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

Abstract

Exclusive photoproduction of ${\rho }^0(770)$ mesons is studied using the H1 detector at the $ep$ collider HERA. A sample of about 900000 events is used to measure single- and double-differential cross sections for the reaction $\gamma p\to {\pi }^{+}{\pi }^{-}Y$. 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 ($m_p<m_Y<10$ 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 $0.5<m_{\pi \pi }<2.2$ GeV, $|t|<1.5$ GeV${}^2$, and $20<W_{\gamma p}<80$ 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 $m_{\pi \pi }$ 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 $m_{\rho }=770.8{}_{-2.7}^{+2.6}$ (tot) MeV and ${\Gamma }_{\rho }=151.3{}_{-3.6}^{+2.7}$ (tot) 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 $\alpha (t=0)=1.0654{}_{-0.0067}^{+0.0098}$ (tot) and the slope ${\alpha }^{\prime }(t=0)=0.233{}_{-0.074}^{+0.067}$ (tot) GeV${}^{-2}$ of the $t$ dependence are extracted for the case $m_Y=m_p$.

Comments: 74 pages, 20 figures, 23 tables, accepted by EPJC. Figure numbering updated. Ancillary material has been updated