2,310 research outputs found
\Delta S=2 and \Delta C=2 bag parameters in the SM and beyond from Nf=2+1+1 twisted-mass LQCD
We present unquenched lattice QCD results for the matrix elements of
four-fermion operators relevant to the description of the neutral K and D
mixing in the Standard Model and its extensions. We have employed simulations
with Nf = 2 + 1 + 1 dynamical sea quarks at three values of the lattice
spacings in the interval 0.06 - 0.09 fm and pseudoscalar meson masses in the
range 210 - 450 MeV. Our results are extrapolated to the continuum limit and to
the physical pion mass. Renormalization constants have been determined
non-perturbatively in the RI-MOM scheme. In particular, for the Kaon
bag-parameter, which is relevant for the \overline{K}^0-K^0 mixing in the
Standard Model, we obtain B_K^{RGI} = 0.717(24).Comment: Added comments to error budget discussion; fig.19 corrected. Version
to appear in PR
Renormalisation of quark bilinears with Nf=2 Wilson fermions and tree-level improved gauge action
We present results for the renormalisation constants of bilinear quark
operators, using the Nf=2 twisted mass Wilson action at maximal twist (which
guarantees automatic O(a) improvement) and the tree-level Symanzik improved
gauge action. The scale-independent renormalisation constants are computed with
a new method, which makes use of both standard twisted mass and
Osterwalder-Seiler fermions. Moreover, the results from an RI-MOM calculation
are presented for both scale independent and scale dependent renormalisation
constants.Comment: 7 pages, talk at The XXV International Symposium on Lattice Field
Theory, July 30 - August 4 2007, Regensbur
Sneutrino condensate as a candidate for the hot big bang cosmology
If inflationary paradigm is correct, then it must create conditions for the
hot big bang model with all observed matter, baryons and the seed perturbations
for the structure formation. In this paper we propose a scenario where the
inflaton energy density is dumped into the bulk in a brane world setup, and all
the required physical conditions are created by the right handed neutrino
sector within supersymmetry. The scalar component of the right handed Majorana
neutrino is responsible for generating the scale invariant fluctuations in the
cosmic microwave background radiation, reheating the Universe at a
temperature~ GeV, and finally generating the lepton/baryon
asymmetry, , with no lepton/baryon isocurvature
fluctuations.Comment: 19 pages, 3 figures. Some discussion on neutrino masses and
baryogenesis, and other small changes adde
Kaon oscillations in the Standard Model and Beyond using Nf=2 dynamical quarks
We compute non-perturbatively the B-parameters of the complete basis of
four-fermion operators needed to study the Kaon oscillations in the SM and in
its supersymmetric extension. We perform numerical simulations with two
dynamical maximally twisted sea quarks at three values of the lattice spacing
on configurations generated by the ETMC. Unwanted operator mixings and O(a)
discretization effects are removed by discretizing the valence quarks with a
suitable Osterwalder-Seiler variant of the Twisted Mass action. Operators are
renormalized non-perturbatively in the RI/MOM scheme. Our preliminary result
for BK(RGI) is 0.73(3)(3).Comment: 7 pages, 3 figures, 1 table, proceedings of the XXVII Int'l Symposyum
on Lattice Field Theory (LAT2009), July 26-31 2009, Peking University,
Beijing (China
On kernels, defaults and even graphs
Extensions in prerequisite-free, disjunction-free default theories have been shown to be in direct correspondence with kernels of directed graphs; hence default theories without odd cycles always have a ``standard'' kind of an extension. We show that, although all ``standard'' extensions can be enumerated explicitly, several other problems remain intractable for such theories: Telling whether a non-standard extension exists, enumerating all extensions, and finding the minimal standard extension. We also present a new graph-theoretic algorithm, based on vertex feedback sets, for enumerating all extensions of a general prerequisite-free, disjunction-free default theory (possibly with odd cycles). The algorithm empirically performs well for quite large theories
K^0-\bar{K}^0 mixing in the Standard Model from Nf=2+1+1 Twisted Mass Lattice QCD
We present preliminary results at {\beta} = 1.95 (a = 0.077 fm) on the first
unquenched N_f=2+1+1 lattice computation of the B_K parameter which controls
the neutral kaon oscillations in the Standard Model. Using N_f=2+1+1 maximally
twisted sea quarks and Osterwalder-Seiler valence quarks we achieve O(a)
improvement and a continuum-like renormalization pattern for the four-fermion
operator. Our results are extrapolated/interpolated to the physical
light/strange quark mass but not yet to the continuum limit. The computation of
the relevant renormalization constants is performed non perturbatively in the
RI'-MOM scheme using dedicated simulations with N_f=4 degenerate sea quark
flavours produced by the ETM collaboration.
We get B_K^{RGI} (a = 0.077) = 0.747(18), which when compared to our previous
unquenched N_f=2 determination and most of the existing results, suggests a
rather weak B_K^{RGI} dependence on the number of dynamical flavours. We are at
the moment analysing lattice data at two additional {\beta} values which will
allow us to perform an extrapolation to the continuum limit.Comment: 7 pages, 8 figures, Proceedings of Lattice 2011, XXIX International
Symposium on Lattice Field Theory, Squaw Valley, Lake Tahoe, Californi
The 4-D Layer Phase as a Gauge Field Localization: Extensive Study of the 5-D Anisotropic U(1) Gauge Model on the Lattice
We study a 4+1 dimensional pure Abelian Gauge model on the lattice with two
anisotropic couplings independent of each other and of the coordinates. A first
exploration of the phase diagram using mean field approximation and monte carlo
techniques has demonstrated the existence of a new phase, the so called Layer
phase, in which the forces in the 4-D subspace are Coulomb-like while in the
transverse direction (fifth dimension) the force is confining. This allows the
possibility of a gauge field localization scheme. In this work the use of
bigger lattice volumes and higher statistics confirms the existence of the
Layer phase and furthermore clarifies the issue of the phase transitions'
order. We show that the Layer phase is separated from the strongly coupled
phase by a weak first order phase transition. Also we provide evidence that the
Layer phase is separated by the five-dimensional Coulomb phase with a second
order phase transition and we give a first estimation of the critical
exponents.Comment: 19 pages, 16 figure
Right-handed-neutrino Majorana mass at the SUSY GUT scale and the solution of the solar-neutrino problem
In the SUSY GUT scenario, it is natural to assume the right-handed-neutrino
Majorana-mass scale to be GeV. This will in principle lead, by the
seesaw mechanism, to a mass of order . This suggests that the solution of
the solar-neutrino puzzle should be either the MSW effect in
-- oscillations, with , or long-wavelength -- oscillations, with . These solutions require
unexpectedly large mixings of with and ,
respectively. I suggest a variation of the Dimopoulos--Hall--Raby model for the
fermion mass matrices which can accomodate such large mixings.Comment: 10 pages, standard LATEX, no figures, Carnegie-Mellon University
Report CMU-HEP93-27 and DOE-ER/40682-5
K^0-\bar{K}^0 Mixing Beyond the SM from Nf=2 tmQCD
We present preliminary results on the of neutral kaon oscillations in
extensions of the Standard Model. Using Nf=2 maximally twisted sea quarks and
Osterwalder-Seiler valence quarks, we achieve both O(a)-improvement and
continuum-like renormalization pattern for the relevant four-fermion operators.
We perform simulations at three values of the lattice spacing and
extrapolate/interpolate our results to the continuum limit and physical
light/strange quark mass. The calculation of the renormalization constants of
the complete operator basis is performed non- perturbatively in the RI-MOM
scheme.Comment: 7 pages, 2 figures; presented at the XXVIII International Symposium
on Lattice Field Theory Villasimius, Sardinia, Ital
SUSY GUTs under Siege : Proton Decay
SO(10) supersymmetric grand unified theories [SUSY GUTs] provide a beautiful
framework for physics beyond the standard model. Experimental measurements of
the three gauge couplings are consistent with unification at a scale GeV. In addition predictive models for fermion masses and
mixing angles have been found which fit the low energy data, including the
recent data for neutrino oscillations. SO(10) boundary conditions can be tested
via the spectrum of superparticles. The simplest models also predict neutron
and proton decay rates. In this paper we discuss nucleon decay rates and obtain
reasonable upper bounds. A clear picture of the allowed SUSY spectra as
constrained by nucleon decay is presented.Comment: 13 page
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