Photon Total Cross-sections

We discuss present predictions for the total $\gamma \gamma$ and $\gamma p$ cross-sections, highlighting why predictions differ. We present results from the Eikonal Minijet Model and improved predictions based on soft gluon resummation.Comment: 7 pages, 10 figures, LaTeX, requires espcrc2.sty, Talk presented by G. Pancheri at PHOTON-2003, International Meeting on Structure and Interactions of the Photon, Frascati, Italy, April 7-11, 200

Infrared gluons, intrinsic transverse momentum and rising total cross-sections

We discuss the infrared limit for soft gluon kt-resummation and relate it to physical observables such as the intrinsic transverse momentum and the high energy limit of total cross-sections.Comment: 8 pages, 6 figures, Presented at Hadron Structure '09, Tatranska Strba, September 2009, Slovacchia, to be published in the Conference Proceeding

Looking for the gluonic EMC effect in associated J/ψJ/\psi + γ\gamma production

We study the associated production of $J/\psi\ + \gamma$ in fixed-target experiments with nuclear targets as well as at the relativistic heavy-ion collider (RHIC). We find that this process affords a very clean probe of $\rho_g$, the ratio of gluon density in a heavy nucleus to that of a proton. The combined $x$ range thus available can be used to discriminate between the predictions of different models of the EMC effect for $\rho_g$.Comment: 10 pages, 4 Figures (not included, available only by p-mail), LaTeX, CERN-TH.6679/92, TIFR/TH/92-59, BU-TH-92/3, Minor revision. Replaced version has the style file equation.sty appended for easy processing, which is the only chang

Total photoproduction cross-section at very high energy

In this paper we apply to photoproduction total cross-section a model we have proposed for purely hadronic processes and which is based on QCD mini-jets and soft gluon re-summation. We compare the predictions of our model with the HERA data as well as with other models. For cosmic rays, our model predicts substantially higher cross-sections at TeV energies than models based on factorization but lower than models based on mini-jets alone, without soft gluons. We discuss the origin of this difference.Comment: 13 pages, 9 figures. Accepted for publication in EPJC. Changes concern added references, clarifications of the Soft Gluon Resummation method used in the paper, and other changes requested by the Journal referee which do not change the results of the original versio

The leading particle effect from light quark fragmentation in charm hadroproduction

The asymmetry of $D^-$ and $D^+$ meson production in $\pi^-N$ scattering observed by the E791 experiment is a typical phenomenon known as the leading particle effect in charm hadroproducton. We show that the phenomenon can be explained by the effect of light quark fragmentation into charmed hadrons (LQF). Meanwhile, the size of the LQF effect is estimated from data of the E791 experiment. A comparison is made with the estimate of the LQF effect from prompt like-sign dimuon rate in neutrino experiments. The influence of the LQF effect on the measurement of nucleon strange distribution asymmetry from charged current charm production processes is briefly discussed.Comment: 6 latex pages, 1 figure, to appear in EPJ

Sizing up the Nuclear Glue in J/ψJ/\psi-production

Nuclear gluon densities are of great importance to the physics of relativistic heavy ion collisions, in particular, in assessing the origin of $J/\psi$-suppression. We describe our attempts to distinguish various models of the gluonic EMC-effect, using the existing $J/\psi$-production data in proton-nucleus collisions. We find that no model is capable of explaining {\em all} the features of the high precision E772 data although the overall trend suggests this to be more a matter of fine-tuning the models than the presence of new physical effects.Comment: Latex, 5 pages, 5 figures, as one uuencoded tarcompressed ps file. espcrc1.sty required. added at the end of the script. TIFR/TH/93-35, BU-TH-93/

Models for Photon-photon Total Cross-sections

We present here a brief overview of recent models describing the photon-photon cross-section into hadrons. We shall show in detail results from the eikonal minijet model, with and without soft gluon summation.Comment: 7 pages, 5 figures, laTeX, requires espcrc2.sty. To appear in the Proceedings of Photon-99, Freiburg, 23 -27 May 99, labels in the figures 1,3,4,5 corrected, one typo in an equation correcte

J/\psi production through resolved photon processes at e+ e- colliders

We consider J/psi photoproduction in e+ e- as well as linear photon colliders. We find that the process is dominated by the resolved photon channel. Both the once-resolved and twice-resolved cross-sections are sensitive to (different combinations of) the colour octet matrix elements. Hence, this may be a good testing ground for colour octet contributions in NRQCD. On the other hand, the once-resolved J/psi production cross-section, particularly in a linear photon collider, is sensitive to the gluon content of the photon. Hence these cross-sections can be used to determine the parton distribution functions, especially the gluon distribution, in a photon, if the colour octet matrix elements are known.Comment: Added a figure on parametrisation dependence of photonic parton densities and some reference

Probing Supersymmetry With Third-Generation Cascade Decays

The chiral structure of supersymmetric particle couplings involving third generation Standard Model fermions depends on left-right squark and slepton mixings as well as gaugino-higgsino mixings. The shapes and intercorrelations of invariant mass distributions of a first or second generation lepton with bottoms and taus arising from adjacent branches of SUSY cascade decays are shown to be a sensitive probe of this chiral structure. All possible cascade decays that can give rise to such correlations within the MSSM are considered. For bottom-lepton correlations the distinctive structure of the invariant mass distributions distinguishes between decays originating from stop or sbottom squarks through either an intermediate chargino or neutralino. For decay through a chargino the spins of the stop and chargino are established by the form of the distribution. When the bottom charge is signed through soft muon tagging, the structure of the same-sign and opposite-sign invariant mass distributions depends on a set function of left-right and gaugino-higgsino mixings, as well as establishes the spins of all the superpartners in the sequential two-body cascade decay. Tau-lepton and tau-tau invariant mass distributions arising from MSSM cascade decays are likewise systematically considered with particular attention to their dependence on tau polarization. All possible tau-lepton and tau-tau distributions are plotted using a semi-analytic model for hadronic one-prong taus. Algorithms for fitting tau-tau and tau-lepton distributions to data are suggested.Comment: 35 pages, 17 .eps figure

The Tevatron Higgs exclusion limits and theoretical uncertainties: a critical appraisal

We examine the exclusion limits set by the CDF and D0 experiments on the Standard Model Higgs boson mass from their searches at the Tevatron in the light of large theoretical uncertainties on the signal and background cross sections. We show that when these uncertainties are consistently taken into account, the sensitivity of the experiments becomes significantly lower and the currently excluded mass range $M_H=158$-175 GeV would be entirely reopened. The necessary luminosity required to recover the current sensitivity is found to be a factor of two higher than the present one.Comment: 11 pages, 5 Figures. Version published in Physics Letter B, including an erratu