1,114 research outputs found
Search for Tau Flavour Violation at the LHC
We explore the prospects for searches at the LHC for sparticle decays that
violate lepton number, in the light of neutrino oscillation data and the
seesaw model for neutrino masses and mixing. We analyse the theoretical and
phenomenological conditions required for tau flavour violation to be observable
in \chi_2 \to \chi + \tau^\pm \mu^\mp decays, for cosmologically interesting
values of the relic neutralino LSP density. We study the relevant
supersymmetric parameter space in the context of the Constrained Minimal
Supersymmetric Extension of the Standard Model (CMSSM) and in SU(5) extensions
of the theory. We pay particular attention to the possible signals from
hadronic tau decays, that we analyse using PYTHIA event simulation. We find
that a signal for \tau flavour-violating \chi_2 decays may be observable if the
branching ratio exceeds about 10%. This may be compatible with the existing
upper limit on \tau \to \mu \gamma decays if there is mixing between
right-handed sleptons, as could be induced in non-minimal SU(5) GUTs.Comment: 24 pages, 10 fig
Finding a Spherically Symmetric Cosmology from Observations in Observational Coordinates -- Advantages and Challenges
One of the continuing challenges in cosmology has been to determine the
large-scale space-time metric from observations with a minimum of assumptions
-- without, for instance, assuming that the universe is almost
Friedmann-Lema\^{i}tre-Robertson-Walker (FLRW). If we are lucky enough this
would be a way of demonstrating that our universe is FLRW, instead of
presupposing it or simply showing that the observations are consistent with
FLRW. Showing how to do this within the more general spherically symmetric,
inhomogeneous space-time framework takes us a long way towards fulfilling this
goal. In recent work researchers have shown how this can be done both in the
traditional Lema\^{i}tre-Tolman-Bondi (LTB) 3 + 1 coordinate framework, and in
the observational coordinate (OC) framework. In this paper we investigate the
stability of solutions, and the use of data in the OC field equations including
their time evolution and compare both approaches with respect to the
singularity problem at the maximum of the angular-diameter distance, the
stability of solutions, and the use of data in the field equations. This allows
a more detailed account and assessment of the OC integration procedure, and
enables a comparison of the relative advantages of the two equivalent solution
frameworks. Both formulations and integration procedures should, in principle,
lead to the same results. However, as we show in this paper, the OC procedure
manifests certain advantages, particularly in the avoidance of coordinate
singularities at the maximum of the angular-diameter distance, and in the
stability of the solutions obtained. This particular feature is what allows us
to do the best fitting of the data to smooth data functions and the possibility
of constructing analytic solutions to the field equations.Comment: 31 page
Electronic dispersion compensation using full optical-field reconstruction in 10Gbit/s OOK based systems
We investigate the design of electronic dispersion compensation (EDC) using full optical-field reconstruction in 10Gbit/s on-off keyed transmission systems limited by optical signal-to-noise ratio (OSNR). By effectively suppressing the impairment due to low- frequency component amplification in phase reconstruction, properly designing the transmission system configuration to combat fiber nonlinearity, and successfully reducing the vulnerability to thermal noise, a 4.8dB OSNR margin can be achieved for 2160km single-mode fiber transmission without any optical dispersion compensation. We also investigate the performance sensitivity of the scheme to various system parameters, and propose a novel method to greatly enhance the tolerance to differential phase misalignment of the asymmetric Mach-Zehnder interferometer. This numerical study provides important design guidelines which will enable full optical-field EDC to become a cost-effective dispersion compensation solution for future transparent optical networks
Impact of optical phase conjugation on the nonlinear Shannon limit
Compensation of the detrimental impacts of nonlinearity on long-haul wavelength division multiplexed system performance is discussed, and the difference between transmitter, receiver and in-line compensation analyzed. We demonstrate that ideal compensation of nonlinear noise could result in an increase in the signal-to-noise ratio (measured in dB) of 50%, and that reaches may be more than doubled for higher order modulation formats. The influence of parametric noise amplification is discussed in detail, showing how increased numbers of optical phase conjugators may further increase the received signal-tonoise ratio. Finally the impact of practical real world system imperfections, such as polarization mode dispersion, are outlined
The impact of parametric noise amplification on long haul transmission throughput
Advanced signal processing, such as multi-channel digital back propagation and mid span optical phase conjugation, can compensate for inter channel nonlinear effects in point to point links. However, once such are effects are compensated, the interaction between the signal and noise fields becomes dominant. We will show that this interaction has a direct impact on the signal to noise ratio improvement, observing that ideal optical phase conjugation offers 1.5 dB more performance benefit than DSP based compensation
On perfect fluid models in non-comoving observational spherical coordinates
We use null spherical (observational) coordinates to describe a class of
inhomogeneous cosmological models. The proposed cosmological construction is
based on the observer past null cone. A known difficulty in using inhomogeneous
models is that the null geodesic equation is not integrable in general. Our
choice of null coordinates solves the radial ingoing null geodesic by
construction. Furthermore, we use an approach where the velocity field is
uniquely calculated from the metric rather than put in by hand. Conveniently,
this allows us to explore models in a non-comoving frame of reference. In this
frame, we find that the velocity field has shear, acceleration and expansion
rate in general. We show that a comoving frame is not compatible with expanding
perfect fluid models in the coordinates proposed and dust models are simply not
possible. We describe the models in a non-comoving frame. We use the dust
models in a non-comoving frame to outline a fitting procedure.Comment: 8 pages, 1 figure. To appear in Phys.Rev.
PMD tolerant nonlinear compensation using in-line phase conjugation
In this paper, we numerically investigate the impact of polarisation mode dispersion on the efficiency of compensation of nonlinear transmission penalties for systems employing one of more inline phase conjugation devices. We will show that reducing the spacing between phase conjugations allows for significantly improved performance in the presence polarisation mode dispersion or a significant relaxation in the acceptable level of polarization mode dispersion. We show that these results are consistent with previously presented full statistical analysis of nonlinear transmission appropriately adjusted for the reduced section length undergoing compensation
Cold Dark Matter detection in SUSY models at large tan(beta)
We study the direct detection rate for SUSY cold dark matter (CDM) predicted
by the minimal supersymmetric standard model with universal boundary conditions
and large values for tan(beta). The relic abundance of the lightest
supersymmetric particle (LSP), assumed to be approximately a bino, is obtained
by including its coannihilations with the next-to-lightest supersymmetric
particle (NLSP), which is the lightest s-tau. The cosmological constraint on
this quantity severely limits the allowed SUSY parameter space, especially in
the case the CP-even Higgs has mass of around 114 GeV. We find that for large
tan(beta) it is possible to find a subsection of the allowed parameter space,
which yields detectable rates in the currently planned experiments.Comment: Changes in text and figure
On Yukawa quasi-unification with mu<0
Although recent data on the muon anomalous magnetic moment strongly disfavor
the constrained minimal supersymmetric standard model with mu<0, they cannot
exclude it because of theoretical ambiguities. We consider this model
supplemented by a Yukawa quasi-unification condition which allows an acceptable
b-quark mass. We find that the cosmological upper bound on the lightest
sparticle relic abundance is incompatible with the data on the branching ratio
of b-->s gamma, which is evaluated by including all the next-to-leading order
corrections. Thus, this scheme is not viable.Comment: 4 pages including 3 figures, Revte
What if Supersymmetry Breaking Unifies beyond the GUT Scale?
We study models in which soft supersymmetry-breaking parameters of the MSSM
become universal at some unification scale, , above the GUT scale,
\mgut. We assume that the scalar masses and gaugino masses have common
values, and respectively, at . We use the
renormalization-group equations of the minimal supersymmetric SU(5) GUT to
evaluate their evolutions down to \mgut, studying their dependences on the
unknown parameters of the SU(5) superpotential. After displaying some generic
examples of the evolutions of the soft supersymmetry-breaking parameters, we
discuss the effects on physical sparticle masses in some specific examples. We
note, for example, that near-degeneracy between the lightest neutralino and the
lighter stau is progressively disfavoured as increases. This has the
consequence, as we show in planes for several different values
of , that the stau coannihilation region shrinks as
increases, and we delineate the regions of the plane
where it is absent altogether. Moreover, as increases, the focus-point
region recedes to larger values of for any fixed and
. We conclude that the regions of the plane that are
commonly favoured in phenomenological analyses tend to disappear at large
.Comment: 24 pages with 11 eps figures; references added, some figures
corrected, discussion extended and figure added; version to appear in EPJ
- …