90 research outputs found
The top induced backgrounds to Higgs production in the WW -> llvv decay channel at NLO in QCD
We present the complete NLO contributions to the pp -> e^+ v_e mu^- vbar_m b
bbar + X process in the four flavour scheme, i.e. with massive b quarks, and
its contribution to the H -> WW -> llvv measurement in the 1-jet bin at the
LHC. This background process includes top pair, single top and non-top
quark-resonant contributions. The uncertainty at NLO from renormalisation and
factorisation scale dependence is about +30% -20%. We show that the NLO
corrections are relatively small, and that separating this background in top
pair, Wt and b-quark associated llvv is a fair approximation.Comment: 4 pages, 5 figure
Extending the MINLO method
We consider improving POWHEG+MINLO simulations, so as to also render them NLO
accurate in the description of observables receiving contributions from events
with lower parton multiplicity than present in their underlying NLO
calculation. On a conceptual level we follow the strategy of the so-called
MINLO' programs. Whereas the existing MINLO' framework requires explicit
analytic input from higher order resummation, here we derive an effective
numerical approximation to these ingredients, by imposing unitarity. This
offers a way of extending the MINLO' method to more complex processes,
complementary to the known route which uses explicit computations of
high-accuracy resummation inputs. Specifically, we have focused on
Higgs-plus-two-jet production (HJJ) and related processes. We also consider how
one can cover three units of multiplicity at NLO accuracy, i.e. we consider how
the HJJ-MINLO simulation may yield NLO accuracy for inclusive H, HJ, and HJJ
quantities. We perform a feasibility study assessing the potential of these
ideas.Comment: 66 pages, 22 figures; v2 references added; v3=JHEP, improved
readabilit
Automation of the Dipole Subtraction Method in MadGraph/MadEvent
We present the implementation of the dipole subtraction formalism for the
real radiation contributions to any next-to-leading order QCD process in the
MadGraph/MadEvent framework. Both massless and massive dipoles are considered.
Starting from a specific (n+1)-particle process the package provides a Fortran
code for all possible dipoles to all Born processes that constitute the
subtraction term to the (n+1)-particle process. The output files are given in
the usual "MadGraph StandAlone" style using helicity amplitudes.Comment: 14 pages, 4 figure
Integrated dipoles with MadDipole in the MadGraph framework
Heading towards a full automation of next-to-leading order (NLO) QCD
corrections, one important ingredient is the analytical integration over the
one-particle phase space of the unresolved particle that is necessary when
adding the subtraction terms to the virtual corrections. We present the
implementation of these integrated dipoles in the MadGraph framework. The
result is a package that allows an automated calculation for the NLO real
emission parts of an arbitrary process.Comment: 22 pages, 1 figure. Published version: few reference updates and a
couple of minor corrections to the tex
A new way of reducing negative weights in MC@NLO
We introduce a new technique, that we dub Born spreading, aimed at reducing
the number of negative-weight events in the MC@NLO matching of NLO
calculations with parton-shower simulations. We show that such a technique,
based on a re-distribution of Born matrix elements in the radiative phase
space, achieves a sizeable reduction of negative-weight events at little
computational cost. The method does not induce any biases in physical
distributions.Comment: 5 pages, 1 tabl
T-odd Asymmetry in W+jet Events at the LHC
W bosons produced at high transverse momentum in hadron collisions can have
polarization along the direction perpendicular to the production plane, which
is odd under na\ddot{\i}ve-T-reversal where both the three-momenta and
angular momenta are reversed. Perturbative QCD predicts non-zero polarization
at the one-loop level, which can be measured as parity-odd components in the
angular distribution of charged leptons from the decay of W bosons. We perform
a detector-level simulation with the generator MadGraph5_aMC@NLO, and
demonstrate that the asymmetry can be observed at the 8 TeV LHC with 20
fb of data. If confirmed, it will be the first experimental measurement
of the sign of the imaginary part of one-loop QCD amplitudes.Comment: 5 pages, 2 figure
NLO QCD corrections to five-jet production at LEP and the extraction of alpha_s(M_Z)
The highest exclusive jet multiplicity studied at LEP experiments is five. In
this paper we compute the next-to-leading order QCD corrections to e+e-
annihilation to five jets, essentially closing the (pure) perturbative QCD
studies of exclusive jetty final states at LEP. We compare fixed-order
perturbative results with ALEPH data. We estimate hadronization corrections to
five-jet observables using the event generator SHERPA, which employs the CKKW
procedure to combine a reliable perturbative treatment of high-multiplicity jet
final states with parton showers. We show that a competitive value of the
strong coupling constant alpha_s(M_Z) = 0.1156 +0.0041 -0.0034 can be extracted
from the distribution of the five-jet resolution parameter and the five-jet
rate at LEP1 and LEP2.Comment: 25 pages, 5 figures. Published version (3 references addded, 1
sentence rephrased
Automatic spin-entangled decays of heavy resonances in Monte Carlo simulations
We present a general method that allows one to decay narrow resonances in Les
Houches Monte Carlo events in an efficient and accurate way. The procedure
preserves both spin correlation and finite width effects to a very good
accuracy, and is therefore particularly suited for the decay of resonances in
production events generated at next-to-leading-order accuracy. The method is
implemented as a generic tool in the MadGraph framework, giving access to a
very large set of possible applications. We illustrate the validity of the
method and the code by applying it to the case of single top and top quark pair
production, and show its capabilities on the case of top quark pair production
in association with a Higgs boson.Comment: 17 pages, 6 figures, published versio
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