100 research outputs found

### Quenching of spin operators in the calculation of radiative corrections for nuclear beta decay

Calculations of the axial-vector component to the radiative correction for
superallowed Fermi $0^+ \rightarrow 0^+$ nuclear beta decay are here modified
with quenched rather than free-nucleon coupling constants for the axial-vector
and electromagnetic interactions with nucleons. The result increases the
deduced value of $V_{ud}$ but does not restore unitarity in the CKM matrix.Comment: LaTeX, 7 pages, no figures, TASCC-P-94-1

### REGULAR SUPPRESSION OF P,T-VIOLATING NUCLEAR MATRIX ELEMENTS

In heavy nuclei there is a parametrical suppression, $\;\sim A^{-1/3}\;$, of
T-odd, P-odd matrix elements as compared to T-even, P-odd ones.Comment: 3 page

### Radiative corrections to low energy neutrino reactions

We show that the radiative corrections to charged current (CC) nuclear
reactions with an electron(positron) in the final state are described by a
universal function. The consistency of our treatment of the radiative
corrections with the procedure used to extract the value of the axial coupling
constant $g_A$ is discussed. To illustrate we apply our results to
(anti)neutrino deuterium disintegration and to $pp$ fusion in the sun. The
limit of vanishing electron mass is considered, and a simple formula valid for
E_{obs}\gsim 1 MeV is obtained. The size of the nuclear structure-dependent
effects is also discussed. Finally, we consider CC transitions with an
electron(positron) in the initial state and discuss some applications to
electron capture reactions.Comment: 23 pages, 5 figure

### Measuring the $\nu_{\mu}$ to $\bar{\nu_{\mu}}$ Ratio in a High Statistics Atmospheric Neutrino Experiment

By exploiting differences in muon lifetimes it is possible to distinguish
$\nu_{\mu}$ from $\bar{\nu_{\mu}}$ charged current interactions in underground
neutrino detectors. Such observations would be a useful tool in understanding
the source of the atmospheric neutrino anomaly.Comment: 6 pages no figure

### Isospin-mixing corrections for fp-shell Fermi transitions

Isospin-mixing corrections for superallowed Fermi transitions in {\it
fp}-shell nuclei are computed within the framework of the shell model. The
study includes three nuclei that are part of the set of nine accurately
measured transitions as well as five cases that are expected to be measured in
the future at radioactive-beam facilities. We also include some new
calculations for $^{10}$C. With the isospin-mixing corrections applied to the
nine accurately measured $ft$ values, the conserved-vector-current hypothesis
and the unitarity condition of the Cabbibo-Kobayashi-Maskawa (CKM) matrix are
tested.Comment: 13 pages plus five tables. revtex macro

### On the Normalization of the Neutrino-Deuteron Cross Section

As is well-known, comparison of the solar neutrino fluxes measured in
SuperKamiokande (SK) by $\nu + e^- \to \nu + e^-$ and in the Sudbury Neutrino
Observatory (SNO) by $\nu_e + d \to e^- + p + p$ can provide a ``smoking gun''
signature for neutrino oscillations as the solution to the solar neutrino
puzzle. This occurs because SK has some sensitivity to all active neutrino
flavors whereas SNO can isolate electron neutrinos. This comparison depends
crucially on the normalization and uncertainty of the theoretical
charged-current neutrino-deuteron cross section. We address a number of effects
which are significant enough to change the interpretation of the SK--SNO
comparison.Comment: 4 pages, 1 figure, submitted to PR

### Effective T-odd P-even hadronic interactions from quark models

Tests of time reversal symmetry at low and medium energies may be analyzed in
the framework of effective hadronic interactions. Here, we consider the quark
structure of hadrons to make a connection to the more fundamental degrees of
freedom. It turns out that for P-even T-odd interactions hadronic matrix
elements evaluated in terms of quark models give rise to factors of 2 to 5.
Also, it is possible to relate the strength of the anomalous part of the
effective rho-type T-odd P-even tensor coupling to quark structure effects.Comment: 6 pages, 1 figure, RevTe

### Weak Transitions in A=6 and 7 Nuclei

The $^6$He beta decay and $^7$Be electron capture processes are studied using
variational Monte Carlo wave functions, derived from a realistic Hamiltonian
consisting of the Argonne $v_{18}$ 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 $\Delta$-isobar excitation of the nucleon--adjusted to reproduce the
Gamow-Teller matrix element in tritium $\beta$-decay. The measured half-life of
$^6$. He is under-predicted by theory by $\simeq$ 8%, while that of $^7$Be for
decay into the ground and first excited states of $^7$Li is over-predicted by
$\simeq$ 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 $\simeq$ 1.7% (4.4%) increase in the value of
the Gamow-Teller matrix element of $^6$He ($^7$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.

### Nuclear muon capture by 3He: meson exchange currents for the triton channel

Exchange current corrections are calculated using currents found from the
hard-pion model and AV14+3BF wavefunctions. Results are given for the rate and
spin observables. Their sensitivity to g_P, the nucleon pseudoscalar form
factor, is reported.Comment: 35 pages, uuencoded gz-compressed tar file 42 Kbyte

### Nuclear Octupole Correlations and the Enhancement of Atomic Time-Reversal Violation

We examine the time-reversal-violating nuclear ``Schiff moment'' that induces
electric dipole moments in atoms. After presenting a self-contained derivation
of the form of the Schiff operator, we show that the distribution of Schiff
strength, an important ingredient in the ground-state Schiff moment, is very
different from the electric-dipole-strength distribution, with the Schiff
moment receiving no strength from the giant dipole resonance in the
Goldhaber-Teller model. We then present shell-model calculations in light
nuclei that confirm the negligible role of the dipole resonance and show the
Schiff strength to be strongly correlated with low-lying octupole strength.
Next, we turn to heavy nuclei, examining recent arguments for the strong
enhancement of Schiff moments in octupole-deformed nuclei over that of 199Hg,
for example. We concur that there is a significant enhancement while pointing
to effects neglected in previous work (both in the octupole-deformed nuclides
and 199Hg) that may reduce it somewhat, and emphasizing the need for
microscopic calculations to resolve the issue. Finally, we show that static
octupole deformation is not essential for the development of collective Schiff
moments; nuclei with strong octupole vibrations have them as well, and some
could be exploited by experiment.Comment: 25 pages, 4 figures embedded in tex

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