2,335 research outputs found
QCD Coherence and the Top Quark Asymmetry
Coherent QCD radiation in the hadroproduction of top quark pairs leads to a
forward--backward asymmetry that grows more negative with increasing transverse
momentum of the pair. This feature is present in Monte Carlo event generators
with coherent parton showering, even though the production process is treated
at leading order and has no intrinsic asymmetry before showering. In addition,
depending on the treatment of recoils, showering can produce a positive
contribution to the inclusive asymmetry. We explain the origin of these
features, compare them in fixed-order calculations and the Herwig++, Pythia and
Sherpa event generators, and discuss their implications.Comment: 28 pages, 11 figures, 2 table
Top pair Asymmetries at Hadron colliders with general couplings
Recently it has been shown that measurement of charge asymmetry of top pair
production at LHC excludes any flavor violating vector gauge boson that
could explain Tevatron forward-backward asymmetry (FBA). We consider the
general form of a gauge boson including left-handed, right-handed vector
and tensor couplings to examine FBA and charge asymmetry. To evaluate top pair
asymmetries at Tevatron and LHC, we consider mixing constraints on
flavor changing couplings and show that this model still explain
forward-backward asymmetry at Tevatron and charge asymmetry can not exclude it
in part of parameters space.Comment: 18 pages, 7 figure
Implementation of electroweak corrections in the POWHEG BOX: single W production
We present a fully consistent implementation of electroweak and strong
radiative corrections to single W hadroproduction in the POWHEG BOX framework,
treating soft and collinear photon emissions on the same ground as coloured
parton emissions. This framework can be easily extended to more complex
electroweak processes. We describe how next-to-leading order (NLO) electroweak
corrections are combined with the NLO QCD calculation, and show how they are
interfaced to QCD and QED shower Monte Carlo. The resulting tool fills a gap in
the literature and allows to study comprehensively the interplay of QCD and
electroweak effects to W production using a single computational framework.
Numerical comparisons with the predictions of the electroweak generator HORACE,
as well as with existing results on the combination of electroweak and QCD
corrections to W production, are shown for the LHC energies, to validate the
reliability and accuracy of the approachComment: 31 pages, 7 figures. Minor corrections, references added and updated.
Final version to appear in JHE
Using gamma+jets Production to Calibrate the Standard Model Z(nunu)+jets Background to New Physics Processes at the LHC
The irreducible background from Z(nunu)+jets, to beyond the Standard Model
searches at the LHC, can be calibrated using gamma+jets data. The method
utilises the fact that at high vector boson pT, the event kinematics are the
same for the two processes and the cross sections differ mainly due to the
boson-quark couplings. The method relies on a precise prediction from theory of
the Z/gamma cross section ratio at high pT, which should be insensitive to
effects from full event simulation. We study the Z/gamma ratio for final states
involving 1, 2 and 3 hadronic jets, using both the leading-order parton shower
Monte Carlo program Pythia8 and a leading-order matrix element program Gambos.
This enables us both to understand the underlying parton dynamics in both
processes, and to quantify the theoretical systematic uncertainties in the
ratio predictions. Using a typical set of experimental cuts, we estimate the
net theoretical uncertainty in the ratio to be of order 7%, when obtained from
a Monte Carlo program using multiparton matrix-elements for the hard process.
Uncertainties associated with full event simulation are found to be small. The
results indicate that an overall accuracy of the method, excluding statistical
errors, of order 10% should be possible.Comment: 22 pages, 14 figures; Accepted for publication by JHE
Z' signals in polarised top-antitop final states
We study the sensitivity of top-antitop samples produced at all energy stages
of the Large Hadron Collider (LHC) to the nature of an underlying Z' boson, in
presence of full tree level standard model (SM) background effects and relative
interferences. We concentrate on differential mass spectra as well as both
spatial and spin asymmetries thereby demonstrating that exploiting combinations
of these observables will enable one to distinguish between sequential Z's and
those pertaining to Left-Right symmetric models as well as E6 inspired ones,
assuming realistic final state reconstruction efficiencies and error estimates.Comment: 21 pages, 6 colour figures, 10 table
Attributes Enhanced Role-Based Access Control Model
Abstract. Attribute-based access control (ABAC) and role-based access control (RBAC) are currently the two most popular access con-trol models. Yet, they both have known limitations and offer features complimentary to each other. Due to this fact, integration of RBAC and ABAC has recently emerged as an important area of research. In this paper, we propose an access control model that combines the two mod-els in a novel way in order to unify their benefits. Our approach provides a fine-grained access control mechanism that not only takes contextual information into account while making the access control decisions but is also suitable for applications where access to resources is controlled by exploiting contents of the resources in the policy
NLO QCD+EW predictions for V + jets including off-shell vector-boson decays and multijet merging
We present next-to-leading order (NLO) predictions including QCD and electroweak (EW) corrections for the production and decay of off-shell electroweak vector bosons in association with up to two jets at the 13 TeV LHC. All possible dilepton final states with zero, one or two charged leptons that can arise from off-shell W and Z bosons or photons are considered. All predictions are obtained using the automated implementation of NLO QCD+EW corrections in the OpenLoops matrix-element generator combined with the Munich and Sherpa Monte Carlo frameworks. Electroweak corrections play an especially important role in the context of BSM searches, due to the presence of large EW Sudakov logarithms at the TeV scale. In this kinematic regime, important observables such as the jet transverse momentum or the total transverse energy are strongly sensitive to multijet emissions. As a result, fixed-order NLO QCD+EW predictions are plagued by huge QCD corrections and poor theoretical precision. To remedy this problem we present an approximate method that allows for a simple and reliable implementation of NLO EW corrections in the MePs@Nlo multijet merging framework. Using this general approach we present an inclusive simulation of vector-boson production in association with jets that guarantees NLO QCD+EW accuracy in all phase-space regions involving up to two resolved jets
Predictions from Heavy New Physics Interpretation of the Top Forward-Backward Asymmetry
We derive generic predictions at hadron colliders from the large
forward-backward asymmetry observed at the Tevatron, assuming the latter arises
from heavy new physics beyond the Standard Model. We use an effective field
theory approach to characterize the associated unknown dynamics. By fitting the
Tevatron t \bar t data we derive constraints on the form of the new physics.
Furthermore, we show that heavy new physics explaining the Tevatron data
generically enhances at high invariant masses both the top pair production
cross section and the charge asymmetry at the LHC. This enhancement can be
within the sensitivity of the 8 TeV run, such that the 2012 LHC data should be
able to exclude a large class of models of heavy new physics or provide hints
for its presence. The same new physics implies a contribution to the
forward-backward asymmetry in bottom pair production at low invariant masses of
order a permil at most.Comment: 11 pages, 6 figures. v2: added remarks on EFT validity range, dijet
bounds and UV completions; matches published versio
Complete off-shell effects in top quark pair hadroproduction with leptonic decay at next-to-leading order
Results for next-to-leading order QCD corrections to the pp(p\bar{p}) -> t
\bar{t} -> W^+W^- b\bar{b} -> e^{+} \nu_{e} \mu^{-} \bar{\nu}_{\mu} b \bar{b}
+X processes with complete off-shell effects are presented for the first time.
Double-, single- and non-resonant top contributions of the order
{\cal{O}}(\alpha_{s}^3 \alpha^4) are consistently taken into account, which
requires the introduction of a complex-mass scheme for unstable top quarks.
Moreover, the intermediate W bosons are treated off-shell. Comparison to the
narrow width approximation for top quarks, where non-factorizable corrections
are not accounted for is performed. Besides the total cross section and its
scale dependence, several differential distributions at the TeVatron run II and
the LHC are given. In case of the TeVatron the forward-backward asymmetry of
the top is recalculated afresh. With inclusive selection cuts, the
forward-backward asymmetry amounts to A^{t}_{FB} = 0.051 +/- 0.0013.
Furthermore, the corrections with respect to leading order are positive and of
the order 2.3% for the TeVatron and 47% for the LHC. A study of the scale
dependence of our NLO predictions indicates that the residual theoretical
uncertainty due to higher order corrections is 8% for the TeVatron and 9% for
the LHC.Comment: 35 pages, 39 figures, 3 tables. References and note added, version to
appear in JHE
Explaining the t tbar forward-backward asymmetry without dijet or flavor anomalies
We consider new physics explanations of the anomaly in the top quark
forward-backward asymmetry measured at the Tevatron, in the context of flavor
conserving models. The recently measured LHC dijet distributions strongly
constrain many otherwise viable models. A new scalar particle in the
antitriplet representation of flavor and color can fit the t tbar asymmetry and
cross section data at the Tevatron and avoid both low- and high-energy bounds
from flavor physics and the LHC. An s-channel resonance in uc to uc scattering
at the LHC is predicted to be not far from the current sensitivity. This model
also predicts rich top quark physics for the early LHC from decays of the new
scalar particles. Single production gives t tbar j signatures with high
transverse momentum jet, pair production leads to t tbar j j and 4 jet final
states.Comment: 7 pages, 6 figures; v2: notation clarified, references adde
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