B. Gabioud

A measurement of the inclusive π0 and η production cross sections at high pT in p̄p and pp collisions at s = 24.3 GeV

Abstract Inclusive π 0 and η production at large transverse momentum were studied in both pp and pp interactions at √ s = 24.3 GeV. The experiment was performed using an internal molecular hydrogen gas jet target in the CERN SPS collider. No significant differences between production in pp and pp were observed in the transverse momentum range 2.5 P T c .

Precise comparison of antiproton-proton and proton-proton forward elastic scattering at √s = 24.3 GeV

Abstract We report results from a measurement of antiproton-proton and proton-proton small-angle elastic scattering at √ s = 24.3 GeV in the range 0.001 ⩽ | t | ⩽ 0.06 (GeV/ c ) 2 . The measurement was performed at the CERN p p Collider by using silicon detectors to observe protons recoiling from a hydrogen cluster-jet target intercepting the stored p and p beams. Fits to the measured differential cross sections yield the ratio of the real to the imaginary part of the forward nuclear scattering amplitude ρ and the nuclear slope parameter b for both p p and pp. We find that the difference Δρ = ρ ( p p ) − ρ( pp ) = 0.031 ± 0.010 agrees with conventional fits and disagrees with the “odderon” fit designed to accommodate the recent UA4 measurement of ρ( p p) at 546 GeV.

Determination of αs and the gluon distribution using direct photon production in pp and pp collisions

Abstract Direct photon cross sections obtained in p p and pp collisions at √s = 24.3 GeV at the CERN SPS, are used in a next-to-leading order QCD analysis. From the cross-section difference σ( p p → γX) − σ (pp → γX) , and quark distributions measured in deep inelastic scattering, a determination of the strong coupling constant, αs, is performed via a measurement of Λ (4) MS . This first measurement of Λ (4) MS using direct photon data yields a value Λ (4) MS = 235 ± 79 (stat.) ± 70 (syst.) MeV. The corresponding value of αs expressed at MZ2 is αs (MZ2) = 0.112±0.006 (stat.)±0.005 (syst.)−0.001+0.009 (theor.). The gluon distribution in the proton for x > 0.2 is determined both from the p p and from the pp data, and the results agree with each other and with previous determinations using the same method.

Measurement of the inclusive J / psi production cross-sections in anti-p p and p p collisions at s**(1/2) = 24.3-GeV

Abstract The inclusive cross section times branching ratio for J/ψ→e + e − has been measured in the forward region in the UA6 experiment for p p and pp collisions at √ s = 24.3 GeV. The ratio of the pp to p p cross sections is found to be 0.76 ± 0.14 ± 0.06. This demonstrates that gluon-gluon fusion dominates over quark-antiquark annihilation in the formation of the c c state.

Direct photon production in pp and pp interactions at √s=24.3 GeV

Abstract Inclusive direct photon invariant cross sections have been measured in both p p and pp collisions at √ s = 24.3 GeV at the CERN SPS, permitting the first measurement of the difference of the p p and pp cross sections. The direct photon cross section in p p collisions has been found to be systematically larger than that in pp collisions, which indicates a significant contribution of the q q annihilation term as predicted by theoretical calculations.

A lead/proportional-tube electromagnetic calorimeter for direct photon detection

Abstract The fine-grained lead/proportional-tube electromagnetic calorimeter used in CERN experiment UA6 is described. Test-beam results demonstrate linear energy response over a wide dynamic range, position resolution for shower localization reaching 1.7 mm for 70 GeV electrons, and good electron/hadron differentiation. Electron shower transverse widths are measured to be less than 9.0 mm for energies between 3 and 70 GeV. This feature allows us to identify neutral-meson decays yielding two close photon showers that constitute the main background to single-photon detection.