Mark W. Paris

Hybrid Stars that Masquerade as Neutron Stars

We show that a hybrid (nuclear+quark matter) star can have a mass-radius relationship very similar to that predicted for a star made of purely nucleonic matter. We show this for a generic parameterization of the quark matter equation of state and also for an MIT bag model, each including a phenomenological correction based on gluonic corrections to the equation of state. We obtain hybrid stars as heavy as 2 M☉ for reasonable values of the bag model parameters. For nuclear matter, we use the equation of state calculated by Akmal and coworkers using many-body techniques. Both mixed and homogeneous phases of nuclear and quark matter are considered.

Unified Chew-Mandelstam SAID analysis of pion photoproduction data

A wealth of N ! �N data, for single- and doublepolarization observables, is anticipated from electromagnetic facilities worldwide over the coming months and years. These data will be pivotal in determining the underlying amplitudes in complete experiments, and in discerning between various microscopic models of multichannel reaction theory. The focus of precision electromagnetic measurements, over the nucleon resonance region, is to more fully map the non-perturbative regime of quantum chromodynamics, the fundamental theory of the strong interaction, to shed light on its confining and chiral symmetry breaking properties. These electromagnetic data take the field to the next and necessary level of precision. This is required in order to obtain a theoretical description of the nucleon that both explains and subsumes the simple constituent quark model, which has provided a qualitative picture of nucleon structure and reactions. The expected data heralds an era of precision hadron spectroscopy, particularly for baryons, and has ushered in a renaissance in hadronic reaction theory. Significant refinements in the quality and quantity of available data offer the opportunity to develop more sophisticated models of hadronic reactions, constrained by fundamental principles of field theory, such as unitarity and gauge invariance, which have model dependencies under better control, if not eliminated. Such a complete and successful phenomenology would appear to be a prerequisite for a deeper understanding in terms of quarks and gluons [1]. The present manuscript details multipole analyses of the single-pion photoproduction data using a parametrization form related to, but an improvement upon, previous SAID parametrizations [2–4]. The energy-dependent (ED) analysis is performed over the center-of-mass energy (W) range from the near-threshold region to about 2.5 GeV, including resonances through

Parity-Violating Interaction Effects in the np System

We investigate parity-violating observables in the np system, including the longitudinal asymmetry and neutron-spin rotation in np elastic scattering, the photon asymmetry in np radiative capture, and the asymmetries in deuteron photo-disintegration d(gamma,n)p in the threshold region and electro-disintegration d(e,e`)np in quasi-elastic kinematics. To have an estimate of the model dependence for the various predictions, a number of different, latest-generation strong-interaction potentials--Argonne v18, Bonn 2000, and Nijmegen I--are used in combination with a weak-interaction potential consisting of pi-, rho-, and omega-meson exchanges--the model known as DDH. The complete bound and scattering problems in the presence of parity-conserving, including electromagnetic, and parity-violating potentials is solved in both configuration and momentum space. The issue of electromagnetic current conservation is examined carefully. We find large cancellations between the asymmetries induced by the parity-violating interactions and those arising from the associated pion-exchange currents. In the np capture, the model dependence is nevertheless quite small, because of constraints arising through the Siegert evaluation of the relevant E1 matrix elements. In quasi-elastic electron scattering these processes are found to be insignificant compared to the asymmetry produced by gamma-Z interference on individual nucleons.

Parameterization dependence of T matrix poles and eigenphases from a fit to

We compare fits to piN elastic scattering data, based on a Chew-Mandelstam K-matrix formalism. Resonances, characterized by T-matrix poles, are compared in fits generated with and without explicit Chew-Mandelstam K-matrix poles. Diagonalization of the S matrix yields the eigenphase representation. While the eigenphases can vary significantly for the different parameterizations, the locations of most T-matrix poles are relatively stable.

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We investigate parity-violating asymmetries in polarized n p radiative capture, and deuteron electro-disintegration in quasi-elastic kinematics, using the DDH model for the parity-violating nucleon-nucleon interaction. We find dramatic cancellations between the asymmetries induced by the parity-violating interaction and those arising from the associated parity-violating pion-exchange currents. In np capture, the model-dependence of the result is nevertheless quite small because of constraints arising through the Siegert evaluation of the relevant E1 matrix element. In quasi-elastic electron scattering these processes are found to be insignificant comared to the asymmetry produced by the gamma-Z interference on individual nucleons. These two experiments, then, provide clean probes of different aspects of weak-interaction physics associated with parity violation in the np system.

N elastic scattering data

Institute for Nuclear Studies, Department of Physics,The George Washington University, Washington, D.C. 20052(Dated: September 6, 2011)Energy-dependent and single-energy fits to the existing pion photoproduction database have beenupdated to cover the region from threshold to 2.7 GeV in the laboratory photon energy. Revisedresonance photo-decay couplings have been extracted and compared to previous determinations.The influence of recent measurements is displayed. Remaining problems and future approaches arediscussed.

Parity violating interactions and currents in the deuteron

We report a new extraction of nucleon resonance couplings using pi- photoproduction cross sections on the neutron. The world database for the process gamma n --> pi- p above 1 GeV has quadrupled with the addition of new differential cross sections from the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab in Hall B. Differential cross sections from CLAS have been improved with a new final-state interaction determination using a diagramatic technique taking into account the NN and piN final-state interaction amplitudes. Resonance couplings have been extracted and compared to previous determinations. With the addition of these new cross sections, significant changes are seen in the high-energy behavior of the SAID cross sections and amplitudes.

Updated SAID analysis of pion photoproduction data

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Amplitude analysis of gamma n --> pi- p data above 1 GeV

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R-matrix description of particle energy spectra produced by low-energy 3H + 3H reactions

-matrix model for three-body final states is presented and applied to a recent measurement of the neutron energy spectrum from the