758 research outputs found
Electroweak lights from Dark Matter annihilations
The energy spectra of Standard Model particles originated from Dark Matter
annihilations can be significantly altered by the inclusion of electroweak
gauge boson radiation from the final state. A situation where this effect is
particularly important is when a Majorana Dark Matter particle annihilates into
two light fermions. This process is in p-wave and hence suppressed by the small
value of the relative velocity of the annihilating particles. The inclusion of
electroweak radiation eludes this suppression and opens up a potentially
sizeable s-wave contribution to the annihilation cross section. I will discuss
the impact of this effect on the fluxes of stable particles resulting from the
Dark Matter annihilations, which are relevant for Dark Matter indirect
searches.Comment: 4 pages, 2 figures. Contribution to the conference proceedings of
TAUP 2011, Munich - Germany (5-9 September 2011
The BFKL Equation at Next-to-Leading Order and Beyond
On the basis of a renormalization group analysis of the kernel and of the
solutions of the BFKL equation with subleading corrections, we propose and
calculate a novel expansion of a properly defined effective eigenvalue
function. We argue that in this formulation the collinear properties of the
kernel are taken into account to all orders, and that the ensuing
next-to-leading truncation provides a much more stable estimate of hard Pomeron
and of resummed anomalous dimensions.Comment: LaTex, 12 pages, 1 eps figur
Towards Collinear Evolution Equations in Electroweak Theory
We consider electroweak radiative corrections to hard inclusive processes at
the TeV scale, and we investigate how collinear logarithms factorize in a
spontaneously broken gauge theory, similarly to the DGLAP analysis in QCD.
Due to the uncancelled double logs noticed previously, we find a
factorization pattern which is qualitatively different from the analogous one
in QCD. New types of splitting functions emerge which are needed to describe
the initial beam charges and are infrared-sensitive, that is dependent on an
infrared cutoff provided, ultimately, by the symmetry breaking scale. We derive
such splitting functions at one-loop level in the example of SU(2) gauge
theory, and we also discuss the structure functions' evolution equations, under
the assumption that isospin breaking terms present in the Ward identities of
the theory are sufficiently subleading at higher orders.Comment: 5 pages, 3 figure
Quantum Tunneling and Unitarity Features of an S-matrix for Gravitational Collapse
Starting from the semiclassical reduced-action approach to transplanckian
scattering by Amati, Veneziano and one of us and from our previous quantum
extension of that model, we investigate the S-matrix expression for inelastic
processes by extending to this case the tunneling features previously found in
the region of classical gravitational collapse. The resulting model exhibits
some non-unitary S-matrix eigenvalues for impact parameters b < b_c, a critical
value of the order of the gravitational radius R = 2 G sqrt(s), thus showing
that some (inelastic) unitarity defect is generally present, and can be studied
quantitatively. We find that S-matrix unitarity for b < b_c is restored only if
the rapidity phase-space parameter y is allowed to take values larger than the
effective coupling G s / hbar itself. Some features of the resulting unitary
model are discussed.Comment: 28 pages, 11 figure
Bloch-Nordsieck Violation in Spontaneously Broken Abelian Theories
We point out that, in a spontaneously broken U(1) gauge theory, inclusive
processes, whose primary particles are mass eigenstates that do not coincide
with the gauge eigenstates, are not free of infrared logarithms. The charge
mixing allowed by symmetry breaking and the ensuing Bloch-Nordsieck violation
are here analyzed in a few relevant cases and in particular for processes
initiated by longitudinal gauge bosons. Of particular interest is the example
of weak hypercharge in the Standard Model where, in addition, left-right mixing
effects arise in transversely polarized fermion beams.Comment: 4 pages, 1 figur
Initial State Radiation in Majorana Dark Matter Annihilations
The cross section for a Majorana Dark Matter particle annihilating into light
fermions is helicity suppressed. We show that, if the Dark Matter is the
neutral Majorana component of a multiplet which is charged under the
electroweak interactions of the Standard Model, the emission of gauge bosons
from the initial state lifts the suppression and allows an s-wave annihilation.
The resulting energy spectra of stable Standard Model particles are importantly
affected. This has an impact on indirect searches for Dark Matter.Comment: 9 pages, 3 figure
Tunneling transition to the Pomeron regime
We point out that, in some models of small-x hard processes, the transition
to the Pomeron regime occurs through a sudden tunneling effect, rather than a
slow diffusion process. We explain the basis for such a feature and we
illustrate it for the BFKL equation with running coupling by gluon rapidity
versus scale correlation plots.Comment: 17 pages, 5 figures, mpeg animations available from
http://www.lpthe.jussieu.fr/~salam/tunneling/ . v2 includes additional
reference
A collinear model for small-x physics
We propose a simple model for studying small-x physics in which we take only
the collinearly enhanced part of leading and subleading kernels, for all
possible transverse momentum orderings. The small-x equation reduces to a
second order differential equation in t=log k^2/Lambda^2 space, whose
perturbative and strong-coupling features are investigated both analytically
and numerically. For two-scale processes, we clarify the transition mechanism
between the perturbative, non Regge regime and the strong-coupling Pomeron
behavior.Comment: 22 pages, 8 figures, LaTeX file, uses JHEP.cl
Electroweak Sudakov Logarithms and Real Gauge-Boson Radiation in the TeV Region
Electroweak radiative corrections give rise to large negative,
double-logarithmically enhanced corrections in the TeV region. These are partly
compensated by real radiation and, moreover, affected by selecting
isospin-noninvariant external states. We investigate the impact of real gauge
boson radiation more quantitatively by considering different restricted final
state configurations. We consider successively a massive abelian gauge theory,
a spontaneously broken SU(2) theory and the electroweak Standard Model. We find
that details of the choice of the phase space cuts, in particular whether a
fraction of collinear and soft radiation is included, have a strong impact on
the relative amount of real and virtual corrections.Comment: 20 pages, 4 figure
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