Measurements of the kinematic distributions of J /ψ mesons produced in p-C, p-Ti and p-W collisions at √ s = 41.6 GeV in the Feynman-x region −0.34 < x F < 0.14 and for transverse momentum up to p T = 5.4 GeV/c are presented. The x F and p T dependencies of the nuclear suppression parameter, α, are also given. The results are based on 2.4 × 10 5 J /ψ mesons reconstructed in both the e + e − and μ + μ − decay channels. The data have been collected by the HERA-B experiment at the HERA proton ring of the DESY laboratory. The measurement explores the negative region of x F for the first time. The average value of α in the measured x F region is 0.981 ± 0.015. The data suggest that the strong nuclear suppression of J /ψ production previously observed at high x F turns into an enhancement at negative x F .

No abstract

Ratios of the ψ ′ over the J/ψ production cross sections in the dilepton channel for C, Ti and W targets have been measured in 920 GeV proton-nucleus interactions with the HERA-B detector at the HERA storage ring. The ψ ′ and J/ψ states were reconstructed in both the µ + µ − and the e + e − decay modes. The measurements covered the kinematic range −0.35 ≤ xF ≤ 0.1 with transverse momentum pT ≤ 4.5 GeV/c. The ψ ′ to J/ψ production ratio is almost constant in the covered xF range and shows a slow increase with pT. The angular dependence of the ratio has been used to measure the difference of the ψ ′ and J/ψ polarization. All results for the muon and electron decay channels are in good agreement: their ratio, averaged over all events, is R ψ ′ (µ)/R ψ ′ (e) = 1.00 ± 0.08 ± 0.04. This result constitutes a new, direct experimental constraint on the double ratio of branching fractions, (B ′ (µ) • B(e)) / (B(µ) • B ′ (e)), of ψ ′ and J/ψ in the two channels.

The inclusive production cross sections of the strange vector mesons K * 0 , K * 0 , and φ have been measured in interactions of 920 GeV protons with C, Ti, and W targets with the HERA-B detector at the HERA storage ring. Differential cross sections as a function of rapidity and transverse momentum have been measured in the central rapidity region and for transverse momenta up to pT = 3.5 GeV/c. The atomic number dependence is parametrised as σpA = σpN * A α , where σpN is the proton-nucleon cross section. Within the phase space accessible, α(K * 0) = 0.86 ± 0.03, α(K * 0) = 0.87 ± 0.03, and α(φ) = 0.96 ± 0.02. The total proton-nucleon cross sections, determined by extrapolating the differential measurements to full phase space, are σ pN→K * 0 = (5.06 ± 0.54) mb, σ pN→K * 0 = (4.02 ± 0.45) mb, and σ pN→φ = (1.17 ± 0.11) mb. For all resonances the Cronin effect is observed; compared to the measurements of Cronin et al. for K ± mesons, the measured values of α for φ mesons coincide with those of K + mesons for all transverse momenta, while the enhancement for K * 0 /K * 0 mesons is smaller.