Threshold Resummation for W-Boson Production at RHIC

We study the resummation of large logarithmic perturbative corrections to the partonic cross sections relevant for the process pp -> W^+- X at the BNL Relativistic Heavy Ion Collider (RHIC). At RHIC, polarized protons are available, and spin asymmetries for this process will be used for precise measurements of the up and down quark and anti-quark distributions in the proton. The corrections arise near the threshold for the partonic reaction and are associated with soft-gluon emission. We perform the resummation to next-to-leading logarithmic accuracy, for the rapidity-differential cross section. We find that resummation leads to relatively moderate effects on the cross sections and spin asymmetries.Comment: 25 pages, 15 figures as eps files. One reference added and typo correcte

Heavy Quark Initiated Contributions to Deep Inelastic Structure Functions

We present O(alpha_s^1) corrections to deep inelastic scattering amplitudes on massive quarks obtained within the scheme of Aivazis, Collins, Olness and Tung (ACOT). After identifying the correct subtraction term the convergence of these contributions towards the analogous coefficient functions for massless quarks, obtained within the modified minimal subtraction scheme (MSbar), is demonstrated. Furthermore, the quantitative relevance of the contributions to neutral current (NC) and charged current (CC) structure functions is investigated for several choices of the factorization scale.Comment: 29 pages, 6 figures; uses epsfig.sty, amssymb.sty, axodraw.sty; minor changes for publication in Phys. Rev.

Has the QCD RG-Improved Parton Content of Virtual Photons been Observed?

It is demonstrated that present $e^+e^-$ and DIS ep data on the structure of the virtual photon can be understood entirely in terms of the standard `naive' quark--parton model box approach. Thus the QCD renormalization group (RG) improved parton distributions of virtual photons, in particular their gluonic component, have not yet been observed. The appropriate kinematical regions for their future observation are pointed out as well as suitable measurements which may demonstrate their relevance.Comment: 24 pages, LaTeX, 5 figure

The curvature of F2p(x,Q2)F_2^p(x,Q^2) as a probe of the range of validity of perturbative QCD evolutions in the small-xx region

Perturbative NLO and NNLO QCD evolutions of parton distributions are studied, in particular in the (very) small-$x$ region, where they are in very good agreement with all recent precision measurements of $F_2^p(x,Q^2)$. These predictions turn out to be also rather insensitive to the specific choice of the factorization scheme ($\bar{\rm MS}$ or DIS). A characteristic feature of perturbative QCD evolutions is a {\em{positive}} curvature of $F_2^p$ which increases as $x$ decreases. This perturbatively stable prediction provides a sensitive test of the range of validity of perturbative QCD.Comment: 17 pages, 6 figures, 2 tables; minor corrections, to appear in EPJ

Dynamical parton distributions of the nucleon and very small-x physics

Utilizing recent DIS measurements (F_{2,L}) and data on dilepton and high-E_{T} jet production we determine the dynamical parton distributions of the nucleon generated radiatively from valence-like positive input distributions at optimally chosen low resolution scales. These are compared with `standard' distributions generated from positive input distributions at some fixed and higher resolution scale. It is shown that up to the next to leading order NLO(\bar{MS}, DIS) of perturbative QCD considered in this paper, the uncertainties of the dynamical distributions are, as expected, smaller than those of their standard counterparts. This holds true in particular in the presently unexplored extremely small-x region relevant for evaluating ultrahigh energy cross sections in astrophysical applications. It is noted that our new dynamical distributions are compatible, within the presently determined uncertainties, with previously determined dynamical parton distributions.Comment: 21 pages, 2 tables, 16 figures, v2: added Ref.[60], replaced Fig.

Canonical active Brownian motion

Active Brownian motion is the complex motion of active Brownian particles. They are active in the sense that they can transform their internal energy into energy of motion and thus create complex motion patterns. Theories of active Brownian motion so far imposed couplings between the internal energy and the kinetic energy of the system. We investigate how this idea can be naturally taken further to include also couplings to the potential energy, which finally leads to a general theory of canonical dissipative systems. Explicit analytical and numerical studies are done for the motion of one particle in harmonic external potentials. Apart from stationary solutions, we study non-equilibrium dynamics and show the existence of various bifurcation phenomena.Comment: 11 pages, 6 figures, a few remarks and references adde

Nonequilibrium Green's function theory for transport and gain properties of quantum cascade structures

The transport and gain properties of quantum cascade (QC) structures are investigated using a nonequilibrium Green's function (NGF) theory which includes quantum effects beyond a Boltzmann transport description. In the NGF theory, we include interface roughness, impurity, and electron-phonon scattering processes within a self-consistent Born approximation, and electron-electron scattering in a mean-field approximation. With this theory we obtain a description of the nonequilibrium stationary state of QC structures under an applied bias, and hence we determine transport properties, such as the current-voltage characteristic of these structures. We define two contributions to the current, one contribution driven by the scattering-free part of the Hamiltonian, and the other driven by the scattering Hamiltonian. We find that the dominant part of the current in these structures, in contrast to simple superlattice structures, is governed mainly by the scattering Hamiltonian. In addition, by considering the linear response of the stationary state of the structure to an applied optical field, we determine the linear susceptibility, and hence the gain or absorption spectra of the structure. A comparison of the spectra obtained from the more rigorous NGF theory with simpler models shows that the spectra tend to be offset to higher values in the simpler theories.Comment: 44 pages, 16 figures, appearing in Physical Review B Dec 200

Complete Next to Leading Order QCD Corrections to the Photon Structure Functions F2γ(x,Q2)F^\gamma_2(x,Q^2) and FLγ(x,Q2)F_L^\gamma(x,Q^2)

We present the complete NLO QCD analysis of the photon structure functions $F_2^\gamma(x,Q^2)$ and $F_L^\gamma(x,Q^2)$ for a real photon target. In particular we study the heavy flavor content of the structure functions which is due to two different production mechanisms, namely collisions of a virtual photon with a real photon, and with a parton. We observe that the charm contributions are noticeable for $F_2^\gamma(x,Q^2)$ as well as $F_L^\gamma(x,Q^2)$ in the x-region studied.Comment: Latex 34 pages, 24 figures, uuencoded, attached at end, ITP-SB-93-46, FERMILAB-Pub-93/240-T, SMU HEP 93-1

Light Gluinos and the Parton Structure of the Nucleon

We study the effects of light gluinos with mass below about 1 GeV on the nucleon parton densities and the running of alpha_(S). It is shown that from the available high-statistics DIS data no lower bound on the gluino mass can be derived. Also in the new kinematical region accessible at HERA the influence of such light gluinos on structure f unctions is found to be very small and difficult to detect. For use in more direct searches involving final state signatures we present a radiative estimate of the gluino distribution in the nucleon.Comment: 23 pages, LateX, 8 figures, MPI-PhT/94-22, LMU-3/9

A Matrix Approach to Numerical Solution of the DGLAP Evolution Equations

A matrix-based approach to numerical integration of the DGLAP evolution equations is presented. The method arises naturally on discretisation of the Bjorken x variable, a necessary procedure for numerical integration. Owing to peculiar properties of the matrices involved, the resulting equations take on a particularly simple form and may be solved in closed analytical form in the variable t=ln(alpha_0/alpha). Such an approach affords parametrisation via data x bins, rather than fixed functional forms. Thus, with the aid of the full correlation matrix, appraisal of the behaviour in different x regions is rendered more transparent and free of pollution from unphysical cross-correlations inherent to functional parametrisations. Computationally, the entire programme results in greater speed and stability; the matrix representation developed is extremely compact. Moreover, since the parameter dependence is linear, fitting is very stable and may be performed analytically in a single pass over the data values.Comment: 13 pages, no figures, typeset with revtex4 and uses packages: acromake, amssym