Recent measurements by the BRAHMS collaboration of high-pT hadron production at forward rapidities at RHIC found the relative production rate (d − Au)/(p − p) to be suppressed, rather than enhanced. Examining other known reactions (forward production of light hadrons, the DrellYan process, heavy flavor production, etc.), one notes that all of these display a similar property, namely, their cross sections in nuclei are suppressed at large xF . Since this is the region where x2 is minimal, it is tempting to interpret this as a manifestation of coherence, or of a color glass condensate, whereas it is actually a simple consequence of energy conservation and takes place even at low energies. We demonstrate that in all these reactions there is a common suppression mechanism that can be viewed, alternatively, as a consequence of a reduced survival probability for large rapidity gap processes in nuclei, Sudakov suppression, an enhanced resolution of higher Fock states by nuclei, or an effective energy loss that rises linearly with energy. Our calculations agree with data.

We study Drell-Yan (DY) dilepton production in proton(deuterium)-nucleus and in nucleus-nucleus collisions within the light-cone color dipole formalism. This approach is especially suitable for predicting nuclear effects in the DY cross section for heavy ion collisions, as it provides the impact parameter dependence of nuclear shadowing and transverse momentum broadening, quantities that are not available from the standard parton model. For p(D)+A collisions we calculate nuclear shadowing and investigate nuclear modification of the DY transverse momentum distribution at RHIC and LHC for kinematics corresponding to coherence length much longer than the nuclear size. Calculations are performed separately for transversely and longitudinally polarized DY photons, and predictions are presented for the dilepton angular distribution. Furthermore, we calculate nuclear broadening of the mean transverse momentum squared of DY dileptons as function of the nuclear mass number and energy. We also predict nuclear effects for the cross section of the DY process in heavy ion collisions. We found a substantial nuclear shadowing for valence quarks, stronger than for the sea.Comment: 46 pages, 18 figures, title changed and some discussion added, accepted for publication in PR

We study the relative contribution of partonic sub-processes to D meson production and D mesontriggered inclusive di-hadrons to lowest order in perturbative QCD. While gluon fusion dominates the creation of large angle DD pairs, charm on light parton scattering determines the yield of single inclusive D mesons. The distinctly different non-perturbative fragmentation of c quarks into D mesons versus the fragmentation of quarks and gluons into light hadrons results in a strong transverse momentum dependence of anticharm content of the away-side charm-triggered jet.In p+A reactions, we calculate and resum the coherent nuclear-enhanced power corrections from the final-state partonic scattering in the medium. We find that single and double inclusive open charm production can be suppressed as much as the yield of neutral pions from dynamical high-twist shadowing. Effects of energy loss in p+A collisions are also investigated phenomenologically and may lead to significantly weaker transverse momentum dependence of the nuclear attenuation.

Broadening of the transverse momentum of a parton propagating through a medium is treated using the color dipole formalism, which has the advantage of being a well developed phenomenology in deep-inelastic scattering and soft processes. Within this approach, nuclear broadening should be treated as color filtering, i.e. absorption of large-size dipoles leading to diminishing (enlarged) transverse separation (momentum). We also present a more intuitive derivation based on the classic scattering theory of Molière. This derivation helps to understand the origin of the dipole cross section, part of which comes from attenuation of the quark, while another part is due to multiple interactions of the quark. It also demonstrates that the lowest-order rescattering term provides an A-dependence very different from the generally accepted A 1/3 behavior. The effect of broadening increases with energy, and we evaluate it using different phenomenological models for the unintegrated gluon density. Although the process is dominated by soft interactions, the phenomenology we use is tested using hadronic cross section data.

We show that the (j, 0) ⊕ (0, j) representation space associated with massive particles is a concrete realisation of a quantum field theory, envisaged many years ago by Bargmann, Wightman and Wigner, in which bosons and antibosons have opposite relative intrinsic parities. Demonstration of the result requires a careful ab initio study of the (j, 0) ⊕ (0, j) representation space for massive particles, introducing a wave equation with well defined transformation properties under C, P and T, and addressing the issue of nonlocality required of such a theory by the work of Lee and Wick.

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We report an analysis of the nuclear dependence of the yield of Drell-Yan dimuons from the 800 GeV/c proton bombardment of 2 H, C, Ca, Fe, and W targets. Employing a new formulation of the Drell-Yan process in the rest frame of the nucleus, this analysis examines the effect of initial-state energy loss and shadowing on the nuclear-dependence ratios versus the incident proton's momentum fraction and dimuon effective mass. The resulting energy loss per unit path length is −dE/dz = 2.32 ± 0.52 ± 0.5 GeV/fm. This is the first observation of a nonzero energy loss of partons traveling in nuclear environment.24.85.+p; 13.85.Qk; 25.40.Ve For many years it has been suggested that quark energy loss might give rise to a nuclear dependence [1][2][3][4] of the cross section of Drell-Yan [5] (DY) production. When a proton enters a nucleus the first (soft) inelastic collision liberates a quark, which then loses energy via hadronization (due to confinement) and interaction in the nuclear medium. A lepton pair created from a subsequent interaction then has reduced energy compared with the DY process on a free nucleon. The goal of the present analysis is to search for this effect in the nuclear dependence of the DY process.Fermilab E772 made a precise measurement of the nuclear dependence of the DY process using 800 GeV/c protons. The experimental details of E772 have been described previously [6][7][8]. Briefly we indicate those germane to the present discussion. Muon pairs were recorded from targets of 2 H, C, Ca, Fe, and W, in the mass range M ≥ 4 GeV/c 2 . Excluding the Υ resonance region, 9 ≤ M ≤ 11 GeV/c 2 , we reconstruct 2.5 × 10 5

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