616 research outputs found
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
Hadroproduction of at NLO accuracy matched with shower Monte Carlo programs
We present the computation of the differential cross section for the process
at NLO QCD accuracy
matched to Shower Monte Carlo (SMC) simulations using PowHel, on the basis of
the interface between HELAC-NLO and POWHEG-BOX. We include all resonant and
non-resonant contributions. This is achieved by fully taking into account the
effect of off-shell t-quarks and off-shell W-bosons in the complex mass scheme.
We also present a program called DECAYER that can be used to let the t-quarks
present in the event files for processes
decay including both the finite width of the t-quarks and spin correlations. We
present predictions for both the Tevatron and the LHC, with emphasis on
differences emerging from three different
hadroproduction computations: (i) full implementation of the process, (ii) generating on-shell t-quarks pushed
off-shell with a Breit-Wigner finite width and decayed by DECAYER, and (iii)
on-shell t-quark production followed by decay in the narrow width
approximation, as described by the SMC.Comment: 40 pages, 26 figures; slightly expanded version matching the one
published in JHE
A model for multifragmentation in heavy-ion reactions
From an experimental point of view, clear signatures of multifragmentation
have been detected by different experiments. On the other hand, from a
theoretical point of view, many different models, built on the basis of totally
different and often even contrasting assumptions, have been provided to explain
them. In this contribution we show the capabilities and the shortcomings of one
of this models, a QMD code developed by us and coupled to the nuclear
de-excitation module taken from the multipurpose transport and interaction code
FLUKA, in reproducing the multifragmentation observations recently reported by
the INDRA collaboration for the reaction Nb + Mg at a 30 MeV/A projectile
bombarding energy. As far as fragment production is concerned, we also briefly
discuss the isoscaling technique by considering reactions characterized by a
different isospin asymmetry, and we explain how the QMD + FLUKA model can be
applied to obtain information on the slope of isotopic yield ratios, which is
crucially related to the symmetry energy of asymmetric nuclear matter.Comment: 8 pages, 2 figures, Proc. 12th International Conference on Nuclear
Reaction Mechanisms, Varenna, Italy, June 15 - 19 200
Top-antitop pair hadroproduction in association with a heavy boson at the NLO QCD accuracy + Parton Shower
The PowHel framework allows to make predictions of total and differential
cross-sections of multiparticle hadroproduction processes at both NLO QCD
accuracy and NLO QCD matched to Parton Shower, on the basis of the interface
between the POWHEG-BOX and HELAC-NLO codes. It has already been applied to
study several processes involving a top-antitop pair in association with a
third particle or hadronic jet. Our most recent predictions concern
top-antitop-V hadroproduction (with V = W or Z), at both parton and hadron
level, by considering different decay channels (hadronic and leptonic) of the
heavy particles. In particular, we show the results of our phenomenological
analyses under the same system of cuts also recently adopted by the CMS
collaboration at LHC.Comment: 4 pages, 2 figures, Proceedings of TOP 2012 - 5th International
Workshop on Top Quark Physics, September 16 - 21 2012, Winchester, U
t tbar W and t tbar Z Hadroproduction at NLO accuracy in QCD with Parton Shower and Hadronization effects
We present theoretical predictions for the hadroproduction of t tbar W+, t
tbar W- and t tbar Z at LHC as obtained by matching numerical computations at
NLO accuracy in QCD with Shower Monte Carlo programs. The calculation is
performed by PowHel, relying on the POWHEG-BOX framework, that allows for the
matching between the fixed order computation, with input of matrix elements
produced by the HELAC-NLO collection of event generators, and the Parton Shower
evolution, followed by hadronization and hadron decays as described by PYTHIA
and HERWIG. We focus on the dilepton and trilepton decay channels, studied
recently by the CMS Collaboration.Comment: 21 pages 12 figure
Z0 - boson production in association with a top anti-top pair at NLO accuracy with parton shower effects
We present predictions for the production cross section of a Standard Model
Z0-boson in association with a top-antitop pair at the next-to-leading order
accuracy in QCD, matched with shower Monte Carlo programs to evolve the system
down to the hadronization energy scale. We adopt a framework based on three
well established numerical codes, namely the POWHEG-BOX, used for computing the
cross section, HELAC-NLO, which generates all necessary input matrix elements,
and finally a parton shower program, such as PYTHIA or HERWIG, which allows for
including t-quark and Z0-boson decays at the leading order accuracy and
generates shower emissions, hadronization and hadron decays.Comment: 10 pages, 5 figures; found and corrected a bug in the
phenomenological analysis, just affecting Fig.4 - 5 that turn out to change
slightly with respect to our previous version and the cross-section values
after all cuts. Conclusions qualitatively unchange
Prompt neutrinos from atmospheric charm in the general-mass variable-flavor-number scheme
We present predictions for the prompt-neutrino flux arising from the decay of
charmed mesons and baryons produced by the interactions of high-energy cosmic
rays in the Earth's atmosphere, making use of a QCD approach on the basis of
the general-mass variable-flavor-number scheme for the description of charm
hadroproduction at NLO, complemented by a consistent set of fragmentation
functions. We compare the theoretical results to those already obtained by our
and other groups with different theoretical approaches. We provide comparisons
with the experimental results obtained by the IceCube Collaboration in two
different analyses and we discuss the implications for parton distribution
functions.Comment: 43 pages, 21 figures, updated version, to be published in JHE
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 Monte Carlo approach to study neutron and fragment emission in heavy-ion reactions
Quantum Molecular Dynamics models (QMD) are Monte Carlo approaches targeted
at the description of nucleon-ion and ion-ion collisions. We have developed a
QMD code, which has been used for the simulation of the fast stage of ion-ion
collisions, considering a wide range of system masses and system mass
asymmetries. The slow stage of the collisions has been described by statistical
methods. The combination of both stages leads to final distributions of
particles and fragments, which have been compared to experimental data
available in literature. A few results of these comparisons, concerning neutron
double-differential production cross-sections for C, Ne and Ar ions impinging
on C, Cu and Pb targets at 290 - 400 MeV/A bombarding energies and fragment
isotopic distributions from Xe + Al at 790 MeV/A, are shown in this paper.Comment: 12 pages, 3 figures, submitted for publication in Adv. Space Re
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