Unpolarized Cross Section Research Articles

Discussion on the recent proton-DVCS results of Jefferson Lab

We present the recent data issued from the Halls A and B of Jefferson Laboratory for the Deep Virtual Compton Scattering Process on the proton. An important set of data for beam spin asymmetries, unpolarized cross sections and differences of polarized cross sections have been obtained. We modestly attempt a first “global” analysis of these three observables at a single <xB>, <Q2> and <−t> kinematic point. We find that it is extremely challenging to describe simultaneously these data in the framework of a few Generalized Parton Distributions models.

Cross sections for charmonia dissociation in collisions with pions, rhos and kaons**Supported by National Natural Science Foundation of China (10675079)

We calculate the unpolarized cross sections for dissociation reactions of charmonia in collisions with π, ρ and K in a potential that is derived from QCD. The reactions are governed by the quark-interchange processes. The mesonic quark-antiquark relative-motion wave functions are determined by the central spinindependent terms of the potential. The numerical wave functions and cross sections are parametrized. The difference of transition amplitudes in the prior form and in the post form is explored by deriving and examining the transition amplitudes of the one-gluon-exchange spin-spin term of the potential in the two forms. We find that the post-prior discrepancy in meson-meson elastic scattering that is governed by quark-interchange processes depends on the difference of quark or antiquark masses and of quark-antiquark spatial distributions of the two mesons.

Phenomenological insight into the space-like proton elastic form factor using the deuteron impulse approximation

The analysis of the JLab proton polarization data on the ratio {mu}{sub p}G{sub Ep}(t)/G{sub Mp}(t) in the framework of the unitary and analytic model of nucleon electromagnetic structure implicates surprising nondipole behavior of the proton electric form factor G{sub Ep}(t) in the space-like region with the zero around the momentum transfer squared value t=-Q{sup 2}=-13 GeV{sup 2}. This result differs from the unpolarized cross-section measurements giving the dipole behavior of G{sub Ep}(t) by means of the Rosenbluth technique. Recent investigations demonstrate that inclusion of a two-photon mechanism could solve the problem, which plays an important role for the unpolarized cross section, but it is negligible for the polarization ratio. Here, investigating a size of a difference in a prediction of the deuteron elastic structure functions A(t),B(t) by both, the nondipole and the old dipole, behaviors of G{sub Ep}(t) in space-like region through the nonrelativistic impulse approximation, we have come to the conclusion that the two-photon mechanism plays much less of a role for the unpolarized electron-deuteron elastic scattering than for the unpolarized electron-proton elastic scattering.

Conversion cross section between photon and axion in external electromagnetic field

We calculate the unpolarized differential cross section in magnetic dipole field, electric dipole field and uniform electrostatic and magnetostatic fields, respectively,by the Feynman perturbation technique. It is found that in electric dipole field, the differential cross section vanishes in both the same and the opposite propagating directions of the photon flux. In the magnetic dipole field the differential cross section generally has non-vanishing values in both directions,but when the propagating direction of photon flux is parallel to the magnetic dipole vector, the differential cross section is equal to zero. In the uniform electrostatic and magnetostatic fields, non-vanishing differential cross sections occur in the same and opposite propagating directions of the photon flux, but in the latter case it is weaker than in the former. When the mass of axion approaches to zero,the above process shows similar characters as the conversion of the photons to the gravitons.

Unitarity constraints on deeply virtual Compton scattering

At moderately low momentum transfer ($\ensuremath{-}t$ up to 1 GeV${}^{2}$) the coupling to the vector meson production channels gives the dominant contribution to real Compton and deeply virtual Compton scattering (DVCS). Starting from a Regge pole approach that successfully describes vector meson production, the singular part of the corresponding box diagrams (where the intermediate vector meson-baryon pair propagates on-shell) is evaluated without any further assumptions (unitarity). Such a treatment explains not only the unexpectedly large DVCS unpolarized cross section that has been recently measured at Jefferson Laboratory (JLab), but also all the beam spin and charge asymmetries that have been measured at JLab and Hermes, without explicit need of Generalized Parton Distributions (GPD). The issue of the relationship between the two approaches is addressed.

Inclusive cross section and double helicity asymmetry forπ0production inp+pcollisions ats=200 GeV: Implications for the polarized gluon distribution in the proton

The PHENIX experiment presents results from the Relativistic Heavy Ion Collider 2005 run with polarized proton collisions at sqrt(s)=200 GeV, for inclusive {pi}{sup 0} production at midrapidity. Unpolarized cross section results are given for transverse momenta p{sub T}=0.5 to 20 GeV/c, extending the range of published data to both lower and higher p{sub T}. The cross section is described well for p{sub T} 2 GeV/c, by perturbative QCD. Double helicity asymmetries ALL are presented based on a factor of 5 improvement in uncertainties as compared to previously published results, due to both an improved beam polarization of 50%, and to higher integrated luminosity. These measurements are sensitive to the gluon polarization in the proton. Using one representative model of gluon polarization it is demonstrated that the gluon spin contribution to the proton spin is significantly constrained.

Quenching of the analyzing power for inclusive quasielastic (p→,p') scattering

Within the context of a nonrelativistic semiclassical distorted wave (SCDW) model, we demonstrate that the experimentally observed reduction of the inclusive quasielastic $^{208}\mathrm{Pb}$($\stackrel{\ensuremath{\rightarrow}}{p},{p}^{'}$) analyzing power relative to the value for free $NN$ scattering, at incident laboratory kinetic energies of 290 and 200 MeV and at the centroid of the quasielastic peak, is mainly attributed to nuclear medium modifications of the $NN$ effective interaction and also nuclear distortion effects on the scattering wave functions. In addition, the SCDW model also provides a satisfactory quantitative description of the unpolarized double differential cross section.

Model and form factor dependence in the reactionep→e'K+Λ

The reaction $\mathit{ep}\ensuremath{\rightarrow}{e}^{'}{K}^{+}\ensuremath{\Lambda}$ was investigated using a tree-level effective Lagrangian model with coupling parameters fit to empirical cross sections for the reaction $\ensuremath{\gamma}p\ensuremath{\rightarrow}{K}^{+}\ensuremath{\Lambda}$. The model incorporates both spin $\frac{1}{2}$ and spin $\frac{3}{2}$ resonances in the $s$ and $u$ channels, as well as $K(892)$ and $K(1270)$ resonances in the $t$ channel. Results for the electroproduction cross sections were obtained with four different photoproduction fits, which yield roughly equivalent descriptions of the empirical photoproduction cross sections but which vary in their predictions for photoproduction polarization observables. Electromagnetic form factors are incorporated in a manner similar to previous calculations except that a multiplicative factor is introduced into the masses associated with the $s$- and $u$-channel transition form factors so that the sensitivity of the calculated cross sections to these form factors can be studied. Results are presented for the unpolarized differential cross section in a variety of kinematical situations and for particular contributions to the cross section in one particular kinematic situation. The sensitivity of the results to both the photoproduction fit employed and to the transition form factors depends to some extent on the kinematics, but in general, the calculated cross sections are more sensitive to the transition form factors than to the photoproduction fit. Our results suggest that electroproduction data, in conjunction with a reaction model that quantitatively fits the photoproduction data, may provide significant constraints on the electromagnetic form factors associated with resonance excitation.

KTfactorization is violated in production of high-transverse-momentum particles in hadron-hadron collisions

We show that hard-scattering factorization is violated in the production of high-${p}_{T}$ hadrons in hadron-hadron collisions, in the case that the hadrons are back-to-back, so that ${k}_{T}$ factorization is to be used. The explicit counterexample that we construct is for the single-spin asymmetry with one beam transversely polarized. The Sivers function needed here has particular sensitivity to the Wilson lines in the parton densities. We use a greatly simplified model theory to make the breakdown of factorization easy to check explicitly. But the counterexample implies that standard arguments for factorization fail not just for the single-spin asymmetry but for the unpolarized cross section for back-to-back hadron production in QCD in hadron-hadron collisions. This is unlike corresponding cases in ${e}^{+}{e}^{\ensuremath{-}}$ annihilation, Drell-Yan, and deeply inelastic scattering. Moreover, the result endangers factorization for more general hadroproduction processes.

Investigating Neutron Polarizabilities through Compton Scattering onHe3

We examine manifestations of neutron electromagnetic polarizabilities in coherent Compton scattering from the helium-3 nucleus. We calculate gamma3He elastic scattering observables using chiral perturbation theory to next-to-leading order [O(e2Q)]. We find that the unpolarized differential cross section can be used to measure neutron electric and magnetic polarizabilities, while two double-polarization observables are sensitive to different linear combinations of the four neutron spin polarizabilities.

Soft gluon corrections to double transverse-spin asymmetries for small- [formula omitted] dilepton production at RHIC and J-PARC

We calculate the double transverse-spin asymmetries, A T T ( Q T ) , in transversely polarized Drell–Yan process at the transverse-momentum Q T of the produced lepton pair. We perform all-order resummation of the logarithmically enhanced contributions in the relevant Drell–Yan cross sections at small Q T , which are due to multiple soft gluon emission in QCD, up to the next-to-leading logarithmic accuracy. The asymmetries A T T ( Q T ) to be observed in polarized experiments at RHIC and J-PARC are studied numerically as a function of Q T . We show that the effects of the soft gluon resummation to the polarized and unpolarized cross sections largely cancel in A T T ( Q T ) , but the significant corrections still remain and are crucial for making a reliable QCD prediction of A T T ( Q T ) . In particular, the soft gluon corrections enhance A T T ( Q T ) considerably in the small Q T region compared with the asymmetry in the fixed-order α s perturbation theory. We also derive a novel asymptotic formula which embodies those remarkable features of A T T ( Q T ) at small Q T in a compact analytic form and is useful to extract the transversity δ q ( x ) from the experimental data.

STUDY OF THE η MESON PRODUCTION IN THE $\vec{d}+d\to{}^4He+\eta$ REACTION USING THE POLARIZED BEAM

The measurement of the η meson production in [Formula: see text] reaction has been performed at the Big Karl magnetic spectograph at COSY synchrotron in Jülich. The vector and tensor polarized deuetron beam at the excess energy of Q=16 MeV was used. The preliminary results for the unpolarized cross section are presented here.

Proton-He3elastic scattering at low energies

We present new accurate measurements of the differential cross section $\ensuremath{\sigma}(\ensuremath{\theta})$ and the proton analyzing power ${A}_{y}$ for proton-$^{3}\mathrm{He}$ elastic scattering at various energies. A supersonic gas-jet target has been employed to obtain these low-energy cross-section measurements. The $\ensuremath{\sigma}(\ensuremath{\theta})$ distributions have been measured at ${E}_{p}=0.99$, 1.59, 2.24, 3.11, and 4.02 MeV. Full angular distributions of ${A}_{y}$ have been measured at ${E}_{p}=1.60$, 2.25, 3.13, and 4.05 MeV. This set of high-precision data is compared to four-body variational calculations employing realistic nucleon-nucleon ($\mathit{NN}$) and three-nucleon ($3N$) interactions. For the unpolarized cross section, the agreement between the theoretical calculation and data is good when a $3N$ potential is used. The comparison between the calculated and measured proton analyzing powers reveals discrepancies of approximately 50% at the maximum of each distribution. This is analogous to the existing ``${A}_{y}$ puzzle'' known for the past 20 years in nucleon-deuteron elastic scattering.

Incoherent pion photoproduction on the deuteron in the first resonance region

Incoherent pion photoproduction on the deuteron is studied in the first resonance region. The unpolarized cross section, the beam asymmetry, and the vector and tensor target asymmetries are calculated in the framework of a diagrammatic approach. Pole diagrams and one-loop diagrams with $\mathit{NN}$ scattering in the final state are taken into account. An elementary operator for pion photoproduction on the nucleon is taken in various on-shell forms and calculated using the SAID and MAID multipole analyses. Model dependence of the obtained results is discussed in some detail. A comparison with predictions of other works is given. Although a reasonable description of many available experimental data on the unpolarized total and differential cross sections and photon asymmetry has been achieved, in some cases a significant disagreement between the theory and experiment has been found. Invoking known information on the reactions $\ensuremath{\gamma}d\ensuremath{\rightarrow}{\ensuremath{\pi}}^{0}d$ and $\ensuremath{\gamma}d\ensuremath{\rightarrow}\mathit{np}$ we predict the total photoabsorption cross section for deuterium. We find that our values strongly overestimate experimental data in the vicinity of the \ensuremath{\Delta} peak.

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 ${\ensuremath{\pi}}^{\ifmmode\pm\else\textpm\fi{}}{p}^{\ensuremath{\uparrow}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{+}{\ensuremath{\mu}}^{\ensuremath{-}}X$ 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.

Production of doubly charged vector bilepton pairs atγγcolliders

The production of pairs of doubly charged vector bileptons is studied at future $\ensuremath{\gamma}\ensuremath{\gamma}$ colliders. The unpolarized cross section for the $\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}{Y}^{--}{Y}^{++}$ subprocess is analytically calculated and convoluted to predict the number of events in the complete ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}\ensuremath{\gamma}\ensuremath{\gamma}\ensuremath{\rightarrow}{Y}^{--}{Y}^{++}$ process. The gauge or nongauge character of the vector bilepton ${Y}^{\ifmmode\pm\else\textpm\fi{}\ifmmode\pm\else\textpm\fi{}}$ is discussed. It is found that, as a consequence of its spectacular signature, as it decays dominantly into two identical charged leptons, and also due to its charge contents, which significantly enhance the cross section, the detection of this class of particles with mass in the sub-TeV region can be at the reach of these colliders. The model-independent nature of our results is stressed.

Study of the angular coefficients and corresponding helicity cross sections of theWboson in hadron collisions

We present the Standard Model prediction for the eight angular coefficients of the W boson, which completely describe its differential cross section in hadron collisions. These coefficients are ratios of the W helicity cross sections and the total unpolarized cross section. We also suggest a technique to experimentally extract the coefficients.

Incoherent pion photoproduction on the deuteron with polarization observables. I. Formal expressions

Formal expressions are developed for the general five-fold differential cross section of incoherent $\pi$-photoproduction on the deuteron including beam and target polarization. The polarization observables of the cross section are described by various beam, target and beam-target asymmetries for polarized photons and/or polarized deuterons. They are given as bilinear hermitean forms in the reaction matrix elements divided by the unpolarized cross section. In addition, the corresponding observables for the semi-exclusive reaction $\vec d(\vec \gamma,\pi)NN$ are also given.

Measurement of Transverse Single-Spin Asymmetries for Midrapidity Production of Neutral Pions and Charged Hadrons in Polarizedp+pCollisions ats=200 GeV

Transverse single-spin asymmetries to probe the transverse-spin structure of the proton have been measured for neutral pions and nonidentified charged hadrons from polarized proton-proton collisions at midrapidity and square root(s) = 200 GeV. The data cover a transverse momentum (pT) range 1.0-5.0 GeV/c for neutral pions and 0.5-5.0 GeV/c for charged hadrons, at a Feynman-x value of approximately zero. The asymmetries seen in this previously unexplored kinematic region are consistent with zero within errors of a few percent. In addition, the inclusive charged hadron cross section at midrapidity from 0.5 < pT < 7.0 GeV/c is presented and compared to next-to-leading order perturbative QCD (pQCD) calculations. Successful description of the unpolarized cross section above approximately 2 GeV/c suggests that pQCD is applicable in the interpretation of the asymmetry results in the relevant kinematic range.

Measurements of the total-cross-section difference Δσ L(np) at 1.39, 1.69, 1.89, and 1.99 GeV

New accurate data of the neutron-proton spin-dependent total-cross-section difference Δσ L(np) at the neutron-beam kinetic energies 1.39, 1.69, 1.89, and 1.99 GeV are presented. In general, these data complete the measurements of energy dependence of Δσ L(np) over the Dubna Synchrophasotron energy region. Measurements were carried out at the Synchrophasotron of the Veksler and Baldin Laboratory of High Energies of the Joint Institute for Nuclear Research. The quasi-monochromatic neutron beam was produced by breakup of extracted polarized deuterons. The deuteron (and hence neutron) polarization direction was flipped every accelerator burst. The initial transverse (with respect to beam momentum) neutron polarization was changed to a longitudinal one and longitudinally polarized neutrons were transmitted through the large proton longitudinally polarized target. The target polarization direction was inverted after one to two days of measurements. Four different combinations of the beam and target parallel and antiparallel polarization directions, both oriented along the neutron-beam momentum, were used at each energy. A fast decrease in −Δσ L(np) with increasing energy above 1.1 GeV and a structure in the energy dependence around 1.8 GeV, first observed from our previous data, seem to be well revealed. The new results are also compared with model predictions and with phase-shift analysis fits. The Δσ L quantities for isosinglet state I = 0, deduced from the measured Δσ L(np) values and known Δσ L(pp) data, are also given. The results of the measurements of unpolarized total cross sections σ 0tot(np) at 1.3, 1.4, and 1.5 GeV and σ 0tot(nC) at 1.4 and 1.5 GeV are presented as well. These data were obtained using the same apparatus and high-intensity unpolarized deuteron beams extracted either from the Synchrophasotron or from the Nuclotron.