Exclusive $$\rho ^0$$ ρ 0 meson photoproduction with a leading neutron at HERA

V Andreev,U Straumann,B Stella,H Zohrabyan,V Chekelian,C Niebuhr,V Brisson,N Gogitidze,J Ferencei,F Sefkow,P Sopicki,A Valkárová,K Begzsuren,R Roosen,L Bystritskaya,J Gayler,K Müller,J Žáček,P Truöl,M Steder,S Mikocki,P D Thompson,M Kapichine,A Lebedev,E Gabathuler,J Cvach,V Dodonov,J E Olsson,D Haidt,J B Dainton,S Ghazaryan,D Traynor,D P C Sankey,R C W Henderson,T Ravdandorj,Y Vazdik,L Goerlich,A Specka,G Grindhammer,C Vallée,A Buniatyan,S Levonian,P Kostka,R Plačakytė,T Greenshaw,V Spaskov,S Egli,S Shushkevich,C Pascaud,G D Patel,J Turnau,M Jacquet,H Pirumov,D Wegener,A Rostovtsev,E Wünsch,E Malinovski,K B Cantun Avila,A Baghdasaryan,K Daum,M Fleischer,A Petrukhin,Y Soloviev,I Tsakov,J Feltesse,B Tseepeldorj,C Kiesling,D Ozerov,H Jung,R Žlebčík,M P J Landon,A Mehta,D Hoffmann,P R Newman,G Nowak,P Laycock,G Brandt,E Sauvan,D Šálek,X Janssen,F Zomer,A Schöning,P Reimer,D South,M Klein,J Kretzschmar,L Favart,P Robmann,B List,R Polifka,C Grab,Z Zhang,W Lange,E Rizvi,V Radescu,S Schmitt,E Elsen,D Britzger,A Grebenyuk,C Diaconu,S J Maxfield,R Kogler,A Morozov,A Bylinkin,M Dobre,M Sauter,L Schoeffel,G Eckerlin,H Meyer,D Pitzl,B Pokorny,A Fedotov,T Sýkora ,J Hladký ,K C̆erný ,S V Rusakov ,E Pérez ,Jenny List ,N Raičević ,P Van Mechelen ,Bogdan Povh ,I Pićurić ,A Meyer ,B Łobodziński ,Florian Huber ,A Campbell ,T Naumann ,H U Martyn ,A E Bolz ,A Fomenko,A Belousov,V Boudry,J Meyer,K Krüger,M Rotaru,R Horisberger,J G Contreras,K Lipka,M Gouzevitch,C Kleinwort,T Hreus

doi:10.1140/epjc/s10052-015-3863-1

Abstract

A first measurement is presented of exclusive photoproduction of $$\rho ^0$$ mesons associated with leading neutrons at HERA. The data were taken with the H1 detector in the years 2006 and 2007 at a centre-of-mass energy of $$\sqrt{s}=319$$ GeV and correspond to an integrated luminosity of 1.16 pb $$^{-1}$$ . The $$\rho ^0$$ mesons with transverse momenta $$p_T<1$$ GeV are reconstructed from their decays to charged pions, while leading neutrons carrying a large fraction of the incoming proton momentum, $$x_L>0.35$$ , are detected in the Forward Neutron Calorimeter. The phase space of the measurement is defined by the photon virtuality $$Q^2 < 2$$ GeV $$^2$$ , the total energy of the photon–proton system $$20 < W_{\gamma p}< 100$$ GeV and the polar angle of the leading neutron $$\theta _n < 0.75$$ mrad. The cross section of the reaction $$\gamma p \rightarrow \rho ^0 n \pi ^+$$ is measured as a function of several variables. The data are interpreted in terms of a double peripheral process, involving pion exchange at the proton vertex followed by elastic photoproduction of a $$\rho ^0$$ meson on the virtual pion. In the framework of one-pion-exchange dominance the elastic cross section of photon-pion scattering, $$\sigma ^\mathrm{el}(\gamma \pi ^+ \rightarrow \rho ^0\pi ^+)$$ , is extracted. The value of this cross section indicates significant absorptive corrections for the exclusive reaction $$\gamma p \rightarrow \rho ^0 n \pi ^+$$ .

Highlights

Measurements of leading baryon production in high energy particle collisions, i.e. the production of protons and neutrons at very small polar angles with respect to the initial hadron beam direction, are important inputs for the theoretical understanding of strong interactions in the soft, non-perturbative regime
The phase space of the measurement is defined by the photon virtuality Q2 < 2 GeV2, the total energy of the photon–proton system 20 < Wγ p < 100 GeV and the polar angle of the leading neutron θn < 0.75 mrad
In ep collisions at HERA, a hard scale may be present in such reactions if the photon virtuality, Q2, is large, or if objects with high transverse momenta, pT, are produced in addition to the leading baryon

Summary

Introduction

Measurements of leading baryon production in high energy particle collisions, i.e. the production of protons and neutrons at very small polar angles with respect to the initial hadron beam direction (forward direction), are important inputs for the theoretical understanding of strong interactions in the soft, non-perturbative regime. Previous HERA measurements [1,2,3,4,5,6,7] have demonstrated that in the semi-inclusive reaction e + p → e + n + X the production of neutrons carrying a large fraction of the proton beam energy is dominated by the pion exchange process. In this picture a virtual photon, emitted from the beam electron, interacts with a pion from the proton cloud, giving access to the γ ∗π cross section and, in the deep-inelastic scattering regime, to the pion structure function.

Methods

Results

Conclusion