Double transverse-spin asymmetries in Drell–Yan processes with antiprotons

We present next-to-leading order predictions for double transverse-spin asymmetries in Drell–Yan dilepton production initiated by proton–antiproton scattering. The kinematic region of the proposed PAX experiment at GSI: 30?s?200 GeV²$30?s?200{\text{GeV}}^2$ and 2?M?7 GeV$2?M?7\text{GeV}$ is examined. The Drell–Yan asymmetries turn out to be large, in the range 20–40%. Measuring these asymmetries would provide the cleanest determination of the quark transversity distributions.     

Manifestations of hidden polarized strangeness of the nucleon in polarization phenomena for the process p +¯p → P0+ V0

The sensitivities of the one-spin and two-spin polarization observables for p+ˉp→P⁰+V⁰, with P⁰=π⁰, η,η′ and V⁰=ρ,ω,φ, based on the triplet enhancement hypothesis for strangeness production in pˉp collisions are analyzed. The analysis is carried out under special kinematical conditions where the P⁰ meson production angle in the CMS is equal to π/2, for which C-invariance of strong interactions reduces the number of independent amplitudes to three. Received: 17 September 1997     

Strange quarks in the nucleon sea

The HAPPEX Collaboration measured parity-violating electron scattering from ⁴He$(e, e)$and H(e, e) in 2004 and 2005 for Q ²≤0.11GeV^2. Results for the strange-quark contributions to the electromagnetic form factors of the nucleon from the 2004 data will be reviewed. Preliminary results from the 2005 data, which have significantly greater statistical precision, are G _E ^s = 0.004±0.014_stat±0.013_syst for Q ² = 0.0772GeV^2 from the helium data and G _E ^s +0.088G _M ^s = 0.004±0.011_stat±0.005_syst±0.004_FF for Q ² = 0.1089GeV^2 from the hydrogen data.     

General formalism in collinear regime for pseudoscalar meson production in NN collisions

The spin structure of the matrix element for the pseudoscalar meson production processes in nucleon-nucleon collisions is established in the collinear kinematical regime in terms of 3 independent scalar amplitudes. This result is valid for any reaction mechanism and for any energy of the colliding and the produced particles. The complete experiment for the full reconstruction of all 3 complex amplitudes must contain two different classes of polarization experiments. The polarization transfer coefficients can be used to determine the moduli of all 3 amplitudes, whereas the spin correlation coefficients for + collisions are sensitive to the relative phases of different amplitudes. Received: 13 August 2001 / Accepted: 20 September 2001     

The nucleon and Δ-resonance masses in relativistic chiral effective-field theory

We study the chiral behavior of the nucleon and Δ-isobar masses within a manifestly covariant chiral effective-field theory, consistent with the analyticity principle. We compute the πN and πΔ one-loop contributions to the mass and field-normalization constant, and find that they can be described in terms of universal relativistic loop functions, multiplied by appropriate spin, isospin and coupling constants. We show that these manifestly relativistic one-loop corrections, when properly renormalized, obey the chiral power-counting and vanish in the chiral limit. The results including only the πN-loop corrections compare favorably with the lattice QCD data for the pion-mass dependence of the nucleon and Δ masses, while inclusion of the πΔ loops tends to spoil this agreement.     

Opportunities with Drell-Yan scattering: Probing sea quarks in the nucleon and nuclei

The large ˉ(x)/ˉ(x) ratio observed by Fermilab E866/NuSea convincingly demonstrated that the sea is not simply a result of pQCD. Moreover, meson cloud models also failed to explain fully the observed kinematic dependence. The Drell-Yan mechanism offers a unique, selective probe of antiquarks in the nucleon. Fermilab has approved a new Drell-Yan experiment, E906, that will exploit this feature to probe ˉ(x)/ˉ(x) by measuring the ratio of cross-sections for the proton-induced Drell-Yan process on hydrogen to deuterium. When the nucleon is contained in a nucleus, the nucleon's parton distributions should to be modified; although this effect was not seen in the sea quark distributions obtained by Fermilab E772 with Drell-Yan scattering. The upcoming E906 Drell-Yan experiment will provide much more precise measurements over a wider kinematic range in order to guide and challenge the theoretical models.     

An Approach to Extract the Spin Transfer to ∧in Polarized Proton--Proton Collision

We introduce an approach to extract the spin transfer to A in polarized proton-proton collision, based on the relation between single spin asymmetry and the polarization of A production. With 4 × 10^6 ∧ events simulated for a PHENIX detector system, DLL = 0.1598 ± 0.0343 is retrieved by the approach and it agrees very well with the input value of 0.1429. The approach is further tested and confirmed with ‘bunch shuffling＇ method for the simulated events as well as experimental events collected at PHENIX in 2003. It is concluded that one can correctly extract the spin transfer without detailed understanding of acceptance correction even if the correction is significantly large in PHENIX experiment by using the approach described here. The method can be generally used for spin transfer study.     

Measurement of the electric form factor of the neutron at Q2 = 0.3-0.8 (GeV/ c) 2 ⋆

The electric form factor of the neutron, G_E,n, has been measured at the Mainz Microtron by recoil polarimetry in the quasielastic D(¯e, e¯n)p reaction. Three data points have been extracted at squared four-momentum transfers Q ² = 0.3, 0.6 and 0.8 (GeV/c)². Corrections for nuclear binding effects have been applied.This revised version was published online in March 2005. In the previous version, the email address of one author was inadvertently assigned to multiple authors.     

Statistical effect in the parton distribution functions of the nucleon

A new and simple statistical approach is performed to calculate the parton distribution functions (PDFs) of the nucleon in terms of light-front kinematic variables. We do not put in any extra arbitrary parameter or corrected term by hand, which guarantees the stringency of our approach. Analytic expressions of the x-dependent PDFs are obtained in the whole x   region [0,1]$[0,1]$, and some features, especially the low-x rise, are more agreeable with experimental data than those in some previous instant-form statistical models in the infinite-momentum frame (IMF). Discussions on heavy-flavored PDFs are also presented.     

Exclusive meson production at HERMES

Hard exclusive production of mesons in deep-inelastic scattering allows one to probe the so-far unknown Generalized Parton Distributions (GPDs) of the nucleon. The HERMES experiment has measured several different observables in exclusive meson production by scattering the 27.6GeV HERA lepton beam off an internal fixed gaseous target. Recent results on exclusive π ⁺, ρ ⁰ and pion pair production will be presented.     

The saturation scale and its x-dependence from Λ polarization studies

The transverse polarization of forward Λ   hyperons produced in high-energy p–A$p\text{–}A$ collisions is expected to display an extremum at a transverse momentum around the saturation scale. This was first observed within the context of the McLerran–Venugopalan model which has an x  -independent saturation scale. The extremum arises due to the kt$k_t$-odd nature of the polarization-dependent fragmentation function, which probes approximately the derivative of the dipole scattering amplitude. The amplitude changes most strongly around the saturation scale, resulting in a peak in the polarization. We find that the observation also extends to the more realistic case in which the saturation scale Qs$Q_s$ is x-dependent. Since a range of x   and therefore Qs$Q_s$ values is probed at a given transverse momentum and rapidity, this result is a priori not expected. Moreover, the measurement of Λ   polarization over a range of xF$x_F$ values actually provides a direct probe of the x-dependence of the saturation scale. This novel feature is demonstrated for typical LHC kinematics and for several phenomenological models of the dipole scattering amplitude. We show that although the measurement will be challenging, it may be feasible at LHC. The situation at RHIC is not favorable, because the peak will likely be at too low transverse momentum of the Λ to be a trustworthy measure of the saturation scale.     

Flavor content of nucleon form factors in a VMD approach

The strange form factors of the nucleon are studied in a two-component model consisting of a three-quark intrinsic structure surrounded by a meson cloud. A comparison with the available experimental world data from the SAMPLE, PVA4, HAPPEX and G0 Collaborations shows a good overall agreement. It is shown that the strangeness contribution to the electric and magnetic form factors is of the order of a few percent. In particular, the strange quark contribution to the charge radius is small 〈r ² _s〉_E = 0.005 fm^2 and to the magnetic moment it is positive μ_s = 0.315 μ_N .     

Left–right asymmetry in semi-inclusive deep inelastic scattering process

We analyze the left–right asymmetry of pion production in semi-inclusive deep inelastic scattering (SIDIS) process of unpolarized charged lepton on transversely polarized nucleon target. Unlike available treatments, in which some specific weighting functions are multiplied to separate theoretically motivated quantities, we do not introduce any weighting function following the analyzing method by the E704 experiment. The advantage is that this basic observable is free of any theoretical bias, although we can perform the calculation under the current theoretical framework. We present numerical calculations at both HERMES kinematics for the proton target and JLab kinematics for the neutron target. We find that with the current theoretical understanding, Sivers effect plays a key role in our analysis.     

Electroweak corrections to the observables of W-boson production at RHIC

The processes of the single W-production in hadron-hadron collisions are suggested for investigation of the nucleon spin. An approach is proposed for the determination of quark spin densities at low x. The lowest order electroweak radiative corrections to the observable quantities are calculated. The numerical calculations of the cross sections and the single spin asymmetries taking into consideration the electroweak corrections at RHIC energies have been made.     

Meson photoproduction on the nucleon with polarized photons

Meson photoproduction with polarized photons has proved to be a powerful tool to identify contributions of baryon resonances that are not evident in the differential cross-sections. It provides information that are complementary to those extracted using pion-nucleon scattering data. Extensive results have been produced in the past on beam asymmetries by the Graal collaboration for η and π ⁰ on the proton. New results are now available for the same reactions on the quasi-free neutron and for the K⁺ photoproduction on the proton. Contributions from hitherto undetected baryon resonances may be important to understand the results.     

Measurement of the beam asymmetry Σ in πeta; production off the proton with the CBELSA/TAPS experiment?

In photoproduction experiments, a large number of final states yielding various resonance contributions are accessible. To extract resonance parameters via partial-wave analysis not only the measurement of differential cross-sections is necessary, but also the determination of polarization observables. At the electron accelerator ELSA (Bonn) the coherent bremsstrahlung method was used to generate a linearly polarized photon beam. Using the CBELSA/TAPS detector setup, the beam asymmetry Σ in the reaction γp → pπ⁰η was determined as a function of various masses and angles for photon energies between 970MeV and 1650MeV.