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Results for “PHOTON-BARYON INTERACTIONS”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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16 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Study of the {gamma}d {yields} K{sup +}K{sup -}np reaction and an alternative explanation for the ''{Theta}{sup +}'' peak

We look for an explanation of the peak in the K{sup +}n invariant mass distribution of the {gamma}d {yields} K{sup +}K{sup -}n p reaction that was associated to the {Theta}{sup +}(1540) pentaquark at LEPS. For this we develop a theoretical model adapted to the experiment. The study shows that the method used in the experiment to associate momenta to the undetected proton and neutron, together with the chosen cuts, necessarily creates an artificial broad peak in the assumed K{sup +}n invariant mass in the region of the claimed {Theta}{sup +}(1540), such that the remaining strength seen for the experimental peak is compatible with a fluctuation of 2{sigma} significance.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Juelich-Georgia meson-baryon model

Meson-baryon scattering and photon-nucleon reactions are described in a coupled-channel approach for energies below 2 GeV. Poles and residues of the reaction amplitudes are obtained by analytical continuation. The dependence of the resonance poles on the pion mass is investigated.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

The Search for Missing Resonances: the Neutron Anomaly

We report the observation of a narrow structure in the invariant mass spectrum of the {eta}-neutron pairs produced in the reaction {gamma}d{yields}{eta}n(p) which is absent in the {eta}-proton pairs produced in the reaction {gamma}d{yields}{eta}p(n). These results are obtained from a liquid deuterium target, the data sample was collected with the Crystal Barrel and TAPS detectors at the electron accelerator ELSA. This structure is also seen in preliminary analyses of the data samples taken by the Crystal Ball and TAPS detectors at the electron accelerator MAMI with a liquid deuterium target and a liquid helium 3 target ruling out a nuclear effect. We report the measurements of the position and width of this narrow structure in the invariant mass spectrum at W{approx_equal}1665 MeV and with a FWHM of {Gamma} = 25 MeV.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Photoproduction of Mesons off Light Nuclei

During the last few years, a series of experiments has been done at the ELSA accelerator in Bonn with the Crystal Barrel/TAPS setup and at the MAMI accelerator in Mainz with the Crystal Ball/TAPS setup. Photoproduction of light mesons off the deuteron and {sup 3}He-nuclei has been studied in detail. We will report some of the most interesting results.A completely unexpected finding, is the pronounced structure in the excitation function of the {gamma}n {yields} n{eta} reaction around 1 GeV, which has no counterpart for the proton. Recent measurements at GRAAL, Sendai, ELSA [1] and MAMI estimate it's width below 50 MeV, which would be extremely narrow for a nucleon resonance at this excitation energy. In the experiment with the {sup 3}He target, special attention was given to the threshold behavior of the {gamma}He{sup 3} reactions in view of possible indications for the formation of (quasi-)bound {eta}-nucleus states, so-called {eta}-mesic nuclei. A very strong threshold enhancement of coherent {eta}-photoproduction off {sup 3}He was found. In a new experiment, this reaction was measured with much better statistical accuracy compared to an earlier experiment at MAMI-B using the TAPS detector [2].

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Disentangling correlations in multiple parton interactions

Multiple Parton Interactions are the tool to obtain information on the correlations between partons in the hadron structure. Partons may be correlated in all degrees of freedom and all different correlation terms contribute to the cross section. The contributions due to the different parton flavors can be isolated, at least to some extent, by selecting properly the final state. In the case of high energy proton-proton collisions, the effects of correlations in the transverse coordinates and in fractional momenta are, on the contrary, unavoidably mixed in the final observables. The standard way to quantify the strength of double parton interactions is by the value of the effective cross section and a small value of the effective cross section may be originated both by the relatively short transverse distance between the pairs of partons undergoing the double interaction and by a large dispersion of the distribution in multiplicity of the multiparton distributions. The aim of the present paper is to show how the effects of longitudinal and transverse correlations may be disentangled by taking into account the additional information provided by double parton interactions in high energy proton-deuteron collisions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Inclusive Measurements of Inelastic Scattering on unpolarized H and D targets at 27.6 GeV

Measurements of the inclusive proton and deuteron structure functions F{sub 2}{sup p} and F{sub 2}{sup d} are presented. The data cover the ranges 0.006{<=}x{<=}0.9 and 0.1 GeV{sup 2}{<=}Q{sup 2}{<=}20 GeV{sup 2} and are in agreement with existing world data in the region of overlap. The measured cross sections were used, in combination with available world data, to perform fits to the total photon-proton cross section.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

The energy dependence of the pp {yields} K{sup +}n{Sigma}{sup +} reaction

The energy dependence of the total cross section for the pp {yields} K{sup +}n{Sigma}{sup +} reaction has been investigated at the magnetic spectrometer COSY-ANKE. Signals from the production of the {Sigma}{sup +} hyperon were searched for in three simultaneously measured spectra. The values obtained for the total production cross section {sigma}({Sigma}{sup +}) are slightly below those of {sigma}({Sigma}{sup 0}) at the same excess energies. They follow a phase space dependence and do not show any evidence for strong threshold effects or a significant n{Sigma}{sup +} final state interaction.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Nucleon polarizabilities and {Delta}-resonance magnetic moment in chiral EFT

Recent chiral EFT calculations of nucleon polarizabilities reveal a problem in the current empirical determination of proton's electromagnetic polarizabilities. We also report on the progress in the empirical determination of the {Delta}(1232)-resonance magnetic moment in the process of {gamma}p{yields}p{pi}{sup 0}{gamma}' measured at MAMI.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Pion photoproduction in a dynamical coupled-channel model

Pion photoproduction reactions are investigated in a dynamical coupled-channel approach based on the Juelich {pi}N model, which presently includes the hadronic {pi}N and {eta}N stable channels as well as the {pi}{Delta}, {sigma}N and {rho}N effective channels. This model has been quite successful in the description of {pi}N{yields}{pi}N scattering for center-of-mass energies up to 1.9 GeV. The full pion photoproduction amplitude is constructed to satisfy the generalized Ward-Takahashi identity and hence, it is fully gauge invariant. The calculated differential cross sections and photon spin asymmetries up to 1.65 GeV center-of-mass energy for the reactions {gamma}p{yields}{pi}{sup +}n, {gamma}p{yields}{pi}{sup 0}p and {gamma}n{yields}{pi}{sup -}p are in good agreement with the experimental data.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Measurement of the {gamma}p{yields}{eta}p Reaction with the Crystal Ball Detector at the Mainz Microtron (MAMI-C)

The {gamma}p{yields}{eta}p reaction has been measured with the Crystal Ball and TAPS multiphoton spectrometers in the energy range from the production threshold to 1.4 GeV (1.49{<=}W{<=}1.87 GeV). Bremsstrahlung photons produced by the 1.5-GeV electron beam of the Mainz Microtron MAMI-C and momentum analyzed by the Glasgow Tagging Spectrometer were used for the {eta}-meson production. Our accumulation of 3.8x10{sup 6} {gamma}p{yields}{eta}p{yields}3{pi}{sup 0}p{yields}6{gamma}p events allows a detailed study of the reaction dynamics. The {gamma}p{yields}{eta}p differential cross sections were determined for 120 energy bins and the full range of the production angles. Our data show a dip near W = 1680 MeV in the total cross section related to a substantial dip in {eta} production at forward angles. The data are compared to predictions of previous SAID and MAID partial-wave analyses and to the latest SAID and MAID fits that included our data.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

The Determination of Pseudoscalar Meson Photoproduction Amplitudes from Complete Experiments

A new generation of complete experiments is currently underway with the goal of performing a high precision extraction of pseudoscalar meson photo-production amplitudes. Here we review the most general analytic form of the cross section, dependent upon the three polarization vectors of the beam, target and recoil baryon, including all single, double and triple-polarization terms involving 16 spin-dependent observables. Analytic expressions that determine the recoil baryon polarization are also presented. Different conventions are in use in the literature and we have used a numerical calculation of cross sections from Chew-Goldberger-Low-Nambu amplitudes with arbitrary spin projections to clarify apparent sign differences. As an illustration of the use of this machinery, we carry out a multipole analysis of the {gamma}p {yields} K{sup +}{Lambda} reaction and examine the impact of recently published polarization measurements. In fitting multipoles, we use a combined Monte Carlo sampling of the amplitude space, with gradient minimization, and find a shallow {chi}{sup 2} valley pitted with a very large number of local minima, despite the inclusion of recent data on 8 different observables. We conclude that, while a mathematical solution to the problem of determining an amplitude free of ambiguities may require 8 observables, as has been pointed out in the literature, experiments with realistically achievable uncertainties will require a significantly larger number.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Exclusive production of {omega} meson in proton-proton collisions at high energies

First we calculate cross section for the {gamma}p{yields}{omega}p reaction from the threshold to very large energies. At low energies the pion exchange is the dominant mechanism. At large energies the experimental cross section can be well described assuming Pomeron exchange within the k{sub t}-factorization approach by adjusting light quark constituent mass. Next we calculate differential distributions for the pp{yields}pp{omega} reaction at RHIC, Tevatron and LHC energies for the first time in the literature. We consider photon-Pomeron (Pomeron-photon), photon-pion (pion-photon) as well as novel diffractive hadronic bremsstrahlung mechanisms. The latter are included in the meson/Reggeon exchange picture with parameters fixed from the known phenomenology. Interesting rapidity distributions are predicted. The hadronic bremsstrahlung contributions dominate at large (forward, backward) rapidities. At small energies the photon-Pomeron contribution is negligible compared to the bremsstrahlung contributions. It could be, however, easily identified at large energies at midrapidities. Absorptions effects are included and discussed. Our predictions are ready for verification at RHIC and LHC.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Photoproduction of K{Sigma}(1385) from the nucleon

The reactions of K{Sigma}(1385) photoproduction, i.e., {gamma}p {yields} K{sup +}{Sigma}{sup 0}(1385) and {gamma} {yields} K{sup +}{Sigma}{sup -}(1385), are investigated in the resonance energy region for studying the role of the nucleon and {Delta} resonances of masses around 2 GeV. The Lagrangians for describing the decays of these resonances into the K{Sigma}(1385) channel are constructed and the decay amplitudes are obtained, which allows us to determine the coupling constants using the predictions of quark models or the data listed by the Particle Data Group. The resulting cross sections are compared to the data from the Thomas Jefferson National Accelerator Facility and the SPring-8, which indicates nontrivial contributions from the two-star-rated resonances in the Particle Data Group as well as from some missing resonances predicted by a quark model.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Physics Prospects with GlueX

The new experiment GlueX is currently being constructed at Jefferson Lab. The experiment was designed to search for hybrid mesons with exotic-quantum-numbers using a beam of linearly polarized photons incident on a liquid hydrogen target. We will discuss the discovery potential of the GlueX experiment and briefly overview its physics program.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Valence quark contributions for the {gamma}N{yields}P{sub 11}(1440) transition

A covariant spectator quark model is applied to estimate the valence quark contributions to the F{sub 1}{sup *}(Q{sup 2}) and F{sub 2}{sup *}(Q{sup 2}) transition form factors for the {gamma}N{yields}P{sub 11}(1440) reaction. The Roper resonance, P{sub 11}(1440), is assumed to be the first radial excitation of the nucleon. The model requires no extra parameters except for those already fixed by the previous studies for the nucleon. The results are consistent with the experimental data in the high Q{sup 2} region, and those from the lattice QCD. Finally the model is also applied to estimate the meson cloud contributions from the CLAS and MAID analysis.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Coherent photoproduction of {pi}{sup +} from {sup 3}He with CLAS at Jefferson Lab

We have measured the differential cross section for the {gamma}{sup 3}He{yields}{pi}{sup +}t reaction. This reaction was studied using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. Real photons produced with the Hall-B bremsstrahlung tagging system in the energy range from 0.5 to 1.55 GeV were incident on a cryogenic liquid {sup 3}He target. The differential cross sections for the {gamma}{sup 3}He{yields}{pi}{sup +}t reaction were measured as a function of photon-beam energy and pion-scattering angle. Theoretical predictions to date cannot explain the large cross sections except at backward angles, showing that additional components must be added to the model.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗