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H1
Abstract:
Exclusive photoproduction of ρ0(770)\rho^0(770)ρ0(770) mesons is studied using the H1 detector at the epepep collider HERA. A sample of about 900000 events is used to measure single- and double-differential cross sections for the reaction γp→π+π−Y\gamma p \to \pi^{+}\pi^{-}Yγp→π+π−Y. Reactions where the proton stays intact (mY=mp{m_Y{=}m_p}mY=mp) are statistically separated from those where the proton dissociates to a low-mass hadronic system (mp<mY<10m_p{<}m_Y{<}10mp<mY<10 GeV). The double-differential cross sections are measured as a function of the invariant mass mππm_{\pi\pi}mππ of the decay pions and the squared 444-momentum transfer ttt at the proton vertex. The measurements are presented in various bins of the photon-proton collision energy WγpW_{\gamma p}Wγp. The phase space restrictions are 0.5<mππ<2.20.5 < m_{\pi\pi} < 2.20.5<mππ<2.2 GeV, ∣t∣<1.5{\vert t\vert < 1.5}∣t∣<1.5 GeV2{}^22, and 20<Wγp<80{20 < W_{\gamma p} < 80}20<Wγp<80 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 mππm_{\pi\pi}mππ dependence with resonant and non-resonant contributions added at the amplitude level leads to a measurement of the ρ0(770)\rho^{0}(770)ρ0(770) meson mass and width at mρ=770.8 −2.7+2.6m_\rho = 770.8\ {}^{+2.6}_{-2.7}mρ=770.8 −2.7+2.6 (tot) MeV and Γρ=151.3 −3.6+2.7\Gamma_\rho = 151.3\ {}^{+2.7}_{-3.6}Γρ=151.3 −3.6+2.7 (tot) MeV, respectively. The model is used to extract the ρ0(770)\rho^0(770)ρ0(770) contribution to the π+π−\pi^{+}\pi^{-}π+π− cross sections and measure it as a function of ttt and WγpW_{\gamma p}Wγp. In a Regge asymptotic limit in which one Regge trajectory α(t)\alpha(t)α(t) dominates, the intercept α(t=0)=1.0654 −0.0067+0.0098\alpha(t{=}0) = 1.0654\ {}^{+0.0098}_{-0.0067}α(t=0)=1.0654 −0.0067+0.0098 (tot) and the slope α′(t=0)=0.233 −0.074+0.067\alpha^\prime(t{=}0) = 0.233\ {}^{+0.067 }_{-0.074 }α′(t=0)=0.233 −0.074+0.067 (tot) GeV−2{}^{-2}−2 of the ttt dependence are extracted for the case mY=mpm_Y{=}m_pmY=mp.
