319 research outputs found
Hyperfine Populations Prior to Muon Capture
It is shown that the 1S level hyperfine populations prior to muon capture
will be statistical when either target or beam are unpolarised independent of
the atomic level at which the hyperfine interaction becomes appreciable. This
assertion holds in the absence of magnetic transitions during the cascade and
is true because of minimal polarisation after atomic capture and selective
feeding during the cascade.Comment: (revtex, 6 preprint pages, no figures
Final state interaction effects in mu-capture induced two-body decay of 3He
The mu-capture process on 3He leading to a neutron, a deuteron and a
mu-neutrino in the final state is studied. Three-nucleon Faddeev wave functions
for the initial 3He bound and the final neutron-deuteron scattering states are
calculated using the BonnB and Paris nucleon-nucleon potentials. The nuclear
weak current operator is restricted to impulse approximation. Large effects on
the decay rates of the final state interaction are found. The comparison to
recent experimental data shows that the inclusion of final state interactions
drastically improves the description of the data.Comment: 14 pages, 6 eps figure
The Axial-Vector Current in Nuclear Many-Body Physics
Weak-interaction currents are studied in a recently proposed effective field
theory of the nuclear many-body problem. The Lorentz-invariant effective field
theory contains nucleons, pions, isoscalar scalar () and vector
() fields, and isovector vector () fields. The theory exhibits a
nonlinear realization of chiral symmetry and has three
desirable features: it uses the same degrees of freedom to describe the
axial-vector current and the strong-interaction dynamics, it satisfies the
symmetries of the underlying theory of quantum chromodynamics, and its
parameters can be calibrated using strong-interaction phenomena, like hadron
scattering or the empirical properties of finite nuclei. Moreover, it has
recently been verified that for normal nuclear systems, it is possible to
systematically expand the effective lagrangian in powers of the meson fields
(and their derivatives) and to reliably truncate the expansion after the first
few orders. Here it is shown that the expressions for the axial-vector current,
evaluated through the first few orders in the field expansion, satisfy both
PCAC and the Goldberger--Treiman relation, and it is verified that the
corresponding vector and axial-vector charges satisfy the familiar chiral
charge algebra. Explicit results are derived for the Lorentz-covariant,
axial-vector, two-nucleon amplitudes, from which axial-vector meson-exchange
currents can be deduced.Comment: 32 pages, REVTeX 4.0 with 12pt.rtx, aps.rtx, revsymb.sty,
revtex4.cls, plus 14 figures; two sentences added in Summary; two references
adde
Theoretical Study of the ^3He(mu^-,nu_mu)^3H Capture
The ^3He(mu^-,nu_mu)^3H weak capture is studied using
correlated-hyperspherical-harmonics wave functions, obtained from realistic
Hamiltonians consisting of the Argonne or Argonne
two-nucleon, and Tucson-Melbourne or Urbana-IX three-nucleon interactions. The
nuclear weak charge and current operators have vector and axial-vector
components that include one- and two-body contributions. The strength of the
leading two-body operator in the axial-vector current is adjusted to reproduce
the Gamow-Teller matrix element in tritium -decay. The calculated total
capture rate is in excellent agreement with the most recent experimental
determination sec, when the PCAC value is adopted for the
induced pseudo-scalar coupling constant . The predictions for the
capture rate and angular correlation parameters , , and
are found to be only very weakly dependent on the model input Hamiltonian. The
variation of these observables with and the theoretical uncertainties
deriving from the model-dependent procedure used to constrain the axial current
are investigated.Comment: 16 pages, 1 figure, submitted to PR
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
Governments, decentralisation, and the risk of electoral defeat
<p>In the last three decades several countries around the world have transferred authority from their national to their regional governments. However, not all their regions have been empowered to the same degree and important differences can be observed between and within countries. Why do some regions obtain more power than others? Current literature argues that variation in the redistribution of power and resources between regions is introduced by demand. Yet these explanations are conditional on the presence of strong regionalist parties or territorial cleavages. This article proposes instead a theory that links the governmentâs risk of future electoral defeat with heterogeneous decentralisation, and tests its effects using data from 15 European countries and 141 regions. The results provide evidence that parties in government protect themselves against the risk of electoral defeat by selectively targeting decentralisation towards regions in which they are politically strong. The findings challenge previous research that overestimates the importance of regionalist parties while overlooking differences between regions.</p
The Collective Action Problem: Within-Group Cooperation and Between-Group Competition in a Repeated Rent-Seeking Game
This paper analyzes the ability of group members to cooperate in rent-seeking activities in a context of between-group competition. For this purpose, we develop an infinitely repeated rent-seeking game between two groups of different size. We first investigate Nash reversion strategies to support cooperative behavior in a given group before analyzing double-edge trigger strategies which have the property
that cheating on the cooperative agreement in a given group is followed by non-cooperation in this group and cooperation in the rival group. The main conclusion is that the set of parameters for which cooperation
can be sustained within the larger group as a subgame perfect outcome is as large as that for which cooperation can be sustained in the smaller group. Hence, in contrast with Olsonâs (1965) celebrated thesis but in accordance with many informal and formal observations, the larger group is as effective as the smaller group in furthering its interest
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An evolutionary approach to international political economy: the case of corporate tax avoidance
Corporate tax avoidance is both widespread and diverse in its practical mechanics. The scope of the phenomenon often leads economists to conclude that in the jungle of economic competition, tax planning (or optimisation) is among the necessary tools to ensure the survival of the fittest. This theory is increasingly associated with a Darwinian theory of economic evolution. In this paper, I develop a contrasting framework of the evolutionary political economy of corporate tax avoidance. Analysing core concepts of Old Institutionalist Economics (OIE), I examine the core drivers of corporate tax avoidance in a globalised system of states. The major contrast, I find, is between that of the corporate and legal personality and the institutional environment in which it operates. Historically, each corporate entity has been considered a separate legal person, yet a series of âmutationsâ of incorporations laws created a widening gap between theory and reality, and these, in turn, give rise to tax arbitrage. Narrowing this gap, however, impinges on another venerable historical institution, the institution of sovereignty and sovereign inequality
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