1,609 research outputs found
The Matrix Element Method and QCD Radiation
Full text linkThe matrix element method (MEM) has been extensively used for the analysis of
top-quark and W-boson physics at the Tevatron, but in general without dedicated
treatment of initial state QCD radiation. At the LHC, the increased center of
mass energy leads to a significant increase in the amount of QCD radiation,
which makes it mandatory to carefully account for its effects. We here present
several methods for inclusion of QCD radiation effects in the MEM, and apply
them to mass determination in the presence of multiple invisible particles in
the final state. We demonstrate significantly improved results compared to the
standard treatment.Comment: 15 pp; v2: references and some clarifications added; v3: discussion
of NLO effects, version published in PR
Quark asymmetries in the proton from a model for parton densities
Full text linkBased on quantum fluctuations in momentum and of the proton into meson-baryon
pairs, we develop a physical model for the non-perturbative x-shape of parton
density functions in the proton. The model describes the proton structure
function and gives a natural explanation of observed quark asymmetries, such as
the difference between the anti-up and anti-down sea quark distributions and
between the up and down valence distributions. An asymmetry in the momentum
distribution of strange and anti-strange quarks in the nucleon is found to
reduce the NuTeV anomaly to a level which does not give a significant
indication of physics beyond the standard model.Comment: 27 pages, 11 figures. Updated with extended discussio
Strange quark asymmetry in the nucleon and the NuTeV anomaly
Full text linkThe NuTeV anomaly of a non-universal value of the fundamental parameter
sin^2\theta_W in the electroweak theory has been interpreted as an indication
for new physics beyond the Standard Model. However, the observed quantity
depends on a possible asymmetry in the momentum distributions of strange quarks
and antiquarks in the nucleon. This asymmetry occurs naturally in a
phenomenologically successful physical model for such parton distributions,
which reduces the NuTeV result to only about two standard deviations from the
Standard Model.Comment: 4 pages, 5 figures, RevTex4. Updated with new references and extended
discussion. v.3: Minor corrections mad
Composite Higgs to two Photons and Gluons
Get PDFWe introduce a simple framework to estimate the composite Higgs boson
coupling to two-photon in Technicolor extensions of the standard model. The
same framework allows us to predict the composite Higgs to two-gluon process.
We compare the decay rates with the standard model ones and show that the
corrections are typically of order one. We suggest, therefore, that the
two-photon decay process can be efficiently used to disentangle a light
composite Higgs from the standard model one. We also show that the Tevatron
results for the gluon-gluon fusion production of the Higgs either exclude the
techniquarks to carry color charges to the 95% confidence level, if the
composite Higgs is light, or that the latter must be heavier than around 200
GeV.Comment: RevTex 7 pages, 6 figure
Light gravitino production in association with gluinos at the LHC
Full text linkWe study the jets plus missing energy signature at the LHC in a scenario
where the gravitino is very light and the gluino is the next-to-lightest
supersymmetric particle and promptly decays into a gluon and a gravitino. We
consider both associated gravitino production with a gluino and gluino pair
production. By merging matrix elements with parton showers, we generate
inclusive signal and background samples and show how information on the gluino
and gravitino masses can be obtained by simple final state observables.Comment: 18 pages, 8 figures, 1 table; v2: typos corrected, version to appear
in JHE
Reach the Bottom Line of the Sbottom Search
Get PDFWe propose a new search strategy for directly-produced sbottoms at the LHC
with a small mass splitting between the sbottom and its decayed stable
neutralino. Our search strategy is based on boosting sbottoms through an
energetic initial state radiation jet. In the final state, we require a large
missing transverse energy and one or two b-jets besides the initial state
radiation jet. We also define a few kinematic variables to further increase the
discovery reach. For the case that the sbottom mainly decays into the bottom
quark and the stable neutralino, we have found that even for a mass splitting
as small as 10 GeV sbottoms with masses up to around 400 GeV can be excluded at
the 95% confidence level with 20 inverse femtobarn data at the 8 TeV LHC.Comment: 16 pages, 6 figure
CP violating anomalous top-quark couplings at the LHC
Full text linkWe study the T odd correlations induced by CP violating anomalous top-quark
couplings at both production and decay level in the process gg --> t t_bar -->
(b mu+ nu_mu) (b_bar mu- nu_mu_bar). We consider several counting asymmetries
at the parton level and find the ones with the most sensitivity to each of
these anomalous couplings at the LHC.Comment: 14 LaTeX Pages, 1 EPS Figure, minor typos correcte
Early search for supersymmetric dark matter models at the LHC without missing energy
Full text linkWe investigate early discovery signals for supersymmetry at the Large Hadron
Collider without using information about missing transverse energy. Instead we
use cuts on the number of jets and isolated leptons (electrons and/or muons).
We work with minimal supersymmetric extensions of the standard model, and focus
on phenomenological models that give a relic density of dark matter compatible
with the WMAP measurements. An important model property for early discovery is
the presence of light sleptons, and we find that for an integrated luminosity
of only 200--300 pb at a center-of-mass energy of 10 TeV models with
gluino masses up to GeV can be tested.Comment: 28 pages, 12 figures; published versio
Supersymmetry and Generic BSM Models in PYTHIA 8
Get PDFWe describe the implementation of supersymmetric models in PYTHIA 8,
including production and decay of superparticles and allowing for violation of
flavour, CP, and R-parity. We also present a framework for importing generic
new-physics matrix elements into PYTHIA 8, in a way suitable for use with
automated tools. We emphasize that this possibility should not be viewed as the
only way to implement new-physics models in PYTHIA 8, but merely as an
additional possibility on top of the already existing ones. Finally we address
parton showers in exotic colour topologies, in particular ones involving colour
epsilon tensors and colour sextets.Comment: 20 page
Where the Sidewalk Ends: Jets and Missing Energy Search Strategies for the 7 TeV LHC
Get PDFThis work explores the potential reach of the 7 TeV LHC to new colored states
in the context of simplified models and addresses the issue of which search
regions are necessary to cover an extensive set of event topologies and
kinematic regimes. This article demonstrates that if searches are designed to
focus on specific regions of phase space, then new physics may be missed if it
lies in unexpected corners. Simple multiregion search strategies can be
designed to cover all of kinematic possibilities. A set of benchmark models are
created that cover the qualitatively different signatures and a benchmark
multiregion search strategy is presented that covers these models.Comment: 30 pages, 8 Figures, 3 Tables. Version accepted at JHEP. Minor
changes. Added figur
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