13,572 research outputs found
't Hooft anomalies of discrete gauge theories and non-abelian group cohomology
We study discrete symmetries of Dijkgraaf-Witten theories and their gauging
in the framework of (extended) functorial quantum field theory. Non-abelian
group cohomology is used to describe discrete symmetries and we derive concrete
conditions for such a symmetry to admit 't Hooft anomalies in terms of the
Lyndon-Hochschild-Serre spectral sequence. We give an explicit realization of a
discrete gauge theory with 't Hooft anomaly as a state on the boundary of a
higher-dimensional Dijkgraaf-Witten theory. This allows us to calculate the
2-cocycle twisting the projective representation of physical symmetries via
transgression. We present a general discussion of the bulk-boundary
correspondence at the level of partition functions and state spaces, which we
make explicit for discrete gauge theories.Comment: 46 pages, 1 figure; v2: minor corrections and clarifying comments
added, references updated; Final version to appear in Communications in
Mathematical Physic
Extended quantum field theory, index theory and the parity anomaly
We use techniques from functorial quantum field theory to provide a geometric
description of the parity anomaly in fermionic systems coupled to background
gauge and gravitational fields on odd-dimensional spacetimes. We give an
explicit construction of a geometric cobordism bicategory which incorporates
general background fields in a stack, and together with the theory of symmetric
monoidal bicategories we use it to provide the concrete forms of invertible
extended quantum field theories which capture anomalies in both the path
integral and Hamiltonian frameworks. Specialising this situation by using the
extension of the Atiyah-Patodi-Singer index theorem to manifolds with corners
due to Loya and Melrose, we obtain a new Hamiltonian perspective on the parity
anomaly. We compute explicitly the 2-cocycle of the projective representation
of the gauge symmetry on the quantum state space, which is defined in a
parity-symmetric way by suitably augmenting the standard chiral fermionic Fock
spaces with Lagrangian subspaces of zero modes of the Dirac Hamiltonian that
naturally appear in the index theorem. We describe the significance of our
constructions for the bulk-boundary correspondence in a large class of
time-reversal invariant gauge-gravity symmetry-protected topological phases of
quantum matter with gapless charged boundary fermions, including the standard
topological insulator in 3+1 dimensions.Comment: 63 pages, 3 figures; v2: clarifying comments and references added;
Final version to be published in Communications in Mathematical Physic
Variations on the Vev Flip-Flop: Instantaneous Freeze-out and Decaying Dark Matter
In this work we consider a simple model for dark matter and identify regions
of parameter space where the relic abundance is set via kinematic thresholds,
which open and close due to thermal effects. We discuss instantaneous
freeze-out, where dark matter suddenly freezes-out when the channel connecting
dark matter to the thermal bath closes, and decaying dark matter, where dark
matter freezes-out while relativistic and later decays when a kinematic
threshold temporarily opens. These mechanisms can occur in the vicinity of a
one-step or a two-step phase transition. In all cases thermal effects provide
this dynamic behaviour, while ensuring that dark matter remains stable until
the present day.Comment: 23 pages, 13 figures; v2: version matched to journal (JHEP), added a
detailed discussion of further two-to-two processe
Moving Five-Branes in Low-Energy Heterotic M-Theory
We construct cosmological solutions of four-dimensional effective heterotic
M-theory with a moving five-brane and evolving dilaton and T modulus. It is
shown that the five-brane generates a transition between two asymptotic
rolling-radii solutions. Moreover, the five-brane motion always drives the
solutions towards strong coupling asymptotically. We present an explicit
example of a negative-time branch solution which ends in a brane collision
accompanied by a small-instanton transition. The five-dimensional origin of
some of our solutions is also discussed.Comment: 16 pages, Latex, 3 eps figure
Consequences of BSE disease outbreaks in the Canadian beef industry
This study examines farm to wholesale prices spreads to measure the impact of the BSE disease outbreak on the Canadian beef industry. The study uses structure break tests developed by Gregory and Hansen (1996) and Hansen (1992) examine possible breaks within co integrating relationships. The study finds evidence that the industry began realignment as a result of the UK BSE disease outbreak, and the Canadian BSE disease outbreak was simply the largest realignment of the process beginning with the UK disease outbreak. However, the only statistically significant break was the BSE disease outbreak itself in May 2003. Stability was not restored until the border was reopened in 2005. Specific results indicated that the processing sector exploited the border closure in May 2003 to enhance its market power and that the system returned to a competitive one after the border re-opened in July 2005.Beef industry, price transmission, BSE, market power, parameter instability, cointegration with structural break, Agribusiness, Agricultural and Food Policy, Agricultural Finance, GA, IN,
A "Single-Photon" Transistor in Circuit Quantum Electrodynamics
We introduce a circuit quantum electrodynamical setup for a "single-photon"
transistor. In our approach photons propagate in two open transmission lines
that are coupled via two interacting transmon qubits. The interaction is such
that no photons are exchanged between the two transmission lines but a single
photon in one line can completely block respectively enable the propagation of
photons in the other line. High on-off ratios can be achieved for feasible
experimental parameters. Our approach is inherently scalable as all photon
pulses can have the same pulse shape and carrier frequency such that output
signals of one transistor can be input signals for a consecutive transistor.Comment: Analysis of pure dephasing, time delays between pulses and gain
added. Word "quantum" dropped from title, to appear in Phys. Rev. Let
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