3,461 research outputs found
Identification of Xenopus SMC protein complexes required for sister chromatid cohesion
The structural maintenance of chromosomes (SMC) family is a growing family of chromosomal ATPases. The founding class of SMC protein complexes, condensins, plays a central role in mitotic chromosome condensation. We report here a new class of SMC protein complexes containing XSMC1 and XSMC3, Xenopus homologs of yeast Smc1p and Smc3p, respectively. The protein complexes (termed cohesins) exist as two major forms with sedimentation coefficients of 9S and 14S. 9S cohesin is a heterodimer of XSMC1 and XSMC3, whereas 14S cohesin contains three additional subunits. One of them has been identified as a Xenopus homolog of the Schizosaccharomyces pombe Rad21p implicated in DNA repair and the Saccharomyces cerevisiae Scc1p/Mcd1p implicated in sister chromatid cohesion. 14S cohesin binds to interphase chromatin independently of DNA replication and dissociates from it at the onset of mitosis. Immunodepletion of cohesins during interphase causes defects in sister chromatid cohesion in subsequent mitosis, whereas condensation is unaffected. These results suggest that proper assembly of mitotic chromosomes is regulated by two distinct classes of SMC protein complexes, cohesins and condensins
Effects of Large CP-violating Soft Phases on Supersymmetric Electroweak Baryogenesis
We revisit the results of recent electroweak baryogenesis calculations and
include all allowed large CP-violating supersymmetric phases. If the phases are
large, the resulting baryon asymmetry can be considerably larger than the
observed value . Much of the asymmetry must
therefore be washed out, and we argue that the upper bound on the light Higgs
mass is larger than the value reported in previous work.Comment: 15 pages, 3 figure
Flavour Matters in Leptogenesis
We give analytic approximations to the baryon asymmetry produced by thermal
leptogenesis with hierarchical right-handed neutrinos. Our calculation includes
flavour-dependent washout processes and CP violation in scattering, and
neglects gauge interactions and finite temperature corrections. Our approximate
formulae depend upon the three CP asymmetries in the individual lepton flavours
as well as on three flavour-dependent efficiency factors. We show that the
commonly used expressions for the lepton asymmetry, which depend on the total
CP asymmetry and one single efficiency factor, may fail to reproduce the
correct lepton asymmetry in a number of cases. We illustrate the importance of
using the flavour-dependent formulae in the context of a two right-handed
neutrino model.Comment: Additional typos corrected (in particular, the plots and captions now
agree
Leptogenesis beyond the limit of hierarchical heavy neutrino masses
We calculate the baryon asymmetry of the Universe in thermal leptogenesis
beyond the usual lightest right-handed (RH) neutrino dominated scenario (N_1DS)
and in particular beyond the hierarchical limit (HL), M_1 << M_2 << M_3, for
the RH neutrino mass spectrum. After providing some orientation among the large
variety of models, we first revisit the central role of the N_1DS, with new
insights on the dynamics of the asymmetry generation and then discuss the main
routes departing from it, focusing on models beyond the HL. We study in detail
two examples of `strong-strong' wash-out scenarios: one with `maximal phase'
and the limit of very large M_3, studying the effects arising when
delta_2=(M_2-M_1)/M_1 is small. We extend analytical methods already applied to
the N_1DS showing, for example, that, in the degenerate limit (DL), the
efficiency factors of the RH neutrinos become equal with the single decay
parameter replaced by the sum. Both cases disprove the misconception that close
RH neutrino masses necessarily lead to a final asymmetry enhancement and to a
relaxation of the lower bounds on M_1 and on the initial temperature of the
radiation-dominated expansion. We also explain why leptogenesis tends to favor
normal hierarchy compared to inverted hierarchy for the left-handed neutrino
masses.Comment: 30 pages, 8 figures; corrected typo in Eq. (67); shortened
Introduction, Section 3 and Conclusions; one figure removed; added 2
references; to appear in JCA
Dark Matter, Light Stops and Electroweak Baryogenesis
We examine the neutralino relic density in the presence of a light top
squark, such as the one required for the realization of the electroweak
baryogenesis mechanism, within the minimal supersymmetric standard model. We
show that there are three clearly distinguishable regions of parameter space,
where the relic density is consistent with WMAP and other cosmological data.
These regions are characterized by annihilation cross sections mediated by
either light Higgs bosons, Z bosons, or by the co-annihilation with the
lightest stop. Tevatron collider experiments can test the presence of the light
stop in most of the parameter space. In the co-annihilation region, however,
the mass difference between the light stop and the lightest neutralino varies
between 15 and 30 GeV, presenting an interesting challenge for stop searches at
hadron colliders. We present the prospects for direct detection of dark matter,
which provides a complementary way of testing this scenario. We also derive the
required structure of the high energy soft supersymmetry breaking mass
parameters where the neutralino is a dark matter candidate and the stop
spectrum is consistent with electroweak baryogenesis and the present bounds on
the lightest Higgs mass.Comment: 24 pages, 8 figures; version published in Phys.Rev.
Flavour Issues in Leptogenesis
We study the impact of flavour in thermal leptogenesis, including the quantum
oscillations of the asymmetries in lepton flavour space. In the Boltzmann
equations we find different numerical factors and additional terms which can
affect the results significantly. The upper bound on the CP asymmetry in a
specific flavour is weaker than the bound on the sum. This suggests that --
when flavour dynamics is included -- there is no model-independent limit on the
light neutrino mass scale,and that the lower bound on the reheat temperature is
relaxed by a factor ~ (3 - 10).Comment: 19 pages, corrected equations for flavour oscillation
Improved Results in Supersymmetric Electroweak Baryogenesis
Electroweak baryogenesis provides a very attractive scenario to explain the
origin of the baryon asymmetry. The mechanism of electroweak baryogenesis makes
use of the baryon number anomaly and relies on physics that can be tested
experimentally. It is today understood that, if the Higgs mass is not larger
than 120 GeV, this mechanism may be effective within supersymmetric extensions
of the Standard Model. In this work, we reconsider the question of baryon
number generation at the electroweak phase transition within the context of the
minimal supersymmetric extension of the Standard Model. We derive the relevant
diffusion equations, give a consistent definition of the sources, and compare
our results with those appearing in the recent literature on this subjectComment: 19 pages, 2 figure
Top-squark searches at the Tevatron in models of low-energy supersymmetry breaking
We study the production and decays of top squarks (stops) at the Tevatron
collider in models of low-energy supersymmetry breaking. We consider the case
where the lightest Standard Model (SM) superpartner is a light neutralino that
predominantly decays into a photon and a light gravitino. Considering the
lighter stop to be the next-to-lightest Standard Model superpartner, we analyze
stop signatures associated with jets, photons and missing energy, which lead to
signals naturally larger than the associated SM backgrounds. We consider both
2-body and 3-body decays of the top squarks and show that the reach of the
Tevatron can be significantly larger than that expected within either the
standard supergravity models or models of low-energy supersymmetry breaking in
which the stop is the lightest SM superpartner. For a modest projection of the
final Tevatron luminosity, L = 4 fb-1, stop masses of order 300 GeV are
accessible at the Tevatron collider in both 2-body and 3-body decay modes. We
also consider the production and decay of ten degenerate squarks that are the
supersymmetric partners of the five light quarks. In this case we find that
common squark masses up to 360 GeV are easily accessible at the Tevatron
collider, and that the reach increases further if the gluino is light.Comment: 32 pages, 9 figures; references adde
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