Measurement of exclusive $${\varvec{{{{\pi ^+\pi ^-}}}}}$$ and $${\varvec{{{{\rho ^0}}}}}$$ meson photoproduction at HERA: H1 Collaboration

Exclusive photoproduction of $${{\\rho ^0}} (770)$$\n 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 $$\\gamma p \\rightarrow \\pi ^{+}\\pi ^{-}Y$$\n . Reactions where the proton stays intact (\n $${{{m_Y}} {=}m_p}$$\n ) are statistically separated from those where the proton dissociates to a low-mass hadronic system (\n $$m_p{\u003c}{{m_Y}} {\u003c}10~{{\\text {GeV}}} $$\n ). The double-differential cross sections are measured as a function of the invariant mass $$m_{\\pi \\pi }$$\n 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 $${{W_{\\gamma p}}} $$\n . The phase space restrictions are $$0.5\\le m_{\\pi \\pi } \\le 2.2~{{\\text {GeV}}} $$\n , $$\\vert t\\vert \\le 1.5~{{\\text {GeV}^2}} $$\n , and $$20 \\le W_{\\gamma p} \\le 80~{{\\text {GeV}}} $$\n . Cross section measurements are presented for both elastic and proton-dissociative scattering. The observed cross section dependencies are described by analytic functions. Parametrising the $${m_{\\pi \\pi }}$$\n dependence with resonant and non-resonant contributions added at the amplitude level leads to a measurement of the $${{\\rho ^0}} (770)$$\n meson mass and width at $$m_\\rho = 770.8{}^{+2.6}_{-2.7}~({\\text {tot.}})~{{\\text {MeV}}} $$\n and $$\\Gamma _\\rho = 151.3 {}^{+2.7}_{-3.6}~({\\text {tot.}})~{{\\text {MeV}}} $$\n , respectively. The model is used to extract the $${{\\rho ^0}} (770)$$\n contribution to the $$\\pi ^{+}\\pi ^{-}$$\n cross sections and measure it as a function of t and $${W_{\\gamma p}}$$\n . In a Regge asymptotic limit in which one Regge trajectory $$\\alpha (t)$$\n dominates, the intercept $$\\alpha (t{=}0) = 1.0654\\ {}^{+0.0098}_{-0.0067}~({\\text {tot.}})$$\n and the slope $$\\alpha ^\\prime (t{=}0) = 0.233 {}^{+0.067 }_{-0.074 }~({\\text {tot.}}) ~{{\\text {GeV}^{-2}}} $$\n of the t dependence are extracted for the case $$m_Y{=}m_p$$\n .