H1 + ZEUS difractive results
Radek Žlebčík1 On behalf of H1 and ZEUS collaborations
1 Deutsches Elektronen-Synchrotron (DESY)
Physics at the Terascale Hamburg November 28, 2017 HERA Collider ± ● e + p The only existing ep collider (1992 - 2007) 27.6 GeV + 920 GeV ● About 0.5 fb-1 of data per experiment ● Two multi-purpose detectors (H1 + ZEUS)
Inelasticity Photon virtuality Photoproduction
Deep-inelastic 2 scatering (DIS) Difraction in ep ● The scatered proton stays Diffractive selection methods intact ● Exchange of vacuum quantum ● Proton spectrometer numbers – Pomeron ● Large rapidity gap method (from Regge theory) HERA: ~10% of low-x DIS events diffractive
q⋅( p− p' ) E ' x = =1− p IP q⋅p beam E p Fractional momentum loss of the scatered proton
2 2 t=( p− p' ) ≈− pT Four-momentum transfer at the proton vertex 3 Large Rapidity Gap Here H1 detector
Hadronic activity in e p Non-difractive forward part event of detector
η max Without hadronic e p Difractive activity in forward part event of detector
4 Factorization in Difraction
● DPDFs determined from inclusive measurement are capable to predict results for other, more exclusive, DDIS processes (dijets, D*), Collins 1997 D D 2 ei 2 σ (ep→ Xp)=∑ f i (β ,Q , x IP ,t)σ (β ,Q ) partoni
Partonic cross section, process dependent, calculable within p-QCD
Universal non-perturbative DPDF
D 2 f i (β ,Q , x IP ,t) DPDFs which obey DGLAP evolution ei 2 5 σ (β ,Q ) Partonic cross section Difractive photoproduction of the isolated photon (ZEUS)
DESY-17-077 [arXiv:1705.10251] Phys. Rev. D96 (2017) 6 Difractive photoproduction of isolated photon
● → photon may dissociate Photon momentum fraction into low mass hadronic system entering the hard subprocess: (structure of such resolved photon described by )
● → (electron leaves detector undetected)
Direct photon interaction Resolved photon interaction
7 Difractive photoproduction of isolated photon
● Difraction → beam proton stays :Pomeron momentum fraction intact and leaves detector undetected entering the hard subprocess: ● Standardly described by exchange of an hadronic object with vacuum quantum numbers (pomeron)
Direct Pomeron Resolved Pomeron
8 Theoretical predictions
Difractive predictions (Resolved pomeron)
● Resolved pomeron model ● The partonic structure of the (Ingelman, Schlein) resolved pomeron described by ● Implemented in the MC generator H1 2006 DPDF Fit B (from fits RAPGAP (LO matrix element + of inclusive difractive DIS) LL parton shower + Lund string ● The partonic structure of the fragmentation) resolved photon described by ● Contains direct and resolved SASGAM-2D ɣPDF photon processes
● Non-difractive background No model for the possible simulated by Pythia 6 direct pomeron interaction available
9 Forward detector’s region Event selection without hadronic activity
● Veto on scatered electron ● Difractive events dominate for small pomeron momentum
fraction wrt proton xIP+ large rapidity gap
Photon Jet
Photoproduction Diffraction
10 Nondiffractive background Extraction of prompt photons signal
● Template fit to obtain the signal and background contribution
● Background mainly from
● Width of the photon candidate Background ~2 cells hit cluster in the beam direction in equally units of cell width
● 90% of photon candidate energy required to be Gamma Gamma+jet measured in EM events events calorimeter HERA I (82 pb-1) 91 76 HERA II (374 pb-1) 366 311 11 Direct pomeron exchange? Direct pomeron interactions? Photon + jet ● The zIP < 0.9 region well described by MC both in shape and normalization
Resolved pomeron model prediction
● The zIP > 0.9 region overshot in data
Resolved pom. Direct pom. Rapgap reweighted: MC reweighted separately for zIP < 0.9 12 and zIP > 0.9 to data Direct vs Resolved pomeron Resolved pomeron Direct pomeron Photon + jet enriched ( ) enriched ( )
Photon momentum Resolved photon Direct photon fraction entering enriched enriched hard process
Gamma/jet pT balance
13 D* Production in Difractive DIS
DESY-17-043 [arXiv:1703.09476] Eur. Phys. J. C77 (2017) 14 D* Production in DDIS Why D*? ● D* is a messenger of difractively produced charm ● At LO charm produced by γg fusion → the probe of gluonic content of Pomeron
What is studied? ● The validity of the difractive factorization theorem, especially gluonic content of DPDFs ● The universality of the charm Why now? fragmentation function ● Ten-fold more statistics
15 D* - Event Topology ● 0.6% of charm quarks decay to the measured D* golden channel
● The mass diference only slightly above pion mass 140 MeV ~23%
16 D* Data sample
● Simultaneous fit of right- and Extended ML fit wrong charge combination of Signal: Crystal Ball (4 pars.) tracks Background: Granet fcn. (2 pars.) ● BG shape assumed to be identical for RC and WC 287 pb-1 of HERA II data (2005 - 2007) Right charge Wrong charge
17 Data vs NLO predictions Parton-level prediction Charm fragmentation NLO QCD in FFNS based on using Kartvellishvili difractive collinear parametrization factorization theorem (using H1 2006 Fit B DPDF)
The measured N(D*) corrected for: - Detector efects - D* branching ratio - QED radiation
The NLO predictions compatible with the theory 18 Data vs NLO predictions Electron variables Diffractive variables
19 Difractive fraction
● We observe that ~7% of D* are produced difractively ● The kinematic dependence of the difractive fraction well described by NLO
20 Conclusion
The difractive program at HERA has long and successful history and is still ongoing (Predictions mostly based on QCD collinear factorization, employing DPDFs extracted from HERA DIS data)
Two recent measurements from 2017 presented ● ZEUS: Photon + jet in photoproduction → Indication for direct Pomeron exchange ● H1 : D* production in DIS → Check of the factorization theorem and universality of charm fragmentation
Emergence of NNLO calculation → plans for new NNLO DPDF fit 21