390 research outputs found
Mass Hierarchy and Vacuum Energy
A hierarchically small weak scale does not generally coincide with enhanced
symmetry, but it may still be exceptional with respect to vacuum energy. By
analyzing the classical vacuum energy as a function of parameters such as the
Higgs mass, we show how near-criticality, i.e. fine-tuning, corresponds
universally to boundaries where the vacuum energy transitions from exactly flat
to concave down. In the presence of quantum corrections, these boundary regions
can easily be perturbed to become maxima of the vacuum energy. After
introducing a dynamical scalar field which scans the Higgs sector
parameters, we propose several possible mechanisms by which this field could be
localized to the maximum. One possibility is that the potential has many
vacua, with those near the maximum vacuum energy expanding faster during a long
period of cosmic inflation and hence dominating the volume of the Universe.
Alternately, we describe scenarios in which vacua near the maximum could be
anthropically favored, due to selection of the late-time cosmological constant
or dark matter density. Independent of these specific approaches, the physical
value of the weak scale in our proposal is generated naturally and dynamically
from loops of heavy states coupled to the Higgs. These states are predicted to
be a loop factor heavier than in models without this mechanism, avoiding
tension with experimental null results.Comment: 45 pages, 10 figures. v2: Additional discussion of inflationary
cosmology scenarios, added reference
Effectively Stable Dark Matter
We study dark matter (DM) which is cosmologically long-lived because of
standard model (SM) symmetries. In these models an approximate stabilizing
symmetry emerges accidentally, in analogy with baryon and lepton number in the
renormalizable SM. Adopting an effective theory approach, we classify DM models
according to representations of , allowing for all operators permitted by symmetry, with
weak scale DM and a cutoff at or below the Planck scale. We identify
representations containing a neutral long-lived state, thus excluding dimension
four and five operators that mediate dangerously prompt DM decay into SM
particles. The DM relic abundance is obtained via thermal freeze-out or, since
effectively stable DM often carries baryon or lepton number, asymmetry sharing
through the very operators that induce eventual DM decay. We also incorporate
baryon and lepton number violation with a spurion that parameterizes hard
breaking by arbitrary units. However, since proton stability precludes certain
spurions, a residual symmetry persists, maintaining the cosmological stability
of certain DM representations. Finally, we survey the phenomenology of
effectively stable DM as manifested in probes of direct detection, indirect
detection, and proton decay.Comment: 7 pages, 1 figure, 4 table
Infrared Consistency and the Weak Gravity Conjecture
The weak gravity conjecture (WGC) asserts that an Abelian gauge theory
coupled to gravity is inconsistent unless it contains a particle of charge
and mass such that . This criterion is obeyed by all
known ultraviolet completions and is needed to evade pathologies from stable
black hole remnants. In this paper, we explore the WGC from the perspective of
low-energy effective field theory. Below the charged particle threshold, the
effective action describes a photon and graviton interacting via
higher-dimension operators. We derive infrared consistency conditions on the
parameters of the effective action using i) analyticity of light-by-light
scattering, ii) unitarity of the dynamics of an arbitrary ultraviolet
completion, and iii) absence of superluminality and causality violation in
certain non-trivial backgrounds. For convenience, we begin our analysis in
three spacetime dimensions, where gravity is non-dynamical but has a physical
effect on photon-photon interactions. We then consider four dimensions, where
propagating gravity substantially complicates all of our arguments, but bounds
can still be derived. Operators in the effective action arise from two types of
diagrams: those that involve electromagnetic interactions (parameterized by a
charge-to-mass ratio ) and those that do not (parameterized by a
coefficient ). Infrared consistency implies that is bounded from
below for small .Comment: 37 pages, 5 figures. Minor typos fixed and equation numbers changed
to match journal. Published in JHE
Hidden Simplicity of the Gravity Action
We derive new representations of the Einstein-Hilbert action in which
graviton perturbation theory is immensely simplified. To accomplish this, we
recast the Einstein-Hilbert action as a theory of purely cubic interactions
among gravitons and a single auxiliary field. The corresponding equations of
motion are the Einstein field equations rewritten as two coupled first-order
differential equations. Since all Feynman diagrams are cubic, we are able to
derive new off-shell recursion relations for tree-level graviton scattering
amplitudes. With a judicious choice of gauge fixing, we then construct an
especially compact form for the Einstein-Hilbert action in which all graviton
interactions are simply proportional to the graviton kinetic term. Our results
apply to graviton perturbations about an arbitrary curved background spacetime.Comment: 20 pages, 1 figur
Non-renormalization Theorems without Supersymmetry
We derive a new class of one-loop non-renormalization theorems that strongly
constrain the running of higher dimension operators in a general
four-dimensional quantum field theory. Our logic follows from unitarity: cuts
of one-loop amplitudes are products of tree amplitudes, so if the latter vanish
then so too will the associated divergences. Finiteness is then ensured by
simple selection rules that zero out tree amplitudes for certain helicity
configurations. For each operator we define holomorphic and anti-holomorphic
weights, , where and are the number and
sum over helicities of the particles created by that operator. We argue that an
operator can only be renormalized by an operator if
and , absent non-holomorphic
Yukawa couplings. These results explain and generalize the surprising
cancellations discovered in the renormalization of dimension six operators in
the standard model. Since our claims rely on unitarity and helicity rather than
an explicit symmetry, they apply quite generally.Comment: 6 pages, 2 figures, and 2 table
Symmetry and Action for Flavor-Kinematics Duality
We propose a new representation of the nonlinear sigma model that exhibits a
manifest duality between flavor and kinematics. The fields couple exclusively
through cubic Feynman vertices which also serve as the structure constants of
an underlying kinematic algebra. The action is invariant under a combination of
internal and spacetime symmetries whose conservation equations imply
flavor-kinematics duality, ensuring that all Feynman diagrams satisfy kinematic
Jacobi identities. Substituting flavor for kinematics, we derive a new cubic
action for the special Galileon theory. In this picture, the vanishing soft
behavior of amplitudes is a byproduct of the Weinberg soft theorem.Comment: 5 pages+refs; matched to published versio
BCFW Recursion Relations and String Theory
We demonstrate that all tree-level string theory amplitudes can be computed
using the BCFW recursion relations. Our proof utilizes the pomeron vertex
operator introduced by Brower, Polchinski, Strassler, and Tan. Surprisingly, we
find that in a particular large complex momentum limit, the asymptotic
expansion of massless string amplitudes is identical in form to that of the
corresponding field theory amplitudes. This observation makes manifest the fact
that field-theoretic Yang-Mills and graviton amplitudes obey KLT-like
relations. Moreover, we conjecture that in this large momentum limit certain
string theory and field theory amplitudes are identical, and provide evidence
for this conjecture. Additionally, we find a new recursion relation which
relates tachyon amplitudes to lower-point tachyon amplitudes.Comment: 36 pages, JHEP3; reference and note added, improved discussion in
section
Bubble Baryogenesis
We propose an alternative mechanism of baryogenesis in which a scalar baryon
undergoes a percolating first-order phase transition in the early Universe. The
potential barrier that divides the phases contains explicit B and CP violation
and the corresponding instanton that mediates decay is therefore asymmetric.
The nucleation and growth of these asymmetric bubbles dynamically generates
baryons, which thermalize after percolation; bubble collision dynamics can also
add to the asymmetry yield. We present an explicit toy model that undergoes
bubble baryogenesis, and numerically study the evolution of the baryon
asymmetry through bubble nucleation and growth, bubble collisions, and washout.
We discuss more realistic constructions, in which the scalar baryon and its
potential arise amongst the color-breaking minima of the MSSM, or in the
supersymmetric neutrino seesaw mechanism. Phenomenological consequences, such
as gravitational waves, and possible applications to asymmetric dark-matter
generation are also discussed.Comment: 15 pages, 13 figures, references added, changes reflect published
versio
Gravitino Freeze-In
We explore an alternative mechanism for the production of gravitino dark
matter whereby relic gravitinos originate from the decays of superpartners
which are still in thermal equilibrium, i.e. via freeze-in. Contributions to
the gravitino abundance from freeze-in can easily dominate over those from
thermal scattering over a broad range of parameter space, e.g. when the scalar
superpartners are heavy. Because the relic abundance from freeze-in is
independent of the reheating temperature after inflation, collider measurements
may be used to unambiguously reconstruct the freeze-in origin of gravitinos. In
particular, if gravitino freeze-in indeed accounts for the present day dark
matter abundance, then the lifetime of the next-to-lightest superpartner is
uniquely fixed by the superpartner spectrum.Comment: 5 pages, 3 figure
Gravity amplitudes from n-space
We identify a hidden GL(n,β) symmetry of the tree level n-point MHV gravity amplitude. Representations of this symmetry reside in an auxiliary n-space whose indices are external particle labels. Spinor helicity variables transform non-linearly under GL(n,β), but linearly under its notable subgroups, the little group and the permutation group S_n. Using GL(n,β) covariant variables, we present a new and simple formula for the MHV amplitude which can be derived solely from geometric constraints. This expression carries a huge intrinsic redundancy which can be parameterized by a pair of reference 3-planes in n-space. Fixing this redundancy in a particular way, we reproduce the S_(nβ3) symmetric form of the MHV amplitude of [1], which is in turn equivalent to the S_(nβ2) symmetric form of [2] as a consequence of the matrix tree theorem. The redundancy of the amplitude can also be fixed in a way that fully preserves S_n, yielding new and manifestly S_n symmetric forms of the MHV amplitude. Remarkably, these expressions need not be manifestly homogenous in spinorial weight or mass dimension. We comment on possible extensions to N^(kβ2)MHV amplitudes and speculate on the deeper origins of GL(n,β)
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