1,227 research outputs found
Symmetryless Dark Matter
It is appealing to stabilize dark matter by the same discrete symmetry that
is used to explain the structure of quark and lepton mass matrices. However, to
generate the observed fermion mixing patterns, any flavor symmetry must
necessarily be broken, rendering dark matter unstable. We study singlet,
doublet and triplet SU(2) multiplets of both scalar and fermion dark matter
candidates and enumerate the conditions under which no d < 6 dark matter decay
operators are generated even in the case if the flavor symmetry is broken to
nothing. We show that the VEVs of flavon scalars transforming as higher
multiplets (e.g. triplets) of the flavor group must be at the electroweak
scale. The most economical way for that is to use SM Higgs boson(s) as flavons.
Such models can be tested by the LHC experiments. This scenario requires the
existence of additional Froggatt-Nielsen scalars that generate hierarchies in
Yukawa couplings. We study the conditions under which large and small flavor
breaking parameters can coexist without destabilizing the dark matter.Comment: 8 pages, no figure
Constraints on the excitations in the Strongly Coupled Standard Model
The Strongly Coupled Standard Model predicts a rich spectrum of excited
states at the Fermi scale. We study the first radial excitations of the vector
bosons. The inclusion of these new states affects the low energy phenomenology
of the model. We put constraints on the effective couplings by performing a
global fit with the electroweak observables, and we find that the excitations
have to be rather decoupled from the low-energy states.Comment: 23 pages, 6 figures, uses RevTeX
Transverse Momentum Broadening and the Jet Quenching Parameter, Redux
We use Soft Collinear Effective Theory (SCET) to analyze the transverse
momentum broadening, or diffusion in transverse momentum space, of an energetic
parton propagating through quark-gluon plasma. Since we neglect the radiation
of gluons from the energetic parton, we can only discuss momentum broadening,
not parton energy loss. The interaction responsible for momentum broadening in
the absence of radiation is that between the energetic (collinear) parton and
the Glauber modes of the gluon fields in the medium. We derive the effective
Lagrangian for this interaction, and we show that the probability for picking
up transverse momentum k_\perp is given by the Fourier transform of the
expectation value of two transversely separated light-like path-ordered Wilson
lines. This yields a field theoretical definition of the jet quenching
parameter \hat q, and shows that this can be interpreted as a diffusion
constant. We close by revisiting the calculation of \hat q for the strongly
coupled plasma of N=4 SYM theory, showing that previous calculations need some
modifications that make them more straightforward and do not change the result.Comment: 18 pages, 7 figures; v2, minor revisions, references added; v3,
version to appear in Phys. Rev. D: Feynman rules corrected, improved
explanations of the gauge invariance of our calculation and of how the
scaling of SCET operators differs from that in other contexts in the
literature; no changes to any result
Gauge invariant definition of the jet quenching parameter
In the framework of Soft-Collinear Effective Theory, the jet quenching
parameter, , has been evaluated by adding the effect of Glauber gluon
interactions to the propagation of a highly-energetic collinear parton in a
medium. The result, which holds in covariant gauges, has been expressed in
terms of the expectation value of two Wilson lines stretching along the
direction of the four-momentum of the parton. In this paper, we show how that
expression can be generalized to an arbitrary gauge by the addition of
transverse Wilson lines. The transverse Wilson lines are explicitly computed by
resumming interactions of the parton with Glauber gluons that appear only in
non-covariant gauges. As an application of our result, we discuss the
contribution to coming from transverse momenta of order in a
medium that is a weakly-coupled quark-gluon plasma.Comment: 31 pages, 7 figures; journal versio
External leg amputation in conformal invariant three-point function
Amputation of external legs is carried out explicitly for the conformal
invariant three-point function involving two spinors and one vector field. Our
results are consistent with the general result that amputing an external leg in
a conformal invariant Green function replaces a field by its conformal partner
in the Green function. A new star-triangle relation, involving two spinors and
one vector field, is derived and used for the calculation.Comment: 16 pages; last paragraph added in Sec. 10, presentation improved, to
appear in Eur. Phys. J.
A non-perturbative contribution to jet quenching
It has been argued by Caron-Huot that infrared contributions to the jet
quenching parameter in hot QCD, denoted by qhat, can be extracted from an
analysis of a certain static-potential related observable within the
dimensionally reduced effective field theory. Following this philosophy, the
order of magnitude of a non-perturbative contribution to qhat from the
colour-magnetic scale, g^2T/pi, is estimated. The result is small; it is
probably below the parametrically perturbative but in practice slowly
convergent contributions from the colour-electric scale, whose all-orders
resummation therefore remains an important challenge.Comment: 4 pages. v2: clarifications, published versio
Virtual signatures of dark sectors in Higgs couplings
Where collider searches for resonant invisible particles loose steam, dark
sectors might leave their trace as virtual effects in precision observables.
Here we explore this option in the framework of Higgs portal models, where a
sector of dark fermions interacts with the standard model through a strong
renormalizable coupling to the Higgs boson. We show that precise measurements
of Higgs-gauge and triple Higgs interactions can probe dark fermions up to the
TeV scale through virtual corrections. Observation prospects at the LHC and
future lepton colliders are discussed for the so-called singlet-doublet model
of Majorana fermions, a generalization of the bino-higgsino scenario in
supersymmetry. We advocate a two-fold search strategy for dark sectors through
direct and indirect observables.Comment: 20 pages, 7 figures, 1 tabl
The Spectrum of Goldstini and Modulini
When supersymmetry is broken in multiple sectors via independent dynamics,
the theory furnishes a corresponding multiplicity of "goldstini" degrees of
freedom which may play a substantial role in collider phenomenology and
cosmology. In this paper, we explore the tree-level mass spectrum of goldstini
arising from a general admixture of F-term, D-term, and almost no-scale
supersymmetry breaking, employing non-linear superfields and a novel gauge
fixing for supergravity discussed in a companion paper. In theories of F-term
and D-term breaking, goldstini acquire a mass which is precisely twice the
gravitino mass, while the inclusion of no-scale breaking renders one of these
modes, the modulino, massless. We argue that the vanishing modulino mass can be
explained in terms of an accidental and spontaneously broken "global"
supersymmetry.Comment: 10 pages, 2 figures; v2: typo corrected, references updated; v3:
version to appear in JHE
The Two Faces of Anomaly Mediation
Anomaly mediation is a ubiquitous source of supersymmetry (SUSY) breaking
which appears in almost every theory of supergravity. In this paper, we show
that anomaly mediation really consists of two physically distinct phenomena,
which we dub "gravitino mediation" and "Kahler mediation". Gravitino mediation
arises from minimally uplifting SUSY anti-de Sitter (AdS) space to Minkowski
space, generating soft masses proportional to the gravitino mass. Kahler
mediation arises when visible sector fields have linear couplings to SUSY
breaking in the Kahler potential, generating soft masses proportional to beta
function coefficients. In the literature, these two phenomena are lumped
together under the name "anomaly mediation", but here we demonstrate that they
can be physically disentangled by measuring associated couplings to the
goldstino. In particular, we use the example of gaugino soft masses to show
that gravitino mediation generates soft masses without corresponding goldstino
couplings. This result naively violates the goldstino equivalence theorem but
is in fact necessary for supercurrent conservation in AdS space. Since
gravitino mediation persists even when the visible sector is sequestered from
SUSY breaking, we can use the absence of goldstino couplings as an unambiguous
definition of sequestering.Comment: 21 pages, 1 table; v2, references added, extended discussion in
introduction and appendix; v3, JHEP versio
Jet quenching in a strongly coupled anisotropic plasma
The jet quenching parameter of an anisotropic plasma depends on the relative
orientation between the anisotropic direction, the direction of motion of the
parton, and the direction along which the momentum broadening is measured. We
calculate the jet quenching parameter of an anisotropic, strongly coupled N=4
plasma by means of its gravity dual. We present the results for arbitrary
orientations and arbitrary values of the anisotropy. The anisotropic value can
be larger or smaller than the isotropic one, and this depends on whether the
comparison is made at equal temperatures or at equal entropy densities. We
compare our results to analogous calculations for the real-world quark-gluon
plasma and find agreement in some cases and disagreement in others.Comment: 22 pages, 10 figures; v2: minor changes, added reference. Extends
arXiv:1202.369
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