54 research outputs found
NLL soft and Coulomb resummation for squark and gluino production at the LHC
We present predictions of the total cross sections for pair production of
squarks and gluinos at the LHC, including the stop-antistop production process.
Our calculation supplements full fixed-order NLO predictions with resummation
of threshold logarithms and Coulomb singularities at next-to-leading
logarithmic (NLL) accuracy, including bound-state effects. The numerical effect
of higher-order Coulomb terms can be as big or larger than that of soft-gluon
corrections. For a selection of benchmark points accessible with data from the
2010-2012 LHC runs, resummation leads to an enhancement of the total inclusive
squark and gluino production cross section in the 15-30 % range. For individual
production processes of gluinos, the corrections can be much larger. The
theoretical uncertainty in the prediction of the hard-scattering cross sections
is typically reduced to the 10 % level.Comment: 45 pages, 16 Figures, LaTex. v2: published version. Grids with
numerical results for the NLL cross sections for squark and gluino production
at the 7/8 TeV LHC are included in the submission and are also available at
http://omnibus.uni-freiburg.de/~cs1010/susy.htm
Gluino-Squark Production at the LHC: The Threshold
An analysis of the cross section for hadronic production of gluino-squark
pairs close to threshold is presented. Within the framework of non-relativistic
QCD a significant enhancement compared to fixed order perturbation theory is
observed which originates from the characteristic remnants of the gluino-squark
resonances below the nominal pair threshold. The analysis includes all colour
configurations of S-wave gluino-squark pairs, i.e. triplet, sextet and 15
representation. Matching coefficients at leading order are separately evaluated
for all colour configurations. The dominant QCD corrections, arising from
initial- and final-state radiation are included. The non-relativistic dynamics
of the gluino pair is solved by calculating the Green's function in
Next-to-Leading Order (NLO). The results are applied to benchmark scenarios,
based on Snowmass Points and Slopes (SPS). As a consequence of the large decay
rate of at least one of the constituents squark or gluino annihilation decays
of the bound state (\tilde{g}\tilde{q})\rightarrow gq, q\gamma, qZ or q'W^{\pm}
are irrelevant. Thus the signatures of gluino-quark production below and above
the nominal threshold are identical. Numerical results for the cross section at
the Large Hadron Collider (LHC) at \sqrt{s}=7 TeV and 14 TeV are presented. The
enhancement of the total cross section through final state interaction amounts
to roughly 3%.Comment: 25 pages, 9 figures, Eq. 10 modified. Reference [30] added.
Discussion of the expected quality of the approximation added before eq. 23.
Some changes in notation. Typos correcte
Probing Colored Particles with Photons, Leptons, and Jets
If pairs of new colored particles are produced at the Large Hadron Collider,
determining their quantum numbers, and even discovering them, can be
non-trivial. We suggest that valuable information can be obtained by measuring
the resonant signals of their near-threshold QCD bound states. If the particles
are charged, the resulting signatures include photons and leptons and are
sufficiently rich for unambiguously determining their various quantum numbers,
including the charge, color representation and spin, and obtaining a precise
mass measurement. These signals provide well-motivated benchmark models for
resonance searches in the dijet, photon+jet, diphoton and dilepton channels.
While these measurements require that the lifetime of the new particles be not
too short, the resulting limits, unlike those from direct searches for pair
production above threshold, do not depend on the particles' decay modes. These
limits may be competitive with more direct searches if the particles decay in
an obscure way.Comment: 39 pages, 9 figures; v2: more recent searches include
Interpreting a 750 GeV diphoton resonance
We discuss the implications of the significant excesses in the diphoton final
state observed by the LHC experiments ATLAS and CMS around a diphoton invariant
mass of 750 GeV. The interpretation of the excess as a spin-zero s-channel resonance implies
model-independent lower bounds on both its branching ratio and its coupling to photons,
which stringently constrain dynamical models. We consider both the case where the
excess is described by a narrow and a broad resonance. We also obtain model-independent
constraints on the allowed couplings and branching fractions to final states other than
diphotons, by including the interplay with 8 TeV searches. These results can guide attempts
to construct viable dynamical models of the resonance. Turning to specific models,
our findings suggest that the anomaly cannot be accounted for by the presence of only an
additional singlet or doublet spin-zero field and the Standard Model degrees of freedom; this
includes all two-Higgs-doublet models. Likewise, heavy scalars in the MSSM cannot explain
the excess if stability of the electroweak vacuum is required, at least in a leading-order analysis.
If we assume that the resonance is broad we find that it is challenging to find a weakly
coupled explanation. However, we provide an existence proof in the form of a model with
vectorlike quarks with large electric charge that is perturbative up to the 100 TeV scale.
For the narrow-resonance case a similar model can be perturbative up to high scales also
with smaller charges. We also find that, in their simplest form, dilaton models cannot
explain the size of the excess. Some implications for flavor physics are briefly discussed
LHC phenomenology of dark matter with a color-octet partner
Colored dark sectors where the dark matter particle is accompanied by colored partners have recently attracted theoretical and phenomenological interest. We explore the possibility that the dark sector consists of the dark matter particle and a color-octet partner, where the interaction with the Standard Model is governed by an effective operator involving gluons. The resulting interactions resemble the color analogues of electric and magnetic dipole moments. Although many phenomenological features of this kind of model only depend on the group representation of the partner under SU(3)c, we point out that interesting collider signatures such as R-hadrons are indeed controlled by the interaction operator between the dark and visible sector. We perform a study of the current constraints and future reach of LHC searches, where the complementarity between different possible signals is highlighted and exploited
Predicting implementation from organizational readiness for change: a study protocol
<p>Abstract</p> <p>Background</p> <p>There is widespread interest in measuring organizational readiness to implement evidence-based practices in clinical care. However, there are a number of challenges to validating organizational measures, including inferential bias arising from the halo effect and method bias - two threats to validity that, while well-documented by organizational scholars, are often ignored in health services research. We describe a protocol to comprehensively assess the psychometric properties of a previously developed survey, the Organizational Readiness to Change Assessment.</p> <p>Objectives</p> <p>Our objective is to conduct a comprehensive assessment of the psychometric properties of the Organizational Readiness to Change Assessment incorporating methods specifically to address threats from halo effect and method bias.</p> <p>Methods and Design</p> <p>We will conduct three sets of analyses using longitudinal, secondary data from four partner projects, each testing interventions to improve the implementation of an evidence-based clinical practice. Partner projects field the Organizational Readiness to Change Assessment at baseline (n = 208 respondents; 53 facilities), and prospectively assesses the degree to which the evidence-based practice is implemented. We will conduct predictive and concurrent validities using hierarchical linear modeling and multivariate regression, respectively. For predictive validity, the outcome is the change from baseline to follow-up in the use of the evidence-based practice. We will use intra-class correlations derived from hierarchical linear models to assess inter-rater reliability. Two partner projects will also field measures of job satisfaction for convergent and discriminant validity analyses, and will field Organizational Readiness to Change Assessment measures at follow-up for concurrent validity (n = 158 respondents; 33 facilities). Convergent and discriminant validities will test associations between organizational readiness and different aspects of job satisfaction: satisfaction with leadership, which should be highly correlated with readiness, versus satisfaction with salary, which should be less correlated with readiness. Content validity will be assessed using an expert panel and modified Delphi technique.</p> <p>Discussion</p> <p>We propose a comprehensive protocol for validating a survey instrument for assessing organizational readiness to change that specifically addresses key threats of bias related to halo effect, method bias and questions of construct validity that often go unexplored in research using measures of organizational constructs.</p
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