393 research outputs found
Electroweak String Configurations with Baryon Number
In the context of electroweak strings, the baryon number anomaly equation may
be reinterpreted as a conservation law for baryon number minus helicity. Since
the helicity is a sum of the link and twist numbers, linked or twisted loops of
electroweak string carry baryon number. We evaluate the change in the baryon
number obtained by delinking loops of electroweak string and show that
twisted electroweak string segments may be regarded as extended sphalerons. We
also suggest an alternative scenario for electroweak baryogenesis.Comment: 11 pages, figure available on request. Added discussion of
string-sphaleron connection for non-vanishing Weinberg angle and shortened
discussion on formation of linked configuration
Probing Primordial Magnetism with Off-Diagonal Correlators of CMB Polarization
Primordial magnetic fields (PMF) can create polarization -modes in the
cosmic microwave background (CMB) through Faraday rotation (FR), leading to
non-trivial 2-point and 4-point correlators of the CMB temperature and
polarization. We discuss the detectability of primordial magnetic fields using
different correlators and evaluate their relative merits. We have fully
accounted for the contamination by weak lensing, which contributes to the
variance, but whose contribution to the 4-point correlations is orthogonal to
that of FR. We show that a Planck-like experiment can detect scale-invariant
PMF of nG strength using the FR diagnostic at 90GHz, while realistic future
experiments at the same frequency can detect 10^{-10} G. Utilizing multiple
frequencies will improve on these prospects, making FR of CMB a powerful probe
of scale-invariant PMF.Comment: 11 pages, 4 figures; unit typos fixed in fig 1 and
Fermions on one or fewer Kinks
We find the full spectrum of fermion bound states on a Z_2 kink. In addition
to the zero mode, there are int[2 m_f/m_s] bound states, where m_f is the
fermion and m_s the scalar mass. We also study fermion modes on the background
of a well-separated kink-antikink pair. Using a variational argument, we prove
that there is at least one bound state in this background, and that the energy
of this bound state goes to zero with increasing kink-antikink separation, 2L,
and faster than e^{-a2L} where a = min(m_s, 2 m_f). By numerical evaluation, we
find some of the low lying bound states explicitly.Comment: 7 pages, 4 figure
Space of kink solutions in SU(N)\times Z_2
We find distinct classes (``generations'') of kink solutions in an
field theory. The classes are labeled by an integer . The
members of one class of kinks will be globally stable while those of the other
classes may be locally stable or unstable. The kink solutions in the
class have a continuous degeneracy given by the manifold ,
where is the unbroken symmetry group and is the group under which the
kink solution remains invariant. The space is found to contain
incontractable two spheres for some values of , indicating the possible
existence of certain incontractable spherical structures in three dimensions.
We explicitly construct the three classes of kinks in an SU(5) model with
quartic potential and discuss the extension of these ideas to magnetic monopole
solutions in the model.Comment: 11 pages, 3 figures. Several minor changes made. Matches the version
accepted to PR
Defect Production in Slow First Order Phase Transitions
We study the formation of vortices in a U(1) gauge theory following a
first-order transition proceeding by bubble nucleation, in particular the
effect of a low velocity of expansion of the bubble walls. To do this, we use a
two-dimensional model in which bubbles are nucleated at random points in a
plane and at random times and then expand at some velocity .
Within each bubble, the phase angle is assigned one of three discrete values.
When bubbles collide, magnetic `fluxons' appear: if the phases are different, a
fluxon--anti-fluxon pair is formed. These fluxons are eventually trapped in
three-bubble collisions when they may annihilate or form quantized vortices. We
study in particular the effect of changing the bubble expansion speed on the
vortex density and the extent of vortex--anti-vortex correlation.Comment: 13 pages, RevTeX, 15 uuencoded postscript figure
On the Detection of Magnetic Helicity
Magnetic fields in various astrophysical settings may be helical and, in the
cosmological context, may provide a measure of primordial CP violation during
baryogenesis. Yet it is difficult, even in principle, to devise a scheme by
which magnetic helicity may be detected, except in some very special systems.
We propose that charged cosmic rays originating from known sources may be
useful for this purpose. We show that the correlator of the arrival momenta of
the cosmic rays is sensitive to the helicity of an intervening magnetic field.
If the sources themselves are not known, the method may still be useful
provided we have some knowledge of their spatial distribution.Comment: 5 pages, 1 figure, discussions and references added, submited to
Phys. Rev.
A New Perspective on Electroweak Strings
The vortex solution (Z-string) of the electroweak interactions can be
interpreted as the 2-dimensional sphaleron at the top of a non-contractible
sphere. The same holds for another type of solution, the W-string.Comment: 13 pages, Latex, NIKHEF-H/94-02 (February 2, 1994), 1 figure
available by fax or mail (send request to [email protected]
Quantized Non-Abelian Monopoles on S^3
A possible electric-magnetic duality suggests that the confinement of
non-Abelian electric charges manifests itself as a perturbative quantum effect
for the dual magnetic charges. Motivated by this possibility, we study vacuum
fluctuations around a non-Abelian monopole-antimonopole pair treated as point
objects with charges g=\pm n/2 (n=1,2,...), and placed on the antipodes of a
three sphere of radius R. We explicitly find all the fluctuation modes by
linearizing and solving the Yang-Mills equations about this background field on
a three sphere. We recover, generalize and extend earlier results, including
those on the stability analysis of non-Abelian magnetic monopoles. We find that
for g \ge 1 monopoles there is an unstable mode that tends to squeeze magnetic
flux in the angular directions. We sum the vacuum energy contributions of the
fluctuation modes for the g=1/2 case and find oscillatory dependence on the
cutoff scale. Subject to certain assumptions, we find that the contribution of
the fluctuation modes to the quantum zero point energy behaves as -R^{-2/3} and
hence decays more slowly than the classical -R^{-1} Coulomb potential for large
R. However, this correction to the zero point energy does not agree with the
linear growth expected if the monopoles are confined.Comment: 18 pages, 5 figures. Minor changes, reference list update
Comment on ``Constraints on the strength of primordial B-fields from big bang nucleosynthesis reexamined''
Recently Cheng, Olinto, Schramm and Truran (COST) reexamined the constraints
from big bang nucleosynthesis (BBN) on the strength of primordial magnetic
fields. Their bottom line agreed with that of an earlier recent paper on the
subject (Kernan, Starkman and Vachaspati (KSV)), both in its final limit on the
magnetic field during BBN, and in its conclusion that for allowed values of the
magnetic field the dominant factor for BBN is the increased expansion rate at a
given temperature caused by the energy density of the magnetic field,
. However, their conclusion that weak interaction rates increased
with increasing B-field at these low field values contradicted the earlier
results of KSV. In this comment we point out that the Taylor series expansion
of the weak interaction rate about B=0 used in COST is not well-defined, while
the Euler-McLaurin expansion of KSV is well-behaved and reliable. Using the
Euler-McLaurin expansion we find that the weak interaction rates decrease
rather than increase with increasing B-field at small values of the B-field.Comment: 4 pages, Latex, submitted to Phys. Rev.
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