119464817
Polishing the lens: I. Pionic final state interactions and HBT correlations: distorted wave emission-function (DWEF) formalism and examples
The emission of pions produced within a dense, strongly interacting system of matter in the presence of strong radial flow and absorption is described using a relativistic optical model formalism, replacing the attenuated or unattenuated plane waves of earlier emission-function approaches with 'distorted wave' solutions to a relativistic wave equation including a complex optical potential. The resulting distorted wave emission-function model (DWEF) is used in numerical calculations to fit HBT correlations and the resonance-corrected pion spectrum from central-collision STAR Au+Au pion data at . Excellent agreement with the STAR data is obtained. The applications are extended by applying linear participant scaling to the spacetime parameters of the model while keeping all other parameters fixed. This allows us to predict HBT radii over a range of centralities for both Au+Au and Cu+Cu collisions. Good agreement is found with STAR HBT data for all but the most peripheral collisions. The squares of pionic distorted wavefunctions, obtained as exact numerical solutions to the wave equation, are displayed and significant differences with the results of using the familiar eikonal approximation are found. Using the eikonal approximation leads to a qualitative accounting for the effects of the imaginary part of the optical potential, but fails entirely to include the effects of the real part of the optical potential. A simple example is used to illustrate that an attractive optical potential can have large effects on extracting radii and can also lead to oscillations in radii measured at low momenta.
120464871
Deuteron photodisintegration: Impulse approximation and gauge invariance
We study the question of whether calculations of deuteron photodisintegration respect gauge invariance, and show that the impulse approximation is in severe disagreement with gauge invariance. The method in which Siegert's theorem is implemented is consistent with gauge invariance to within ten percent. We introduce a modification of the impulse approximation in which the current is conserved exactly. For deuteron photodisintegration this method yields essentially the same results as that of Partovi. Our method can easily be applied to electron- or neutrino-nucleus scattering.
122904929
Isobar dynamics and pion-nucleus elastic scattering
Abstract A new formalism is developed for studying pion-nucleus scattering in a model which takes into account the dynamics of the (3, 3) pion-nucleon resonance, or Δ isobar. This treatment is used to calculate π + elastic scattering from 16 O, 40 Ca, 48 Ca and 208 Pb at energies from 114 to 240 MeV. Some results for π − elastic scattering are also given. From fits to π + scattering data it is found that the Δ-nucleus interaction is well described by a spherical local complex potential proportional to the nuclear density. The central strength of this potential depends on energy but not on nuclear mass number. Some difficulties in determining the parameters of this potential from elastic scattering are discussed.
118854850
Polarized quark distributions in nuclear matter
We compute the polarized quark distribution function of a bound nucleon. The chiral quark-soliton model provides the quark and antiquark substructure of the nucleon embedded in nuclear matter. Nuclear effects cause significant modifications to the polarized distributions, including an enhancement of the axial coupling constant.
119513589
Shapes of the proton
A model proton wave function, constructed using Poincar\'e invariance and constrained by recent electromagnetic form factor data, is used to study the shape of the proton. Spin-dependent quark densities are defined as matrix elements of density operators in proton states of definite polarization, and shown to have an infinite variety of nonspherical shapes. For high momentum quarks with spin parallel to that of the proton, the shape resembles that of a peanut, but for quarks with antiparallel spin the shape is that of a bagel.
356245
Current algebra and the cloudy-bag model.
Two versions of the cloudy-bag model (surface and volume coupling) satisfy the commutation relations of current algebra. Perturbation-theory evaluations of various observables are compared for the two treatments. The electromagnetic properties are essentially the same in both versions. The volume-coupling model contains, in lowest order, a pion-quark contact interaction that contributes to the axial-vector current. Including the effect of this term leads to a computed value of g/sub A/ of about 1.25, if the radius of the nucleon bag is around 1.0 fm.
121623246
Semiconductor Particle Detectors
The operation, properties, and fabrication of semiconductor detectors are reviewed. Materials other than Ge and Si are mentioned only briefly. The electronics involved in use of semiconductors as particle detectors is discussed. Application and possible applications to the following are summarized: nuclear reaction chargedparticle spectroscopy, particle identification, mosaics and special structures, alpha -particle spectroscopy, fission studies, neutron spectroscopy, BETA spectroscopy, gamma spectroscopy, detection of minimum- ionizing particles, low-temperature physics, space vehicles, industry, and medicine.
119233886
On the origin of gravitational Lorentz covariance
We provide evidence that general relativity (GR) is the unique spatially covariant effective field theory of the transverse, traceless graviton degrees of freedom. The Lorentz covariance of GR, having not been assumed in our analysis, is thus plausibly interpreted as an accidental or emergent symmetry of the gravitational sector.
37692138
Dirac-Born-Infeld genesis: an improved violation of the null energy condition.
We show that the Dirac-Born-Infeld conformal galileons, derived from the world-volume theory of a 3-brane moving in an anti-de Sitter bulk, admit a background, stable under quantum corrections, which violates the null energy condition. The perturbations around this background are stable and propagate subluminally. Unlike other known examples of null energy condition violation, such as ghost condensation and conformal galileons, this theory also admits a stable, Poincaré-invariant vacuum. The 2 → 2 amplitude satisfies standard analyticity conditions. The full S matrix is likely not analytic, however, since perturbations around deformations of the Poincaré invariant vacuum propagate superluminally.