2,870 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
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
hadroproduction with massive bottom quarks with PowHel
The associated production of top-antitop-bottom-antibottom quarks is a
relevant irreducible background for Higgs boson analyses in the
top-antitop-Higgs production channel, with Higgs decaying into a
bottom-antibottom quark pair. We implement this process in the PowHel event
generator, considering the bottom quarks as massive in all steps of the
computation which involves hard-scattering matrix-elements in the 4-flavour
number scheme combined with 4-flavour Parton Distribution Functions.
Predictions with NLO QCD + Parton Shower accuracy, as obtained by PowHel +
PYTHIA, are compared to those which resulted from a previous PowHel
implementation with hard-scattering matrix-elements in the 5-flavour number
scheme, considering as a baseline the example of a realistic analysis of
top-antitop hadroproduction with additional -jet activity, performed by the
CMS collaboration at the Large Hadron Collider.Comment: 9 pages, 6 figure
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
Assault on the NLO Wishlist: pp -> tt bb
We present the results of a next-to-leading order calculation of QCD
corrections to the production of an on-shell top-anti-top quark pair in
association with two flavored b-jets. Besides studying the total cross section
and its scale dependence, we give several differential distributions. Where
comparable, our results agree with a previous analysis. While the process under
scrutiny is of major relevance for Higgs boson searches at the LHC, we use it
to demonstrate the ability of our system built around Helac-Phegas to tackle
complete calculations at the frontier of current studies for the LHC. On the
technical side, we show how the virtual corrections are efficiently computed
with Helac-1Loop, based on the OPP method and the reduction code CutTools,
using reweighting and Monte Carlo over color configurations and polarizations.
As far as the real corrections are concerned, we use the recently published
Helac-Dipoles package. In connection with improvements of the latter, we give
the last missing integrated dipole formulae necessary for a complete
implementation of phase space restriction dependence in the massive dipole
subtraction formalism.Comment: 19 pages, 11 figures, 2 tables. References added, version to appear
in JHE
A room-temperature alternating current susceptometer - Data analysis, calibration, and test
An AC susceptometer operating in the range of 10 Hz to 100 kHz and at room
temperature is designed, built, calibrated and used to characterize the
magnetic behaviour of coated magnetic nanoparticles. Other weakly magnetic
materials (in amounts of some millilitres) can be analyzed as well. The setup
makes use of a DAQ-based acquisition system in order to determine the amplitude
and the phase of the sample magnetization as a function of the frequency of the
driving magnetic field, which is powered by a digital waveform generator. A
specific acquisition strategy makes the response directly proportional to the
sample susceptibility, taking advantage of the differential nature of the coil
assembly. A calibration method based on conductive samples is developed.Comment: 8 pages, 7 figures, 19 ref
Helac-nlo
Based on the OPP technique and the HELAC framework, HELAC-1LOOP is a program
that is capable of numerically evaluating QCD virtual corrections to scattering
amplitudes. A detailed presentation of the algorithm is given, along with
instructions to run the code and benchmark results. The program is part of the
HELAC-NLO framework that allows for a complete evaluation of QCD NLO
corrections.Comment: minor text revisions, version to appear in Comput.Phys.Commu
A new class of sum rules for products of Bessel functions
In this paper we derive a new class of sum rules for products of the Bessel
functions of first kind. Using standard algebraic manipulations we extend some
of the well known properties of . Some physical applications of the
results are also discussed. A comparison with the Newberger[J. Math. Phys.
\textbf{23} (1982) 1278] sum rules is performed on a typical example.Comment: Published in Journal of Mathematical Physics, 9 pages, no picture
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