Scattering amplitudes with open loops Cascioli, F; Maierhöfer, P; Pozzorini, S
Physical review letters,
2012-Mar-16, Letnik:
108, Številka:
11
Journal Article
Recenzirano
Odprti dostop
We introduce a new technique to generate scattering amplitudes at one loop. Traditional tree algorithms, which handle diagrams with fixed momenta, are promoted to generators of loop-momentum ...polynomials that we call open loops. Combining open loops with tensor-integral and Ossola-Papadopoulos-Pittau reduction results in a fully flexible, very fast, and numerically stable one-loop generator. As demonstrated with nontrivial applications, the open-loop approach will permit us to obtain precise predictions for a very wide range of collider processes.
A
bstract
We present the first combination of NNLO QCD and NLO EW corrections for vector-boson pair production at the LHC. We consider all final states with two, three and four charged leptons, ...including resonant and non-resonant diagrams, spin correlations and off-shell effects. Detailed predictions are discussed for three representative channels corresponding to
W
+
W
−
,
W
±
Z
and
Z Z
production. Both QCD and EW corrections are very significant, and the details of their combination can play a crucial role to achieve the level of precision demanded by experimental analyses. In this context we point out nontrivial issues that arise at large transverse momenta, where the EW corrections are strongly enhanced by Sudakov logarithms and the QCD corrections can feature so-called giant
K
-factors. Our calculations have been carried out in the M
atrix
+O
pen
L
oops
framework and can be extended to the production of an arbitrary colour singlet in hadronic collisions, provided that the required two-loop QCD amplitudes are available. Combined NNLO QCD and NLO EW predictions for the full set of massive diboson processes will be made publicly available in the next release of M
atrix
and will be instrumental in advancing precision diboson studies and new-physics searches at the LHC and future hadron colliders.
A
bstract
We present next-to-leading order (NLO) predictions including QCD and electroweak (EW) corrections for the production and decay of off-shell electroweak vector bosons in association with up ...to two jets at the 13 TeV LHC. All possible dilepton final states with zero, one or two charged leptons that can arise from off-shell W and Z bosons or photons are considered. All predictions are obtained using the automated implementation of NLO QCD+EW corrections in the O
pen
Loops matrix-element generator combined with the Munich and Sherpa Monte Carlo frameworks. Electroweak corrections play an especially important role in the context of BSM searches, due to the presence of large EW Sudakov logarithms at the TeV scale. In this kinematic regime, important observables such as the jet transverse momentum or the total transverse energy are strongly sensitive to multijet emissions. As a result, fixed-order NLO QCD+EW predictions are plagued by huge QCD corrections and poor theoretical precision. To remedy this problem we present an approximate method that allows for a simple and reliable implementation of NLO EW corrections in the MePs@Nlo multijet merging framework. Using this general approach we present an inclusive simulation of vector-boson production in association with jets that guarantees NLO QCD+EW accuracy in all phase-space regions involving up to two resolved jets.
A
bstract
We present next-to-leading order (NLO) calculations including QCD and electroweak (EW) corrections for 2
ℓ
2
ν
diboson signatures with two opposite-charge leptons and two neutrinos. ...Specifically, we study the processes
pp
→
e
+
μ
−
ν
e
ν
¯
μ
and
pp
→
e
+
e
−
ν
ν
¯
, including all relevant off-shell diboson channels,
W
+
W
−
,
ZZ
, γ
Z
, as well as non-resonant contributions. Photon-induced processes are computed at NLO EW, and we discuss subtle differences related to the definition and the renormalisation of the coupling
α
for processes with initial- and final-state photons. All calculations are performed within the automated M
unich
/S
herpa
+O
pen
L
oops
frameworks, and we provide numerical predictions for the LHC at 13 TeV. The behaviour of the corrections is investigated with emphasis on the high-energy regime, where NLO EW effects can amount to tens of percent due to large Sudakov logarithms. The interplay between
W W
and
ZZ
contributions to the same-flavour channel,
pp
→
e
+
e
−
ν
ν
¯
, is discussed in detail, and a quantitative analysis of photon-induced contributions is presented. Finally, we consider approximations that account for all sources of large logarithms, at high and low energy, by combining virtual EW corrections with a YFS soft-photon resummation or a QED parton shower.
A
bstract
We present the first NLO QCD+EW predictions for Higgs boson production in association with a
ℓν
ℓ
or
ℓ
+
ℓ
−
pair plus zero or one jets at the LHC. Fixed-order NLO QCD+EW calculations are ...combined with a QCD+QED parton shower using the recently developed resonance-aware method in the POWHEG framework. Moreover, applying the improved MiNLO technique to
Hℓν
ℓ
+jet and
Hℓ
+
ℓ
−
+jet production at NLO QCD+EW, we obtain predictions that are NLO accurate for observables with both zero or one resolved jet. This approach permits also to capture higher-order effects associated with the interplay of EW corrections and QCD radiation. The behavior of EW corrections is studied for various kinematic distributions, relevant for experimental analyses of Higgsstrahlung processes at the 13 TeV LHC. Exact NLO EW corrections are complemented with approximate analytic formulae that account for the leading and next-to-leading Sudakov logarithms in the high-energy regime. In the tails of transverse-momentum distributions, relevant for analyses in the boosted Higgs regime, the Sudakov approximation works well, and NLO EW effects can largely exceed the ten percent level. Our predictions are based on the POWHEG BOX RES+OpenLoops framework in combination with the Pythia 8.1 parton shower.
A
bstract
We present a fully automated implementation of next-to-leading order electroweak (NLO EW) corrections in the O
pen
L
oops
matrix-element generator combined with the
Sherpa
and M
unich
Monte ...Carlo frameworks. The process-independent character of the implemented algorithms opens the door to NLO QCD + EW simulations for a vast range of Standard Model processes, up to high particle multiplicity, at current and future colliders. As a first application, we present NLO QCD + EW predictions for the production of positively charged on-shell
W
bosons in association with up to three jets at the Large Hadron Collider. At the TeV energy scale, due to the presence of large Sudakov logarithms, EW corrections reach the 20-40% level and play an important role for searches of physics beyond the Standard Model. The dependence of NLO EW effects on the jet multiplicity is investigated in detail, and we find that
W
+ multijet final states feature genuinely different EW effects as compared to the case of
W
+ 1 jet.
A
bstract
We present details of a calculation of the cross section for hadronic top-antitop production in next-to-leading order (NLO) QCD, including the decays of the top and antitop into bottom ...quarks and leptons. This calculation is based on matrix elements for
ν
e
e
+
μ
−
production and includes all non-resonant diagrams, interferences, and off-shell effects of the top quarks. Such contributions are formally suppressed by the top-quark width and turn out to be small in the inclusive cross section. However, they can be strongly enhanced in exclusive observables that play an important role in Higgs and new-physics searches. Also non-resonant and off-shell effects due to the finite W-boson width are investigated in detail, but their impact is much smaller than naively expected. We also introduce a matching approach to improve NLO calculations involving intermediate unstable particles. Using a fixed QCD scale leads to perturbative instabilities in the high-energy tails of distributions, but an appropriate dynamical scale stabilises NLO predictions. Numerical results for the total cross section, several distributions, and asymmetries are presented for Tevatron and the LHC at 7 TeV, 8 TeV, and 14 TeV.
We present an NLO simulation of WWbb production with massive b-quarks at the LHC. Off-shell and non-resonant contributions associated with top-pair and single-top channels and with leptonic W-boson ...decays are consistently taken into account using the complex-mass scheme. Thanks to the finite b-quark mass, WWbb predictions can be extended to the whole b-quark phase space, thereby including Wt-channel single-top contributions that originate from collinear g
→
bb splittings in the four-flavour scheme. This provides a consistent NLO description of tt and Wt production and decay, including quantum interference effects. The simulation is also applicable to exclusive 0- and 1-jet bins, which is of great importance for Higgs-boson studies in the H
→
WW channel and for any other analysis with large top backgrounds and jet vetoes or jet bins.
A
bstract
We present next-to-leading order QCD and electroweak (EW) theory predictions for
V
+ 2 jet production, with
V
=
Z, W
±
, considering both the QCD and EW production modes and their ...interference. We focus on phase-space regions where
V
+ 2 jet production is dominated by vector-boson fusion, and where these processes yield the dominant irreducible backgrounds in searches for invisible Higgs boson decays. Predictions at parton level are provided together with detailed prescriptions for their implementation in experimental analyses based on the reweighting of Monte Carlo samples. The key idea is that, exploiting accurate data for
W
+ 2 jet production in combination with a theory-driven extrapolation to the
Z
+ 2 jet process can lead to a determination of the irreducible background at the few-percent level. Particular attention is devoted to the estimate of the residual theoretical uncertainties due to unknown higher-order QCD and EW effects and their correlation between the different
V
+ 2 jet processes, which is key to improve the sensitivity to invisible Higgs decays.
Theoretical uncertainties in the simulation of tt¯bb¯ production represent one of the main obstacles that still hamper the observation of Higgs-boson production in association with top-quark pairs in ...the H→bb¯ channel. In this Letter we present a next-to-leading order (NLO) simulation of tt¯bb¯ production with massive b-quarks matched to the Sherpa parton shower. This allows one to extend NLO predictions to arbitrary tt¯bb¯ kinematics, including the case where one or both b-jets arise from collinear g→bb¯ splittings. We find that this splitting mechanism plays an important role for the tt¯H(bb¯) analysis.
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