Search

A new measurement of the b ¯ b production cross section in 920 GeV proton-nucleus collisions is presented by the HERA-B Collaboration. The b ¯ b production is tagged via inclusive bottom quark decays into J / ψ mesons by exploiting the longitudinal separation of J / ψ → l + l − decay vertices from the primary proton-nucleus interaction point. Both e + e − and μ + μ − channels are reconstructed for a total of 83 ± 12 inclusive b → J / ψ X events found. The combined analysis yields a b ¯ b to prompt J / ψ cross section ratio of Δ σ ( b ¯ b ) Δ σ J / ψ = 0.032 ± 0.005 stat ± 0.004 sys measured in the x F acceptance ( − 0.35 < x F < 0.15 ), extrapolated to σ ( b ¯ b ) = 14.9 ± 2.2 stat ± 2.4 sys nb/nucleon in the total phase space.

A measurement of the ratio R χ c = ( χ c → J / ψ + γ ) / J / ψ in p C , p Ti , and p W interactions at 920 GeV / c ( √ s = 41.6 GeV ) in the Feynman-x range − 0.35 < x J / ψ F < 0.15 is presented. Both μ + μ − and e + e − J / ψ decay channels are observed with an overall statistics of about 15 000 χ c events, which is by far the largest available sample in pA collisions. The result is R χ c = 0.188 ± 0.013 st + 0.024 − 0.022 sys averaged over the different materials, when no J / ψ and χ c polarizations are considered. The χ c 1 to χ c 2 production ratio R 12 = R χ c 1 / R χ c 2 is measured to be 1.02 ± 0.40 , leading to a cross section ratio σ ( χ c 1 ) σ ( χ c 2 ) = 0.57 ± 0.23 . The dependence of R χ c on the Feynman-x of the J / ψ , x J / ψ F , and its transverse momentum, p J / ψ T , is studied, as well as its dependence on the atomic number, A, of the target. For the first time, an extensive study of possible biases on R χ c and R 12 due to the dependence of acceptance on the polarization states of J / ψ and χ c is performed. By varying the polarization parameter, λ obs , of all produced J / ψ 's by two sigma around the value measured by HERA-B, and considering the maximum variation due to the possible χ c 1 and χ c 2 polarizations, it is shown that R χ c could change by a factor between 1.02 and 1.21 and R 12 by a factor between 0.89 and 1.16.

We have searched for Θ + (1540) and Ξ −− (1862) pentaquark candidates in proton-induced reactions on C, Ti, and W targets at midrapidity and √ s = 41.6 G e V . In 2 × 10 8 inelastic events we find no evidence for narrow ( σ ≈ 5 M e V ) signals in the Θ + → p K 0 S and Ξ −− → Ξ − π − channels; our 95% C.L. upper limits (UL) for the inclusive production cross section times branching fraction B d σ / d y | y ≈ 0 are ( 4 - 16 ) μ b / N for a Θ + mass between 1521 and 1555 MeV, and 2.5 μ b / N for the Ξ −− . The UL of the yield ratio of Θ + / Λ ( 1520 ) < ( 3 - 12 ) % is significantly lower than model predictions. Our UL of B Ξ −− / Ξ ( 1530 ) 0 < 4 % is at variance with the results that have provided the first evidence for the Ξ −− .