Structure functions at large x

Structure function data together with other measurements from fixed-target deep inelastic scattering and hadron-hadron collider experiments which contribute to our knowledge of the parton density functions are reviewed. The inclusive cross-section measurements of neutral and charged current interactions at HERA are presented and their impact on the parton density functions is discussed. Future prospects for an improved knowledge of the parton density functions at large x are briefly mentioned.Comment: Invited talk given at the workshop on ``New Trends in HERA Physics 2001'', Ringberg castle, Germany, June 17-22, 200

Examination of direct-photon and pion production in proton-nucleon collisions

We present a study of inclusive direct-photon and pion production in hadronic interactions, focusing on a comparison of the ratio of gamma/pi0 yields with expectations from next-to-leading order perturbative QCD (NLO pQCD). We also examine the impact of a phenomenological model involving k_T smearing (which approximates effects of additional soft-gluon emission) on absolute predictions for photon and pion production and their ratio.Comment: 20 pages, 12 figures. Minor changes in wording and in figure

kt Effects in Direct-Photon Production

We discuss the phenomenology of initial-state parton-kt broadening in direct-photon production and related processes in hadron collisions. After a brief summary of the theoretical basis for a Gaussian-smearing approach, we present a systematic study of recent results on fixed-target and collider direct-photon production, using complementary data on diphoton and pion production to provide empirical guidance on the required amount of kt broadening. This approach provides a consistent description of the observed pattern of deviation of next-to-leading order QCD calculations relative to the direct-photon data, and accounts for the shape and normalization difference between fixed-order perturbative calculations and the data. We also discuss the uncertainties in this phenomenological approach, the implications of these results on the extraction of the gluon distribution of the nucleon, and the comparison of our findings to recent related work.Comment: LaTeX, uses revtex and epsf, 37 pages, 15 figure

Direct photon cross sections in proton-proton and antiproton-proton interactions at s=24.3\sqrt{s} = 24.3 GeV

We report results on inclusive direct photon $(\gamma), \pi_0$, and $\eta$ production in both pp and \={p}p interactions at $\sqrt{s} = 24.3$ GeV in the transverse momentum range {$4.1 \le p_T \le 7 .7$ GeV/$c$} and rapidity range \mbox{$-0.1 \le y \le 0.9$}. The data were collected between 1988 and 1990 by the UA6 experiment at CERN, which employed an internal $\mathrm{H_2}$ gas jet target in the S\={p}pS collider. The inclusive direct photon cross sections and the cross section difference \mbox{$\sigma(\overline{p}p) - \sigma(pp)$} expressed as functions of $p_T(\gamma)$ are compared w ith next-to-leading order QCD predictions

Search for a new gauge boson in π0\pi^{0} decays

A search was made for a new light gauge boson $X$ which might be produced in $\pi^{0}\to\gamma + X$ decay from neutral pions generated by 450-GeV protons in the CERN SPS neutrino target. The X's would penetrate the downstream shielding and be observed in the NOMAD detector via the Primakoff effect, in the process of $X \to\pi^{0}$ conversion in the external Coulomb field of a nucleus. With $1.45\times10^{18}$ protons on target, 20 candidate events with energy between 8 and 140 GeV were found from the analysis of neutrino data. This number is in agreement with the expectation of 18.1$\pm$2.8 background events from standard neutrino processes. A new 90% C.L. upper limit on the branching ratio $Br(\pi^{0}\to\gamma + X)< (3.3 to 1.9) \times10^{-5}$ for $X$ masses ranging from 0 to 120 MeV/c^2 is obtained.Comment: 15 pages, LaTex, 6 eps figures included, submitted to Physics Letters