Local analytic sector subtraction at NNLO Magnea, L.; Maina, E.; Pelliccioli, G. ...
The journal of high energy physics,
12/2018, Letnik:
2018, Številka:
12
Journal Article
Recenzirano
Odprti dostop
A
bstract
We present a new method for the local subtraction of infrared divergences at next-to-next-to-leading order (NNLO) in QCD, for generic infrared-safe observables. Our method attempts to ...conjugate the minimal local counterterm structure arising from a sector partition of the radiation phase space with the simplifications following from analytic integration of the counterterms. In this first implementation, the method applies to final-state massless particles. We show how our method compactly organises infrared subtraction at NLO, we deduce in detail the general structure of the subtraction terms at NNLO, and we provide a proof of principle with a complete application to a simple process at NNLO.
Factorisation and subtraction beyond NLO Magnea, L.; Maina, E.; Pelliccioli, G. ...
The journal of high energy physics,
12/2018, Letnik:
2018, Številka:
12
Journal Article
Recenzirano
Odprti dostop
A
bstract
We provide a general method to construct local infrared subtraction counterterms for unresolved radiative contributions to differential cross sections, to any order in perturbation theory. ...We start from the factorised structure of virtual corrections to scattering amplitudes, where soft and collinear divergences are organised in gauge-invariant matrix elements of fields and Wilson lines, and we define radiative eikonal form factors and jet functions which are fully differential in the radiation phase space, and can be shown to cancel virtual poles upon integration by using completeness relations and general theorems on the cancellation of infrared singularities. Our method reproduces known results at NLO and NNLO, and yields substantial simplifications in the organisation of the subtraction procedure, which will help in the construction of efficient subtraction algorithms at higher orders.
A
bstract
We show how a careful analysis of the behaviour of a parton shower Monte Carlo in the vicinity of the soft and collinear regions allows one to formulate a modified MC@NLO-matching ...prescription that reduces the number of negative-weight events with respect to that stemming from the standard MC@NLO procedure. As a first practical application of such a prescription, that we dub MC@NLO-∆, we have implemented it in the M
ad
G
raph
5_
a
MC@NLO framework, by employing the P
ythia
8 Monte Carlo. We present selected MC@NLO-∆ results at the 13 TeV LHC, and compare them with MC@NLO ones. We find that the former predictions are consistent with the latter ones within the typical matching systematics, and that the reduction of negative-weight events is significant.
A
bstract
We discuss the theoretical bases that underpin the automation of the computations of tree-level and next-to-leading order cross sections, of their matching to parton shower simulations, and ...of the merging of matched samples that differ by light-parton multiplicities. We present a computer program, MadGraph5 aMC@NLO, capable of handling all these computations — parton-level fixed order, shower-matched, merged — in a unified framework whose defining features are flexibility, high level of parallelisation, and human intervention limited to input physics quantities. We demonstrate the potential of the program by presenting selected phenomenological applications relevant to the LHC and to a 1-TeV
e
+
e
−
collider. While next-to-leading order results are restricted to QCD corrections to SM processes in the first public version, we show that from the user viewpoint no changes have to be expected in the case of corrections due to any given renormalisable Lagrangian, and that the implementation of these are well under way.
We present predictions for the SM-Higgs-pair production channels of relevance at the LHC: gluon–gluon fusion, VBF, and top-pair, W, Z and single-top associated production. All these results are at ...the NLO accuracy in QCD, and matched to parton showers by means of the MC@NLO method; hence, they are fully differential. With the exception of the gluon–gluon fusion process, for which a special treatment is needed in order to improve upon the infinite-top-mass limit, our predictions are obtained in a fully automatic way within the publicly available MadGraph5_aMC@NLO framework. We show that for all channels in general, and for gluon–gluon fusion and top-pair associated production in particular, NLO corrections reduce the theoretical uncertainties, and are needed in order to arrive at reliable predictions for total rates as well as for distributions.
A
bstract
We study the production of a Higgs boson in association with bottom quarks in hadronic collisions, and present phenomenological predictions relevant to the 13 TeV LHC. Our results are ...accurate to the next-to-leading order in QCD, and matched to parton showers through the MC@NLO method; thus, they are fully differential and based on unweighted events, which we shower by using both Herwig++ and Pythia8. We perform the computation in both the four-flavour and the five-flavour schemes, whose results we compare extensively at the level of exclusive observables. In the case of the Higgs transverse momentum, we also consider the analytically-resummed cross section up to the NNLO+NNLL accuracy. In addition, we analyse at
O
α
S
3
the effects of the interference between the
b
b
¯
H
and gluon-fusion production modes.
A framework for Higgs characterisation Artoisenet, P.; de Aquino, P.; Demartin, F. ...
The journal of high energy physics,
11/2013, Letnik:
2013, Številka:
11
Journal Article
Recenzirano
Odprti dostop
A
bstract
We introduce a framework, based on an effective field theory approach, that allows one to perform characterisation studies of the boson recently discovered at the LHC, for all the relevant ...channels and in a consistent, systematic and accurate way. The production and decay of such a boson with various spin and parity assignments can be simulated by means of multi-parton, tree-level matrix elements and of next-to-leading order QCD calculations, both matched with parton showers. Several sample applications are presented which show, in particular, that beyond-leading-order effects in QCD have nontrivial phenomenological implications.
We present the calculation of scalar and pseudoscalar Higgs production in association with a top–antitop pair to the next-to-leading order (NLO) accuracy in QCD, interfaced with parton showers ...according to the MC@NLO formalism. We apply our results to the cases of light and very light Higgs boson production at the LHC, giving results for total rates as well as for sample differential distributions, relevant to the Higgs, to the top quarks, and to their decay products. This work constitutes the first phenomenological application of aMC@NLO, a fully automated approach to complete event generation at NLO in QCD.
A
bstract
We study the hadroproduction of a
W b
pair in association with a light jet, focusing on the dominant
t
-channel contribution and including exactly at the matrix-element level all ...non-resonant and off-shell effects induced by the finite top-quark width. Our simulations are accurate to the next-to-leading order in QCD, and are matched to the Herwig6 and Pythia8 parton showers through the MC@NLO method. We present phenomenological results relevant to the 8 TeV LHC, and carry out a thorough comparison to the case of on-shell
t
-channel single-top production. We formulate our approach so that it can be applied to the general case of matrix elements that feature coloured intermediate resonances and are matched to parton showers.
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