A. Bianconi

Monte Carlo simulation of events with Drell-Yan lepton pairs from antiproton-proton collisions

The complete knowledge of the nucleon spin structure at leading twist requires also addressing the transverse spin distribution of quarks, or transversity, which is yet unexplored because of its chiral-odd nature. Transversity can be best extracted from single-spin asymmetries in fully polarized Drell-Yan processes with antiprotons, where valence contributions are involved anyway. Alternatively, in single-polarized Drell-Yan the transversity happens convoluted with another chiral-odd function, which is likely to be responsible for the well known (and yet unexplained) violation of the Lam-Tung sum rule in the corresponding unpolarized cross section. We present Monte Carlo simulations for the unpolarized and single-polarized Drell-Yan pp{sup ({up_arrow}}{sup )}{yields}{mu}{sup +}{mu}{sup -}X at different center-of-mass energies in both configurations where the antiproton beam hits a fixed proton target or it collides on another proton beam. The goal is to estimate the minimum number of events needed to extract the above chiral-odd distributions from future measurements at the high energy storage ring (HESR) at GSI. It is important to study the feasibility of such experiments at HESR in order to demonstrate that interesting spin physics can be explored already using unpolarized antiprotons.

Monte Carlo simulation of events with Drell-Yan lepton pairs from antiproton-proton collisions: The fully polarized case

In this paper, we extend the study of Drell-Yan processes with antiproton beams already presented in a previous work. We consider the fully polarized p{sup {up_arrow}}p{sup {up_arrow}}{yields}{mu}{sup +}{mu}{sup -}X process, because this is the simplest scenario for extracting the transverse spin distribution of quarks, or transversity, which is the missing piece to complete the knowledge of the nucleon spin structure at leading twist. We perform Monte Carlo simulations for transversely polarized antiproton and proton beams colliding at a center-of-mass energy of interest for the future HESR at GSI. The goal is to possibly establish feasibility conditions for an unambiguous extraction of the transversity from data on double spin asymmetries.

A scheme for fast exploratory simulation of azimuthal asymmetries in Drell–Yan experiments at intermediate energies: The DY_AB Monte Carlo event generator

Abstract In this note I report and discuss the physical scheme and the main approximations used by the event generator code DY_AB. This Monte Carlo code is aimed at preliminary simulation, during the stage of apparatus planning, of Drell–Yan events characterized by azimuthal asymmetries, in experiments with moderate center of mass energy s ⪡ 100 GeV .

Monte Carlo simulation of single spin asymmetries in pion-proton collisions

We present Monte Carlo simulations of both the Sivers and the Boer-Mulders effects in the polarized Drell-Yan {pi}{sup {+-}}p{sup {up_arrow}}{yields}{mu}{sup +}{mu}{sup -}X process at a kinematics suitable to the COMPASS setup. For the Sivers effect, we adopt two different parametrizations for the Sivers function to explore the statistical accuracy required to extract unambiguous information on this parton density. In particular, we verify the possibility of checking its predicted sign change between semi-inclusive deep-inelastic scattering (SIDIS) and Drell-Yan processes, a crucial test of nonperturbative QCD. For the Boer-Mulders effect, because of the lack of parametrizations we can make only guesses. The goal is to explore the possibility of extracting information on the transversity distribution, the missing piece necessary to complete the knowledge of the nucleon spin structure at leading twist, and the Boer-Mulders function, which is related to the long-standing problem of the violation of the Lam-Tung sum rule in the unpolarized Drell-Yan cross section.

Monte Carlo simulation of the Sivers effect in high-energy proton-proton collisions

We present Monte Carlo simulations of the Sivers effect in the polarized Drell-Yan pp{sup {up_arrow}}{yields}{mu}{sup +}{mu}{sup -}X process at the center-of-mass energy {radical}(s)=200 GeV reachable at the Relativistic Heavy-Ion Collider (RHIC) of BNL. We use two different parametrizations for the Sivers function, one deduced from the analysis of semi-inclusive deep-inelastic scattering (SIDIS) data at much lower energies, and another one constrained by the RHIC data for the pp{sup {up_arrow}}{yields}{pi}X process at the same energy. For a given luminosity of 10{sup 32} cm{sup -2} s{sup -1}, we explore the necessary conditions to reach a statistical accuracy that allows one to extract unambiguous information on the structure of the Sivers function. In particular, we consider the feasibility of the test on its predicted universality property of changing sign when switching from SIDIS to Drell-Yan processes.

Hadronic dynamics of colour transparency

Abstract We elaborate a theoretical approach to the dynamics of nuclear transparency in hard nuclear reactions, based on the hadronic eigenstate expansion of the correlator of the hard scattering operator. It appears that the kinematical treatment of the effects related to Fermi motion is of great importance in exclusive measurements; this is relevant for the interpretation of the BNL experiment on hard ( p ,2 p ) reaction. Colour transparency manifests itself even at moderate momentum transfers if one chooses the missing momentum of the struck nucleon by appropriate kinematics. In general, the role of the Fermi motion is found to be crucial; without it, any effect of anomalous nuclear transparency would be impossible.

Periodic Interference Structures in the Timelike Proton Form Factor.

An intriguing and elusive feature of the timelike hadron form factor is the possible presence of an imaginary part associated to rescattering processes. We find evidence of that in the recent and precise data on the proton timelike form factor measured by the BABAR Collaboration. By plotting these data as a function of the 3-momentum of the relative motion of the final proton and antiproton, a systematic sinusoidal modulation is highlighted in the near-threshold region. Our analysis attributes this pattern to rescattering processes at a relative distance of 0.7-1.5 fm between the centers of the forming hadrons. This distance implies a large fraction of inelastic processes in pp interactions, and a large imaginary part in the related e(+)e(-)→pp reaction because of unitarity.

Enhancement of annihilation cross sections by electric interactions between the antineutron and the field of a large nucleus

Data relative to antineutron and antiproton annihilation on large nuclei in the range 75-200MeV/c present two unexpected features: a) antineutron and antiproton cross sections have a similar size, b) the rise of the antineutron cross section at decreasing energy is much steeper than predictable for an inelastic process of purely strong nature at that energy. The observed behavior of

Extraction of proton form factors in the timelike region from single-polarized e + e − → p → p ¯ events

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