Elastic and proton-dissociative ρ0 photoproduction at HERA

Elastic and proton-dissociative p(o) photoproduction (gamma p --> rho(o)p, gamma p --> rho(o)N, respectively, with rho(o) --> 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 M-N(2) < 0.1W(2). 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(o) 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 W-a gives a = 0.16 +/- 0.06 (stat.)(-0.15)(+0.11) (syst.). The resonant part of the gamma p --> pi(+)pi(-) p cross section is 11.2 +/- 0.1 (stat.)(-1.2)(+1.1)(syst.) mu b at [W] = 71.7 GeV. The t dependence of the cross section can be described by a function of the type A(rho) exp (-b(rho)\t\ + c(p)t(2)) with b(rho) = 10.9 +/- 0.3 (stat.)(-0.5)(+1.0) (syst.) GeV-2 and c(rho) = 2.7 +/- 0.9 (stat.)(-1.7)(+1.9) (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 alpha(IP)' = 0.23 +/- 0.15 (stat.)(-0.07)(+0.10), (syst.) GeV-2 has been deduced. The rho(o) spin density matrix elements r(00)(04), r(1-1)(04) and Re[r(10)(04)] 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(o) mass range, the distributions of the two-pion invariant mass, Wi 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 +/- 0.3 (stat.) +/- 0.5 (syst.) GeV-2. The ratio of the elastic to proton-dissociative rho(o) photoproduction cross section is 2.0 +/- 0.2 (stat.) +/- 0.7 (syst.).