9,476 research outputs found
Dark matter allowed scenarios for Yukawa-unified SO(10) SUSY GUTs
Simple supersymmetric grand unified models based on the gauge group SO(10)
require --in addition to gauge and matter unification-- the unification of
t-b-\tau Yukawa couplings. Yukawa unification, however, only occurs for very
special values of the soft SUSY breaking parameters. We perform a search using
a Markov Chain Monte Carlo (MCMC) technique to investigate model parameters and
sparticle mass spectra which occur in Yukawa-unified SUSY models, where we also
require the relic density of neutralino dark matter to saturate the
WMAP-measured abundance. We find the spectrum is characterizd by three mass
scales: first/second generation scalars in the multi-TeV range, third
generation scalars in the TeV range, and gauginos in the \sim 100 GeV range.
Most solutions give far too high a relic abundance of neutralino dark matter.
The dark matter discrepancy can be rectified by 1. allowing for neutralino
decay to axino plus photon, 2. imposing gaugino mass non-universality or 3.
imposing generational non-universality. In addition, the MCMC approach finds 4.
a compromise solution where scalar masses are not too heavy, and where
neutralino annihilation occurs via the light Higgs h resonance. By imposing
weak scale Higgs soft term boundary conditions, we are also able to generate 5.
low \mu, m_A solutions with neutralino annihilation via a light A resonance,
though these solutions seem to be excluded by CDF/D0 measurements of the B_s\to
\mu^+\mu^- branching fraction. Based on the dual requirements of Yukawa
coupling unification and dark matter relic density, we predict new physics
signals at the LHC from pair production of 350--450 GeV gluinos. The events are
characterized by very high b-jet multiplicity and a dilepton mass edge around
mz2-mz1 \sim 50-75 GeV.Comment: 35 pages with 21 eps figure
Gyromagnetic Factors and Atomic Clock Constraints on the Variation of Fundamental Constants
We consider the effect of the coupled variations of fundamental constants on
the nucleon magnetic moment. The nucleon g-factor enters into the
interpretation of the measurements of variations in the fine-structure
constant, alpha, in both the laboratory (through atomic clock measurements) and
in astrophysical systems (e.g. through measurements of the 21 cm transitions).
A null result can be translated into a limit on the variation of a set of
fundamental constants, that is usually reduced to alpha. However, in specific
models, particularly unification models, changes in alpha are always
accompanied by corresponding changes in other fundamental quantities such as
the QCD scale, Lambda_QCD. This work tracks the changes in the nucleon
g-factors induced from changes in Lambda_QCD and the light quark masses. In
principle, these coupled variations can improve the bounds on the variation of
alpha by an order of magnitude from existing atomic clock and astrophysical
measurements. Unfortunately, the calculation of the dependence of g-factors on
fundamental parameters is notoriously model-dependent.Comment: 35 pages, 3 figures. Discussions of the effects of the polarization
of the non-valence nucleons, spin-spin interaction and nuclear radius on the
nuclear g-factor are added. References added. Matches published versio
Mechanistic unity of the predictive mind
It is often recognized that cognitive science employs a diverse explanatory toolkit. It has also been argued that cognitive scientists should embrace this explanatory diversity rather than pursue search for some grand unificatory framework or theory. This pluralist stance dovetails with the mechanistic view of cognitive-scientific explanation. However, one recently proposed theory – based on an idea that the brain is a predictive engine – opposes the spirit of pluralism by unapologetically wearing unificatory ambitions on its sleeves. In this paper, my aim is to investigate those pretentions to elucidate what sort of unification is on offer. I challenge the idea that explanatory unification of cognitive science follows from the Free Energy Principle. I claim that if the predictive story is to provide an explanatory unification, it is rather by proposing that many distinct cognitive mechanisms fall under the same functional schema that pertains to prediction error minimization. Seen this way, the brain is not simply a predictive mechanism – it is a collection of predictive mechanisms. I also pursue a more general aim of investigating the value of unificatory power for mechanistic explanations. I argue that even though unification is not an absolute evaluative criterion for mechanistic explanations, it may play an epistemic role in evaluating the credibility of an explanation relative to its direct competitors
Quantum Geometry and Gravity: Recent Advances
Over the last three years, a number of fundamental physical issues were
addressed in loop quantum gravity. These include: A statistical mechanical
derivation of the horizon entropy, encompassing astrophysically interesting
black holes as well as cosmological horizons; a natural resolution of the
big-bang singularity; the development of spin-foam models which provide
background independent path integral formulations of quantum gravity and
`finiteness proofs' of some of these models; and, the introduction of
semi-classical techniques to make contact between the background independent,
non-perturbative theory and the perturbative, low energy physics in Minkowski
space. These developments spring from a detailed quantum theory of geometry
that was systematically developed in the mid-nineties and have added a great
deal of optimism and intellectual excitement to the field.
The goal of this article is to communicate these advances in general physical
terms, accessible to researchers in all areas of gravitational physics
represented in this conference.Comment: 24 pages, 2 figures; report of the plenary talk at the 16th
International Conference on General Relativity and Gravitation, held at
Durban, S. Africa in July 200
Development of a high sensitivity torsional balance for the study of the Casimir force in the 1-10 micrometer range
We discuss a proposal to measure the Casimir force in the parallel plate
configuration in the m range via a high-sensitivity torsional balance.
This will allow to measure the thermal contribution to the Casimir force
therefore discriminating between the various approaches discussed so far. The
accurate control of the Casimir force in this range of distances is also
required to improve the limits to the existence of non-Newtonian forces in the
micrometer range predicted by unification models of fundamental interactions.Comment: 10 pages, 2 figure
With Grand Unification Signals in, Can Proton Decay be Far Behind?
It is noted that one is now in possession of a set of facts, which may be
viewed as the matching pieces of a puzzle ; in that all of them can be resolved
by just one idea - that is grand unification. These include : (i) the observed
family-structure, (ii) quantization of electric charge, (iii) meeting of the
three gauge couplings, (iv) neutrino oscillations; in particular the mass of
suggested by SuperK), (v) the intricate pattern of the masses and
mixings of the fermions, including the smallness of and the largeness
of , and (vi) the need for - to
implement baryogenesis (via leptogenesis). All these pieces fit beautifully
together within a single puzzle board framed by supersymmetric unification,
based on SO(10) or a string-unified G(224)-symmetry. The one and the most
notable piece of the puzzle still missing, however, is proton decay. A concrete
proposal is presented, within a predictive SO(10)/G(224)-framework, that
successfully describes the masses and mixings of all fermions, including the
neutrinos - with eight predictions, all in agreement with observation. An
updated study of proton decay is carried out within this framework, which shows
that a conservative upper limit on its lifetime is about (1/2-1)\times 10^34
yrs
- …