The inclusive polarized structure functions of the proton and deuteron, g1p and g1d, were measured with high statistical precision using polarized 6 GeV electrons incident on a polarized ammonia ...target in Hall B at Jefferson Laboratory. Electrons scattered at lab angles between 18 and 45 degrees were detected using the CEBAF Large Acceptance Spectrometer (CLAS). For the usual DIS kinematics, Q^2>1 GeV^2 and the final-state invariant mass W>2 GeV, the ratio of polarized to unpolarized structure functions g1/F1 is found to be nearly independent of Q^2 at fixed x. Significant resonant structure is apparent at values of W up to 2.3 GeV. In the framework of perturbative QCD, the high-W results can be used to better constrain the polarization of quarks and gluons in the nucleon, as well as high-twist contributions.
Exclusive neutral-pion electroproduction (\(ep\to e^\prime p^\prime \pi^0\)) was measured at Jefferson Lab with a 5.75-GeV electron beam and the CLAS detector. Differential cross sections ...\(d^4\sigma/dtdQ^2dx_Bd\phi_\pi\) and structure functions \(\sigma_T+\epsilon\sigma_L, \sigma_{TT}\) and \(\sigma_{LT}\) as functions of \(t\) were obtained over a wide range of \(Q^2\) and \(x_B\). The data are compared with Regge and handbag theoretical calculations. Analyses in both frameworks find that a large dominance of transverse processes is necessary to explain the experimental results. For the Regge analysis it is found that the inclusion of vector meson rescattering processes is necessary to bring the magnitude of the calculated and measured structure functions into rough agreement. In the handbag framework, there are two independent calculations, both of which appear to roughly explain the magnitude of the structure functions in terms of transversity generalized parton distributions.
The beam-helicity asymmetry in associated electroproduction of real photons, \(ep\to e\gamma \pi N\), in the \(\Delta\)(1232)-resonance region is measured using the longitudinally polarized HERA ...positron beam and an unpolarized hydrogen target. Azimuthal Fourier amplitudes of this asymmetry are extracted separately for two channels, \(ep\to e\gamma \pi^0 p\) and \(ep\to e\gamma \pi^+ n\), from a data set collected with a recoil detector. All asymmetry amplitudes are found to be consistent with zero.
Exclusive electroproduction of $\omega $ mesons on unpolarized hydrogen and deuterium targets is studied in the kinematic region of $Q^2> 1.0$ GeV$^2$ , 3.0 GeV $< W
We discuss polarizing a proton beam in a storage ring, either by selective removal or by spin flip of the stored ions. Prompted by recent, conflicting calculations, we have carried out a measurement ...of the spin flip cross section in low-energy electron-proton scattering. The experiment uses the cooling electron beam at COSY as an electron target. The measured cross sections are too small for making spin flip a viable tool in polarizing a stored beam. This invalidates a recent proposal to use co-moving polarized positrons to polarize a stored antiproton beam.
Phys.Rev.Lett. 94 (2005) 014801 Polarized antiprotons can be produced in a storage ring by spin--dependent
interaction in a purely electron--polarized hydrogen gas target. The polarizing
process is ...based on spin transfer from the polarized electrons of the target
atoms to the orbiting antiprotons. After spin filtering for about two beam
lifetimes at energies $T\approx 40-170$ MeV using a dedicated large acceptance
ring, the antiproton beam polarization would reach $P=0.2-0.4$. Polarized
antiprotons would open new and unique research opportunities for spin--physics
experiments in $\bar{p}p$ interactions.
We propose to use an internal polarized hydrogen storage cell gas target in the AD ring to determine for the first time the two total spin-dependent pbar-p cross sections sigma_1 and sigma_2 at ...antiproton beam energies in the range from 50 to 450 MeV. The data obtained are of interest by themselves for the general theory of pbar-p interactions since they will provide a first experimental constraint of the spin-spin dependence of the nucleon-antinucleon potential in the energy range of interest. In addition, measurements of the polarization buildup of stored antiprotons are required to define the optimum parameters of a future, dedicated Antiproton Polarizer Ring (APR), intended to feed a double-polarized asymmetric pbar-p collider with polarized antiprotons. Such a machine has recently been proposed by the PAX collaboration for the new Facility for Antiproton and Ion Research (FAIR) at GSI in Darmstadt, Germany. The availability of an intense stored beam of polarized antiprotons will provide access to a wealth of single- and double-spin observables, thereby opening a new window on QCD spin physics.
A polarized antiproton beam at the Facility for Antiproton and Ion Research,
proposed by the PAX collaboration, will open a window to new physics uniquely
accessible at the new High Energy Storage ...Ring. Our proposal to realize an
asymmetric collider, in which polarized protons with momenta of about 3.5 GeV/c
collide with polarized antiprotons with momenta up to 15 GeV/c, is well--suited
to perform a direct measurement of the transversity distribution function
$h_1$. In this report we summarize the outcome of various working group
meetings within the PAX collaboration. The overall machine setup at the HESR,
proposed by the PAX collaboration, is described along with the associated PAX
experimental program.
PDF
