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Measurement of Exclusive $\pi^{+}\pi^{-}$ and $\rho^0$ Meson Photoproduction at HERA

V. Andreev A. Baghdasaryan A. Baty K. Begzsuren A. Belousov A. Bolz V. Boudry 1 G. Brandt D. Britzger A. Buniatyan L. Bystritskaya A.J. Campbell K.B. Cantun Avila K. Cerny V. Chekelian Z. Chen J.G. Contreras J. Cvach J.B. Dainton K. Daum A. Deshpande C. Diaconu 2 G. Eckerlin S. Egli E. Elsen L. Favart A. Fedotov J. Feltesse 3 M. Fleischer A. Fomenko C. Gal J. Gayler L. Goerlich N. Gogitidze M. Gouzevitch 4 C. Grab A. Grebenyuk T. Greenshaw G. Grindhammer D. Haidt R.C.W. Henderson J. Hladky D. Hoffmann 2 R. Horisberger T. Hreus F. Huber M. Jacquet 5 X. Janssen A.W. Jung H. Jung M. Kapichine J. Katzy C. Kiesling M. Klein C. Kleinwort R. Kogler P. Kostka J. Kretzschmar D. Krücker K. Krüger M.P.J. Landon W. Lange P. Laycock A. Lebedev S. Levonian K. Lipka B. List J. List W. Li B. Lobodzinski E. Malinovski H.-U. Martyn S.J. Maxfield A. Mehta A.B. Meyer H. Meyer J. Meyer S. Mikocki M.M. Mondal A. Morozov K. Müller Th. Naumann P.R. Newman C. Niebuhr G. Nowak J.E. Olsson D. Ozerov S. Park C. Pascaud 5 G.D. Patel Emile Perez A. Petrukhin 4 I. Picuric D. Pitzl R. Polifka V. Radescu N. Raicevic T. Ravdandorj P. Reimer E. Rizvi P. Robmann R. Roosen A. Rostovtsev M. Rotaru D.P.C. Sankey M. Sauter E. Sauvan 2, 6 S. Schmitt B.A. Schmookler L. Schoeffel 3 A. Schöning F. Sefkow S. Shushkevich Y. Soloviev P. Sopicki D. South V. Spaskov A. Specka 1 M. Steder B. Stella U. Straumann T. Sykora P.D. Thompson D. Traynor P. Truöl B. Tseepeldorj Z. Tu A. Valkárová C. Vallée 2 P. van Mechelen D. Wegener E. Wünsch J. Žáček J. Zhang Z. Zhang 5 R. Žlebčík H. Zohrabyan F. Zomer 5
Abstract : Exclusive photoproduction of ${{\rho ^0}} (770)$ 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 $\gamma p \rightarrow \pi ^{+}\pi ^{-}Y$. Reactions where the proton stays intact (${{{m_Y}} {=}m_p}$) are statistically separated from those where the proton dissociates to a low-mass hadronic system ($m_p{<}{{m_Y}} {<}10~{{\text {GeV}}} $). The double-differential cross sections are measured as a function of the invariant mass $m_{\pi \pi }$ 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 ${{W_{\gamma p}}} $. The phase space restrictions are $0.5\le m_{\pi \pi } \le 2.2~{{\text {GeV}}} $, $\vert t\vert \le 1.5~{{\text {GeV}^2}} $, and $20 \le W_{\gamma p} \le 80~{{\text {GeV}}} $. Cross section measurements are presented for both elastic and proton-dissociative scattering. The observed cross section dependencies are described by analytic functions. Parametrising the ${m_{\pi \pi }}$ dependence with resonant and non-resonant contributions added at the amplitude level leads to a measurement of the ${{\rho ^0}} (770)$ meson mass and width at $m_\rho = 770.8{}^{+2.6}_{-2.7}~({\text {tot.}})~{{\text {MeV}}} $ and $\Gamma _\rho = 151.3 {}^{+2.7}_{-3.6}~({\text {tot.}})~{{\text {MeV}}} $, respectively. The model is used to extract the ${{\rho ^0}} (770)$ contribution to the $\pi ^{+}\pi ^{-}$ cross sections and measure it as a function of t and ${W_{\gamma p}}$. In a Regge asymptotic limit in which one Regge trajectory $\alpha (t)$ dominates, the intercept $\alpha (t{=}0) = 1.0654\ {}^{+0.0098}_{-0.0067}~({\text {tot.}})$ and the slope $\alpha ^\prime (t{=}0) = 0.233 {}^{+0.067 }_{-0.074 }~({\text {tot.}}) ~{{\text {GeV}^{-2}}} $ of the t dependence are extracted for the case $m_Y{=}m_p$.
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V. Andreev, A. Baghdasaryan, A. Baty, K. Begzsuren, A. Belousov, et al.. Measurement of Exclusive $\pi^{+}\pi^{-}$ and $\rho^0$ Meson Photoproduction at HERA. Eur.Phys.J.C, 2020, 80 (12), pp.1189. ⟨10.1140/epjc/s10052-020-08587-3⟩. ⟨hal-02886864⟩

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