172,701 research outputs found
Lepton Flavour Violating top decays at the LHC
We consider lepton flavour violating decays of the top quark, mediated by
four-fermion operators. We compile constraints on a complete set of
SU(3)*U(1)-invariant operators, arising from their loop contributions to rare
decays and from HERA's single top search. The bounds on e-mu flavour change are
more restrictive than l-tau; nonetheless the top could decay to a jet with a branching ratio of order . We estimate that the
currently available LHC data (20 inverse-fb at 8 TeV) could be sensitive to
+ jet) , and extrapolate that 100
inverse-fb at 13 TeV could reach a sensitivity of .Comment: 10 pages + Appendice
Beyond MFV in family symmetry theories of fermion masses
Minimal Flavour Violation (MFV) postulates that the only source of flavour
changing neutral currents and CP violation, as in the Standard Model, is the
CKM matrix. However it does not address the origin of fermion masses and mixing
and models that do usually have a structure that goes well beyond the MFV
framework. In this paper we compare the MFV predictions with those obtained in
models based on spontaneously broken (horizontal) family symmetries, both
Abelian and non-Abelian. The generic suppression of flavour changing processes
in these models turns out to be weaker than in the MFV hypothesis. Despite
this, in the supersymmetric case, the suppression may still be consistent with
a solution to the hierarchy problem, with masses of superpartners below 1 TeV.
A comparison of FCNC and CP violation in processes involving a variety of
different family quantum numbers should be able to distinguish between various
family symmetry models and models satisfying the MFV hypothesis.Comment: 34 pages, no figure
Small-Scale Structure of Spacetime: Bounds and Conjectures
This review consists of two parts. The first part establishes certain
astrophysical bounds on the smoothness of classical spacetime. Some of the best
bounds to date are based on the absence of vacuum Cherenkov radiation in
ultrahigh-energy cosmic rays. The second part discusses possible implications
of these bounds for the quantum structure of spacetime. One conjecture is that
the fundamental length scale of quantum spacetime may be different from the
Planck length.Comment: 20 pages; invited talk at the Third Mexican Meeting on Mathematical
and Experimental Physics, September 10-14, 2007, El Colegio Nacional, Mexico
City; v3: final versio
Bounds on length scales of classical spacetime foam models
Simple models of a classical spacetime foam are considered, which consist of
identical static defects embedded in Minkowski spacetime. Plane-wave solutions
of the vacuum Maxwell equations with appropriate boundary conditions at the
defect surfaces are obtained in the long-wavelength limit. The corresponding
dispersion relations \omega^2=\omega^2(\vec{k}) are calculated, in particular,
the coefficients of the quadratic and quartic terms in \vec{k}. Astronomical
observations of gamma-ray bursts and ultra-high-energy cosmic rays then place
bounds on the coefficients of the dispersion relations and, thereby, on
particular combinations of the fundamental length scales of the static
spacetime-foam models considered. Spacetime foam models with a single length
scale are excluded, even models with a length scale close to the Planck length
(as long as a classical spacetime remains relevant).Comment: 22 pages with revtex4, v5: published versio
Carving Out the Space of 4D CFTs
We introduce a new numerical algorithm based on semidefinite programming to
efficiently compute bounds on operator dimensions, central charges, and OPE
coefficients in 4D conformal and N=1 superconformal field theories. Using our
algorithm, we dramatically improve previous bounds on a number of CFT
quantities, particularly for theories with global symmetries. In the case of
SO(4) or SU(2) symmetry, our bounds severely constrain models of conformal
technicolor. In N=1 superconformal theories, we place strong bounds on
dim(Phi*Phi), where Phi is a chiral operator. These bounds asymptote to the
line dim(Phi*Phi) <= 2 dim(Phi) near dim(Phi) ~ 1, forbidding positive
anomalous dimensions in this region. We also place novel upper and lower bounds
on OPE coefficients of protected operators in the Phi x Phi OPE. Finally, we
find examples of lower bounds on central charges and flavor current two-point
functions that scale with the size of global symmetry representations. In the
case of N=1 theories with an SU(N) flavor symmetry, our bounds on current
two-point functions lie within an O(1) factor of the values realized in
supersymmetric QCD in the conformal window.Comment: 60 pages, 22 figure
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