Duljina koherencije i nuklearno zasjenjivanje za poprečne i uzdužne fotone

We study nuclear shadowing for transverse and longitudinal photons. The coherence length, which controls the onset of nuclear shadowing at small Bjorken-x, xBj, is longer for longitudinal than for transverse photons. The light-cone Green function technique properly treats the finite coherence length in all multiple scattering terms. This is especially important in the region xBj > 0.01, where most of the data exist. NMC data on shadowing in deep inelastic scattering are well reproduced in this approach. We also incorporate nonperturbative effects, in order to extrapolate this approach to small photon virtualities Q2 , where perturbative QCD cannot be applied. This way, we achieve a description of shadowing that is based only on quark and gluon degrees of freedom, even at low Q2 .Proučavamo nuklearno zasjenjivanje za poprečne i uzdužne fotone. Duljina koherencije, koja određuje nastanak nuklearnog zasjenjivanja za male Bjorkenove x, xBj, je dulja za uzdužne nego za poprečne fotone. Metodom Greenovih funkcija na svjetlosnom konusu ispravno se opisuje konačna duljina koherencije za sve članove višestrukog raspršenja. To je posebno važno u području xBj > 0.01 u kojem postoji najviše podataka. Podaci NMC o zasjenjivanju u duboko neelastičnom raspršenju dobro se opisuju ovim pristupom. Uključujemo također neperturbativne efekte kako bi ovaj pristup ekstrapolirali do malih virtualnosti Q2 , za koje se ne može primijeniti perturbativni QCD. Tako postižemo opis zasjenjivanja zasnovan samo na stupnjevima slobode kvarkova i gluona, čak i za male Q2

Relating different approaches to nuclear broadening

Transverse momentum broadening of fast partons propagating through a large nucleus is proportional to the average color field strength in the nucleus. In this work, the corresponding coefficient is determined in three different frameworks, namely in the color dipole approach, in the approach of Baier et al. and in the higher twist factorization formalism. This result enables one to use a parametrization of the dipole cross section to estimate the values of the gluon transport coefficient and of the higher twist matrix element, which is relevant for nuclear broadening. A considerable energy dependence of these quantities is found. In addition, numerical calculations are compared to data for nuclear broadening of Drell-Yan dileptons, J/psi and Upsilon mesons. The scale dependence of the strong coupling constant leads to measurable differences between the higher twist approach and the other two formalisms.Comment: 11 pages, 3 figures; v2: some changes in presentation, reference added, accepted for publication in PL

Statistical Physics and Light-Front Quantization

Light-front quantization has important advantages for describing relativistic statistical systems, particularly systems for which boost invariance is essential, such as the fireball created in a heavy ion collisions. In this paper we develop light-front field theory at finite temperature and density with special attention to quantum chromodynamics. We construct the most general form of the statistical operator allowed by the Poincare algebra and show that there are no zero-mode related problems when describing phase transitions. We then demonstrate a direct connection between densities in light-front thermal field theory and the parton distributions measured in hard scattering experiments. Our approach thus generalizes the concept of a parton distribution to finite temperature. In light-front quantization, the gauge-invariant Green's functions of a quark in a medium can be defined in terms of just 2-component spinors and have a much simpler spinor structure than the equal-time fermion propagator. From the Green's function, we introduce the new concept of a light-front density matrix, whose matrix elements are related to forward and to off-diagonal parton distributions. Furthermore, we explain how thermodynamic quantities can be calculated in discretized light-cone quantization, which is applicable at high chemical potential and is not plagued by the fermion-doubling problem.Comment: 30 pages, 3 figures; v2: Refs. added, minor changes, accepted for publication in PR

Relating parton model and color dipole formulation of heavy quark hadroproduction

At high center of mass energies, hadroproduction of heavy quarks can be expressed in terms of the same color dipole cross section as low Bjorken-x deep inelastic scattering. We show analytically that at leading order, the dipole formulation is equivalent to the gluon-gluon fusion mechanism of the conventional parton model. In phenomenological application, we employ a parameterization of the dipole cross section which also includes higher order and saturation effects, thereby going beyond the parton model. Numerical calculations in the dipole approach agree well with experimental data on open charm production over a wide range of energy. Dipole approach and next to leading order parton model yield similar values for open charm production, but for open bottom production, the dipole approach tends to predict somewhat higher cross sections than the parton model.Comment: 16 pages, 4 figure

Parton model versus color dipole formulation of the Drell-Yan process

In the kinematical region where the center of mass energy is much larger than all other scales, the Drell-Yan process can be formulated in the target rest frame in terms of the same color dipole cross section as low Bjorken-x deep inelastic scattering. Since the mechanisms for heavy dilepton production appear very different in the dipole approach and in the conventional parton model, one may wonder whether these two formulations really represent the same physics. We perform a comparison of numerical calculations in the color dipole approach with calculations in the next-to-leading order parton model. For proton-proton scattering, the results are very similar at low x_2 from fixed target to RHIC energies, confirming the close connection between these two very different approaches. We also compare the transverse momentum distributions of Drell-Yan dileptons predicted in both formulations. The range of applicability of the dipole formulation and the impact of future Drell-Yan data from RHIC for determining the color dipole cross section are discussed. A detailed derivation of the dipole formulation of the Drell-Yan process is also included.Comment: 20 pages, 5 figure