147 research outputs found
Relative space-time asymmetries in pion and nucleon production in non-central nucleus-nucleus collisions at high energies
We propose to use the ratio of the pion-proton correlation functions
evaluated under different conditions to study the relative space-time
asymmetries in pion and proton emission (pion and nucleon source relative
shifts) in high energy heavy ion collision. We address the question of the
non-central collisions, where the sources can be shifted spatially both in the
longitudinal and in the transverse directions in the reaction plane. We use the
RQMD event generator to illustrate the effect and the technique.Comment: RevTex, 4 pages, 3 figures included as eps file
Towards the solution of the anomaly in shell-model calculations of muon capture
Recently many authors have performed shell-model calculations of nuclear
matrix elements determining the rates of the ordinary muon capture in light
nuclei. These calculations have employed well-tested effective interactions in
large scale shell-model studies. For one of the nuclei of interest, namely
Si, there exists recent experimental data which can be used to deduce
the value of the ratio by using the calculated matrix elements.
Surprisingly enough, all the abovementioned shell-model results suggest a very
small value () for , quite far from the PCAC prediction
and recent data on muon capture in hydrogen. We show that this rather
disturbing anomaly is solved by employing effective transition operators. This
finding is also very important in studies of the scalar coupling of the weak
charged current of leptons and hadrons.Comment: Revtex, 6 pages, 2 figs include
Elastic Pion Scattering on the Deuteron in a Multiple Scattering Model
Pion elastic scattering on deuterium is studied in the KMT multiple
scattering approach developed in momentum space. Using a Paris wave function
and the same methods and approximations as commonly used in pion scattering on
heavier nuclei excellent agreement with differential cross section data is
obtained for a wide range of pion energies. Only for MeV and very
backward angles, discrepancies appear that are reminiscent of disagreements in
pion scattering on He, H, and He. At low energies the second order
corrections have been included. Polarization observables are studied in detail.
While tensor analyzing powers are well reproduced, vector analyzing powers
exhibit dramatic discrepancies.Comment: 25 pages LATEX and 9 postscript figures in a self-extracting uufile
archiv
Pion interaction with the trinucleon up to the eta production threshold
Pion elastic, charge exchange scattering and induced eta production on the
trinucleon systems are investigated in a coupled-channels approach in momentum
space with Fadeev wave functions. The channel is
included using an isobar model with S-, P-, and D-wave resonances. While the
coherent reactions like He(He can be reasonably well reproduced
up to =500 MeV, large discrepancies appear for the incoherent
processes, He(H and He(H at backward
angles and energies above -resonance. In the forward direction the
calculations underestimate the experimental measurements very
close to threshold but agreement with the data improves with increasing pion
energy. Predictions are made for the asymmetries of the various reactions on
polarized He.Comment: 40 pages, 12 figures (available from the authors), Mainz preprint
MKPH-T-92-1
Weak Transitions in A=6 and 7 Nuclei
The He beta decay and Be electron capture processes are studied using
variational Monte Carlo wave functions, derived from a realistic Hamiltonian
consisting of the Argonne two-nucleon and Urbana-IX three-nucleon
interactions. The model for the nuclear weak axial current includes one- and
two-body operators with the strength of the leading two-body term--associated
with -isobar excitation of the nucleon--adjusted to reproduce the
Gamow-Teller matrix element in tritium -decay. The measured half-life of
. He is under-predicted by theory by 8%, while that of Be for
decay into the ground and first excited states of Li is over-predicted by
9%. However, the experimentally known branching ratio for these latter
processes is in good agreement with the calculated value. Two-body axial
current contributions lead to a 1.7% (4.4%) increase in the value of
the Gamow-Teller matrix element of He (Be), obtained with one-body
currents only, and slightly worsen (appreciably improve) the agreement between
the calculated and measured half-life. Corrections due to retardation effects
associated with the finite lepton momentum transfers involved in the decays, as
well as contributions of suppressed transitions induced by the weak vector
charge and axial current operators, have also been calculated and found to be
negligible.Comment: 23 pages 8 tables. submitted to Phys. Rev.
Microscopic Calculation of Total Ordinary Muon Capture Rates for Medium - Weight and Heavy Nuclei
Total Ordinary Muon Capture (OMC) rates are calculated on the basis of the
Quasiparticle Random Phase Approximation for several spherical nuclei from
90^Zr to 208^Pb. It is shown that total OMC rates calculated with the free
value of the axial-vector coupling constant g_A agree well with the
experimental data for medium-size nuclei and exceed considerably the
experimental rates for heavy nuclei. The sensitivity of theoretical OMC rates
to the nuclear residual interactions is discussed.Comment: 27 pages and 3 figure
Quasifree Pion Electroproduction from Nuclei in the Region
We present calculations of the reaction in the
distorted wave impulse approximation. The reaction allows for the study of the
production process in the nuclear medium without being obscured by the details
of nuclear transition densities. First, a pion electroproduction operator
suitable for nuclear calculations is obtained by extending the Blomqvist-Laget
photoproduction operator to the virtual photon case. The operator is gauge
invariant, unitary, reference frame independent, and describes the existing
data reasonably well. Then it is applied in nuclei to predict nuclear cross
sections under a variety of kinematic arrangements. Issues such as the effects
of gauge-fixing, the interference of the resonance with the
background, sensitivities to the quadrupole component of the
excitation and to the electromagnetic form factors, the role of final-state
interactions, are studied in detail. Methods on how to experimentally separate
the various pieces in the coincidence cross section are suggested. Finally, the
model is compared to a recent SLAC experiment.Comment: 27 pages in REVTEX, plus 22 PS figures embedded using psfig.sty
(included), uuencode
Polarized photons in radiative muon capture
We discuss the measurement of polarized photons arising from radiative muon
capture. The spectrum of left circularly polarized photons or equivalently the
circular polarization of the photons emitted in radiative muon capture on
hydrogen is quite sensitive to the strength of the induced pseudoscalar
coupling constant . A measurement of either of these quantities, although
very difficult, might be sufficient to resolve the present puzzle resulting
from the disagreement between the theoretical prediction for and the
results of a recent experiment. This sensitivity results from the absence of
left-handed radiation from the muon line and from the fact that the leading
parts of the radiation from the hadronic lines, as determined from the chiral
power counting rules of heavy-baryon chiral perturbation theory, all contain
pion poles.Comment: 10 pages, 6 figure
Induced pseudoscalar coupling of the proton weak interaction
The induced pseudoscalar coupling is the least well known of the weak
coupling constants of the proton's charged--current interaction. Its size is
dictated by chiral symmetry arguments, and its measurement represents an
important test of quantum chromodynamics at low energies. During the past
decade a large body of new data relevant to the coupling has been
accumulated. This data includes measurements of radiative and non radiative
muon capture on targets ranging from hydrogen and few--nucleon systems to
complex nuclei. Herein the authors review the theoretical underpinnings of
, the experimental studies of , and the procedures and uncertainties
in extracting the coupling from data. Current puzzles are highlighted and
future opportunities are discussed.Comment: 58 pages, Latex, Revtex4, prepared for Reviews of Modern Physic
Finite-size effect on two-particle production in continuous and discrete spectrum
The formalism allowing one to account for the effect of a finite space-time
extent of particle production region is given. Its applications to the lifetime
measurement of hadronic atoms produced by a high-energy beam in a thin target,
as well as to the femtoscopy techniques widely used to measure space-time
characteristics of the production processes, are discussed. Particularly, it is
found that the neglect of the finite-size effect on the pionium lifetime
measurement in the experiment DIRAC at CERN could lead to the lifetime
overestimation comparable with the 10% statistical error. The theoretical
systematic errors arising in the calculation of the finite-size effect due to
the neglect of non-equal emission times in the pair center-of-mass system, the
space-time coherence and the residual charge are shown to be negligible.Comment: LaTeX, 77 pages including 5 tables and 18 figures. Somewhat extended
version to be published in Phys. El. Part. At. Nuc
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