729 research outputs found
The role of the top mass in b-production at future lepton colliders
We compute the one loop contribution coming from vertex and box diagrams,
where virtual top quarks are exchanged, to the asymptotic energy behaviour of
pair production at future lepton colliders. We find that the effect
of the top mass is an extra linear logarithmic term of Sudakov type that is not
present in the case of (u,d,s,c) production. This appears to be particularly
relevant in the case of the cross section.Comment: 9 pages and 3 figures; version submitted to Phys.ReV.D,Rapid. e-mail:
[email protected]
Spherically Symmetric Solutions in Ghost-Free Massive Gravity
Recently, a class of theories of massive gravity has been shown to be
ghost-free. We study the spherically symmetric solutions in the bigravity
formulation of such theories. In general, the solutions admit both a Lorentz
invariant and a Lorentz breaking asymptotically flat behaviour and also fall in
two branches. In the first branch, all solutions can be found analitycally and
are Schwarzschild-like, with no modification as is found for other classes of
theories. In the second branch, exact solutions are hard to find, and relying
on perturbation theory, Yukawa-like modifications of the static potential are
found. The general structure of the solutions suggests that the bigravity
formulation of massive gravity is crucial and more than a tool.Comment: 15 pages. Some change in the reference
Logarithmic expansion of electroweak corrections to four-fermion processes in the TeV region
Starting from a theoretical representation of the electroweak component of
four-fermion neutral current processes that uses as theoretical input the
experimental measurements at the Z peak, we consider the asymptotic high energy
behaviour in the Standard Model at one loop of those gauge-invariant
combinations of self-energies, vertices and boxes that contribute all the
different observables. We find that the logarithmic contribution due to the
renormalization group running of the various couplings is numerically
overwhelmed by single and double logarithmic terms of purely electroweak
(Sudakov-type) origin, whose separate relative effects grow with energy,
reaching the 10% size at about one TeV. We then propose a simple "effective"
parametrization that aims at describing the various observables in the TeV
region, and discuss its validity both beyond and below 1 TeV, in particular in
the expected energy range of future linear electron-positron (LC) and muon-muon
colliders.Comment: 23 pages and 9 figures; version submitted to Phys.Rev.D. e-mail:
[email protected]
Stars and (Furry) Black Holes in Lorentz Breaking Massive Gravity
We study the exact spherically symmetric solutions in a class of
Lorentz-breaking massive gravity theories, using the effective-theory approach
where the graviton mass is generated by the interaction with a suitable set of
Stuckelberg fields. We find explicitly the exact black hole solutions which
generalizes the familiar Schwarzschild one, which shows a non-analytic hair in
the form of a power-like term r^\gamma. For realistic self-gravitating bodies,
we find interesting features, linked to the effective violation of the Gauss
law: i) the total gravitational mass appearing in the standard 1/r term gets a
multiplicative renormalization proportional to the area of the body itself; ii)
the magnitude of the power-like hairy correction is also linked to size of the
body. The novel features can be ascribed to presence of the goldstones fluid
turned on by matter inside the body; its equation of state approaching that of
dark energy near the center. The goldstones fluid also changes the matter
equilibrium pressure, leading to an upper limit for the graviton mass, m <~
10^-28 - 10^29 eV, derived from the largest stable gravitational bound states
in the Universe.Comment: 22 pages, 4 Figures. Final version to be published in PRD. Typos
corrected, comments adde
Exact Spherically Symmetric Solutions in Massive Gravity
A phase of massive gravity free from pathologies can be obtained by coupling
the metric to an additional spin-two field. We study the gravitational field
produced by a static spherically symmetric body, by finding the exact solution
that generalizes the Schwarzschild metric to the case of massive gravity.
Besides the usual 1/r term, the main effects of the new spin-two field are a
shift of the total mass of the body and the presence of a new power-like term,
with sizes determined by the mass and the shape (the radius) of the source.
These modifications, being source dependent, give rise to a dynamical violation
of the Strong Equivalence Principle. Depending on the details of the coupling
of the new field, the power-like term may dominate at large distances or even
in the ultraviolet. The effect persists also when the dynamics of the extra
field is decoupled.Comment: 24 pages, Latex JHEP style, added clarifications. Version accepted in
JHE
Spontaneous Lorentz Breaking and Massive Gravity
We study a theory where the presence of an extra spin-two field coupled to
gravity gives rise to a phase with spontaneously broken Lorentz symmetry. In
this phase gravity is massive, and the Weak Equivalence Principle is respected.
The newtonian potentials are in general modified, but we identify an
non-perturbative symmetry that protects them. The gravitational waves sector
has a rich phenomenology: sources emit a combination of massless and massive
gravitons that propagate with distinct velocities and also oscillate. Since
their velocities differ from the speed of light, the time of flight difference
between gravitons and photons from a common source could be measured.Comment: 4 page
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