Electroproduction Of Rho Mesons Research Articles

Unintegrated gluon distribution from forward polarized rho -electroproduction

We present here some arguments to support our suggestion that data on the helicity structure for the hard exclusive electroproduction of rho mesons at HERA (and in possible future high-energy electron-proton colliders) provide useful information to constrain the kappa -shape of the unintegrated gluon distribution in the proton.

Elastic electroproduction of $\rho$ mesons at HERA

The elastic electroproduction of rho mesons is studied at HERA with the H1 detector for a photon virtuality in the range 1 Q^2 60 GeV^2 and for a hadronic centre of mass energy in the range 30 W 140 GeV. The shape of the pipi mass distribution in the rho resonance region is measured as a function of Q^2. The full set of rho spin density matrix elements is determined, and evidence is found for a helicity flip amplitude at the level of 8 +- 3 % of the non-flip amplitudes. Measurements are presented of the dependence of the cross section on Q^2, W and t (the four-momentum transfer squared to the proton). They suggest that, especially at large Q^2, the gamma^*p cross section develops a stronger W dependence than that expected from the behaviour of elastic and total hadron-hadron cross sections.

Diffractive electroproduction ofρmeson excitations

We use perturbative QCD and parton-hadron duality to estimate the cross sections for the diffractive electroproduction of rhoprime(1-) and rho(3-) resonances which occur in the 1.3-1.8 GeV mass interval. We present the cross sections and the ratios sigma_L/sigma_T as a function of Q^2. We compare the predictions with those for the diffractive electroproduction of rho mesons. We show how such diffractive electroproduction measurements at HERA can probe features of the perturbative QCD 'Pomeron'.

Proton dissociative $\rho$ and elastic $\phi$ electroproduction at HERA

The electroproduction of rho mesons with proton diffractive dissociation for Q^2 > 7 GeV^2 and the elastic electroproduction of Phi mesons for Q^2 > 6 Gev^2 are studied in e^+ p collisions at HERA with the H1 detector, for an integrated luminosity of 2.8 pb-1. The dependence of the cross sections on P_t^2 and Q^2 is measured, and the vector meson polarisation obtained. The cross section ratio between proton dissociative and elastic production of rho mesons is measured and discussed in the framework of the factorisation hypothesis of diffractive vertices. The ratio of the elastic cross section for Phi and rho meson production is investigated as a function of Q^2.

Helicity conservation and meson diffraction dissociation

In a Deck-model description of meson diffraction dissociation, we study the contribution made to the low-mass enhancement by the vector-meson-exchange Deck amplitudes. The vector-exchange amplitudes are parametrized with the help of data on the electroproduction of rho mesons. Cross sections for vector exchange are compared to the standard pseudoscalar-meson-exchange Deck results are are found to be comparable in specific cases. A detailed spin and helicity amplitude analysis shows that the vector-exchange Deck amplitudes provide a low-mass system which satisfies approximate s-channel helicity conservation, whereas the pseudoscalar-exchange terms yield approximate t-channel helicity conservation. Combining the two contributions, we show that approximate s-channel helicity conservation holds in Kp ..-->.. (rhoK) p and in Kp ..-->.. (..omega..K) p. However, t-channel helicity conservation is expected in Kp ..-->.. (K*..pi..) p, ..pi..p ..-->.. (rho..pi..) p, and Kp ..-->.. (phiK) p. These results agree with data. The relevance of these results to the interpretation of the Q-meson-resonance region is discussed.