Measurement of exclusive π+π- and ρ meson photoproduction at HERA: H1 Collaboration

Andreev, V. and Baghdasaryan, A. and Baty, A. and Begzsuren, K. and Belousov, A. and Bolz, A. and Boudry, V. and Brandt, G. and Britzger, D. and Buniatyan, A. and Bystritskaya, L. and Campbell, A.J. and Cantun Avila, K.B. and Cerny, K. and Chekelian, V. and Chen, Z. and Contreras, J.G. and Cvach, J. and Dainton, J.B. and Daum, K. and Deshpande, A. and Diaconu, C. and Eckerlin, G. and Egli, S. and Elsen, E. and Favart, L. and Fedotov, A. and Feltesse, J. and Fleischer, M. and Fomenko, A. and Gal, C. and Gayler, J. and Goerlich, L. and Gogitidze, N. and Gouzevitch, M. and Grab, C. and Grebenyuk, A. and Greenshaw, T. and Grindhammer, G. and Haidt, D. and Henderson, R.C.W. and Hladkỳ, J. and Hoffmann, D. and Horisberger, R. and Hreus, T. and Huber, F. and Jacquet, M. and Janssen, X. and Jung, A.W. and Jung, H. and Kapichine, M. and Katzy, J. and Kiesling, C. and Klein, M. and Kleinwort, C. and Kogler, R. and Kostka, P. and Kretzschmar, J. and Krücker, D. and Krüger, K. and Landon, M.P.J. and Lange, W. and Laycock, P. and Lebedev, A. and Levonian, S. and Lipka, K. and List, B. and List, J. and Li, W. and Lobodzinski, B. and Malinovski, E. and Martyn, H.-U. and Maxfield, S.J. and Mehta, A. and Meyer, A.B. and Meyer, H. and Meyer, J. and Mikocki, S. and Mondal, M.M. and Morozov, A. and Müller, K. and Naumann, T. and Newman, P.R. and Niebuhr, C. and Nowak, G. and Olsson, J.E. and Ozerov, D. and Park, S. and Pascaud, C. and Patel, G.D. and Perez, E. and Petrukhin, A. and Picuric, I. and Pitzl, D. and Polifka, R. and Radescu, V. and Raicevic, N. and Ravdandorj, T. and Reimer, P. and Rizvi, E. and Robmann, P. and Roosen, R. and Rostovtsev, A. and Rotaru, M. and Sankey, D.P.C. and Sauter, M. and Sauvan, E. and Schmitt, S. and Schmookler, B.A. and Schoeffel, L. and Schöning, A. and Sefkow, F. and Shushkevich, S. and Soloviev, Y. and Sopicki, P. and South, D. and Spaskov, V. and Specka, A. and Steder, M. and Stella, B. and Straumann, U. and Sykora, T. and Thompson, P.D. and Traynor, D. and Truöl, P. and Tseepeldorj, B. and Tu, Z. and Valkárová, A. and Vallée, C. and Van Mechelen, P. and Wegener, D. and Wünsch, E. and Žáček, J. and Zhang, J. and Zhang, Z. and Žlebčík, R. and Zohrabyan, H. and Zomer, F. (2020) Measurement of exclusive π+π- and ρ meson photoproduction at HERA: H1 Collaboration. European Physical Journal C: Particles and Fields, 80 (12): 1189. ISSN 1434-6044

Abstract

Exclusive photoproduction of ρ(770) mesons is studied using the H1 detector at the ep collider HERA. A sample of about 900,000 events is used to measure single- and double-differential cross sections for the reaction γp→ π+π-Y. Reactions where the proton stays intact (mY= mp) are statistically separated from those where the proton dissociates to a low-mass hadronic system (mp<mY<10GeV). The double-differential cross sections are measured as a function of the invariant mass mππ of the decay pions and the squared 4-momentum transfer t at the proton vertex. The measurements are presented in various bins of the photon–proton collision energy Wγp. The phase space restrictions are 0.5≤mππ≤2.2GeV, |t|≤1.5GeV2, and 20≤Wγp≤80GeV. Cross section measurements are presented for both elastic and proton-dissociative scattering. The observed cross section dependencies are described by analytic functions. Parametrising the mππ dependence with resonant and non-resonant contributions added at the amplitude level leads to a measurement of the ρ(770) meson mass and width at mρ=770.8-2.7+2.6(tot.)MeV and Γρ=151.3-3.6+2.7(tot.)MeV, respectively. The model is used to extract the ρ(770) contribution to the π+π- cross sections and measure it as a function of t and Wγp. In a Regge asymptotic limit in which one Regge trajectory α(t) dominates, the intercept α(t=0)=1.0654-0.0067+0.0098(tot.) and the slope α′(t=0)=0.233-0.074+0.067(tot.)GeV-2 of the t dependence are extracted for the case mY= mp.