Measurement of Elastic ${\rho }^0$ Photoproduction at HERA

Abstract

Elastic ${\rho }^0$ photoproduction has been measured using the ZEUS detector at HERA. Untagged photoproduction events from $ep$ interactions were used to measure the reaction $\gamma p\to {\rho }^{0}p$ (${\rho }^0\to {\pi }^{+}{\pi }^{-}$) at photon-proton centre-of-mass energies between 60 and 80~GeV and $|t|<0.5$~GeV${}^2$, where $t$ is the square of the four-momentum transferred at the proton vertex. The differential cross section $d\sigma /d{M}_{\pi \pi }$, where $M_{\pi \pi }$ is the invariant mass of the two pions, and the integrated cross section, ${\sigma }_{\gamma p\to {\rho }^{0}p}$, are presented; the latter was measured to be $14.7\pm 0.4~(\mbox{stat.})~\pm2.4~(\mbox{syst.})~μ\mbox{b}$. The differential cross section $d\sigma /dt$ has an approximately exponential shape; a fit of the type $A_t^{\prime }\exp (-b_t^{\prime }|t|+c_t^{\prime }{t}^2)$ yields a $t$-slope $b^{\prime}_{t}= 9.9~\pm~1.2~(\mbox{stat.})~\pm 1.4~(\mbox{syst.})~μ\mbox{b}$. The results, when compared to low energy data, show a weak energy dependence of both ${\sigma }_{\gamma p\to {\rho }^{0}p}$ and of the $t$-slope. The ${\rho }^0$ is produced predominantly with transverse polarisation, demonstrating that $s$-channel helicity conservation holds at these energies.

38 pages, latex, 10 figures appended as uuencoded file. This manuscript replaces the one which had a non standard style file

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