4,219 research outputs found
(Non-)BPS bound states and D-brane instantons
We study non-perturbative effects in four-dimensional N=1 supersymmetric
orientifold compactifications due to D-brane instantons which are generically
not invariant under the orientifold projection. We show that they can yield
superpotential contributions via a multi-instanton process at threshold. Some
constituents of this configuration form bound states away from the wall of
marginal stability which can decay in other regions of moduli space. A
microscopic analysis reveals how contributions to the superpotential are
possible when new BPS states compensate for their decay. We study this
concretely for D2-brane instantons along decaying special Lagrangians in Type
IIA and for D5-branes instantons carrying holomorphic bundles in Type I theory.Comment: 39 pages, 4 tables, 2 figures; v2: Equ. (20) corrected, related
discussion adjusted, 1 reference added; v3: JHEP version with more refs adde
Suppression of weak-localization (and enhancement of noise) by tunnelling in semiclassical chaotic transport
We add simple tunnelling effects and ray-splitting into the recent
trajectory-based semiclassical theory of quantum chaotic transport. We use this
to derive the weak-localization correction to conductance and the shot-noise
for a quantum chaotic cavity (billiard) coupled to leads via
tunnel-barriers. We derive results for arbitrary tunnelling rates and arbitrary
(positive) Ehrenfest time, . For all Ehrenfest times, we show
that the shot-noise is enhanced by the tunnelling, while the weak-localization
is suppressed. In the opaque barrier limit (small tunnelling rates with large
lead widths, such that Drude conductance remains finite), the weak-localization
goes to zero linearly with the tunnelling rate, while the Fano factor of the
shot-noise remains finite but becomes independent of the Ehrenfest time. The
crossover from RMT behaviour () to classical behaviour
() goes exponentially with the ratio of the Ehrenfest time
to the paired-paths survival time. The paired-paths survival time varies
between the dwell time (in the transparent barrier limit) and half the dwell
time (in the opaque barrier limit). Finally our method enables us to see the
physical origin of the suppression of weak-localization; it is due to the fact
that tunnel-barriers ``smear'' the coherent-backscattering peak over reflection
and transmission modes.Comment: 20 pages (version3: fixed error in sect. VC - results unchanged) -
Contents: Tunnelling in semiclassics (3pages), Weak-localization (5pages),
Shot-noise (5pages
Three- and Four-point correlators of excited bosonic twist fields
We compute three- and four-point correlation functions containing excited
bosonic twist fields. Our results can be used to determine properties, such as
lifetimes and production rates, of massive string excitations localised at
D-brane intersections, which could be signatures of a low string scale even if
the usual string resonances are inaccessible to the LHC.Comment: 42 pages, no figure
Discrete symmetries from hidden sectors
We study the presence of abelian discrete symmetries in globally consistent
orientifold compactifications based on rational conformal field theory. We
extend previous work [1] by allowing the discrete symmetries to be a linear
combination of U(1) gauge factors of the visible as well as the hidden sector.
This more general ansatz significantly increases the probability of finding a
discrete symmetry in the low energy effective action. Applied to globally
consistent MSSM-like Gepner constructions we find multiple models that allow
for matter parity or Baryon triality.Comment: 20 page
Shot noise in semiclassical chaotic cavities
We construct a trajectory-based semiclassical theory of shot noise in clean
chaotic cavities. In the universal regime of vanishing Ehrenfest time \tE, we
reproduce the random matrix theory result, and show that the Fano factor is
exponentially suppressed as \tE increases. We demonstrate how our theory
preserves the unitarity of the scattering matrix even in the regime of finite
\tE. We discuss the range of validity of our semiclassical approach and point
out subtleties relevant to the recent semiclassical treatment of shot noise in
the universal regime by Braun et al. [cond-mat/0511292].Comment: Final version, to appear in Physical Review Letter
Precision Muon Tracking Detectors for High-Energy Hadron Colliders
Small-diameter muon drift tube (sMDT) chambers with 15 mm tube diameter are a
cost-effective technology for high-precision muon tracking over large areas at
high background rates as expected at future high-energy hadron colliders
including HL-LHC. The chamber design and construction procedures have been
optimized for mass production and provide sense wire positioning accuracy of
better than 10 ?m. The rate capability of the sMDT chambers has been
extensively tested at the CERN Gamma Irradiation Facility. It exceeds the one
of the ATLAS muon drift tube (MDT) chambers, which are operated at
unprecedentedly high background rates of neutrons and gamma-rays, by an order
of magnitude, which is sufficient for almost the whole muon detector acceptance
at FCC-hh at maximum luminosity. sMDT operational and construction experience
exists from ATLAS muon spectrometer upgrades which are in progress or under
preparation for LHC Phase 1 and 2
- …