2021 Impact factor 4.991
Particles and Fields
Eur. Phys. J. C 2, 247-267

Elastic and proton-dissociative $\rho^0$ photoproduction at HERA

The ZEUS Collaboration, J. Breitweg et al.

Argonne National Laboratory, Argonne, IL, USA

Received: 3 December 1997

Elastic and proton-dissociative $\rho^0$ photoproduction ($\gamma p \rightarrow \rho^0 p$, $\gamma p \rightarrow \rho^0 N$,respectively, with $\rho^0 \rightarrow \pi^+\pi^-$)has been studied in ep interactions at HERA for photon-proton centre-of-mass energies in the range 50<W<100 GeV and for |t|<0.5 GeV2, where t is the square of the four-momentum transfer at the proton vertex; the results on the proton-dissociative reaction are presented for masses of the dissociated proton system in the range MN2<0.1 W2. For the elastic process, the $\pi^+\pi^-$invariant mass spectrum has been investigated as a function of t. As in fixed target experiments, the $\rho^0$ resonance shape is asymmetric; this asymmetry decreases with increasing |t|, as expected in models in which the asymmetry is ascribed to the interference of resonant and non-resonant $\pi^+\pi^-$ production. The cross section has been studied as a function of W; a fit to the resonant part with the form Wa gives $a=0.16\pm 0.06~\mbox{(stat.)}~^{+0.11}_{-0.15}~\mbox{(syst.)}$.The resonant part of the $\gamma p \rightarrow \pi^+\pi^- p$ cross section is $11.2\pm 0.1~\mbox{(stat.)}~^{+1.1}_{-1.2}~\mbox{(syst.)}$ $\mu$b at $\langle W\rangle=71.7$ GeV. The t dependence of the cross section can be described by a function of the type $A_{\rho} \exp{(-b_{\rho} \vert t\vert + c_{\rho} t^2)}$ with $b_{\rho}= 10.9 \pm 0.3$ (stat.) +1.0-0.5 (syst.) GeV-2 and $c_{\rho}= 2.7 \pm 0.9$ (stat.) +1.9-1.7 (syst.) GeV-4. The t dependence has also been studied as a function of W and a value of the slope of the pomeron trajectory $\mbox{$\alpha_{_{I\hspace{-0.2em}P}}$}^{\prime}=
0.23~\pm 0.15~\mbox{(stat.)}~^{+0.10}_{-0.07}~\mbox{(syst.)}$ GeV-2 has been deduced. The $\rho^0$ spin density matrix elements r0400, r041-1 and $\Re{e[r^{04}_{10}]}$ have been measured and found to be consistent with expectations based on s-channel helicity conservation. For proton-dissociative $\pi^+\pi^-$ photoproduction in the $\rho^0$mass range, the distributions of the two-pion invariant mass, W and the polar and azimuthal angles of the pions in the helicity frame are the same within errors as those for the elastic process. The t distribution has been fitted to an exponential function with a slope parameter $5.8 \pm~0.3~\mbox{(stat.)} \pm 0.5~\mbox{(syst.)}$ GeV-2. The ratio of the elastic to proton-dissociative $\rho^0$ photoproduction cross section is $2.0~\pm~0.2~\mbox{(stat.)}~\pm 0.7~\mbox{(syst.)}$.

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