Tests of QCD factorisation in the diffractive production of dijets in deep-inelastic scattering and photoproduction at HERA

A Aktas,A Zhelezov,D Mladenov,G Weber,F Tomasz,K Krastev,M Fleischer,S Levonian,S Egli,S Mikocki,A Valkárová,A Perieanu,S Essenov,Z Zhang,A.-I Lucaci-Timoce,R.N Shaw-West,T Greenshaw,K Wacker,T Kluge,A Petrukhin,A Bunyatyan,D Wegener,R Felst,I Sheviakov,A Mehta,F Sefkow,S Baudrand,M.-O Boenig,U Straumann,D Traynor,S Vinokurova,D Haidt,T Hreus,K Meier,L Schoeffel,D Ozerov,L Lytkin,J Žáček,T Anthonis,G Heinzelmann,J Gayler,L Goerlich,G Laštovička-Medin,W Lange,M Kapichine,F Salvaire,A Eliseev,B Stella,B Naroska,O Behnke,M Del Degan,A Liptaj,A Specka,R Roosen,G Buschhorn,A De Roeck,J Turnau,H Peng,A Astvatsatourov,T Klimkovich,B List,J Ferencei,K Lipka,A Stoilov,K Urban,N Gogitidze,K Krüger,K Nankov,M Steder,R Eichler,J Stiewe,A Usik,A Lebedev,V Chekelian,G Brandt,E Rizvi,E Gabathuler,C Wissing,V Yeganov,G Cozzika,C Pascaud,K Daum,R Oganezov,O Behrendt,A Morozov,C Kleinwort,V Jemanov,G Flucke,T Frisson,E Barrelet,A Hovhannisyan,L Jönsson,A Vargas Trevino,R Weber,E Tzamariudaki,J Meyer,D Bruncko,P Marage,S Habib,Y Soloviev,T Toll,V Brisson,A Falkewicz,I Tsurin,G Franke,S Gorbounov,A Rimmer,V Dodonov,S Ghazaryan,C Grab,G Leibenguth,L Finke,H Zohrabyan,G Knies,J Feltesse,G Thompson,B Delcourt,S Ginzburgskaya,A Makankine,A Rostovtsev,P Reimer,C Vallée,V Efremenko,D Utkin,A Dubak,V Boudry,F Zomer,V Lubimov,J Cvach,P Van Mechelen,D Sunar,M Gregori,F Moreau,K Begzsuren,J.C Bizot,C Risler,A Knutsson,G Herrera,R Plačakytė,L Favart,A Kropivnitskaya,A Glazov,A Nikiforov,Y De Boer,F Cassol-Brunner,C Diaconu,E Wünsch,P Baranov,M Jacquet,H Lueders,H Henschel,I Milcewicz-Mika,F Eisele,M Klein,M Beckingham,W Bartel,C Kiesling,V Palichik,A Cholewa,E Malinovski,K Nowak,V Tchoulakov,T Sloan,A Asmone,A Schöning,I Tsakov,L Lindfeld,N Loktionova,F.W Büsser,A Baghdasaryan,Y Vazdik,V Lendermann,P Prideaux,M Goettlich,M Gouzevitch,C Niebuhr,P Kostka,V Spaskov,M Hildebrandt,K Müller,S Schätzel,V Korbel,M Kraemer,P Truöl,G Grindhammer,V Volchinski,L Bystritskaya,T Zimmermann,Volker Michels ,J V Morris ,V Andreev,T N Trinh ,M Martišíková ,G Tsipolitis,P Laycock,L Marti,H Meyer,P R Newman ,Z Rurikova,A Zhokin,I Božović-Jelisavc̆ić ,I Pićurić ,B Olivier,G Eckerlin,A Fomenko,A Babaev,B R Grell ,M P J Landon ,S Osman,I R Kenyon ,D Pitzl,J B Dainton ,J A Coughlan ,N Berger,A J Rahmat ,P D Thompson ,D Johnson ,J Braciník ,E Sauvan,E Garutti,Y Zhu ,H.‐C Schultz‐Coulon ,J Kretzschmar,M Pandurović ,J E Olsson ,Peter Faulkner ,Jens Zimmermann ,S Backović ,T Sýkora ,Magnus Hansson ,G D Patel ,B Roland,A Belousov,S Aplin,P Robmann,D South,K B Cantun Avila ,Th D Papadopoulou ,W Yan,X Janssen,L N Shtarkov ,K Černý ,G Nowak,A Fedotov,K H Hiller ,E Pérez ,R Horisberger,N Raičević ,E Elsen,Martin Brinkmann ,M U Mozer ,S Schmidt,D Pérez-Astudillo ,J G Contreras ,Jenny List ,M Wessels,I Glushkov,C Werner,A Meyer ,S V Rusakov ,S Schmitt,E A De Wolf ,D P C Sankey ,Syed Asad Hussain ,I Smiljanić ,C Schmitz,I Panagoulias,H U Martyn ,T Naumann,J Zálešâk ,Bogdan Povh ,П Мурін ,C Helebrant,R C W Henderson ,A W Jung ,D Hoffmann,A Mohamed,H Jung,R López-Fernández ,S Xella,M Sauter,M Nožička ,V Černý ,S M Piec ,B Antunović ,R Wolf,P Smirnov,J Katzy,A Campbell ,S J Maxfield

doi:10.1140/epjc/s10052-007-0325-4

Abstract

Measurements are presented of differential dijet cross sections in diffractive photoproduction (Q^2<0.01 GeV^2) and deep-inelastic scattering processes (DIS, 4<Q^2<80 GeV^2). The event topology is given by ep-> e X Y, in which the system X, containing at least two jets, is separated from a leading low-mass proton remnant system Y by a large rapidity gap. The dijet cross sections are compared with NLO QCD predictions based on diffractive parton densities previously obtained from a QCD analysis of inclusive diffractive DIS cross sections by H1. In DIS, the dijet data are well described, supporting the validity of QCD factorisation. The diffractive DIS dijet data are more sensitive to the diffractive gluon density at high fractional parton momentum than the measurements of inclusive diffractive DIS. In photoproduction, the predicted dijet cross section has to be multiplied by a factor of approximately 0.5 for both direct and resolved photon interactions to describe the measurements. The ratio of measured dijet cross section to NLO prediction in photoproduction is a factor 0.5+-0.1 smaller than the same ratio in DIS. This suppression is the first clear observation of QCD hard scattering factorisation breaking at HERA. The measurements are also compared to the two soft colour neutralisation models SCI and GAL. The SCI model describes diffractive dijet production in DIS but not in photoproduction. The GAL model fails in both kinematic regions.

Highlights

It can be shown in Quantum Chromodynamics (QCD) that the cross section for diffractive processes in deep-inelastic Ô scattering (DIS) factorises into universal diffractive parton density functions (DPDFs) of the proton and process-dependent hard scattering cross sections (QCD factorisation) [1]
Previous measurements of diffractive dijet production in DIS [4, 5] have been found to be described by leading order (LO) Monte Carlo (MC) QCD calculations based on the factorisation approach that use the diffractive parton densities from [2] and include parton showers to simulate higher order effects
The H1 Collaboration has determined diffractive parton densities from QCD fits to inclusive diffractive DIS data in [2, 3]

Summary

Introduction

It can be shown in Quantum Chromodynamics (QCD) that the cross section for diffractive processes in deep-inelastic Ô scattering (DIS) factorises into universal diffractive parton density functions (DPDFs) of the proton and process-dependent hard scattering cross sections (QCD factorisation) [1]. Final state configurations for which a partonic cross section is perturbatively calculable include dijet and heavy quark production, which are directly sensitive to the diffractive gluon distribution. Using the same diffractive parton densities in LO QCD calculations overestimates the cross section for single-diffractive dijet production in ÔÔ collisions at the Tevatron by approximately one order of magnitude [6]. This discrepancy has been attributed to the presence of the additional beam hadron remnant in ÔÔ collisions, which leads to secondary interactions. The suppression, often characterised by a ‘rapidity gap survival probability,’ cannot be calculated perturbatively but has been parameterised in various ways (see, e.g., [7,8,9,10,11])

Methods

Results

Conclusion