69 research outputs found
Softness and Amplitudes' Positivity for Spinning Particles
We derive positivity bounds for scattering amplitudes of particles with
arbitrary spin using unitarity, analyticity and crossing symmetry. The bounds
imply the positivity of certain low-energy coefficients of the effective action
that controls the dynamics of the light degrees of freedom. We show that
low-energy amplitudes strictly softer than do not admit unitary
ultraviolet completions unless the theory is free. This enforces a bound on the
energy growth of scattering amplitudes in the region of validity of the
effective theory. We discuss explicit examples including the Goldstino from
spontaneous supersymmetry breaking, and the theory of a spin-1/2 fermion with a
shift symmetry.Comment: 28 pages + refs and 3 appendices, 2 figures; v2: extra refs, improved
discussions, typos fixed, accepted on JHE
Effective perfect fluids in cosmology
We describe the cosmological dynamics of perfect fluids within the framework
of effective field theories. The effective action is a derivative expansion
whose terms are selected by the symmetry requirements on the relevant
long-distance degrees of freedom, which are identified with comoving
coordinates. The perfect fluid is defined by requiring invariance of the action
under internal volume-preserving diffeomorphisms and general covariance. At
lowest order in derivatives, the dynamics is encoded in a single function of
the entropy density that characterizes the properties of the fluid, such as the
equation of state and the speed of sound. This framework allows a neat
simultaneous description of fluid and metric perturbations. Longitudinal fluid
perturbations are closely related to the adiabatic modes, while the transverse
modes mix with vector metric perturbations as a consequence of vorticity
conservation. This formalism features a large flexibility which can be of
practical use for higher order perturbation theory and cosmological parameter
estimation.Comment: Matches JCAP versio
Composite Higgses
We present an overview of composite Higgs models in light of the discovery of
the Higgs boson. The small value of the physical Higgs mass suggests that the
Higgs quartic is likely loop generated, thus models with tree-level quartics
will generically be more tuned. We classify the various models (including bona
fide composite Higgs, little Higgs, holographic composite Higgs, twin Higgs and
dilatonic Higgs) based on their predictions for the Higgs potential, review the
basic ingredients of each of them, and quantify the amount of tuning needed,
which is not negligible in any model. We explain the main ideas for generating
flavor structure and the main mechanisms for protecting against large flavor
violating effects, and present a summary of the various coset models that can
result in realistic pseudo-Goldstone Higgses. We review the current
experimental status of such models by discussing the electroweak precision,
flavor and direct search bounds, and comment on UV completions and on ways to
incorporate dark matter.Comment: 58 Pages, 5 Figures, 5 Tables. Invited review article accepted for
publication in The European Physical Journal
Symmetries, Sum Rules and Constraints on Effective Field Theories
Using unitarity, analyticity and crossing symmetry, we derive universal sum
rules for scattering amplitudes in theories invariant under an arbitrary
symmetry group. The sum rules relate the coefficients of the energy expansion
of the scattering amplitudes in the IR to total cross sections integrated all
the way up to the UV. Exploiting the group structure of the symmetry, we
systematically determine all the independent sum rules and positivity
conditions on the expansion coefficients. For effective field theories the
amplitudes in the IR are calculable and hence the sum rules set constraints on
the parameters of the effective Lagrangian. We clarify the impact of gauging on
the sum rules for Goldstone bosons in spontaneously broken gauge theories. We
discuss explicit examples that are relevant for WW-scattering, composite Higgs
models, and chiral perturbation theory. Certain sum rules based on custodial
symmetry and its extensions provide constraints on the Higgs boson coupling to
the electroweak gauge bosons.Comment: 50 pages, 5 figures, 5 appendices; several typos fixed, discussions
improved, references added; results unchange
Off-critical Luttinger Junctions
We investigate Luttinger junctions of quantum wires away from criticality.
The one-body scattering matrix, corresponding to the off-critical boundary
conditions at the junction, admits in general antibound and/or bound states.
Their contribution to the theory is fixed by causality. The presence/absence of
bound states determines the existence of two different regimes with
inequivalent physical properties. A scattering matrix without bound states
defines an isolated equilibrium system. Bound states instead drive the system
away from equilibrium, giving raise to non-trivial incoming or outgoing energy
flows in the junction. We derive in both regimes and in explicit form the
electromagnetic conductance tensor, pointing out the different impact of bound
and antibound states.Comment: LaTex, 9 pages, 1 figur
Photophilic Higgs from sgoldstino mixing
The spontaneous breaking of linearly realized N=1 supersymmetry implies the
existence of a pseudo-Goldstone fermion, the goldstino, and of its complex
scalar superpartner, the sgoldstino. The latter has generically sizable
tree-level couplings to Standard Model gauge bosons while its couplings to SM
fermions are suppressed. We consider a light sgoldstino, with a mass around 1
TeV, that mixes with a SM-like Higgs scalar at around 125 GeV. We show that
such a mixing can enhance the Higgs to di-photon signal rate while evading all
the relevant experimental bounds and without significantly affecting the other
decay channels.Comment: 8 pages, 6 figures and 1 tabl
Quantum Gravity Constraints from Unitarity and Analyticity
We derive rigorous bounds on corrections to Einstein gravity using unitarity
and analyticity of graviton scattering amplitudes. In spacetime
dimensions, these consistency conditions mandate positive coefficients for
certain quartic curvature operators. We systematically enumerate all such
positivity bounds in and before extending to . Afterwards,
we derive positivity bounds for supersymmetric operators and verify that all of
our constraints are satisfied by weakly-coupled string theories. Among
quadratic curvature operators, we find that the Gauss-Bonnet term in
is inconsistent unless new degrees of freedom enter at the natural cutoff scale
defined by the effective theory. Our bounds apply to perturbative ultraviolet
completions of gravity.Comment: 26 page
Charming Higgs
We present a simple supersymmetric model where the dominant decay mode of the
lightest Higgs boson is h->2eta->4c where eta is a light pseudoscalar and c is
the charm quark. For such decays the Higgs mass can be smaller than 100 GeV
without conflict with experiment. Together with the fact that both the Higgs
and the pseudoscalar eta are pseudo-Goldstone bosons, this resolves the little
hierarchy problem.Comment: 6 pages, 2 figure
SUSY Digs up a Buried Higgs
The Higgs boson may dominantly decay to 4 light jets through a light
pseudo-scalar intermediary: h -> 2 eta -> 4j, making reconstruction at the LHC
particularly challenging. We explore the phenomenology of such "Buried Higgs"
scenarios in which the primary discovery channel of the Higgs is in cascade
decays of superpartners. QCD backgrounds that would otherwise overwhelm the
Higgs decay are suppressed by the requirement of high p_T jets and large
missing transverse momentum that are the typical signatures of TeV scale
supersymmetry. Utilizing jet substructure techniques, we find that for buried
Higgses in the 100-120 GeV range, a 5-sigma discovery can be expected with
roughly 10-25 inverse fb of data at E_CM = 14 TeV. For lighter Higgs bosons,
the signal is contaminated by hadronically decaying W bosons, and discovery
remains an unsolved challenge.Comment: 10 pages, 7 figures, references adde
A Higgslike Dilaton
We examine the possibility that the recently discovered 125 GeV higgs like
resonance actually corresponds to a dilaton: the Goldstone boson of scale
invariance spontaneously broken at a scale f. Comparing to LHC data we find
that a dilaton can reproduce the observed couplings of the new resonance as
long as f ~ v, the weak scale. This corresponds to the dynamical assumption
that only operators charged under the electroweak gauge group obtain VEVs. The
more difficult task is to keep the mass of the dilaton light compared to the
dynamical scale, Lambda ~ 4 pi f, of the theory. In generic, non-supersymmetric
theories one would expect the dilaton mass to be similar to Lambda. The mass of
the dilaton can only be lowered at the price of some percent level (or worse)
tuning and/or additional dynamical assumptions: one needs to suppress the
contribution of the condensate to the vacuum energy (which would lead to a
large dilaton quartic coupling), and to allow only almost marginal deformations
of the CFT.Comment: 30 pages, 4 figures; v2: references added, typos fixed, discussions
clarified, accepted for publication in EPJ
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