Search Results (27 total)

We study the transverse target spin dependence of the cross section for inclusive electron-nucleon scattering with unpolarized beam. Such dependence is absent in the one-photon exchange approximation (Christ-Lee theorem) and arises only in higher orders of the QED expansion, from the interference of one-photon and absorptive two-photon exchange amplitudes as well as from real photon emission (bremsstrahlung). We demonstrate that the transverse spin-dependent two-photon exchange cross section is free of QED infrared and collinear divergences. We argue that in DIS kinematics the transverse spin dependence should be governed by a “partonlike” mechanism in which the two-photon exchange couples mainly to a single quark. We calculate the normal spin asymmetry in an approximation where the dominant contribution arises from quark-helicity flip due to interactions with nonperturbative vacuum fields (constituent quark picture) and is proportional to the quark transversity distribution in the nucleon. Such helicity-flip processes are not significantly Sudakov-suppressed if the infrared scale for gluon emission in the photon-quark subprocess is of the order of the chiral symmetry breaking scale, μ_chiral²≫Λ_QCD². We estimate the asymmetry in the kinematics of the planned Jefferson Lab Hall A experiment to be of the order 10^-4, with different sign for proton and neutron. We also comment on the spin dependence in the limit of soft high-energy scattering.

We calculate, in a model, the beam spin asymmetry in semi-inclusive jet production in deep inelastic scattering. This twist-3, T-odd observable is nonzero due to final state strong interactions. With reasonable choices for the parameters, one finds an asymmetry of several percent, about the size seen experimentally. We present the result both as an explicit asymmetry calculation and as a model calculation of the new transverse-momentum dependent distribution function g^⊥.

We estimate the two-photon exchange contribution to elastic electron-proton scattering at large momentum transfer by using a quark-parton representation of virtual Compton scattering. We thus can relate the two-photon exchange amplitude to the generalized parton distributions which also enter in other wide-angle scattering processes. We find that the interference of one- and two-photon exchange contribution is able to substantially resolve the difference between electric form factor measurements from Rosenbluth and polarization transfer experiments. Two-photon exchange has additional consequences which could be experimentally observed, including nonzero polarization effects and a positron-proton/electron-proton scattering asymmetry. The predicted Rosenbluth plot is no longer precisely linear; it acquires a measurable curvature, particularly at large laboratory angle.

Higher-order QED effects play an important role in precision measurements of nucleon elastic form factors in electron scattering. Here we introduce a two-photon-exchange QED correction to the parity-violating polarization asymmetry of elastic electron-proton scattering. We calculate this correction in the parton model using the formalism of generalized parton distributions, and demonstrate that it can reach several percent in certain kinematics, becoming comparable in size with existing experimental measurements of strange-quark effects in the proton neutral weak current.

The three-body photodisintegration of ³He has been measured with the CLAS detector at Jefferson Lab, using tagged photons of energies between 0.35 GeV and 1.55  GeV. The large acceptance of the spectrometer allowed us for the first time to cover a wide momentum and angular range for the two outgoing protons. Three kinematic regions dominated by either two- or three-body contributions have been distinguished and analyzed. The measured cross sections have been compared with results of a theoretical model, which, in certain kinematic ranges, have been found to be in reasonable agreement with the data.

We study a parity-conserving single-spin beam asymmetry of elastic electron-proton scattering induced by an absorptive part of the two-photon exchange amplitude. It is demonstrated that excitation of inelastic hadronic intermediate states by the consecutive exchange of two photons leads to logarithmic and double-logarithmic enhancement due to contributions of hard collinear quasireal photons. The asymmetry at small electron scattering angles is expressed in terms of the total photoproduction cross section on the proton and is predicted to reach the magnitude of 20–30 ppm. At these conditions and fixed 4-momentum transfers, the asymmetry is rising logarithmically with increasing electron beam energy, following the high-energy diffractive behavior of total photoproduction cross section on the proton.

We estimate the two-photon exchange contribution to elastic electron-proton scattering at large momentum transfer through the scattering off a parton in the proton. We relate the two-photon exchange amplitude to the generalized parton distributions which appear in hard exclusive processes. We find that when taking the polarization transfer determinations of the form factors as input, adding in the 2-photon correction does reproduce the Rosenbluth cross sections.

We report the first evidence for a nonzero beam-spin azimuthal asymmetry in the electroproduction of positive pions in the deep-inelastic kinematic region. Data for the reaction ep→e^′π⁺X have been obtained using a polarized electron beam of 4.3 GeV with the CEBAF Large Acceptance Spectrometer at the Thomas Jefferson National Accelerator Facility. The amplitude of the sin φ modulation increases with the momentum of the pion relative to the virtual photon, z. In the range z=0.5–0.8 the average amplitude is 0.038±0.005±0.003 for a missing mass M_X>1.1 GeV and 0.037±0.007±0.004 for M_X>1.4 GeV.

The first complete measurements of the angular distributions of the two-body deuteron photodisintegration differential cross section at photon energies above 1.6 GeV were performed at the Thomas Jefferson National Accelerator Facility. The results show a persistent forward-backward asymmetry up to E_γ=2.4 GeV, the highest-energy measured in this experiment. The Hard Rescattering and the Quark-Gluon string models are in fair agreement with the results.

A formalism for radiative correction (RC) calculation in exclusive pion electroproduction on the proton is presented. A FORTRAN code EXCLURAD is developed for the RC procedure. The numerical analysis is done in the kinematics of current Jefferson Lab experiments.

We present measurements of the recoil proton polarization for the ¹H(γ→,p→)π⁰ reaction for θ_c.m.^π=60°–135° and for photon energies up to 4.1 GeV. These are the first data in this reaction for polarization transfer with circularly polarized photons. Various theoretical models are compared with the results. No evidence for hadron helicity conservation is observed. Models that employ factorization are not favored. It appears from the strong angular dependence of the induced polarization at photon energies of 2.5 and 3.1 GeV that a relatively high spin resonance or background amplitude might exist in this energy region.

Using a method of electron structure functions, we calculated a model independent radiative correction to the recoil proton polarization for elastic electron-proton scattering. Explicit expressions for the recoil proton polarization are represented as a convolution of the electron structure functions and the hard part of the polarization-dependent contribution into the cross section. The hard part is calculated with first order radiative corrections. The obtained representation includes leading radiative corrections in all orders of perturbation theory and the main part of the second order next-to-leading radiative corrections. Numerical calculations illustrate our analytical results.

Explicit formulas for radiative correction (RC) calculations for elastic ep scattering are presented. Two typical measurements of polarization observables, such as beam-target asymmetry or recoil proton polarization, are considered. The possibilities of taking into account realistic experimental acceptances are discussed. The FORTRAN code MASCARAD for providing the RC procedure is presented. A numerical analysis is done for the kinematical conditions of CEBAF.

We present measurements of the ratio of the proton elastic electromagnetic form factors, μ_pG_Ep/G_Mp. The Jefferson Lab Hall A Focal Plane Polarimeter was used to determine the longitudinal and transverse components of the recoil proton polarization in ep elastic scattering; the ratio of these polarization components is proportional to the ratio of the two form factors. These data reproduce the observation of Jones et al. [Phys. Rev. Lett. 84, 1398 (2000)], that the form factor ratio decreases significantly from unity above Q²=1 GeV².

We present measurements of the recoil proton polarization for the d(γ→,p→)n reaction at θ_c.m.  =  90° for photon energies up to 2.4 GeV. These are the first data in this reaction for polarization transfer with circularly polarized photons. The induced polarization p_y vanishes above 1 GeV, contrary to meson-baryon model expectations, in which resonances lead to large polarizations. However, the polarization transfer C_x does not vanish above 1 GeV, inconsistent with hadron helicity conservation. Thus, we show that the scaling behavior observed in the d(γ,p)n cross sections is not a result of perturbative QCD. These data should provide important tests of new nonperturbative calculations in the intermediate energy regime.

We discuss extending scaling and duality studies to semiexclusive processes. We show that semiexclusive hard pion photoproduction should exhibit scaling behavior in kinematic regions where the photon and pion both interact directly with the same quark. We show that such kinematic regions exist. We also show that the constancy with changing momentum transfer of the resonance peak/scaling curve ratio, familiar for many resonances in deep inelastic scattering, is also expected in the semiexclusive case.

We analyze the processes γ→+p→ρ⁰π⁺n at low momentum transfer focusing on the possibility of the production of an exotic J^PC=1^-+ meson state. In particular we discuss polarization observables and conclude that linear photon polarization is instrumental for separating the exotic wave.

Hard, or high transverse momentum, pion photoproduction can be a tool for probing the parton structure of the beam and target. We estimate the soft contributions to this process, with an eye toward delineating the region where perturbatively calculable processes dominate. Our soft process estimate is based on vector meson dominance and data based parametrizations of semiexclusive hadronic cross sections. We find that soft processes dominate in single pion photoproduction somewhat past 2 GeV transverse momentum at a few times 10 GeV incoming energy. The recent polarization asymmetry data are consistent with the perturbative asymmetry being diluted by polarization insensitive soft processes. Determining the polarized gluon distribution using hard pion photoproduction appears feasible with a few hundred GeV of incoming energy (in the target rest frame).

We investigate spatial instabilities (induced by small angular perturbations) of counterpropagating waves in nonlinear distributed feedback (DFB) structures. We determined the DFB-structure threshold length at which an absolute instability occurs and a nonhomogeneous spatial intensity distribution is generated. The evolution of the transverse intensity distribution is studied for counterpropagating waves as a function of the control parameters.

The local-field effects are considered which are feasible in a dense ensemble of resonant atoms modeled by multilevel quantum systems. Our approach is based on the generalized two-level system [V.S. Butylkin, A.E. Kaplan, and Yu.G. Khronopulo, Zh. Éksp. Teor. Fiz. 59, 921 (1970)]. Making use of this model, we take account of the nonresonant polarization and Stark shift of the absorption line due to the difference in linear polarizabilities of atoms in ground and excited states. With near dipole-dipole interaction and quadratic Stark effect included simultaneously, new features of the hysteresis behavior of the population difference as a function of the external-field intensity are predicted.

We calculate polarization asymmetries in the photoproduction of high transverse momentum mesons, focusing on charged pions, considering the direct, fragmentation, and resolved photon processes. The results at very high meson momentum measure the polarized quark distributions and are sensitive to differences among the existing models. The results at moderate meson momentum are sensitive to the polarized gluon distribution and can provide a good way to measure it. Suitable data may come as a by-product of deep inelastic experiments to measure g₁ or from dedicated experiments.

We estimate the kinematic dependence of the exclusive photo- and electro-production of J^PC=1^-+ exotic mesons due to π exchange. We show that the kinematic dependence is largely independent of the exotic meson form factor, which is explicitly derived for a 1^-+ isovector hybrid meson in the flux-tube model of Isgur and Paton. The relevance to experiments currently planned at Jefferson Lab is indicated.

In the present study the general equations which describe the absorption Mössbauer spectra under radio-frequency (rf) magnetic field excitation with arbitrary frequency and field strength have been derived. Within our model chosen for a ferromagnet as a system of exchange-coupled (interacting) Stoner-Wohlfarth particles, the evolution of the magnetization and the corresponding Mössbauer spectra as a function of temperature and initial magnetization relaxation parameters are traced. It is found that the collapse effect is of a pronounced threshold character with respect to the rf field strength and does not need strong rf fields for its realization. The necessary condition for the observation of a collapse effect is only a rf amplitude causing the corresponding magnetization curves to be symmetrical in time reversal. The theory developed allows us to perform calculations of Mössbauer spectra under rf magnetic field excitation and the corresponding magnetization curves (also for multiphase systems such as modern nanostructured magnetic alloys). The results are also useful in a situation when the hyperfine field at the nuclei does not follow the rf magnetic field. This circumstance determines the rather nontrivial transition from the collapsed line (in strong enough rf field) to the well-resolved hyperfine structure (in weak rf field) and contributes therefore in understanding the selective partial collapse effect.

A calculation of the semileptonic decays of the kaon (K_l3) is presented. The results are direct predictions of a covariant model of the pion and kaon introduced earlier by Ito, Buck, and Gross. The weak form factors for K_l3 are predicted with absolutely no parameter adjustments of the model. We obtained, for the form factor parameters, f_-(q²=m_l²)/f₊(q²=m_l²)=-0.28 and λ₊= 0.028, both within experimental error bars. The connections of this approach to heavy quark symmetry are discussed.

We describe a type of dissipative structure, namely, triadic Hopf-static patterns, which are in fact drifting spots in a rhombic or rhomboidal arrangement. They are generated by a resonant interaction between simultaneously unstable Hopf and static modes. We analyze triadic Hopf-static patterns in a nonlinear optical system consisting of a thin layer of two-level material and a feedback mirror with explicit inclusion of delay effects.