17 research outputs found
Exact factorization of the time-dependent electron-nuclear wavefunction
We present an exact decomposition of the complete wavefunction for a system
of nuclei and electrons evolving in a time-dependent external potential. We
derive formally exact equations for the nuclear and electronic wavefunctions
that lead to rigorous definitions of a time-dependent potential energy surface
(TDPES) and a time-dependent geometric phase. For the molecular ion
exposed to a laser field, the TDPES proves to be a useful interpretive tool to
identify different mechanisms of dissociation.Comment: 4 pages, 2 figure
Nonequilibrium Dynamics in Noncommutative Spacetime
We study the effects of spacetime noncommutativity on the nonequilibrium
dynamics of particles in a thermal bath. We show that the noncommutative
thermal bath does not suffer from any further IR/UV mixing problem in the sense
that all the finite-temperature non-planar quantities are free from infrared
singularities. We also point out that the combined effect of finite temperature
and noncommutative geometry has a distinct effect on the nonequilibrium
dynamics of particles propagating in a thermal bath: depending on the momentum
of the mode of concern, noncommutative geometry may switch on or switch off
their decay and thermalization. This momentum dependent alternation of the
decay and thermalization rates could have significant impacts on the
nonequilibrium phenomena in the early universe at which spacetime
noncommutativity may be present. Our results suggest a re-examination of some
of the important processes in the early universe such as reheating after
inflation, baryogenesis and the freeze-out of superheavy dark matter
candidates.Comment: 24 pages, 2 figure
The Effect of Gravitational Tidal Forces on Renormalized Quantum Fields
The effect of gravitational tidal forces on renormalized quantum fields
propagating in curved spacetime is investigated and a generalisation of the
optical theorem to curved spacetime is proved. In the case of QED, the
interaction of tidal forces with the vacuum polarization cloud of virtual e^+
e^- pairs dressing the renormalized photon has been shown to produce several
novel phenomena. In particular, the photon field amplitude can locally increase
as well as decrease, corresponding to a negative imaginary part of the
refractive index, in apparent violation of unitarity and the optical theorem.
Below threshold decays into e^+ e^- pairs may also occur. In this paper, these
issues are studied from the point of view of a non-equilibrium initial-value
problem, with the field evolution from an initial null surface being calculated
for physically distinct initial conditions and for both scalar field theories
and QED. It is shown how a generalised version of the optical theorem, valid in
curved spacetime, allows a local increase in amplitude while maintaining
consistency with unitarity. The picture emerges of the field being dressed and
undressed as it propagates through curved spacetime, with the local
gravitational tidal forces determining the degree of dressing and hence the
amplitude of the renormalized quantum field. These effects are illustrated with
many examples, including a description of the undressing of a photon in the
vicinity of a black hole singularity.Comment: 76 pages, jheppub.sty, 10 figures, small corrections. arXiv admin
note: text overlap with arXiv:1006.014
New Constraints (and Motivations) for Abelian Gauge Bosons in the MeV-TeV Mass Range
We survey the phenomenological constraints on abelian gauge bosons having
masses in the MeV to multi-GeV mass range (using precision electroweak
measurements, neutrino-electron and neutrino-nucleon scattering, electron and
muon anomalous magnetic moments, upsilon decay, beam dump experiments, atomic
parity violation, low-energy neutron scattering and primordial
nucleosynthesis). We compute their implications for the three parameters that
in general describe the low-energy properties of such bosons: their mass and
their two possible types of dimensionless couplings (direct couplings to
ordinary fermions and kinetic mixing with Standard Model hypercharge). We argue
that gauge bosons with very small couplings to ordinary fermions in this mass
range are natural in string compactifications and are likely to be generic in
theories for which the gravity scale is systematically smaller than the Planck
mass - such as in extra-dimensional models - because of the necessity to
suppress proton decay. Furthermore, because its couplings are weak, in the
low-energy theory relevant to experiments at and below TeV scales the charge
gauged by the new boson can appear to be broken, both by classical effects and
by anomalies. In particular, if the new gauge charge appears to be anomalous,
anomaly cancellation does not also require the introduction of new light
fermions in the low-energy theory. Furthermore, the charge can appear to be
conserved in the low-energy theory, despite the corresponding gauge boson
having a mass. Our results reduce to those of other authors in the special
cases where there is no kinetic mixing or there is no direct coupling to
ordinary fermions, such as for recently proposed dark-matter scenarios.Comment: 49 pages + appendix, 21 figures. This is the final version which
appears in JHE
Composite gauge fields in renormalizable models
A generalization of the non-Abelian version of the CP{sup {ital N}{minus}1} models (also known as Grassmannian models) is presented. The generalization helps accommodate a partial breaking of the non-Abelian gauge symmetry. Constituents of the composite gauge fields, in many cases, are naturally constrained to belong to an anomaly-free representation which in turn generates a composite scalar-simulating Higgs mechanism to break the gauge symmetry dynamically for large {ital N}. Two cases are studied in detail: one based on the SU(2) gauge group and the other on SO(10). Breakings such as SU(2){r_arrow}U(1) or SO(10){r_arrow}SU(5){times}U(1) are found feasible. The properties of the composites fields and gauge boson masses are computed by doing a derivative expansion of the large {ital N} effective action
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CP violation from finite groups
We discuss the origin of CP violation in settings with a discrete (flavor) symmetry G. We show that physical CP transformations always have to be class-inverting automorphisms of G. This allows us to categorize finite groups into three types: (i) Groups that do not exhibit such an automorphism and, therefore, in generic settings, explicitly violate CP. In settings based on such groups, CP violation can have pure group-theoretic origin and can be related to the complexity of some Clebsch-Gordan coefficients. (ii) Groups for which one can find a CP basis in which all the Clebsch-Gordan coefficients are real. For such groups, imposing CP invariance restricts the phases of coupling coefficients. (iii) Groups that do not admit real Clebsch-Gordan coefficients but possess a class-inverting automorphism that can be used to define a proper (generalized) CP transformation. For such groups, imposing CP invariance can lead to an additional symmetry that forbids certain couplings. We make use of the so-called twisted Frobenius-Schur indicator to distinguish between the three types of discrete groups. With ÎŽ(27), T ', and Ï(72) we present one explicit example for each type of group, thereby illustrating the CP properties of models based on them. We also show that certain operations that have been dubbed generalized CP transformations in the recent literature do not lead to physical CP conservation. © 2014 The Authors