The exclusive electroproduction of J /ψ mesons, ep → epJ /ψ , has been studied with the ZEUS detector at HERA for virtualities of the exchanged photon in the ranges 0.15 < Q2 < 0.8 GeV2 and 2 < Q2 < 100 GeV2 using integrated luminosities of 69 and 83 pb−1, respectively. The photon– proton centre-of-mass energy was in the range 30 < W < 220 GeV and the squared four-momentum transfer at the proton vertex2|t| < 1 GeV2. The cross sections and decay angular distributions are presented as functions of Q , W and t. The effective parameters of the pomeron trajectory are in agreement with those found in J/ψ photoproduction. The spin-density matrix elements, calculated from the decay angular distributions, are consistent with the hypothesis of s-channel helicity conservation. The ratio of the longitudinal to transverse cross sections, σL/σT , grows with Q2, whilst no dependence on W or t is observed. The results are in agreement with perturbative QCD calculations and exhibit a strong sensitivity to the gluon distribution in the proton.
Exclusive electroproduction of J/psi mesons at HERA
BASILE, MAURIZIO;CIFARELLI, LUISA;CONTIN, ANDREA;PALMONARI, FEDERICO;RINALDI, LORENZO;SARTORELLI, GABRIELLA;ZICHICHI, ANTONINO;
2004
Abstract
The exclusive electroproduction of J /ψ mesons, ep → epJ /ψ , has been studied with the ZEUS detector at HERA for virtualities of the exchanged photon in the ranges 0.15 < Q2 < 0.8 GeV2 and 2 < Q2 < 100 GeV2 using integrated luminosities of 69 and 83 pb−1, respectively. The photon– proton centre-of-mass energy was in the range 30 < W < 220 GeV and the squared four-momentum transfer at the proton vertex2|t| < 1 GeV2. The cross sections and decay angular distributions are presented as functions of Q , W and t. The effective parameters of the pomeron trajectory are in agreement with those found in J/ψ photoproduction. The spin-density matrix elements, calculated from the decay angular distributions, are consistent with the hypothesis of s-channel helicity conservation. The ratio of the longitudinal to transverse cross sections, σL/σT , grows with Q2, whilst no dependence on W or t is observed. The results are in agreement with perturbative QCD calculations and exhibit a strong sensitivity to the gluon distribution in the proton.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
