664 research outputs found
The Quest for an Intermediate-Scale Accidental Axion and Further ALPs
The recent detection of the cosmic microwave background polarimeter
experiment BICEP2 of tensor fluctuations in the B-mode power spectrum basically
excludes all plausible axion models where its decay constant is above
GeV. Moreover, there are strong theoretical, astrophysical, and cosmological
motivations for models involving, in addition to the axion, also axion-like
particles (ALPs), with decay constants in the intermediate scale range, between
GeV and GeV. Here, we present a general analysis of models
with an axion and further ALPs and derive bounds on the relative size of the
axion and ALP photon (and electron) coupling. We discuss what we can learn from
measurements of the axion and ALP photon couplings about the fundamental
parameters of the underlying ultraviolet completion of the theory. For the
latter we consider extensions of the Standard Model in which the axion and the
ALP(s) appear as pseudo Nambu-Goldstone bosons from the breaking of global
chiral (Peccei-Quinn (PQ)) symmetries, occuring accidentally as low
energy remnants from exact discrete symmetries. In such models, the axion and
the further ALP are protected from disastrous explicit symmetry breaking
effects due to Planck-scale suppressed operators. The scenarios considered
exploit heavy right handed neutrinos getting their mass via PQ symmetry
breaking and thus explain the small mass of the active neutrinos via a seesaw
relation between the electroweak and an intermediate PQ symmetry breaking
scale. We show some models that can accommodate simultaneously an axion dark
matter candidate, an ALP explaining the anomalous transparency of the universe
for -rays, and an ALP explaining the recently reported 3.55 keV gamma
line from galaxies and clusters of galaxies, if the respective decay constants
are of intermediate scale.Comment: 43pp, 4 figures. v2: version accepted for publication in JHE
Neutrino spin oscillations in gravitational fields
We study neutrino spin oscillations in gravitational fields. The
quasi-classical approach is used to describe the neutrino spin evolution. First
we examine the case of a weak gravitational field. We obtain the effective
Hamiltonian for the description of neutrino spin oscillations. We also receive
the neutrino transition probability when a particle propagates in the
gravitational field of a rotating massive object. Then we apply the general
technique to the description of neutrino spin oscillations in the Schwarzschild
metric. The neutrino spin evolution equation for the case of the neutrino
motion in the vicinity of a black hole is obtained. The effective Hamiltonian
and the transition probability are also derived. We examine the neutrino
oscillations process on different circular orbits and analyze the frequencies
of spin transitions. The validity of the quasi-classical approach is also
considered.Comment: RevTeX4, 9 pages, 1 esp figure; article was revised, some misprints
were corrected, 6 references added; accepted for publication in
Int.J.Mod.Phys.
Nondegenerate Fermions in the Background of the Sphaleron Barrier
We consider level crossing in the background of the sphaleron barrier for
nondegenerate fermions. The mass splitting within the fermion doublets allows
only for an axially symmetric ansatz for the fermion fields. In the background
of the sphaleron we solve the partial differential equations for the fermion
functions. We find little angular dependence for our choice of ansatz. We
therefore propose a good approximate ansatz with radial functions only. We
generalize this approximate ansatz with radial functions only to fermions in
the background of the sphaleron barrier and argue, that it is a good
approximation there, too.Comment: LATEX, 20 pages, 11 figure
The Sphaleron Barrier in the Presence of Fermions
We calculate the minimal energy path over the sphaleron barrier in the
pre\-sen\-ce of fermions, assuming that the fermions of a doublet are
degenerate in mass. This allows for spherically symmetric ans\"atze for the
fields, when the mixing angle dependence is neglected. While light fermions
have little influence on the barrier, the presence of heavy fermions ( TeV) strongly deforms the barrier, giving rise to additional sphalerons
for very heavy fermions ( 10 TeV). Heavy fermions form
non-topological solitons in the vacuum sector.Comment: 19 pages, latex, 18 figures in 3 seperate uuencoded postscript files
THU-93/1
On the abundance of Lithium in T Coronae Borealis
We have obtained high resolution echelle spectroscopy of the recurrent nova T
CrB. We find that the surface lithium abundance in T CrB is signifcantly
enhanced compared to field M giants, where it is not detectable. We offer
possible explanations for this in terms of either a delay in the onset of
convection in the giant star, enhanced coronal activity due to star-spots or
the enhancement of Li resulting from the nova explosion(s).Comment: 3 pages, 1 figure (a and b), accepted by MNRA
Instanton propagator and instanton induced processes in scalar model
The propagator in the instanton background in the
scalar model in four dimensions is studied.Leading and sub-leading terms of its
asymptotics for large momenta and its on-shell double residue are calculated
analytically. These results are applied to the analysis of the initial-state
and initial-final-state corrections and the calculation of the next-to-leading
(propagator) correction to the exponent of the cross section of instanton
induced multiparticle scattering processes.Comment: 44 pages, 7 postscript figures, LaTe
Schwinger Pair Production at Finite Temperature in Scalar QED
In scalar QED we study the Schwinger pair production from an initial ensemble
of charged bosons when an electric field is turned on for a finite period
together with or without a constant magnetic field. The scalar QED Hamiltonian
depends on time through the electric field, which causes the initial ensemble
of bosons to evolve out of equilibrium. Using the Liouville-von Neumann method
for the density operator and quantum states for each momentum mode, we
calculate the Schwinger pair-production rate at finite temperature, which is
the pair-production rate from the vacuum times a thermal factor of the
Bose-Einstein distribution.Comment: RevTex 10 pages, no figure; replaced by the version accepted in Phys.
Rev. D; references correcte
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