766 research outputs found
Long-lived stops in MSSM scenarios with a neutralino LSP
This work investigates the possibility of a long-lived stop squark in
supersymmetric models with the neutralino as the lightest supersymmetric
particle (LSP). We study the implications of meta-stable stops on the sparticle
mass spectra and the dark matter density. We find that in order to obtain a
sufficiently long stop lifetime so as to be observable as a stable R-hadron at
an LHC experiment, we need to fine tune the mass degeneracy between the stop
and the LSP considerably. This increases the stop-neutralino coanihilation
cross section, leaving the neutralino relic density lower than what is expected
from the WMAP results for stop masses ~1.5 TeV/c^2. However, if such scenarios
are realised in nature we demonstrate that the long-lived stops will be
produced at the LHC and that stop-based R-hadrons with masses up to 1 TeV/c^2
can be detected after one year of running at design luminosity
Superconformal Flavor Simplified
A simple explanation of the flavor hierarchies can arise if matter fields
interact with a conformal sector and different generations have different
anomalous dimensions under the CFT. However, in the original study by Nelson
and Strassler many supersymmetric models of this type were considered to be
'incalculable' because the R-charges were not sufficiently constrained by the
superpotential. We point out that nearly all such models are calculable with
the use of a-maximization. Utilizing this, we construct the simplest
vector-like flavor models and discuss their viability. A significant constraint
on these models comes from requiring that the visible gauge couplings remain
perturbative throughout the conformal window needed to generate the
hierarchies. However, we find that there is a small class of simple flavor
models that can evade this bound.Comment: 43 pages, 1 figure; V3: small corrections and clarifications,
references adde
LHC Discovery Potential for Non-Standard Higgs Bosons in the 3b Channel
In a variety of well motivated models, such as two Higgs Doublet Models
(2HDMs) and the Minimal Supersymmetric Standard Model (MSSM), there are neutral
Higgs bosons that have significantly enhanced couplings to b-quarks and tau
leptons in comparison to those of the SM Higgs. These so called non-standard
Higgs bosons could be copiously produced at the LHC in association with b
quarks, and subsequently decay into b-quark pairs. However, this production
channel suffers from large irreducible QCD backgrounds. We propose a new search
strategy for non-standard neutral Higgs bosons at the 7 TeV LHC in the 3b's
final state topology. We perform a simulation of the signal and backgrounds,
using state of the art tools and methods for different sets of selection cuts,
and conclude that neutral Higgs bosons with couplings to b-quarks of about 0.3
or larger, and masses up to 400 GeV, could be seen with a luminosity of 30
fb^{-1}. In the case of the MSSM we also discuss the complementarity between
the 3b channel and the inclusive tau pair channel in exploring the
supersymmetric parameter space.Comment: 14 pages, 3 figures, 4 tables, references added, published versio
The flavor puzzle in multi-Higgs models
We reconsider the flavor problem in the models with two Higgs doublets. By
studying two generation toy models, we look for flavor basis independent
constraints on Yukawa couplings that will give us the mass hierarchy while
keeping all Yukawa couplings of the same order. We then generalize our findings
to the full three generation Standard Model. We find that we need two
constraints on the Yukawa couplings to generate the observed mass hierarchy,
and a slight tuning of Yukawa couplings of order 10%, much less than the
Standard Model. We briefly study how these constraints can be realized, and
show how flavor changing currents are under control for mixing in
the near-decoupling limit.Comment: 26 pages, typos are corrected, references are added, the final
versio
Light Stop NLSPs at the Tevatron and LHC
How light can the stop be given current experimental constraints? Can it
still be lighter than the top? In this paper, we study this and related
questions in the context of gauge-mediated supersymmetry breaking, where a stop
NLSP decays into a W, b and gravitino. Focusing on the case of prompt decays,
we simulate several existing Tevatron and LHC analyses that would be sensitive
to this scenario, and find that they allow the stop to be as light as 150 GeV,
mostly due to the large top production background. With more data, the existing
LHC analyses will be able to push the limit up to at least 180 GeV. We hope
this work will motivate more dedicated experimental searches for this simple
scenario, in which, for most purposes, the only free parameters are the stop
mass and lifetime.Comment: 31 pages, 11 figures; v2: added minor clarifications and reference
Revealing the footprints of squark gluino production through Higgs search experiments at the Large Hadron Collider at 7 TeV and 14 TeV
The invariant mass distribution of the di-photons from the decay of the
lighter scalar Higgs boson(h) to be carefully measured by dedicated h search
experiments at the LHC may be distorted by the di-photons associated with the
squark-gluino events with much larger cross sections in Gauge Mediated
Supersymmetry Breaking (GMSB) models. This distortion if observed by the
experiments at the Large Hadron Collider at 7 TeV or 14 TeV, would disfavour
not only the standard model but various two Higgs doublet models with
comparable h - masses and couplings but without a sector consisting of new
heavy particles decaying into photons. The minimal GMSB (mGMSB) model
constrained by the mass bound on h from LEP and that on the lightest neutralino
from the Tevatron, produce negligible effects. But in the currently popular
general GMSB(GGMSB) models the tail of the above distribution may show
statistically significant excess of events even in the early stages of the LHC
experiments with integrated luminosity insufficient for the discovery of h. We
illustrate the above points by introducing several benchmark points in various
GMSB models - minimal as well as non-minimal. The same conclusion follows from
a detailed parameter scan in a simplified GGMSB model recently employed by the
CMS collaboration to interpret their searches in the di-photon + \etslash
channel. Other observables like the effective mass distribution of the
di-photon + X events may also reveal the presence of new heavy particles beyond
the Higgs sector. The contamination of the h mass peak and simple remedies are
also discussed.Comment: 23 pages, 7 figures, title and organization of the paper is changed,
detailed parameter scan in a simplified GGMSB model is added, conclusions and
old numerical results remain unchange
Mu-Tau Production at Hadron Colliders
Motivated by large nu_mu-nu_tau flavor mixing, we consider mu-tau production
at hadron colliders via dimension-6 effective operators, which can be
attributed to new physics in the flavor sector at a higher scale Lambda.
Current bounds on many of these operators from low energy experiments are very
weak or nonexistent, and they may lead to clean mu+tau- and mu-tau+ signals at
hadron colliders. At the Tevatron with 8 inverse femtobarns, one can exceed
current bounds for most operators, with most 2 sigma sensitivities being in the
6-24 TeV range. We find that at the LHC with 1 (100) inverse femtobarns
integrated luminosity, one can reach a 2 sigma sensitivity for Lambda \sim 3-10
TeV (Lambda =\sim 6-21 TeV), depending on the Lorentz structure of the
operator. For some operators, an improvement of several orders of magnitude in
sensitivity can be obtained with only a few tens of inverse picobarns at the
LHC.Comment: 16 pages, 9 figure
Flavor Violating Higgs Decays
We study a class of nonstandard interactions of the newly discovered 125 GeV
Higgs-like resonance that are especially interesting probes of new physics:
flavor violating Higgs couplings to leptons and quarks. These interaction can
arise in many frameworks of new physics at the electroweak scale such as two
Higgs doublet models, extra dimensions, or models of compositeness. We rederive
constraints on flavor violating Higgs couplings using data on rare decays,
electric and magnetic dipole moments, and meson oscillations. We confirm that
flavor violating Higgs boson decays to leptons can be sizeable with, e.g., h ->
tau mu and h -> tau e branching ratios of order 10% perfectly allowed by low
energy constraints. We estimate the current LHC limits on h -> tau mu and h ->
tau e decays by recasting existing searches for the SM Higgs in the tau-tau
channel and find that these bounds are already stronger than those from rare
tau decays. We also show that these limits can be improved significantly with
dedicated searches and we outline a possible search strategy. Flavor violating
Higgs decays therefore present an opportunity for discovery of new physics
which in some cases may be easier to access experimentally than flavor
conserving deviations from the Standard Model Higgs framework.Comment: 39 pages, 12 figures, 3 tables; v2: Improved referencing, updated mu
-> 3e bounds to include large loop contributions, corrected single top
constraints; conclusions unchanged; matches version to be published in JHEP;
v3: included 2-loop contributions in mu -> e conversion, improved discussion
of tau -> 3 mu and of EDM constraints on FV top-Higgs couplings; conclusions
unchange
What traits are carried on mobile genetic elements, and why?
Although similar to any other organism, prokaryotes can transfer genes vertically from mother cell to daughter cell, they can also exchange certain genes horizontally. Genes can move within and between genomes at fast rates because of mobile genetic elements (MGEs). Although mobile elements are fundamentally self-interested entities, and thus replicate for their own gain, they frequently carry genes beneficial for their hosts and/or the neighbours of their hosts. Many genes that are carried by mobile elements code for traits that are expressed outside of the cell. Such traits are involved in bacterial sociality, such as the production of public goods, which benefit a cell's neighbours, or the production of bacteriocins, which harm a cell's neighbours. In this study we review the patterns that are emerging in the types of genes carried by mobile elements, and discuss the evolutionary and ecological conditions under which mobile elements evolve to carry their peculiar mix of parasitic, beneficial and cooperative genes
Charged-Higgs phenomenology in the Aligned two-Higgs-doublet model
The alignment in flavour space of the Yukawa matrices of a general
two-Higgs-doublet model results in the absence of tree-level flavour-changing
neutral currents. In addition to the usual fermion masses and mixings, the
aligned Yukawa structure only contains three complex parameters, which are
potential new sources of CP violation. For particular values of these three
parameters all known specific implementations of the model based on discrete
Z_2 symmetries are recovered. One of the most distinctive features of the
two-Higgs-doublet model is the presence of a charged scalar. In this work, we
discuss its main phenomenological consequences in flavour-changing processes at
low energies and derive the corresponding constraints on the parameters of the
aligned two-Higgs-doublet model.Comment: 46 pages, 19 figures. Version accepted for publication in JHEP.
References added. Discussion slightly extended. Conclusions unchange
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