Search

The interaction of a high-current $O$(100~\textmu A), medium energy $O$(10\,GeV) electron beam with a thick target $O$(1m) produces an overwhelming shower of standard matter particles in addition to hypothetical Light Dark Matter particles. While most of the radiation (gamma, electron/positron, and neutron) is contained in the thick target, deep penetrating particles (muons, neutrinos, and light dark matter particles) propagate over a long distance, producing high-intense secondary beams. Using sophisticated Monte Carlo simulations based on FLUKA and GEANT4, we explored the characteristics of secondary muons and neutrinos and (hypothetical) dark scalar particles produced by the interaction of Jefferson Lab 11 GeV intense electron beam with the experimental Hall-A beam dump. Considering the possible beam energy upgrade, this study was repeated for a 20 GeV CEBAF beam.

Data on hyperon form factors recently obtained from annihilation reactions are compared each other, and to proton and neutron form factors, in terms of two kinematical variables: the transferred momentum square, $q^2$, and the modulus $P$ of the relative momentum between the outgoing baryons. They are critically discussed in terms of possible correlated structures. The present status of the time-like form factor data for baryons is described and suggestions are given on the reactions and the kinematical range where data are desirable in order to clarify the arisen questions., Comment: 20 pages, 10 figures

The data on the proton form factors in the time-like region from the BaBar, BESIII and CMD-3 Collaborations are examined to have coherent pieces of information on the proton structure. Oscillations in the annihilation cross section, previously observed, are determined with better precision. The moduli of the individual form factors, determined for the first time, their ratio and the angular asymmetry of the annihilation reaction $e^+e^-\to\bar p p$ are discussed. Fiits of the available data on the cross section, the effective form factor, and the form factor ratio, allow to propose a description of the electric and magnetic time-like form factors from the threshold up to the highest momenta., Comment: 20 pages, 7 figures

The annihilation of a lepton pair into a proton-antiproton pair, and the reverse process, allow for a measurement of the timelike proton form factors. This work aims at studying the corrections due to direct soft interactions between the proton and the antiproton before their annihilation or after their creation. The analysis is carried on in the spacetime formalism used by us in a series of recent works on timelike FFs. In particular the effect of $\bar{p}p$ annihilation into many-meson states is considered. We expect it to determine similar real and imaginary parts for the form factor near the $\bar{p}p$ channel threshold. We also discuss the possibility that the timelike FF differ by a phase in the hadron pair production and annihilation., Comment: 19 pages, 10 figures

As well known, spacelike proton form factors expressed in the Breit frame may be interpreted as the Fourier transform of static space distributions of electric charge and current. In particular, the electric form factor is simply the Fourier transform of the charge distribution $F(q)=\int e^{i\vec q \cdot \vec r} \rho(r)d^3r$. We don't have an intuitive interpretation of the same level of simplicity for the proton timelike form factor appearing in the reactions $e^+e^-\leftrightarrow \bar{p}p$. However, one may suggest that in the center of mass (CM) frame, where $q_\mu x^\mu =qt$, a timelike electric form factor is the Fourier transform $F(q) =\int e^{iqt} R(t)dt$ of a function $R(t)$ expressing how the electric properties of the forming (or annihilating) proton-antiproton pair evolve in time. Here we analyze in depth this idea, show that the functions $\rho(r)$ and $R(t)$ can be formally written as the time and space integrals of a unique correlation function depending on both time and space coordinates., Comment: 8 figures, 15 pages

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 $\bar{p}p$ interactions, and a large imaginary part in the related $e^+e^- \rightarrow \bar{p}p$ reaction because of unitarity., Comment: 5 pages 3 figures - Discussion modified. To appear in Phys Rev Letters

Background: The epidemiology of the slightly radioactive contrast agent named Thorotrast presents a very long latency period between the injection and the development of the related pathologies. It is an example of the more general problem posed by a radioactive internal contaminant whose effects are not noteworthy in the short term but become dramatic in the long period. A point that is still to be explored is fluctuations (in space and time) in the localized absorption of radiation by the tissues. Methods: A Monte Carlo simulation code has been developed to study over a 30 year period the daily absorption of alpha radiation by micrometer sized portions of tissue placed at a distance of 0-100 micrometers from a model source, that approximates a compact thorium dioxide source in liver or spleen whose size is larger or equal to 20 micrometers. The biological depletion of the daughter nuclei of the thorium series is taken into account. The initial condition assumes chemically purified natural thorium. Results: Most of the absorbed dose is concentrated in a 25 micrometer thick layer of tissue, adjacent to the source boundary. Fluctuations where a target region with a volume of 1 cube micrometer is hit by 3-5 alpha particles in a day or in a shorter period of time are relevant in a 1-10 micrometer thick layer of tissue adjacent to the source boundary, where their frequency is larger than the Poisson law prediction., Comment: In press on Physica Medica, available online at the journal site since february 21th, 2014

The transverse polarization of a quark is a degree of freedom that is not taken into account in the most commonly used Monte Carlo generators. For the case "e^+e^- -> hadrons" I show that it is possible to use these generators to simulate processes where the parent quark and antiquark are transversely polarized and the fragmentation process is affected by this polarization. The key point is that it is possible to obtain this without touching the generator code at all. One only works on the parton-level and hadron-level outputs that the Monte Carlo code has produced, modifying them in a correlated way. A group of techniques is presented to obtain this, matching the most obvious needs of a user (in particular: reproducing a pre-assigned final distribution). As an example these methods are applied to modify Pythia-generated events to obtain a nonzero Collins function and a consequent cos(Phi1+Phi2)-asymmetry of pion pairs., Comment: 22 pages, 9 figures

T-odd distributions like Sivers and Boer-Mulders functions are normally modeled using few-body models. In the present work I want to follow the completely opposite point of view, and study a high-energy proton-proton Drell-Yan where both the annihilating partons are wee, and one (at least) comes from a high-order Fok state of the parent proton, i.e. a state where a large number N of partons is present. I show that rescattering between active and spectator partons is modified, with suppression of those terms that are needed to build T-odd parton distributions., Comment: 12 pages, 2 figures

I study hard collisions between unpolarized protons and antiprotons where a lepton-antilepton pair is detected in coincidence with a final proton-antiproton pair, and no more particles are produced, in the regime 10 GeV^2 << s << 1000 GeV^2, M > 4 GeV, q_T < 3 GeV/c. The present work is centered on azimuthal asymmetries. Because of momentum conservation, a Boer-Mulders term in the momentum distribution of a quark implies a balancing effect in the momentum distribution of some spectators. This produces azimuthal asymmetries of the final hadrons. To analyze this, I have organized a parton-level MonteCarlo generator where a standard cos(2phi)-asymmetry of the dilepton distribution is produced, thanks to a soft rescattering process between an active quark coming from a hadron and a spectator anti-diquark coming from the other hadron. This produces cos(2phi)-asymmetries of the final hadron pair. Hadron and lepton asymmetries have the same size., Comment: final version

Here a preliminary study is presented concerning the detection of the normally unseen Drell-Yan fragments, possible in the PANDA experiment. To work as a multi-purpose apparatus, this experiment will record all the particles produced in the collisions between the antiproton beam and the target, with a rather wide acceptance. So detecting Drell-Yan dileptons with or without analyzing the other fragments is just a matter of applying cutoffs in the data analysis stage. The distribution of the products of 50,000 typical Drell-Yan events is here simulateded using a well-known generator code (Pythia-8). The resulting distributions are inserted within the PANDA acceptance region, to analyze the chances of missing some searched fragment combinations, or of confusing different sets of particles. The most interesting result is that, due to the reduced phase space, the produced states are much simpler than one could imagine: (i) almost 50 % of the events just consist of a dilepton plus a nucleon-antinucleon pair; (ii) practically all events present a nucleon-antinucleon pair; (iii) the number of light particles (photons over an infrared cutoff and pions) is pretty small. The presented simulations show that it is possible to study experimentally some, or some aspects, of the most relevant final states, with good statistics and precision., Comment: 16 pages, 5 figures

I study the effect of scalar and spin-orbit rescattering terms, in the production of a nonzero Sivers-like asymmetry in proton-proton collisions (nclusive production and Drell-Yan) at moderate center of mass energies $\sqrt{s}$ $<$ 15 GeV and transverse momentum up to 3 GeV/c. An ultrarelativistic generalization of the Glauber formalism is here used to (i) fit the scalar and spin-orbit interaction terms on proton-proton elastic scattering data, including analyzing power, (ii) transfer such information to inclusive proton-proton scattering. It is shown that the phenomenological interactions responsible for the nonzero analyzing power in proton-proton elastic scattering produce a relevant nonzero analyzing power in inclusive processes associated with proton-proton collisions. This could represent a relevant (possibly higher-twist) contribution to the Sivers asymmetry., Comment: 35 pages, 11 figures

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 $\sqrt{s}$ $<<$ 100 GeV., Comment: 26 pages, 10 figures. In print on Nuncl. Instr. and Methods in Phys. Res. A

In order to illustrate the capabilities of COMPASS using a hadronic beam, I review some of the azimuthal asymmetries in hadronic collisions, that allow for the extraction of transversity, Sivers and Boer-Mulders functions, necessary to explore the partonic spin structure of the nucleon. I also report on some Monte Carlo simulations of such asymmetries for the production of Drell-Yan lepton pairs from the collision of high-energy pions on a transversely polarized proton target., Comment: talk delivered to the "International Workshop on Structure and Spectroscopy", Freiburg, March 19-21, 2007; 18 pages, RevTeX4 style, 8 figures with 10 .eps files

Recent works have discussed the violation of factorization and universality in hadronic hard scattering processes aimed at measurements of T-odd distributions. We use simple arguments to show that it is possible to restore an approximate factorization involving T-odd contributions if the factorization breaking interactions present a frequency spectrum dominated by by a narrow and regular peak whose maximum value corresponds to a respected factorization., Comment: 5 pages, no figures

I study the effect of scalar and spin-orbit absorption potentials, in the production of a nonzero Sivers-like asymmetry in hadron-hadron high energy collisions (Drell-Yan and single spin asymmetries). A basic model is built for the intrinsic state of a quark in the projectile hadron. S-wave and P-wave 2-component states are considered. Before the hard event, this quark is subject to absorbing scalar and spin-orbit local mean fields simulating interactions with a composite target. The relevant interaction terms are found to be the imaginary diagonal spin-orbit ones. Spin rotating terms, and scalar absorption, are not decisive. For $x$ $=$ 0 the found Sivers asymmetry vanishes, while at larger $x$ its qualitative dependence on $x$, $K_T$ follows the usual trends met in available models and parameterizations. Given the present-day knowledge of the considered phenomenological interactions, it is not possible to establish whether the related Sivers-like asymmetry is a leading twist one., Comment: 11 printed pages, 6 figures

T-odd parton distribution functions in a Drell-Yan process are here studied by examining the evolution of the internal statistical properties of the interacting hadrons. T-odd functions are shown to be a signature of the irreversible process in which a hadronic state characterized by long range correlation properties (hadronic phase) decays to produce a cloud of independent partons (partonic phase) because of initial/final state interactions. The relevant considered variable is the rate of increase of the entropy of the hadronic system. This quantity is shown to be roughly equal to the decay rate of the hadronic state. Conditions for getting a leading twist T-odd effect are established on this basis. Last, the relevant case of a large entropy increase associated with transverse-dominated initial/final state interactions is analyzed., Comment: 18 printed pages, no figures. Revised version, with added subsections to sections 4 and 5, and added references to the introduction. In print on Phys.Rev.D

We perform numerical simulations of the Sivers effect from single spin asymmetries in Drell-Yan processes on transversely polarized protons. We consider colliding antiprotons and pions at different kinematic conditions of interest for the future planned experiments. We conventionally name "framework I" the results obtained when properly accounting for the various flavor dependent polarized valence contributions in the numerator of the asymmetry, and for the unpolarized nonvalence contribution in its denominator. We name "framework II" the results obtained when taking a suitable flavor average of the valence contributions and neglecting the nonvalence ones. We compare the two methods, also with respect to the input parametrization of the Sivers function which is extracted from data with approximations sometimes intermediate between frameworks I and II. Deviations between the two approaches are found to be small except for dilepton masses below 3 GeV. The Sivers effect is used as a test case; the arguments can be generalized to other interesting azimuthal asymmetries in Drell-Yan processes, such as the Boer-Mulders effect., Comment: 13 pages, 9 figures in eps format

We have performed numerical simulations of the single-polarized e+ e- --> \vec p anti p process in kinematic conditions under discussion for a possible upgrade of the existing DAFNE facility. By fitting the cross section and spin asymmetry angular distributions with typical Born expressions, we can study the conditions for extracting information on moduli and phases of the proton electromagnetic form factors in the timelike region, which are poorly known and whose preliminary data show puzzling features. We have explored also non-Born contributions by introducing a further component in the angular fit, which is related to two-photon exchange diagrams. Using a dipole parametrization, we show that these corrections can be identified if larger than 5% of the Born contribution; we also explore the conditions for extracting information on the phase and, consequently, on the relative weight between their real and imaginary parts, which are presently unknown., Comment: 17 pages, 6 figures, version to appear in Phys. Rev. D

Recently, a reconsideration of Drell-Yan cross sections at moderate energies and masses has suggested the possibility of relevant enhancements of the cross sections in some kinematical regions. If confirmed, these predictions could largely affect the planning of Drell-Yan experiments aimed at transverse spin measurements after 2010. More in general, the problem is present of a precision measurement of the $\tau$ dependence of Drell-Yan cross sections. Here we discuss the feasibility of such a measurement within short time at the COMPASS apparatus, and its relevance for the PANDA experiment., Comment: 4 pages, 4 figures, in print on Physical Review D

We present Monte Carlo simulations of both the Sivers and the Boer-Mulders effects in the polarized Drell-Yan $\pi^\pm p^\uparrow \to \mu^+ \mu^- X$ process at the center-of-mass energy $\sqrt{s} \sim 14$ GeV reachable at COMPASS with pion beams of energy 100 GeV. 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., Comment: RevTeX, 13 pages, 7 figures included in .eps format

We present Monte Carlo simulations of the Sivers effect in the polarized Drell-Yan $pp^\uparrow \to \mu^+ \mu^- X$ process at the center-of-mass energy $\sqrt{s}=200$ 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 $pp^\uparrow \to \pi X$ process at the same energy. For a given luminosity of $10^{32}$ cm$^{-2}$ sec$^{-1}$, we explore the necessary conditions to reach a statistical accuracy that allows 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., Comment: 17 pages in RevTeX; 7 figures in eps format; improved text, added reference

A short summary of results of recent simulations of (un)polarized Drell-Yan experiments is presented here. Dilepton production in $pp$, $\bar{p}p$, $\pi^-p$ and $\pi^+p$ scattering is considered, for several kinematics corresponding to interesting regions for experiments at GSI, CERN-Compass and RHIC. A table of integrated cross sections, and a set of estimated error bars on measurements of azimuthal asymmetries (associated with collection of 5, 20 or 80 Kevents) are reported., Comment: 8 pages, 7 figures in .eps format; to appear in the Proceedings of the International Workshop on Transverse Polarisation Phenomena in Hard Processes (Transversity 2005), Villa Olmo (Como), 7-10th September 2005; World Scientific style files included

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 $\bar{p}^\uparrow p^\uparrow \to \mu^+ \mu^- X$ 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., Comment: Produced in RevTeX 4, 10 figures in .eps format