35 research outputs found
Color Octet Scalar Production at the LHC
New physics at the weak scale that can couple to quarks typically gives rise
to unacceptably large flavor changing neutral currents. An attractive way to
avoid this problem is to impose the principal of minimal flavor violation
(MFV). Recently it was noted that in MFV only scalars with the same gauge
quantum numbers as the standard model Higgs doublet or color octet scalars with
the same weak quantum numbers as the Higgs doublet can couple to quarks. In
this paper we compute the one-loop rate for production of a single color octet
scalar through gluon fusion at the LHC, which can become greater than the tree
level pair production rate for octet scalar masses around a TeV. We also
calculate the precision electroweak constraint from Z decays to a b and anti-b
quark; this constraint on color octet mass and Yukawa coupling affects the
allowed range for single octet scalar production through gluon fusion.Comment: 15 pages, 5 figure
On the Effect of Nuclear Response Functions in Dark Matter Direct Detection
We examine the effect of nuclear response functions, as laid out in
[Fitzpatrick et al, arXiv:1203.3542], on dark matter (DM) direct detection in
the context of well-motivated UV completions, including electric and magnetic
dipoles, anapole, spin-orbit, and pseudoscalar-mediated DM. Together, these
encompass five of the six nuclear responses extracted from the non-relativistic
effective theory of [Fitzpatrick et al, arXiv:1203.3542] (with the sixth
difficult to UV complete), with two of the six combinations corresponding to
standard spin-independent and -dependent responses. For constraints from
existing direct detection experiments, we find that only the COUPP constraint,
due to its heavy iodine target with large angular momentum and an unpaired
spin, and its large energy range sensitivity, is substantially modified by the
new responses compared to what would be inferred using the standard form
factors to model the energy dependence of the response. For heavy targets such
as xenon and germanium, the behavior of the new nuclear responses as recoil
energy increases can be substantially different than that of the standard
responses, but this has almost no impact on the constraints derived from
experiments such as LUX, XENON100 and CDMS since the maximum nuclear recoil
energy detected in these experiments is relatively low. We simulate mock data
for 80 and 250 GeV DM candidates utilizing the new nuclear responses to
highlight how they might affect a putative signal, and find the new responses
are most important for momentum-suppressed interactions such as the magnetic
dipole or pseudoscalar-mediated interaction when the target is relatively heavy
(such as xenon and iodine).Comment: 42 pages, 12 figures, 5 table
Asymmetric Dark Stars and Neutron Star Stability
We consider gravitationally bound states of asymmetric dark matter (ADM
stars), and the impact of ADM capture on the stability of neutron stars. We
derive and interpret the equation of state for ADM with both attractive and
repulsive interactions, and solve the Tolman-Oppenheimer-Volkoff equations to
find equilibrium sequences and maximum masses of ADM stars. Gravitational wave
searches can utilize our solutions to model exotic compact objects (ECOs). Our
results for attractive interactions differ substantially from those in the
literature, where fermionic ADM with attractive self-interactions was employed
to destabilize neutron stars more effectively than non-interacting fermionic
ADM. By contrast, we argue that fermionic ADM with an attractive force is no
more effective in destabilizing neutron stars than fermionic ADM with no
self-interactions.Comment: 9 pages plus 2 appendices (15 pages total), 7 figures, 1 tabl
Light Dark Matter Anomalies After LUX
We examine the consistency of light dark matter (DM) elastic scattering in
CoGeNT, DAMA, and CDMS-Silicon in light of constraints from XENON, CDMS, LUX,
PICASSO and COUPP. We consider a variety of operators that have been employed
to reconcile anomalies with constraints, including anapole, magnetic dipole,
momentum-dependent, and isospin-violating DM. We find that elastic scattering
through these alternative operators does not substantially reduce the tension
between the signals and the null constraints for operators where at least two
of the three purported signals map onto a common space in the DM
mass--scattering cross-section plane. Taking a choice of the scintillation
efficiency that lies at the region of the Manzur et al measurement
relieves tension between signals and the LUX constraint---in particular for a
magnetic dipole interaction and a xenophobic interaction (though for the latter
the signal regions do not substantially overlap). We also find that modest
changes in the halo model does not alter this result. We conclude that, even
relaxing the assumption about the type of elastic scattering interaction and
taking a conservative choice for the scintillation efficiency, LUX and the
results from other null experiments remain in tension with a light DM elastic
scattering explanation of direct detection anomalies.Comment: 27 pages, 8 figures; v2: typos corrected, a few references added, and
in the appendix: discussion of LUX analysis expanded and clarifications made
on XENON100 analysi
Primordial Power Spectra from Anisotropic Inflation
We examine cosmological perturbations in a dynamical theory of inflation in
which an Abelian gauge field couples directly to the inflaton, breaking
conformal invariance. When the coupling between the gauge field and the
inflaton takes a specific form, inflation becomes anisotropic and anisotropy
can persist throughout inflation, avoiding Wald's no-hair theorem. After
discussing scenarios in which anisotropy can persist during inflation, we
calculate the dominant effects of a small persistent anisotropy on the
primordial gravitational wave and curvature perturbation power spectra using
the "in-in" formalism of perturbation theory. We find that the primordial power
spectra of cosmological perturbations gain significant direction dependence and
that the fractional direction dependence of the tensor power spectrum is
suppressed in comparison to that of the scalar power spectrum.Comment: 25 pages, 2 figures; References added, typos corrected and some
discussion expanded; version submitted for publication in PR
Classical stability of a homogeneous, anisotropic inflating space-time
We study the classical stability of an anisotropic space-time seeded by a
spacelike, fixed norm, dynamical vector field in a vacuum-energy-dominated
inflationary era. It serves as a model for breaking isotropy during the
inflationary era. We find that, for a range of parameters, the linear
differential equations for small perturbations about the background do not have
a growing mode. We also examine the energy of fluctuations about this
background in flat-space. If the kinetic terms for the vector field do not take
the form of a field strength tensor squared then there is a negative energy
mode and the background is unstable. For the case where the kinetic term is of
the form of a field strength tensor squared we show that perturbations about
the background have positive energy at lowest order.Comment: 12 pages, no figures; references added, content in section V revised
and some clarification made in text; minor typos corrected, v4 closely
resembles version published in Phys. Rev. D; in v5 - incorrect argument in
section V removed and one reference adde
On Models of New Physics for the Tevatron Top A_FB
CDF has observed a top forward-backward asymmetry discrepant with the
Standard Model prediction at 3.4 \sigma. We analyze models that could generate
the asymmetry, including flavor-violating W's, horizontal Z'_Hs, triplet and
sextet diquarks, and axigluons. We consider the detailed predictions of these
models for the invariant mass and rapidity distributions of the asymmetry at
the parton level, comparing against the unfolded parton-level CDF results.
While all models can reproduce the asymmetry with the appropriate choice of
mass and couplings, it appears at first examination that the extracted
parton-level invariant mass distribution for all models are in conflict with
Tevatron observations. We show on closer examination, however, that t tbar
events in Z'_H and W' models have considerably lower selection efficiencies in
high invariant mass bins as compared to the Standard Model, so that W', Z'_H,
and axigluon models can generate the observed asymmetry while being consistent
with the total cross-section and invariant mass spectrum. Triplet and sextet
models have greater difficulty producing the observed asymmetry while remaining
consistent with the total cross-section and invariant mass distribution. To
more directly match the models and the CDF results, we proceed to decay and
reconstruct the tops, comparing our results against the "raw" CDF asymmetry and
invariant mass distributions. We find that the models that successfully
generate the corrected CDF asymmetry at the parton level reproduce very well
the more finely binned uncorrected asymmetry. Finally, we discuss the early LHC
reach for discovery of these models, based on our previous analysis
[arXiv:1102.0018].Comment: 29 pages, 14 figures, 2 table
Nuclear Structure of Bound States of Asymmetric Dark Matter
Models of Asymmetric Dark Matter (ADM) with a sufficiently attractive and
long-range force gives rise to stable bound objects, analogous to nuclei in the
Standard Model, called nuggets. We study the properties of these nuggets and
compute their profiles and binding energies. Our approach, applicable to both
elementary and composite fermionic ADM, utilizes relativistic mean field
theory, and allows a more systematic computation of nugget properties, over a
wider range of sizes and force mediator masses, compared to previous
literature. We identify three separate regimes of nugget property behavior
corresponding to (1) non-relativistic and (2) relativistic constituents in a
Coulomb-like limit, and (3) saturation in an anti-Coulomb limit when the
nuggets are large compared to the force range. We provide analytical
descriptions for nuggets in each regime. Through numerical calculations, we are
able to confirm our analytic descriptions and also obtain smooth transitions
for the nugget profiles between all three regimes. We also find that over a
wide range of parameter space, the binding energy in the saturation limit is an
fraction of the constituent's mass, significantly larger than
expectations in the non-relativistic case. In a companion paper, we apply our
results to synthesis of ADM nuggets in the early Universe.Comment: 20 pages, 8 figures, 1 appendi
Making Asymmetric Dark Matter Gold: Early Universe Synthesis of Nuggets
We compute the mass function of bound states of Asymmetric Dark
Matter--nuggets--synthesized in the early Universe. We apply our results for
the nugget density and binding energy computed from a nuclear model to obtain
analytic estimates of the typical nugget size exiting synthesis. We numerically
solve the Boltzmann equation for synthesis including two-to-two fusion
reactions, estimating the impact of bottlenecks on the mass function exiting
synthesis. These results provide the basis for studying the late Universe
cosmology of nuggets in a future companion paper.Comment: 27 pages, 11 figures, modified discussions in Section I