26 research outputs found
Initial determination of the spins of the gluino and squarks at LHC
In principle particle spins can be measured from their production cross
sections once their mass is approximately known. The method works in practice
because spins are quantized and cross sections depend strongly on spins. It can
be used to determine, for example, the spin of the top quark. Direct
application of this method to supersymmetric theories will have to overcome the
challenge of measuring mass at the LHC, which could require high statistics. In
this article, we propose a method of measuring the spins of the colored
superpatners by combining rate information for several channels and a set of
kinematical variables, without directly measuring their masses. We argue that
such a method could lead to an early determination of the spin of gluino and
squarks. This method can be applied to the measurement of spin of other new
physics particles and more general scenarios.Comment: 23 pages, 8 figures, minor change
Stop the Top Background of the Stop Search
The main background for the supersymmetric stop direct production search
comes from Standard Model ttbar events. For the single-lepton search channel,
we introduce a few kinematic variables to further suppress this background by
focusing on its dileptonic and semileptonic topologies. All are defined to have
end points in the background, but not signal distributions. They can
substantially improve the stop signal significance and mass reach when combined
with traditional kinematic variables such as the total missing transverse
energy. Among them, our variable M^W_T2 has the best overall performance
because it uses all available kinematic information, including the on-shell
mass of both W's. We see 20%-30% improvement on the discovery significance and
estimate that the 8 TeV LHC run with 20 fb-1 of data would be able to reach an
exclusion limit of 650-700 GeV for direct stop production, as long as the stop
decays dominantly to the top quark and a light stable neutralino. Most of the
mass range required for the supersymmetric solution of the naturalness problem
in the standard scenario can be covered.Comment: 16 pages, 5 figure
LHC String Phenomenology
We argue that it is possible to address the deeper LHC Inverse Problem, to
gain insight into the underlying theory from LHC signatures of new physics. We
propose a technique which may allow us to distinguish among, and favor or
disfavor, various classes of underlying theoretical constructions using
(assumed) new physics signals at the LHC. We think that this can be done with
limited data , and improved with more data. This is because of
two reasons -- a) it is possible in many cases to reliably go from
(semi)realistic microscopic string construction to the space of experimental
observables, say, LHC signatures. b) The patterns of signatures at the LHC are
sensitive to the structure of the underlying theoretical constructions. We
illustrate our approach by analyzing two promising classes of string
compactifications along with six other string-motivated constructions. Even
though these constructions are not complete, they illustrate the point we want
to emphasize. We think that using this technique effectively over time can
eventually help us to meaningfully connect experimental data to microscopic
theory.Comment: 50 Pages, 13 Figures, 3 Tables, v2: minor changes, references adde
Top Quarks as a Window to String Resonances
We study the discovery potential of string resonances decaying to
final state at the LHC. We point out that top quark pair production is a
promising and an advantageous channel for studying such resonances, due to
their low Standard Model background and unique kinematics. We study the
invariant mass distribution and angular dependence of the top pair production
cross section via exchanges of string resonances. The mass ratios of these
resonances and the unusual angular distribution may help identify their
fundamental properties and distinguish them from other new physics. We find
that string resonances for a string scale below 4 TeV can be detected via the
channel, either from reconstructing the semi-leptonic
decay or recent techniques in identifying highly boosted tops.Comment: 22 pages, 6 figure
Determining Supersymmetric Parameters With Dark Matter Experiments
In this article, we explore the ability of direct and indirect dark matter
experiments to not only detect neutralino dark matter, but to constrain and
measure the parameters of supersymmetry. In particular, we explore the
relationship between the phenomenological quantities relevant to dark matter
experiments, such as the neutralino annihilation and elastic scattering cross
sections, and the underlying characteristics of the supersymmetric model, such
as the values of mu (and the composition of the lightest neutralino), m_A and
tan beta. We explore a broad range of supersymmetric models and then focus on a
smaller set of benchmark models. We find that by combining astrophysical
observations with collider measurements, mu can often be constrained far more
tightly than it can be from LHC data alone. In models in the A-funnel region of
parameter space, we find that dark matter experiments can potentially determine
m_A to roughly +/-100 GeV, even when heavy neutral MSSM Higgs bosons (A, H_1)
cannot be observed at the LHC. The information provided by astrophysical
experiments is often highly complementary to the information most easily
ascertained at colliders.Comment: 46 pages, 76 figure
Measurements of the Production, Decay and Properties of the Top Quark: A Review
With the full Tevatron Run II and early LHC data samples, the opportunity for
furthering our understanding of the properties of the top quark has never been
more promising. Although the current knowledge of the top quark comes largely
from Tevatron measurements, the experiments at the LHC are poised to probe
top-quark production and decay in unprecedented regimes. Although no current
top quark measurements conclusively contradict predictions from the standard
model, the precision of most measurements remains statistically limited.
Additionally, some measurements, most notably the forward-backward asymmetry in
top quark pair production, show tantalizing hints of beyond-the-Standard-Model
dynamics. The top quark sample is growing rapidly at the LHC, with initial
results now public. This review examines the current status of top quark
measurements in the particular light of searching for evidence of new physics,
either through direct searches for beyond the standard model phenomena or
indirectly via precise measurements of standard model top quark properties
Simplified Models for LHC New Physics Searches
This document proposes a collection of simplified models relevant to the
design of new-physics searches at the LHC and the characterization of their
results. Both ATLAS and CMS have already presented some results in terms of
simplified models, and we encourage them to continue and expand this effort,
which supplements both signature-based results and benchmark model
interpretations. A simplified model is defined by an effective Lagrangian
describing the interactions of a small number of new particles. Simplified
models can equally well be described by a small number of masses and
cross-sections. These parameters are directly related to collider physics
observables, making simplified models a particularly effective framework for
evaluating searches and a useful starting point for characterizing positive
signals of new physics. This document serves as an official summary of the
results from the "Topologies for Early LHC Searches" workshop, held at SLAC in
September of 2010, the purpose of which was to develop a set of representative
models that can be used to cover all relevant phase space in experimental
searches. Particular emphasis is placed on searches relevant for the first
~50-500 pb-1 of data and those motivated by supersymmetric models. This note
largely summarizes material posted at http://lhcnewphysics.org/, which includes
simplified model definitions, Monte Carlo material, and supporting contacts
within the theory community. We also comment on future developments that may be
useful as more data is gathered and analyzed by the experiments.Comment: 40 pages, 2 figures. This document is the official summary of results
from "Topologies for Early LHC Searches" workshop (SLAC, September 2010).
Supplementary material can be found at http://lhcnewphysics.or
Heavy Flavor Simplified Models at the LHC
We consider a comprehensive set of simplified models that contribute to final
states with top and bottom quarks at the LHC. These simplified models are used
to create minimal search strategies that ensure optimal coverage of new heavy
flavor physics involving the pair production of color octets and triplets. We
provide a set of benchmarks that are representative of model space, which can
be used by experimentalists to perform their own optimization of search
strategies. For data sets larger than 1/fb, same-sign dilepton and 3b search
regions become very powerful. Expected sensitivities from existing and
optimized searches are given.Comment: 33 pages, 17 figures, 5 table
New Physics from the Top at the LHC
The top quark may hold the key to new physics associated with the electroweak
symmetry-breaking sector, given its large mass and enhanced coupling to the
Higgs sector. We systematically categorize generic interactions of a new
particle that couples to the top quark and a neutral particle, which is assumed
to be heavy and stable, thus serving as a candidate for cold dark matter. The
experimental signatures for new physics involving top quarks and its partners
at the Large Hadron Collider (LHC) may be distinctive, yet challenging to
disentangle. We optimize the search strategy at the LHC for the decay of the
new particle to a top quark plus missing energy and propose the study of its
properties, such as its spin and couplings. We find that, at 14 TeV with an
integrated luminosity of 100 fb^-1, a spin-zero top partner can be observed at
the 5-sigma level for a mass of 675 GeV. A spin-zero particle can be
differentiated from spin-1/2 and spin-1 particles at the 5-sigma level with a
luminosity of 10 fb^-1.Comment: 19 pages, 7 figures; v2: journal versio
Search for squarks and gluinos in events with isolated leptons, jets and missing transverse momentum at s√=8 TeV with the ATLAS detector
The results of a search for supersymmetry in final states containing at least one isolated lepton (electron or muon), jets and large missing transverse momentum with the ATLAS detector at the Large Hadron Collider are reported. The search is based on proton-proton collision data at a centre-of-mass energy s√=8 TeV collected in 2012, corresponding to an integrated luminosity of 20 fb−1. No significant excess above the Standard Model expectation is observed. Limits are set on supersymmetric particle masses for various supersymmetric models. Depending on the model, the search excludes gluino masses up to 1.32 TeV and squark masses up to 840 GeV. Limits are also set on the parameters of a minimal universal extra dimension model, excluding a compactification radius of 1/R c = 950 GeV for a cut-off scale times radius (ΛR c) of approximately 30