41 research outputs found
A Thermodynamic Interpretation of Time for Superstring Rolling Tachyons
Rolling tachyon backgrounds, arising from open strings on unstable branes in
bosonic string theory, can be related to a simple statistical mechanical model
- Coulomb gas of point charges in two dimensions confined to a circle, the
Dyson gas. In this letter we describe a statistical system that is dual to
non-BPS branes in superstring theory. We argue that even though the concept of
time is absent in the statistical dual sitting at equilibrium, the notion of
time can emerge at the large number of particles limit.Comment: 6 pages, 3 figures, v2: reference added, v3: minor clarification,
version to appear in journa
Scattering theory of the chiral magnetic effect in a Weyl semimetal: Interplay of bulk Weyl cones and surface Fermi arcs
We formulate a linear response theory of the chiral magnetic effect in a
finite Weyl semimetal, expressing the electrical current density induced by
a slowly oscillating magnetic field or chiral chemical potential in
terms of the scattering matrix of Weyl fermions at the Fermi level. Surface
conduction can be neglected in the infinite-system limit for , but not for : The chirally circulating surface Fermi
arcs give a comparable contribution to the bulk Weyl cones no matter how large
the system is, because their smaller number is compensated by an increased flux
sensitivity. The Fermi arc contribution to has the
universal value , protected by chirality against impurity scattering
--- unlike the bulk contribution of opposite sign.Comment: 8 pages, 8 figures; V2: added references with discussion; V3: To be
published in the Focus Issue on "Topological semimetals" of New Journal of
Physic
Topologically protected charge transfer along the edge of a chiral -wave superconductor
The Majorana fermions propagating along the edge of a topological
superconductor with pairing deliver a shot noise power of
per eV of voltage bias. We calculate the full
counting statistics of the transferred charge and find that it becomes
trinomial in the low-temperature limit, distinct from the binomial statistics
of charge- transfer in a single-mode nanowire or charge- transfer
through a normal-superconductor interface. All even-order correlators of
current fluctuations have a universal quantized value, insensitive to disorder
and decoherence. These electrical signatures are experimentally accessible,
because they persist for temperatures and voltages large compared to the
Thouless energy.Comment: 5 pages, 4 figures. v3 [post-publication]: added an appendix on the
effect of a tunnel barrier at the normal-superconductor contac
Dynamics of disentanglement, density matrix and coherence in neutrino oscillations
In charged current weak interaction processes, neutrinos are produced in an
entangled state with the charged lepton. This correlated state is disentangled
by the measurement of the charged lepton in a detector at the production site.
We study the dynamical aspects of disentanglement, propagation and detection,
in particular the conditions under which the disentangled state is a coherent
superposition of mass eigenstates. The appearance and disappearance
far-detection processes are described from the time evolution of this
disentangled "collapsed" state. The familiar quantum mechanical interpretation
and factorization of the detection rate emerges when the quantum state is
disentangled on time scales \emph{much shorter} than the inverse oscillation
frequency, in which case the final detection rate factorizes in terms of the
usual quantum mechanical transition probability provided the final density of
states is insensitive to the neutrino energy difference. We suggest
\emph{possible} corrections for short-baseline experiments. If the charged
lepton is unobserved, neutrino oscillations and coherence are described in
terms of a reduced density matrix obtained by tracing out an un-observed
charged lepton. The diagonal elements in the mass basis describe the production
of mass eigenstates whereas the off diagonal ones provide a measure of
coherence. It is shown that coherences are of the same order of the diagonal
terms on time scales up to the inverse oscillation frequency, beyond which the
coherences oscillate as a result of the interference between mass eigenstates.Comment: 19 pages, v.2: discussions adde
Systematics of Moduli Stabilization, Inflationary Dynamics and Power Spectrum
We study the scalar sector of type IIB superstring theory compactified on
Calabi-Yau orientifolds as a place to find a mechanism of inflation in the
early universe. In the large volume limit, one can stabilize the moduli in
stages using perturbative method. We relate the systematics of moduli
stabilization with methods to reduce the number of possible inflatons, which in
turn lead to a simpler inflation analysis. Calculating the order-of-magnitude
of terms in the equation of motion, we show that the methods are in fact valid.
We then give the examples where these methods are used in the literature. We
also show that there are effects of non-inflaton scalar fields on the scalar
power spectrum. For one of the two methods, these effects can be observed with
the current precision in experiments, while for the other method, the effects
might never be observable.Comment: 20 pages, JHEP style; v.2 and v.3: typos fixed, discussion and
references adde
Holographic models of de Sitter QFTs
We describe the dynamics of strongly coupled field theories in de Sitter
spacetime using the holographic gauge/gravity duality. The main motivation for
this is to explore the possibility of dynamical phase transitions during
cosmological evolution. Specifically, we study two classes of theories: (i)
conformal field theories on de Sitter in the static patch which are maintained
in equilibrium at temperatures that may differ from the de Sitter temperature
and (ii) confining gauge theories on de Sitter spacetime. In the former case we
show the such states make sense from the holographic viewpoint in that they
have regular bulk gravity solutions. In the latter situation we add to the
evidence for a confinement/deconfinement transition for a large N planar gauge
theory as the cosmological acceleration is increased past a critical value. For
the field theories we study, the critical acceleration corresponds to a de
Sitter temperature which is less than the Minkowski space deconfinement
transition temperature by a factor of the spacetime dimension.Comment: 35 pages, LaTeX, 4 figures, v2: refs adde
Neutrino oscillations and uncertainty relations
We show that coherent flavor neutrino states are produced (and detected) due
to the momentum-coordinate Heisenberg uncertainty relation. The Mandelstam-Tamm
time-energy uncertainty relation requires non-stationary neutrino states for
oscillations to happen and determines the time interval (propagation length)
which is necessary for that. We compare different approaches to neutrino
oscillations which are based on different physical assumptions but lead to the
same expression for the neutrino transition probability in standard neutrino
oscillation experiments. We show that a Moessbauer neutrino experiment could
allow to distinguish different approaches and we present arguments in favor of
the 163Ho-163Dy system for such an experiment.Comment: Some small changes in section 2, results unchanged. Added referenc
Enhanced Non-Gaussianity from Excited Initial States
We use the techniques of effective field theory in an expanding universe to
examine the effect of choosing an excited inflationary initial state built over
the Bunch-Davies state on the CMB bi-spectrum. We find that even for Hadamard
states, there are unexpected enhancements in the bi-spectrum for certain
configurations in momentum space due to interactions of modes in the early
stages of inflation. These enhancements can be parametrically larger than the
standard ones and are potentially observable in current and future data. These
initial state effects have a characteristic signature in -space which
distinguishes them from the usual contributions, with the enhancement being
most pronounced for configurations corresponding to flattened triangles for
which two momenta are collinear.Comment: 33 pages, 1 figure. Refs added and minor addition
Astrophysical and Cosmological Implications of Large Volume String Compactifications
We study the spectrum, couplings and cosmological and astrophysical
implications of the moduli fields for the class of Calabi-Yau IIB string
compactifications for which moduli stabilisation leads to an exponentially
large volume V ~ 10^{15} l_s^6 and an intermediate string scale m_s ~
10^{11}GeV, with TeV-scale observable supersymmetry breaking. All K\"ahler
moduli except for the overall volume are heavier than the susy breaking scale,
with m ~ ln(M_P/m_{3/2}) m_{3/2} ~ (\ln(M_P/m_{3/2}))^2 m_{susy} ~ 500 TeV and,
contrary to standard expectations, have matter couplings suppressed only by the
string scale rather than the Planck scale. These decay to matter early in the
history of the universe, with a reheat temperature T ~ 10^7 GeV, and are free
from the cosmological moduli problem (CMP). The heavy moduli have a branching
ratio to gravitino pairs of 10^{-30} and do not suffer from the gravitino
overproduction problem. The overall volume modulus is a distinctive feature of
these models and is an M_{planck}-coupled scalar of mass m ~ 1 MeV and subject
to the CMP. A period of thermal inflation can help relax this problem. This
field has a lifetime ~ 10^{24}s and can contribute to dark matter. It may be
detected through its decays to 2\gamma or e^+e^-. If accessible the e^+e^-
decay mode dominates, with Br(\chi \to 2 \gamma) suppressed by a factor
(ln(M_P/m_{3/2}))^2. We consider the potential for detection of this field
through different astrophysical sources and find that the observed gamma-ray
background constrains \Omega_{\chi} <~ 10^{-4}. The decays of this field may
generate the 511 keV emission line from the galactic centre observed by
INTEGRAL/SPI.Comment: 31 pages, 2 figures; v2. refs adde