283 research outputs found
On the ratio of ttbb and ttjj cross sections at the CERN Large Hadron Collider
Triggered by ongoing experimental analyses, we report on a study of the cross
section ratio sigma(pp -> ttbb)/sigma(pp -> ttjj) at the next-to-leading order
in QCD, focusing on both present and future collider energies: sqrt{s}= 7, 8,
13 TeV. In particular, we provide a comparison between our predictions and the
currently available CMS data for the 8 TeV run. We further analyse the
kinematics and scale uncertainties of the two processes for a single set of
parton distribution functions, with the goal of assessing possible correlations
that might help to reduce the theoretical error of the ratio and thus enhance
the predictive power of this observable. We argue that the different jet
kinematics makes the ttbb and ttjj processes uncorrelated in several
observables, and show that the scale uncertainty is not significantly reduced
when taking the ratio of the cross sections.Comment: 23 pages, 10 figures, 3 tables, some issues clarified,
acknowledgement and references added, version to appear in JHE
Probing the CP nature of the Higgs boson at linear colliders with tau spin correlations; the case of mixed scalar--pseudoscalar couplings
The prospects for the measurement of the pseudoscalar admixture in the
h-tau-tau coupling to a Standard Model Higgs boson of 120 GeV mass are
discussed in a quantitative manner e+ e- collisions of 350 GeV centre-of-mass
energy. Specific angular distributions in the h --> tau tau; tau --> rho nu
decay chain can be used to probe mixing angles of scalar--pseudoscalar
h-tau-tau couplings. In the discussion of the feasibility of the method,
assumptions on the properties of a future detector for an e+ e- linear collider
such as TESLA are used. The Standard Model Higgsstrahlung production process is
taken as an example. For the expected performance of a typical Linear Collider
set-up, the sensitivity of a measurement of the scalar--pseudoscalar mixing
angle turned out to be 6 degree. It will be straightforward to apply our
results to estimate the sensitivity of a measurement, in cases another scenario
of the Higgs boson sector (Standard Model or not) is chosen by nature. The
experimental error of the method is expected to be limited by the statistics.Comment: 10 pages, 7 figures, LaTeX. Corrections of published version include
Theoretical improvements for luminosity monitoring at low energies
A comparison of theoretical results on NNLO leptonic and hadronic corrections
to Bhabha scattering with the Monte Carlo generator BabaYaga@NLO used at meson
factories is given. Complete NLO virtual corrections to the process are discussed.Comment: 8 pages, 4 figs, presented by J.Gluza at Radcor 201
Matching the Nagy-Soper parton shower at next-to-leading order
We present an MC@NLO-like matching of next-to-leading order QCD calculations
with the Nagy-Soper parton shower. An implementation of the algorithm within
the HELAC-DIPOLES Monte Carlo generator is used to address the uncertainties
and ambiguities of the matching scheme. First results obtained using the
Nagy-Soper parton shower implementation in DEDUCTOR in conjunction with the
HELAC-NLO framework are given for the pp -> top anti-top j + X process at the
LHC with sqrt(s)=8 TeV. Effects of resummation are discussed for various
observables.Comment: 53 pages, 18 figures, 3 tables. References and a few typos corrected,
acknowledgments added, dependence on the variation of the starting shower
time corrected, version to appear in JHE
Off-shell Top Quarks with One Jet at the LHC: A comprehensive analysis at NLO QCD
We present a comprehensive study of the production of top quark pairs in
association with one hard jet in the di-lepton decay channel at the LHC. Our
predictions, accurate at NLO in QCD, focus on the LHC Run II with a
center-of-mass energy of 13 TeV. All resonant and non-resonant contributions at
the perturbative order are taken into account,
including irreducible backgrounds to production, interferences and
off-shell effects of the top quark and the gauge boson. We extensively
investigate the dependence of our results upon variation of renormalisation and
factorisation scales and parton distribution functions in the quest for an
accurate estimate of the theoretical uncertainties. Additionally, we explore a
few possibilities for a dynamical scale choice with the goal of stabilizing the
perturbative convergence of the differential cross sections far away from the
threshold. Results presented here are particularly relevant for
searches of new physics as well as for precise measurements of the top-quark
fiducial cross sections and top-quark properties at the LHC.Comment: 51 pages, 36 figures, 6 tables, version to appear in JHE
Physics Opportunities for Vector-Boson Scattering at a Future 100 TeV Hadron Collider
Vector-boson scattering (VBS) processes provide particularly promising means
for probing the mechanism of electroweak symmetry breaking and to search for
new physics in the weak sector. In the environment of a future proton-proton
collider operating at a center-of-mass energy of 100 TeV, unprecedented
opportunities arise for the investigation of this important class of reactions.
We highlight the prominent features of VBS processes in this energy regime and
discuss how the VBS signal can be isolated in the presence of a priori large
QCD backgrounds. We find excellent opportunities for the analysis of VBS
reactions in a kinematic range that is inaccessible to present colliders.Comment: 26 pages, 14 figures; matches version published by Phys. Rev.
Dominant QCD Backgrounds in Higgs Boson Analyses at the LHC: A Study of pp -> t anti-t + 2 jets at Next-To-Leading Order
We report the results of a next-to-leading order simulation of top quark pair
production in association with two jets. With our inclusive cuts, we show that
the corrections with respect to leading order are negative and small, reaching
11%. The error obtained by scale variation is of the same order. Additionally,
we reproduce the result of a previous study of top quark pair production in
association with a single jet.Comment: 4 pages, 5 figures, 1 tabl
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