Quenching of hadron spectra in DIS on nuclear targets

The multiple scatterings incurred by a hard quark produced in a nuclear medium induce the emission of soft gluons which carry a fraction of the quark energy and eventually affect the hadronization process. Here, the depletion of semi-inclusive hadron spectra in DIS on various nuclei (N, Ne, Cu, Kr) is computed as a function of nu and z to leading order in alpha_s through medium-modified fragmentation functions. Using the transport coefficient q previously determined from Drell-Yan production, the predictions are found to be in good agreement with EMC and HERMES preliminary data. Calculations on Xe targets are also presented and discussed.Comment: 20 pages, 8 figure

Top quark total and differential cross sections at NNLO and NNLL

I present recent NNLO-NNLL results for top quark hadroproduction at the Tevatron. The total cross section as well as transverse momentum and rapidity distributions are shown.Comment: 4 pages LaTeX, 2 figures; presented at DPF2000, to appear in the Proceeding

Fully differential QCD corrections to single top quark final states

A new next-to-leading order Monte Carlo program for calculation of fully differential single top quark final states is described and first results presented. Both the s- and t-channel contributions are included.Comment: 3 pages, 3 figures, talk presented at DPF2000, August 9-12, 2000. To appear in International Journal of Modern Physics

Uncertainties on parton distribution functions from the ZEUS NLO QCD fit to data on deep inelastic scattering

An NLO QCD analysis of the ZEUS data on $e^+ p$ deep inelastic scattering together with fixed-target data has been performed from which the gluon and quark densities of the proton and the value of the strong coupling parameter, $\alpha_s(M_Z^2)$, have been extracted. The study includes a full treatment of the experimental systematic uncertainties, including point-to-point correlations. Different ways of incorporating correlated systematic uncertainties into the fit are discussed and compared.Comment: 8 pages, 1 figure. Invited talk at the Conference on Advanced Statistical Techniques in Particle Physics, March 18-22 2002, Durham, U

Energy Loss Effect in High Energy Nuclear Drell-Yan Process

The energy loss effect in nuclear matter, which is another nuclear effect apart from the nuclear effect on the parton distribution as in deep inelastic scattering process, can be measured best by the nuclear dependence of the high energy nuclear Drell-Yan process. By means of the nuclear parton distribution studied only with lepton deep inelastic scattering experimental data, measured Drell-Yan production cross sections for 800GeV proton incident on a variety of nuclear targets are analyzed within Glauber framework which takes into account energy loss of the beam proton. It is shown that the theoretical results with considering the energy loss effect are in good agreement with the FNAL E866

Small-x QCD Effects in Particle Collisions at High Energies

Recent theoretical developments to calculate cross sections of hadronic objects in the high energy limit are summarised and experimental attempts to establish the need for new QCD effects connected with a resummation of small hadron momentum fractions x are reviewed. The relation between small-$x$ parton dynamics and the phenomenon of diffraction is briefly out-lined. In addition, a search for a novel, non-perturbative QCD effect, the production of QCD instanton induced events, is presented.Comment: Invited talk at the XX. Int. Sym. on Lepton and Photon Interactions at High Energies, Rome, Italy, July 200

Production of the neutral top-pion πt0\pi_{t}^{0} in association with a high-pTp_{T} jet at the LHCLHC

In the framework of the topcolor-assisted technicolor $(TC2)$ model, we study production of the neutral top-pion $\pi_{t}^{0}$ in association with a high-$p_{T}$ jet at the $LHC$, which proceeds via the partonic processes $gg\longrightarrow \pi_{t}^{0}g$, $gq\longrightarrow \pi_{t}^{0}q$, $q\bar{q}\longrightarrow \pi_{t}^{0}g$, $gb(\bar{b})\longrightarrow \pi_{t}^{0}b(\bar{b})$, and $b\bar{b}\longrightarrow \pi_{t}^{0}g$. We find that it is very challenging to detect the neutral top-pion $\pi_{t}^{0}$ via the process $pp\longrightarrow \pi_{t}^{0}+jet+X\to t\bar{t}+jet+X$, while the possible signatures of $\pi_{t}^{0}$ might be detected via the process $pp\longrightarrow \pi_{t}^{0}+jet+X\to(\bar{t}c+t\bar{c})+jet+X$ at the $LHC$.Comment: 13 pages, 4 figures; typos correcte

Signals from R-parity violating top quark decays at LHC

We evaluate the potential of the CERN LHC collider to observe rare decays of the top quark in channels involving R-parity violating (RPV) interactions. We stress the importance of calculating top quark production and decay simultaneously as a true 2->4 process. The process of tt-bar pair production followed by RPV decay of one of the top quarks is analyzed with fast detector simulation. We show that intermediate supersymmetric particles can be observed as resonances even if they are heavier than the top quark due to the significant off-shell top-quark mass effects. The approach where the top quark is produced on-mass-shell and then decays into 2- or 3-body final state would in general lead to incorrect kinematical distributions and rates. The rates of the 2 -> 4 process with top quark production and RPV 3-body decay depend on the total width of the heavy intermediate sfermion which could,therefore, be measured indirectly. We find that the LHC collider offers a unique potential to study rare top quark decays in the framework of supersymmetry with broken R-parity for branching fractions of RPV top decays as low as 10^{-6}Comment: 23 pages, 22 figure

The charged-hadron/pion ratio at the Relativistic Heavy Ion Collider

The hadron/pion ratio is calculated in 200 GeV AuAu collisions at midrapidity, applying pQCD and non-universal transverse-momentum broadening. Arguments are presented for such non-universality, and the idea is implemented in a model, which explains the enhancement of the hadron/pion ratio in central AuAu collisions. The model also describes the qualitative difference between the recently-measured dAu nuclear enhancement factors for pions and charged hadrons.Comment: 4 pages, 3 figure

Charm-sea Contribution to High-p_T \psi Production at the Fermilab Tevatron

The direct production of $J/\psi(\psi')$ at large transverse momentum, $p_T \gg M_{J/\psi}$, at the Fermilab Tevatron is revisited. It is found that the sea-quark initiated processes dominate in the high-$p_T$ region within the framework of color-singlet model, which is not widely realized. We think this finding is enlightening for further investigation on the charmonium production mechanism.Comment: Conclusions not changed, to appear in J. of Phys.