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 , E. 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)

1 LLR - Laboratoire Leprince-Ringuet
2 CPPM - Centre de Physique des Particules de Marseille
3 DPhP - Département de Physique des Particules (ex SPP)
4 IPNL - Institut de Physique Nucléaire de Lyon
5 LAL - Laboratoire de l'Accélérateur Linéaire
6 LAPP - Laboratoire d'Annecy de Physique des Particules

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 $m_Y{=}m_p$.

Mots clés: experimental results; H1; rho(770): photoproduction; pi: pair production; exclusive reaction; electron p: scattering; resonance: production; p: dissociation; DESY HERA Stor; Regge poles; phase space; slope; photon p --> pi+ pi- p; data analysis method; Monte Carlo; pi+ pi-; cross section: measured; M001M; M009M0P; M009W0P

Domaines: Physique des Hautes Energies - Expérience [hep-ex]; Physique des Hautes Energies - Phénoménologie [hep-ph]

V. Andreev, A. Baghdasaryan, A. Baty, K. Begzsuren, A. Belousov, et al.. Measurement of Exclusive $\pi^{+}\pi^{-}$ and $\rho^0$ Meson Photoproduction at HERA. European Physical Journal C: Particles and Fields, 2020, 80 (12), pp.1189. ⟨10.1140/epjc/s10052-020-08587-3⟩. ⟨hal-02886864⟩