010

Deep inelastic scattering: Neutral Current 10.1.2019 AFP meeting, Yerevan 2010

Diffraction 10% of DIS events at HERA are of diffractive type colour singlet exchange – rapidity gap 10.1.2019 AFP meeting, Yerevan 2010

Introduction – why to study diffraction? To understand the nature of diffraction, high energy limit of QCD. Diffraction is a significant fraction of the inclusive cross section. Can pQCD be used to describe diffraction? Can we think of diffraction in terms of a factorisable structure function and a hard scattering process (QCD factorisation)? If so, what are the parton distributions (DPDFs) of the structure functions…. Practical motivation: to study factorisation properties of diffraction – try to transport to hh scattering (e.g.predict diffractive Higgs production at LHC) 10.1.2019 AFP meeting, Yerevan 2010

Diffraction and diffraction kinematics Two kinematic regions of diffractive events: Q2~0 → photoproduction Q2>>0 → deep inelastic scattering (DIS) e‘ W Q2 ´ e MX xIP p p‘ momentum fraction of color singlet exchange My t My =mp proton stays intact, needs detector setup to detect protons My › mp proton dissociates, contribution should be understood fraction of exchange momentum, coupling to γ 4-momentum transfer squared 24.06.2010 10.1.2019 AFP meeting, Yerevan 2010 Low x workshop, Kavala 2010 9

Methods of diffractive ev.selection Proton spectrometers ZEUS: LPS (1993-2000) H1: FPS (1995-2007) VFPS (2002-2007) t measurement access to high xIP range free of p-dissociation background at low xIP small acceptance  low statistics ☠ Large Rapidity Gap, H1, ZEUS: require no activity beyond ηmax t not measured, very good acceptance at low xIP p-diss background about 20% ☠ Different systematics – non-trivial to compare! 10.1.2019 24.06.2010 Low x workshop, Kavala 2010 AFP meeting, Yerevan 2010 10 10

Diffractive cross section at low y → diffractive reduced cross section if Longitudinal part of diffractive structure funcion Integrate over t when proton is not measured → σRD(3)(β,Q2,xP) 10.1.2019 AFP meeting, Yerevan 2010

Two types of factorisation QCD factorisation holds for inclusive and non-inclusive processes: photon is point-like (Q2 is high enough) higher twist corrections are negligible (Mx is high enough) QCD factorisation theoretically proven for DIS (Collins 1998) → DPDFs – obey DGLAP, universal for diff. ep DIS (inclusive,dijet,charm) → universal hard scattering cross section (same as in inclusive DIS) It allows the extraction of DPDFs from the (DIS) data H1 and ZEUS –QCD fits assuming Regge factorisation for DPDF conjecture, e.g. Resolved Pomeron Model by Ingelman, Schlein pomeron flux factor pomeron PDF 24.06.2010 10.1.2019 Low x workshop, Kavala 2010 AFP meeting, Yerevan 2010 12

QCD and diffraction 2 steps process: parametrise the IP flux (xIP,t) & factorise it (hypothesis) fit diffractive PDfs (β,Q2) 10.1.2019 AFP meeting, Yerevan 2010

f(xIP) from data 10.1.2019 AFP meeting, Yerevan 2010

Soft pomeron, Regge model Regge model: analytic model of HADRONIC scattering Exchange of collective states: linear trajectories in the spin- energy (α,t) plane, σpp IP IR αj(t)= αj(0)+α´j(t) (j=IR,IP,..) In γ*p→XY virtual photon resolves structure of exchanged object dominant contribution looks similar to soft pomeron, we can extract Regge trajectory 10.1.2019 AFP meeting, Yerevan 2010

xIP dependence for two t values Reduced cross section measured for the first time at two t values Low xIP and high β, decrease with increasing xIP →IP-like behaviour High xIP and low β, increase with increasing xIP → IR-like behaviour Lines – results of Regge fit Regge fit, IP+IR 10.1.2019 Not soft pomeron, different multi-pomeron absorption effects? AFP meeting, Yerevan 2010

f(xIP) from data 10.1.2019 AFP meeting, Yerevan 2010

Proton tagged data – t dependence Fitting to e-bt → b ~6-7GeV-2 independent of β, Q2 Almost no xIP dependence → 10.1.2019 AFP meeting, Yerevan 2010

Proton vertex factorisation Regge fit in different Q2 bins, no strong evidence for αIP(0) dependence Variables (xIP,t) describing proton vertex factorise from those at photon vertex (Q2,β) to good approximation. Q2 and β dependence interpreted in terms of Diffractive Parton Densities (DPDF) → measurement of partonic structure of exchange, NLO QCD fits 10.1.2019 AFP meeting, Yerevan 2010

Diffractive PDfs f(β,Q2) 10.1.2019 AFP meeting, Yerevan 2010

DPDF functions H1 dijet DIS measurement: H1 fit Jets Low sensitivity of fits to inclusive cross section to gluon PDF especially at large zIP → use jets to combined fits! H1 dijet DIS measurement: H1 fit Jets 10.1.2019 AFP meeting, Yerevan 2010

DPDF functions New QCD fit of ZEUS ZEUS fit SJ uses inclusive & DIS dijet data Not very different from H1 fit jets… 10.1.2019 AFP meeting, Yerevan 2010

Gluon fraction 10.1.2019 AFP meeting, Yerevan 2010

Comparison of H1 and ZEUS DPDFs Differences in H1/ZEUS fits: Q2>8.5 GeV2 / Q2>5 GeV2 My< 1.6 GeV / My< mp, scaling by 0.81 Reflects the consistency between H1 and ZEUS data 10.1.2019 AFP meeting, Yerevan 2010

Comparison between methods Are „rapidity gap“ and „forward proton“ methods compatible? LRG selection contains about 20% events of proton diss. no significant dependence on any variable well controlled, precise measurements ratio ZEUS, LPS/LRG = 0.76  0.01   0.03 0.08 0.02 0.05 Precise knowledge and corrections for proton dissociation background - key point in H1- ZEUS data comparison H1, LRG/FPS = 1.18  0.01 0.06 0.10 10.1.2019 AFP meeting, Yerevan 2010

Comparison H1 FPS & ZEUS LPS The cleanest comparison, all available data used by both collaborations, H1 HERA II 20 times improved statistics, higher Q2 Fair agreement, normalisation uncertainty → LPS~10%, FPS ~6% 10.1.2019 AFP meeting, Yerevan 2010

H1 & ZEUS inclusive diffraction,LRG Q2 (GeV2) Q2 (GeV2) Good agreement between H1 and ZEUS in most of the phase space, (except low Q2), ZEUS scaled by 0.87, covered by normalisation uncertainty 10.1.2019 AFP meeting, Yerevan 2010

Complete H1 data set Combined FPS,VFPS and LRG results. 10.1.2019 AFP meeting, Yerevan 2010

FLD extraction 10.1.2019 AFP meeting, Yerevan 2010

FLD extraction The linear fits to Rosenbluths plots for two highest bins are shown, allowing two new measurements at low Q2 to be made. 10.1.2019 AFP meeting, Yerevan 2010

Longitudinal part of DPDF A new measurement in low Q2 together with previously measured FLD at medium Q2. The data measured at the energy of protons: 920,820,575 and 460 GeV. The measurement is consistent with the extrapolated fit B. 10.1.2019 AFP meeting, Yerevan 2010

Photoproduction, γp, Q2→0 In LO! direct resolved Similarity with hadron-hadron collisions xγ - fraction of photon’s momentum in hard subprocess direct resolved hadron –like component photon fluctuates into hadronic system, which takes part in hadronic scattering xγ<0.2 (at parton level) point –like component of resolved photon dominates in the region of 0.2<xγ<1 direct photoproduction photon directly involved in hard scattering xγ=1 (at parton level) 24.06.2010 10.1.2019 AFP meeting, Yerevan 2010 Low x workshop, Kavala 2010 32

Tests of QCD factorisation Basic strategy: measure a particular diffractive final state compare the measurement with NLO calculation using DPDFs previously extracted What kind of final states? processes with a hard scale sensitive to gluons (gluons contribute by up to 80% to the DPDFs, mainly for high zIIP) Dijets and D* in DIS/photoproduction are the best candidates! 10.1.2019 24.06.2010 Low x workshop, Kavala 2010 AFP meeting, Yerevan 2010 33

Factorisation in hadron-hadron collisions Exporting DPDFs from HERA to Tevatron does not work S2= σ (data) σ (theory) suppression factor Factorisation broken by β-dependent factor ~ 10, S2 ~ 0.1. Succesfully explained by terms of rescattering and absorption (see Kaidalov,Khoze,Martin,Ryskin:Phys.Lett.B567 (2003),61) KKMR predicted suppression factor for HERA resolved photoproduction S2 ~ 0.34 In 2010 new theoretical prediction by KKMR: (European Journal of Physics 66,373 (2010)) Suppression 0.34 is present only for hadronic part of photon PDF, (dominant for xγ<0.2 ), for point-like component which is dominant in our phase space – suppression: quarks GRV 0.75(0.71) ETjet1 >7.5 (5.) gluons GRV 0.58(0.53) GeV 10.1.2019 24.06.2010 AFP meeting, Yerevan 2010 Low x workshop, Kavala 2010 34

What we learned from HERA data? DIS dijets – factorisation theoretically predicted. Both H1 and ZEUS confirmed experimentally and used for QCD fits („H1 fit jets“, „ZEUS fit SJ“). D* in DIS & photoproduction- data within large errors not in contradiction with factorisation New ZEUS fits compared to published DIS D* data. (Nucl.Phys. B672 (2003),3.) (Nucl.Phys. B831 (2010), 1) 24.06.2010 10.1.2019 Low x workshop, Kavala 2010 AFP meeting, Yerevan 2010 35

What we learned from HERA data? Photoproduction dijets – factorisation not predicted theoretically, experimentally not fully understood…. different conclusions made by H1 and ZEUS, H1 observed suppression about 0.5-0.6, ZEUS negligible suppression (in different phase space, e.g.larger ET of jets.) ETjet1 > 7.5 GeV, ETjet2 > 5 GeV Published ZEUS dijet photoproduction data (Eur.Phys.J.C 55 (2008),177) compared to NLO with „H1 fit Jets“ and „ZEUS fit SJ“ 10.1.2019 AFP meeting, Yerevan 2010

Dijet photoproduction H1 data 1999-2000 DESY10-043 (2010) Data compared to RAPGAP MC and NLO GRV photon structure function NLO calculations – Frixione/Ridolfi 3 sets of DPDFs H1 2006 fit B - using inclusive data H1 2007 fit jets ZEUS SJ fit using DIS dijets zIIP < 0.8 10.1.2019 24.06.2010 Low x workshop, Kavala 2010 AFP meeting, Yerevan 2010 37

Dijets in photoproduction SS NLO with „H1 fit B“ larger cross section than data. Shapes of distributions are described. RAPGAP describes data satisfactorily. 24.06.2010 10.1.2019 Low x workshop, Kavala 2010 AFP meeting, Yerevan 2010 38

Shapes of distributions are described succesfully, (with the exception of Etjet1), global suppression of about 0.6 observed…. DPDF uncertainty Scale variation uncertainty 24.06.2010 10.1.2019 Low x workshop, Kavala 2010 AFP meeting, Yerevan 2010 39

New ZEUS fit-comparison with old data Published data: EPJ C55 (2008) 177 Very good description → no evidence for suppression for ZEUS combined fit and H1 fit jets → ISP ~ 1….. 10.1.2019 AFP meeting, Yerevan 2010

Double differential cross sections Clear evidence for decrease of resolved component with increasing Et The suppression in resolved enriched region of xγ may be Et dependent. (suppression less for higher Et.) ZEUS ZEUS photoproduction dijets Nucl.Phys. B831 (2010), 1 10.1.2019 24.06.2010 AFP meeting, Yerevan 2010 Low x workshop, Kavala 2010 41

Double differential cross sections Shapes of distributions described by NLO well. Gap survival probabability is insensitive to the presence or nature of remnant (either the photon or diffractive exchange) 10.1.2019 AFP meeting, Yerevan 2010

Ratio data/theory – 3 DPDFs „H1 fit jets“ and „ZEUS fit SJ“ give similar agreement in shape. „ZEUS fit SJ“ gives larger prediction by about 15-20 % than „H1 fit B“. Differences are covered by theor. uncertainties. Global suppression: O.58 for „H1 fit B“ 0.64 for „H1 fit jets“ 0.70 for „ZEUS fit SJ“ 24.06.2010 10.1.2019 Low x workshop, Kavala 2010 AFP meeting, Yerevan 2010 43

Comparison with KKMR models NLO calculations Model KKMR 2003: resolved part suppressed by 0.34. Model KKMR 2010: quarks suppressed by 0.71 gluons suppressed by 0.53 Model KKMR 2010 agrees with H1 data better than model 2003 but shape description is still better with global suppression. 24.06.2010 10.1.2019 Low x workshop, Kavala 2010 AFP meeting, Yerevan 2010 44

Ratio diffractive to inclusive Proposed by Kaidalov et al. Phys.Lett B567 (2003) 61 Full or partial cancellation of PDF uncertainties, scales….. Distribution of xγ sensitive to gap survival. H1 - measured in same kinematic range with same method as diffractive cross sections Acceptance corrections – PYTHIA Problem → describes low Et inclusive data with inclusion of multiple interactions only, large hadronisation corrections! Such a low Et jets also not properly described by NLO – see eg. H1 inclusive jet paper (EPJ C 129 (2003) 497) 10.1.2019 AFP meeting, Yerevan 2010

Ratio diffractive to inclusive Ep=820 GeV Ep=930 GeV zIP < 0.8 comparison to MC models - RAPGAP/PYTHIA very different phase space for incl.& diffractive large sensitivity to multiple interactions (MI) for inclusive dijets better agreement of data ratio with PYTHIA MI due to these facts - interpretation difficult 10.1.2019 AFP meeting, Yerevan 2010

Diffractive forward jets Jets measured in H1, protons in Forward Proton Spectrometer Direct and accurate measurement of diffractive variables, free of proton diss.background, small acceptance – low statistics Data compared to NLOJET++ with H1 fit B, hadronization uncertainties about 8% Cuts: 10.1.2019 AFP meeting, Yerevan 2010

Comparison to previous analysis Diffractive dijet analysis with LRG – JHEP 0710:042 Data scaled by 1.23 (no proton dissociation background) Very good consistency, phase space extension in xIP by factor 3 10.1.2019 AFP meeting, Yerevan 2010

Diffractive forward jets Within error, the NLO describes data well. RAPGAP describes the shape but not normalisation 10.1.2019 AFP meeting, Yerevan 2010

Conclusions-factorisation DPDFs measured by H1 and ZEUS using assumption of Regge factorisation (inclusive and DIS dijet measurement). DIS dijets, D* in DIS & photoproduction – factorisation holds. Dijets in photoproduction: conclusions about factorisation using H1 and ZEUS data (with the identical DPDFs) are different…. H1 – data suppressed by global factor 0.57-0.7 (depending on DPDF). ZEUS – compatible with no suppression possible explanation suppression is decreasing with increasing ET of the jets. Longitudinal part of DPDF measured, measurement consistent with H1 fit B. Forward jets in DIS measured in new kinematic region, NLO describe data well. 10.1.2019 24.06.2010 Low x workshop, Kavala 2010 AFP meeting, Yerevan 2010 50