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

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 $$m_Y{=}m_p$$ .