A study of the angular distributions of leptons from decays of J/psi's produced in p-C and p-W collisions at sqrt{s}=41.6 GeV has been performed in the Feynman-x region -0.34 < x_F < 0.14 and for transverse momentum up to 5.4 GeV/c. The data were collected by the HERA-B experiment at the HERA proton ring of the DESY laboratory. The results, based on a clean selection of 2.3 x 10^5 J/psi's reconstructed in both the e^+ e^- and mu^+ mu^- decay channels, indicate that J/psi's are produced with longitudinal polarization. The magnitude of the effect is maximal at low p_T. For p_T >1 GeV/c a significant dependence on the reference frame is found: the polar anisotropy is more pronounced in the Collins-Soper frame and almost vanishes in the helicity frame, where, instead, a significant azimuthal anisotropy arises.

Measurements of the kinematic distributions of J/ψ mesons produced in p–C, p–Ti and p–W collisions at \(\sqrt{s}=41.6~\mathrm{GeV}\) 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 2.4×105 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 .

The spin-parity analysis of the data on the ¯pp → 2π+ 2π − annihilation reaction at rest in liquid and in gaseous hydrogen at 3 bar pressure and in flight at p momentum of 50 MeV/c, collected by the Obelix spectrometer at the LEAR complex of CERN, is presented. The relative branching ratios (a1(1260) → σπ)/(a1(1260) → ρπ) = 0.06 ± 0.05 and (π(1300) → σπ)/(π(1300) → ρπ) = 2.2 ± 0.4 are obtained. It is also shown that the inclusion of the exotic meson π1(1400), JPC= 1−+, mass and width M = 1.384 ± 0.028, Γ = 0.378 ± 0.058 GeV/c2, in its decay to ρπ, improves the fit and some implications of these results are briefly discussed. The relative S and P-wave annihilation percentages in four charged pions at two target densities are obtained

The p¯ stopping power in helium from 1 keV kinetic energy is evaluated. Contrary to the effect observed around and below the maximum, Obelix data indicate a p¯ stopping power higher than that for proton, the difference being of the order of 15±5% at ≈700 keV. The result contributes to assert the fundamental difference between p¯ stoppings in the simplest gases (He, H2) and in solid targets below some MeV.

Data on (p) over bar slowing down and capture in helium at 1 and 0.2 mb at room temperature are presented and compared to the corresponding previously publicated data in molecular hydrogen and deuterium. A Monte Carlo simulation containing a low-energy extrapolation of measured (p) over bar electronic stopping power in helium gas, screened Rutherford collisions, and simple cascade mechanisms is able to reproduce the gross features of the data, but cannot explain some nontrivial details of the measured distributions.

Inclusive differential cross sections dσpA/dxF and dσpA/dpt 2 for the production of Ks 0, Λ, and Λ̄ particles are measured at HERA in proton-induced reactions on C, Al, Ti, and W targets. The incident beam energy is 920 GeV, corresponding to √s = 41.6 GeV in the proton-nucleon system. The ratios of differential cross sections dσpA(Ks 0)/dσpA(Λ) and dσpA(Λ̄)/dσpA(Λ) are measured to be 6.2 ± 0.5 and 0.66 ± 0.07, respectively, for xF ≈ -0.06. No significant dependence upon the target material is observed. Within errors, the slopes of the transverse momentum distributions dσpA/dpt 2 also show no significant dependence upon the target material. The dependence of the extrapolated total cross sections σpA on the atomic mass A of the target material is discussed, and the deduced cross sections per nucleon σPN are compared with results obtained at other energies.

The computing cluster built at Bologna to provide the LHCb Collaboration with a powerful Monte Carlo production tool is presented. It is a performance oriented Beowulf-class cluster, made of rack mounted commodity components, designed to minimize operational support requirements and to provide full and continuous availability of the computing resources. In this paper we describe the architecture of the cluster, and discuss the technical solutions adopted for each specialized sub-system.

We report the stopping power of molecular hydrogen for antiprotons of kinetic energy above the maximum (approximately 100 keV) with the purpose of comparing with the proton one. Our result is consistent with a positive difference in antiproton-proton stopping powers above approximately 250 keV and with a maximum difference between the stopping powers of 21%+/-3% at around 600 keV.

A spin-parity analysis of a sample of (p) over barp --> K(+)K(-)pi(+)pi(-)pi(0) annihilation events taken at rest in gaseous H-2 at NTP is reported. The invariant mass spectrum of the K(+)K(-)n(0) system shows the presence of the E/t resonance pattern at 1.42 GeV. The analysis of this signal confirms the results obtained by the OBELIX Collaboration in previous works, namely: the existence of two pseudoscalar states at 1.413 +/- 0.002 and 1.460 +/- 0.012 GeV with widths 0.051 +/- 0.004 and 0.120 +/- 0.015 GeV, respectively. The (p) over barp system in the P-3(1) wave shows also a signal corresponding to the axial vector f(1) (1420) decaying to (KK)-K-* with mass 1.420 +/- 0.003 GeV and width 0.061 +/- 0.005 GeV. A study of the incoherent phase space background shows that its origin is mostly due to the reflection of a resonant state in the K(+)K(-)pi(+)pi(-) system. The isobar decomposition of this resonant state is mainly f(0)(1370)(pipi)S, its parameters are: J(PC) = 0(++), m = 1.670 +/- 0.02 GeV and width 0.267 +/- 0.036 GeV. This signal can be probably identified with another decay mode of the f0(1710). (C) 2002 Elsevier Science B.V. All rights reserved.

The annihilation frequency of the reaction (p) over barp --> pi(0)pi(0) at rest in liquid hydrogen has been measured by the Obelix experiment by using different apparatus configurations and trigger conditions. The value obtained is f(pi(0)pi(0), LH) = (2.8 +/- 0.1(stat)+/-0.4(syst)) x 10(-4). With the same data samples, the pi(0) eta annihilation frequency has been determined to be, f(pi(0) eta, LH) = (0.9+/-0.2(stat)+/-0.1(syst)) x 10(-4). The results are discussed within the frame of the present experimental situation.

The results of an investigation of the Pontecorvo reaction (p) over bard -->phin with stopped antiprotons in a gaseous deuterium target at NTP are presented. A very selective dedicated trigger was used to collect a sample of about 8.5 x 10(6) annihilations, It is found that the annihilation rate is Y((p) over bard -->phin) = (3.56 +/- 0.20(-0.1)(+0.2)) x 10(-6). Comparison with the corresponding value for the omega-meson production demonstrates a large (a factor 40) apparent violation of the OZI rule, even larger than that found in the annihilation on a free nucleon. (C) 2002 Elsevier Science B.V. All fights reserved.

A direct physical interpretation of the K-matrix and P-vector approach is proposed; the aspects relevant to the understanding of the formalism are commented in detail. The procedures developed are extended to include the general case of non-resonant terms in K-matrix and P-vector expressions currently required by experimental data.