A
bstract
The observation of double and triple scalar boson production at hadron colliders could provide key information on the Higgs self couplings and the potential. As for single Higgs production ...the largest rates for multiple Higgs production come from gluon gluon fusion processes mediated by a top-quark loop. However, at variance with single Higgs production, top-quark mass and width effects from the loops cannot be neglected. Computations including the exact top-quark mass dependence are only available at the leading order, and currently predictions at higher orders are obtained by means of approximations based on the Higgs-gluon effective field theory (HEFT). In this work we present a reweighting technique that, starting from events obtained via the MC@NLO method in the HEFT, allows to exactly include the top-quark mass and width effects coming from one- and two-loop amplitudes. We describe our approach and apply it to double Higgs production at NLO in QCD, computing the needed one-loop amplitudes and using approximations for the unknown two-loop ones. The results are compared to other approaches used in the literature, arguing that they provide more accurate predictions for distributions and for total rates as well. As a novel application of our procedure we present predictions at NLO in QCD for triple Higgs production at hadron colliders.
A
bstract
We introduce P
ine
APPL, a library that produces fast-interpolation grids of physical cross sections, computed with a general-purpose Monte Carlo generator, accurate to fixed order in the ...strong, electroweak, and combined strong-electroweak couplings. We demonstrate this unique ability, that distinguishes PineAPPL from similar software available in the literature, by interfacing it to M
ad
G
raph
5_
a
MC@NLO. We compute fast-interpolation grids, accurate to next-to-leading order in the strong and electroweak couplings, for a representative set of LHC processes for which EW corrections may have a sizeable effect on the accuracy of the corresponding theoretical predictions. We formulate a recommendation on the format of the experimental deliverables in order to consistently compare them with computations that incorporate EW corrections, and specifically to determine parton distribution functions to the same accuracy.
A
bstract
We study the production of jets in hadronic collisions, by computing all contributions proportional to
α
S
n
α
m
, with
n
+
m
= 2 and
n
+
m
= 3. These correspond to leading and ...next-to-leading order results, respectively, for single-inclusive and dijet observables in a perturbative expansion that includes both QCD and electroweak effects. We discuss issues relevant to the definition of hadronic jets in the context of electroweak corrections, and present sample phenomenological predictions for the 13-TeV LHC. We find that both the leading and next-to-leading order contributions largely respect the relative hierarchy established by the respective coupling-constant combinations.
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.
A
bstract
We illustrate how electron Parton Distribution Functions (PDFs) with next-to-leading collinear logarithmic accuracy must be employed in the context of perturbative predictions for ...high-energy
e
+
e
−
-collision processes. In particular, we discuss how the renormalisation group equation evolution of such PDFs is affected by the presence of multiple fermion families and their respective mass thresholds, and by the dependences on the choices of the factorisation and renormalisation schemes. We study the impact of the uncertainties associated with the PDFs on physical cross sections, in order to arrive at realistic precision estimates for observables computed with collinear-factorisation formulae. We do so by presenting results for the production of a heavy neutral object as well as for
t
t
¯
and
W
+
W
−
pairs, including next-to-leading-order effects of electroweak origin.
A
bstract
We compute the contribution of order
α
S
2
α
2
to the cross section of a top-antitop pair in association with at least one heavy Standard Model boson —
Z
,
W
±
, and Higgs — by including ...all effects of QCD, QED, and weak origin and by working in the automated M
ad
G
raph
5_
a
MC@NLO framework. This next-to-leading order contribution is then combined with that of order
α
S
3
α
, and with the two dominant lowest-order ones,
α
S
2
α
and
α
S
α
2
, to obtain phenomenological results relevant to a 8, 13, and 100 TeV
pp
collider.
A
bstract
We analyse the sensitivity to beyond-the-Standard-Model effects of hadron-collider processes involving the interaction of two electroweak and two Higgs bosons, VVHH, with V being either a W ...or a Z boson. We examine current experimental results by the CMS collaboration in the context of a dimension-8 extension of the Standard Model in an effective-field-theory formalism. We show that constraints from vector-boson-fusion Higgs-pair production on operators that modify the Standard Model VVHH interactions are already comparable with or more stringent than those quoted in the analysis of vector-boson-scattering final states. We study the modifications of such constraints when introducing unitarity bounds, and investigate the potential of new experimental final states, such as ZHH associated production. Finally, we show perspectives for the high-luminosity phase of the LHC.
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.
We study the charge asymmetry between the t and t¯ quarks at the LHC, when they are produced in association with a W boson. Though sizably reducing the cross section with respect to the inclusive ...production, requiring a W boson in the final state has two important implications. First, at leading order in QCD, tt¯W± production can only occur via qq¯ annihilation. As a result, the asymmetry between the t and t¯ generated at NLO in QCD is significantly larger than that of inclusive tt¯ production, which is dominated by gluon fusion. Second, the top quarks tend to inherit the polarization of the initial-state quarks as induced by the W-boson emission. Hence, the decay products of the top quarks display a sizable asymmetry already at the leading order in QCD. We study the relevant distributions and their uncertainties in the standard model, compare them to those obtained in a simple axigluon model and discuss prospects for measurements at the LHC and beyond.
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.
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