142 research outputs found
TASI Lectures on Jet Substructure
Jet physics is a rich and rapidly evolving field, with many applications to
physics in and beyond the Standard Model. These notes, based on lectures
delivered at the June 2012 Theoretical Advanced Study Institute, provide an
introduction to jets at the Large Hadron Collider. Topics covered include
sequential jet algorithms, jet shapes, jet grooming, and boosted Higgs and top
tagging.Comment: 38 pages, 6 figures. v2: non-compiling references fixe
Polarized tops from new physics: signals and observables
Top quarks may be produced in large numbers in association with new physics
at the LHC. The polarization of these top quarks probes the chiral structure of
the new physics. We discuss several kinematic distributions which are sensitive
to the polarization of single top quarks and can be used without full event
reconstruction. For collimated tops we construct polarization-sensitive
observables for both hadronic and leptonic decay modes and plot their
distributions. We compute the observable polarization signals from top quarks
produced in the on-shell cascade decay of a stop squark into a top quark and a
neutralino, as well as top quarks produced in the analogous decay chain in
same-spin partner models.Comment: 6 pages, 4 .eps figures. v2: version accepted to PR
A Solar System Test of Self-Interacting Dark Matter
Dark matter (DM) self-interactions affect the gravitational capture of DM in
the Sun and Earth differently as a simple consequence of the differing
kinematics of collisions within the two potential wells: the dominant effect of
self-interactions in the Sun is to provide an additional channel for capture,
while the dominant effect in the Earth is to eject previously captured DM. We
point out that this simple observation can be used to deduce the existence of
DM self-interactions by comparing the annihilation rates of DM gravitationally
bound within the Sun and Earth. We compute the Sun and Earth annihilation
fluxes for DM with spin-independent nuclear cross-sections and thermal
annihilation cross-sections and demonstrate that, for cross-sections allowed by
direct detection, self-interactions can easily suppress the expected Earth flux
by multiple orders of magnitude. This suppression is potentially significant
even for self-interaction cross-sections orders of magnitude below the Bullet
Cluster bounds, making this solar system comparison a leading test of dark
matter self-interactions. Additionally, we consider thermalization of the
captured DM population with the nuclei of the capturing body in some detail,
accounting for both nuclear and self-interactions, and point out some
consequential and broadly applicable considerations.Comment: 37 pages, 16 figures. v2: updated direct detection bounds, version to
appear in JCA
Hunting Asymmetric Stops
We point out that in the irreducible natural SUSY spectrum, stops have
comparable branching fractions to chargino-bottom and neutralino-top in the
vast bulk of parameter space, provided only that both decay modes are
kinematically accessible. The total stop pair branching fractions into ttbar +
MET can therefore be reduced to O(50%), while b bbar + X branching fractions
are typically much smaller, O(10%), thus limiting the reach of traditional stop
searches. We propose a new stop search targeting the asymmetric final state
\~t\~t* --> t chi^0 b chi^pm, which can restore sensitivity to natural stops in
the 7 and 8 TeV LHC runs. In addition we present a new variable, topness, which
efficiently suppresses the dominant top backgrounds to semi-leptonic top
partner searches. We demonstrate the utility of topness in both our asymmetric
search channel and traditional \~t\~t* --> ttbar + MET searches and show that
it matches or out-performs existing variables.Comment: 5 pages, 4 figures. V2: version accepted for publication in PR
Darkogenesis
In standard models of baryogenesis and of dark matter, the mechanisms which
generate the densities in both sectors are unrelated to each other. In this
paper we explore models which generate the baryon asymmetry through the dark
matter sector, simultaneously relating the baryon asymmetry to the dark matter
density. In the class of models we explore, a dark matter asymmetry is
generated in the hidden sector through a first order phase transition. Within
the hidden sector, it is easy to achieve a sufficiently strong first order
phase transition and large enough violation to generate the observed
asymmetry. This can happen above or below the electroweak phase transition, but
in both cases significantly before the dark matter becomes non-relativistic. We
study examples where the Asymmetric Dark Matter density is then transferred to
the baryons both through perturbative and non-perturbative communication
mechanisms, and show that in both cases cosmological constraints are satisfied
while a sufficient baryon asymmetry can be generated.Comment: 20 pages, 2 figure
Direct and indirect detection of dissipative dark matter
We study the constraints from direct detection and solar capture on dark
matter scenarios with a subdominant dissipative component. This dissipative
dark matter component in general has both a symmetric and asymmetric relic
abundance. Dissipative dynamics allow this subdominant dark matter component to
cool, resulting in its partial or total collapse into a smaller volume inside
the halo (e.g., a dark disk) as well as a reduced thermal velocity dispersion
compared to that of normal cold dark matter. We first show that these features
considerably relax the limits from direct detection experiments on the
couplings between standard model (SM) particles and dissipative dark matter. On
the other hand, indirect detection of the annihilation of the symmetric
dissipative dark matter component inside the Sun sets stringent and robust
constraints on the properties of the dissipative dark matter. In particular,
IceCube observations force dissipative dark matter particles with mass above 50
GeV to either have a small coupling to the SM or a low local density in the
solar system, or to have a nearly asymmetric relic abundance. Possible
helioseismology signals associated with purely asymmetric dissipative dark
matter are discussed, with no present constraints.Comment: 28 pages, 5 figures, minor modifications, references added and to
appear in JCA
Fitting the Galactic Center Gamma-Ray Excess with Cascade Annihilations
The apparent excess of gamma rays in an extended region in the direction of
the galactic center has a spatial distribution and amplitude that are
suggestive of dark matter annihilations. If this excess is indeed due to dark
matter annihilations, it would indicate the presence of both dark matter and an
additional particle beyond the Standard Model that mediates the interactions
between the dark matter and Standard Model states. We introduce reference
models describing dark matter annihilation to pairs of these new mediators,
which decouples the SM-mediator coupling from the thermal annihilation cross
section and easily explains the lack of direct detection signals. We determine
the parameter regions that give good descriptions of the gamma ray excess for
several motivated choices of mediator couplings to the SM. We find fermion dark
matter with mass 7-26 GeV and a dark vector mediator, or scalar dark matter in
the 10-50 GeV range (Higgs portal mediator) or 10-65 GeV range (gluophilic
mediator) can provide a comparable or improved fit, compared to the case of
direct annihilation. We demonstrate that these models can easily satisfy all
constraints from collider experiments, direct detection, and cosmology.Comment: 11 pages. v3: version to appear in PR
Kinematics of Top Quark Final States: A Snowmass White Paper
This is the summary report of the Top Quark Kinematics working group prepared
for Snowmass 2013. We survey the current state of theoretical predictions for
top pair differential distributions, in both boosted and un-boosted regimes,
and present an overview of uncertainties and prospects for top spin
correlations. We study the prospects for measuring the inclusive SM top pair
production asymmetry AFC at LHC 14 as a function of systematic error, and show
that some improvement over current systematic uncertainties, as customarily
handled, is required for observing a SM-size asymmetry. Cuts on top pair
invariant mass and rapidity do not substantially alter this conclusion. We
summarize the conclusions of contributed studies on alternate LHC measurements
of the ttbar production asymmetry, in ttbar+jet final states and in forward top
production at LHCb, both of which show good prospects for observing SM-size
asymmetries in 50 fb-1 of data at LHC14.Comment: Contributed paper to US Snowmass process for the Top quark group, Top
quark kinematics subgrou
Tadpoles and Closed String Backgrounds in Open String Field Theory
We investigate the quantum structure of Witten's cubic open bosonic string
field theory by computing the one-loop contribution to the open string tadpole
using both oscillator and conformal field theory methods. We find divergences
and a breakdown of BRST invariance in the tadpole diagram arising from
tachyonic and massless closed string states, and we discuss ways of treating
these problems. For a Dp-brane with sufficiently many transverse dimensions,
the tadpole can be rendered finite by analytically continuing the closed string
tachyon by hand; this diagram then naturally incorporates the (linearized)
shift of the closed string background due to the presence of the brane. We
observe that divergences at higher loops will doom any straightforward attempt
at analyzing general quantum effects in bosonic open string field theory on a
Dp-brane of any dimension, but our analysis does not uncover any potential
obstacles to the existence of a sensible quantum open string field theory in
the supersymmetric case.Comment: 51 pages, 11 eps figures, Latex; v2: References adde
Dark Matter Through the Neutrino Portal
We consider a model of dark matter whose most prominent signature is a
monochromatic flux of TeV neutrinos from the galactic center. As an example of
a general scenario, we consider a specific model where the dark matter is a
fermion in the adjoint representation of a hidden SU(N) gauge group that
confines at GeV energies. The absence of light fermionic states in the dark
sector ensures stability of dark matter on cosmological time scales. Dark
matter couples to the standard model via the neutrino portal, that is, the
singlet operator H L constructed from the Higgs and lepton doublets, which is
the lowest dimensional fermionic singlet operator in the standard model. This
coupling prompts dark matter decay where the dominant decay channel has one
neutrino (and at least one dark glueball) in the final state. Other decay
channels with charged standard model particles involve more particles in the
final state and are therefore suppressed by phase space. In consequence, the
standard indirect detection signals like gamma-ray photons, antiprotons and
positrons are suppressed with respect to the neutrino signal. This coupling via
the neutrino portal is most robustly constrained by Super-Kamiokande, which
restricts the dark matter lifetime to be larger than 10^25 seconds. In the near
future, the scenario will be probed by the new generation of neutrino
telescopes. ANTARES will be sensitive to a dark matter lifetime of order 10^26
seconds, while IceCube/DeepCore can probe a lifetime as large as 10^27 seconds.Comment: 37 pages, 9 figure
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