69 research outputs found

    Softness and Amplitudes' Positivity for Spinning Particles

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    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 O(p4)O(p^4) 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

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    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

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    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

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    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

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    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

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    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

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    We derive rigorous bounds on corrections to Einstein gravity using unitarity and analyticity of graviton scattering amplitudes. In D≥4D\geq 4 spacetime dimensions, these consistency conditions mandate positive coefficients for certain quartic curvature operators. We systematically enumerate all such positivity bounds in D=4D=4 and D=5D=5 before extending to D≥6D\geq 6. 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 D≥5D\geq 5 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

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    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

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    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

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    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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