Higher Order QCD Corrections Research Articles

Prophecy4f 3.0: A Monte Carlo program for Higgs-boson decays into four-fermion final states in and beyond the Standard Model

The Monte Carlo generator Prophecy4f provides a PROPer description of the Higgs dECaY into 4Fermions within the Standard Model, the Standard Model with a fourth fermion generation, a simple Higgs-singlet extension of the Standard Model, and the Two-Higgs-Doublet Model. The fully differential predictions include the full QCD and electroweak next-to-leading-order corrections, all interference contributions between different WW/ZZ channels, and all off-shell effects of intermediate W/Z bosons. Prophecy4f computes the inclusive partial decay widths and allows for the computation of binned differential distributions of the decay products. For leptonic final states also unweighted events are provided. Program summaryProgram Title:Prophecy4f, version 3.0Program Files doi:http://dx.doi.org/10.17632/jc85b6vkzx.1Licensing provisions: GPLv3Programming language: Fortran 77, Fortran 95External routines/libraries:Collier (https://collier.hepforge.org/).Nature of problem: Precision calculation of partial decay widths and differential distributions for Higgs-boson decays into four-fermion final states as described in Refs. [1–7].Solution method: Multi-channel Monte Carlo integration of perturbative matrix elements including higher-order QCD and electroweak corrections which are based on a Feynman-diagrammatic calculation.Restrictions: No unweighted events for semileptonic and hadronic final states.References•[[1]] A. Bredenstein, A. Denner, S. Dittmaier and M. M. Weber, Phys. Rev. D 74 (2006) 013004 [hep-ph/0604011].•[[2]] A. Bredenstein, A. Denner, S. Dittmaier and M. M. Weber, Nucl. Phys. Proc. Suppl. 160 (2006) 131 [hep-ph/0607060].•[[3]] A. Bredenstein, A. Denner, S. Dittmaier and M. M. Weber, JHEP 0702 (2007) 080 [hep-ph/0611234].•[[4]] L. Altenkamp, S. Dittmaier and H. Rzehak, JHEP 1709 (2017) 134 [arXiv:1704.02645 [hep-ph]].•[[5]] L. Altenkamp, S. Dittmaier and H. Rzehak, JHEP 1803 (2018) 110 [arXiv:1710.07598 [hep-ph]].•[[6]] L. Altenkamp, M. Boggia and S. Dittmaier, JHEP 1804 (2018) 062 [arXiv:1801.07291 [hep-ph]].•[[7]] A. Denner, S. Dittmaier and J. N. Lang, JHEP 1811 (2018) 104 [arXiv:1808.03466 [hep-ph]].

Higher-order QCD corrections to hadronic τ decays from Padé approximants

The mathematical method of Padé approximants is put forward to reconstruct the Borel transformed series of the Adler function and extract higher-order corrections in a model-independent way. The use of the D-log Padé approximants provides valuable information to its analytic structure and yields, together with the standard Padé approximants, reliable model-independent predictions of the higher-order coefficients, as for example, c5,1=277±51. Such reconstruction strongly favours the use of fixed-order perturbation theory (FOPT) for the renormalization-scale setting.

Phenomenology of diffractive DIS in the framework of fracture functions and determination of diffractive parton distribution functions

The goal of this study is to determine a set of diffractive parton distribution function (diffractive PDFs) from a QCD analysis of all available and up-to-date diffractive deep inelastic scattering (diffractive DIS) data sets from HERA $ep$ collider, including the most recent H1 and ZUES combined inclusive diffractive cross section measurements. This extraction of diffractive PDFs, referred to as {\tt HK19-DPDF}, is performed at next-to-leading (NLO) and next-to-next-to-leading (NNLO) in perturbative QCD. This new determination of diffractive PDFs is based on the fracture functions methodology, a QCD framework designed to provide a statistically sound representation of diffractive DIS processes. Heavy quark contributions to the diffractive DIS are considered within the framework of the FONLL general mass variable flavor number scheme (GM-VFNS) and the "Hessian approach" is used to determine the uncertainties of diffractive PDFs. We discuss the novel aspects of the approach used in the present analysis, namely an optimized and flexible parametrization of the diffractive PDFs as well as a strategy based on the fully factorization theorem for diffractive hard processes. We then present the diffractive PDFs, and discuss the fit quality and the stability upon variations of the kinematic cuts and the fitted data sets. We find that the systematic inclusion of higher-order QCD corrections could improves the description of the data. We compare the extracted sets of diffractive PDFs based on the fracture functions approach to other recent sets of diffractive PDFs, finding in general very good agreements.

Higgs boson production at the LHC using the qT subtraction formalism at N3LO QCD

We consider higher-order QCD corrections to Higgs boson production through gluon-gluon fusion in the large top quark mass limit in hadron collisions. We extend the transverse-momentum (qT ) subtraction method to next-to-next-to-next-to-leading order (N3LO) and combine it with the NNLO Higgs-plus-jet calculation to numerically compute differential infrared-safe observables at N3LO for Higgs boson production in gluon fusion. To cancel the infrared divergences, we exploit the universal behaviour of the associated qT distributions in the small-qT region. We document all the necessary ingredients of the transverse-momentum subtraction method up to N3LO. The missing third-order collinear functions, which contribute only at qT = 0, are approximated using a prescription which uses the known result for the total Higgs boson cross section at this order. As a first application of the third-order qT subtraction method, we present the N3LO rapidity distribution of the Higgs boson at the LHC.

Higher-order QCD corrections to hadronic \u03c4 decays from Pad\xe9 approximants

Perturbative QCD corrections to hadronic τ decays and e+e− annihilation into hadrons below charm are obtained from the Adler function, which at present is known in the chiral limit to five-loop accuracy. Extractions of the strong coupling, αs, from these processes suffer from an ambiguity related to the treatment of unknown higher orders in the perturbative series. In this work, we exploit the method of Padé approximants and its convergence theorems to extract information about higher-order corrections to the Adler function in a systematic way. First, the method is tested in the large-β0 limit of QCD, where the perturbative series is known to all orders. We devise strategies to accelerate the convergence of the method employing renormalization scheme variations and the so-called D-log Padé approximants. The success of these strategies can be understood in terms of the analytic structure of the series in the Borel plane. We then apply the method to full QCD and obtain reliable model-independent predictions for the higher-order coefficients of the Adler function. For the six-, seven-, and eight-loop coefficients we find c5,1 = 277 ± 51, c6,1 = 3460±690, and c7,1 = (2.02±0.72)×104, respectively, with errors to be understood as lower and upper bounds. Our model-independent reconstruction of the perturbative QCD corrections to the τ hadronic width strongly favours the use of fixed-order perturbation theory (FOPT) for the renormalization-scale setting.

Constraining anomalous gluon self-interactions at the LHC: a reappraisal

Anomalous self-interactions of non-abelian gauge fields can be described by higher dimensional operators featuring gauge-invariant combinations of the field strengths. In the case of QCD, the gluon self-interactions start to be modified at dimension six by operators of the type GGG, with G the gluon field strength tensor, possibly leading to deviations in all observables and measurements that probe strong interactions at very small distances. In this work we consider the sensitivity to the triple gluon operator of a series of observables at the LHC in di-jet, three- and multi-jet final states and heavy-quark production. We critically re-examine the robustness of long-standing as well as more recent proposals addressing issues such as the validity of the EFT expansion and the impact of higher order QCD corrections. Our results support the conclusion that multi-jet observables can reliably bound these anomalous interactions to the level that their impact on other key observables at the LHC, involving for example top quark and Higgs production, can be safely neglected. We also highlight the potential of using previously suggested angular observables in three-jet events at the LHC to further constrain these interactions.

N3LO corrections to jet production in deep inelastic scattering using the Projection-to-Born method

Computations of higher-order QCD corrections for processes with exclusive final states require a subtraction method for real-radiation contributions. We present the first-ever generalisation of a subtraction method for third-order (N3LO) QCD corrections. The Projection-to-Born method is used to combine inclusive N3LO coefficient functions with an exclusive second-order (NNLO) calculation for a final state with an extra jet. The input requirements, advantages, and potential applications of the method are discussed, and validations at lower orders are performed. As a test case, we compute the N3LO corrections to kinematical distributions and production rates for single-jet production in deep inelastic scattering in the laboratory frame, and compare them with data from the ZEUS experiment at HERA. The corrections are small in the central rapidity region, where they stabilize the predictions to sub per-cent level. The corrections increase substantially towards forward rapidity where large logarithmic effects are expected, thereby yielding an improved description of the data in this region.

The C2HDM revisited

The complex two-Higgs doublet model is one of the simplest ways to extend the scalar sector of the Standard Model to include a new source of CP-violation. The model has been used as a benchmark model to search for CP-violation at the LHC and as a possible explanation for the matter-antimatter asymmetry of the Universe. In this work, we re-analyse in full detail the softly broken ℤ2 symmetric complex two-Higgs doublet model (C2HDM). We provide the code C2HDM_HDECAY implementing the C2HDM in the well-known HDECAY program which calculates the decay widths including the state-of-the-art higher order QCD corrections and the relevant off-shell decays. Using C2HDM_HDECAY together with the most relevant theoretical and experimental constraints, including electric dipole moments (EDMs), we review the parameter space of the model and discuss its phenomenology. In particular, we find cases where large CP-odd couplings to fermions are still allowed and provide benchmark points for these scenarios. We examine the prospects of discovering CP-violation at the LHC and show how theoretically motivated measures of CP-violation correlate with observables.

NNLO QCD corrections to event orientation in e+e− annihilation

We present a new implementation of the NNLO QCD corrections to three-jet final states and related event-shape observables in electron–positron annihilation. Our implementation is based on the antenna subtraction method, and is performed in the NNLOjet framework. The calculation improves upon earlier results by taking into account the full kinematical information on the initial state momenta, thereby allowing the event orientation to be computed to NNLO accuracy. We find the event-orientation distributions at LEP and SLC to be very robust under higher order QCD corrections.

A determination of the fragmentation functions of pions, kaons, and protons with faithful uncertainties

We present NNFF1.0, a new determination of the fragmentation functions (FFs) of charged pions, charged kaons, and protons/antiprotons from an analysis of single-inclusive hadron production data in electron–positron annihilation. This determination, performed at leading, next-to-leading, and next-to-next-to-leading order in perturbative QCD, is based on the NNPDF methodology, a fitting framework designed to provide a statistically sound representation of FF uncertainties and to minimise any procedural bias. We discuss novel aspects of the methodology used in this analysis, namely an optimised parametrisation of FFs and a more efficient chi ^2 minimisation strategy, and validate the FF fitting procedure by means of closure tests. We then present the NNFF1.0 sets, and discuss their fit quality, their perturbative convergence, and their stability upon variations of the kinematic cuts and the fitted dataset. We find that the systematic inclusion of higher-order QCD corrections significantly improves the description of the data, especially in the small-z region. We compare the NNFF1.0 sets to other recent sets of FFs, finding in general a reasonable agreement, but also important differences. Together with existing sets of unpolarised and polarised parton distribution functions (PDFs), FFs and PDFs are now available from a common fitting framework for the first time.

The N2HDM under theoretical and experimental scrutiny

The N2HDM is based on the CP-conserving 2HDM extended by a real scalar singlet field. Its enlarged parameter space and its fewer symmetry conditions as compared to supersymmetric models allow for an interesting phenomenology compatible with current experimental constraints, while adding to the 2HDM sector the possibility of Higgs-to-Higgs decays with three different Higgs bosons. In this paper the N2HDM is subjected to detailed scrutiny. Regarding the theoretical constraints we implement tests of tree-level perturbativity and vacuum stability. Moreover, we present, for the first time, a thorough analysis of the global minimum of the N2HDM. The model and the theoretical constraints have been implemented in ScannerS, and we provide N2HDECAY, a code based on HDECAY, for the computation of the N2HDM branching ratios and total widths including the state-of-the-art higher order QCD corrections and off-shell decays. We then perform an extensive parameter scan in the N2HDM parameter space, with all theoretical and experimental constraints applied, and analyse its allowed regions. We find that large singlet admixtures are still compatible with the Higgs data and investigate which observables will allow to restrict the singlet nature most effectively in the next runs of the LHC. Similarly to the 2HDM, the N2HDM exhibits a wrong-sign parameter regime, which will be constrained by future Higgs precision measurements.

Higher-order QCD predictions for dark matter production in mono-Z searches at the LHC

We present theoretical predictions for mono-$Z$ production in the search for dark matter in Run-II at the LHC, including next-to-leading order QCD corrections and parton-shower effects. We consider generic simplified models with vector and scalar $s$-channel mediators. The calculation is performed by implementing the simplified models in the FeynRules/MadGraph5_aMC@NLO framework, which allows us to include higher-order QCD corrections and parton-shower effects in an automated way. We find that these corrections are sizeable and help to reduce the theoretical uncertainties. We also investigate the discovery potential in several benchmark scenarios in the 13 TeV run at the LHC.

Global fit of top quark effective theory to data

In this paper we present a global fit of beyond the Standard Model (BSM) dimension six operators relevant to the top quark sector to currently available data. Experimental measurements include parton-level top-pair and single top production from the LHC and the Tevatron. Higher order QCD corrections are modelled using differential and global K-factors, and we use novel fast-fitting techniques developed in the context of Monte Carlo event generator tuning to perform the fit. This allows us to provide new, fully correlated and model-independent bounds on new physics effects in the top sector from the most current direct hadron-collider measurements in light of the involved theoretical and experimental systematics. As a by-product, our analysis constitutes a proof-of-principle that fast fitting of theory to data is possible in the top quark sector, and paves the way for a more detailed analysis including top quark decays, detector corrections and precision observables.

Higher-order QCD predictions for dark matter production at the LHC in simplified models with s-channel mediators.

Weakly interacting dark matter particles can be pair-produced at colliders and detected through signatures featuring missing energy in association with either QCD/EW radiation or heavy quarks. In order to constrain the mass and the couplings to standard model particles, accurate and precise predictions for production cross sections and distributions are of prime importance. In this work, we consider various simplified models with s-channel mediators. We implement such models in the FeynRules/MadGraph5_aMC@NLO framework, which allows to include higher-order QCD corrections in realistic simulations and to study their effect systematically. As a first phenomenological application, we present predictions for dark matter production in association with jets and with a top-quark pair at the LHC, at next-to-leading order accuracy in QCD, including matching/merging to parton showers. Our study shows that higher-order QCD corrections to dark matter production via s-channel mediators have a significant impact not only on total production rates, but also on shapes of distributions. We also show that the inclusion of next-to-leading order effects results in a sizeable reduction of the theoretical uncertainties.

NLO QCD corrections to Higgs pair production including dimension-6 operators

New Physics that becomes relevant at some high scale Λ beyond the experimental reach, can be described in the effective theory approach by adding higher-dimensional operators to the Standard Model (SM) Lagrangian. In Higgs pair production through gluon fusion, which gives access to the trilinear Higgs self-coupling, this leads not only to modifications of the SM couplings but also induces novel couplings not present in the SM. For a proper prediction of the cross section, higher order QCD corrections that are important for this process, have to be taken into account. The various higher-dimensional contributions are affected differently by the QCD corrections. In this paper, we provide the next-to-leading order (NLO) QCD corrections to Higgs pair production including dimension-6 operators in the limit of large top quark masses. Depending on the dimension-6 coefficients entering the Lagrangian, the new operators affect the relative NLO QCD corrections by several per cent, while modifying the cross section by up to an order of magnitude.

Constraints on the off-shell Higgs boson signal strength in the high-mass ZZ and WW final states with the ATLAS detector.

Measurements of the ZZ and WW final states in the mass range above the 2m_Z and 2m_W thresholds provide a unique opportunity to measure the off-shell coupling strength of the Higgs boson. This paper presents constraints on the off-shell Higgs boson event yields normalised to the Standard Model prediction (signal strength) in the ZZ rightarrow 4ell , ZZrightarrow 2ell 2nu and WWrightarrow enu mu nu final states. The result is based on pp collision data collected by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of 20.3 fb^{-1} at a collision energy of sqrt{s}=8 TeV. Using the CL_s method, the observed 95 % confidence level (CL) upper limit on the off-shell signal strength is in the range 5.1–8.6, with an expected range of 6.7–11.0. In each case the range is determined by varying the unknown ggrightarrow ZZ and ggrightarrow WW background K-factor from higher-order quantum chromodynamics corrections between half and twice the value of the known signal K-factor. Assuming the relevant Higgs boson couplings are independent of the energy scale of the Higgs boson production, a combination with the on-shell measurements yields an observed (expected) 95 % CL upper limit on Gamma _H/Gamma _H^{mathrm {SM}} in the range 4.5–7.5 (6.5–11.2) using the same variations of the background K-factor. Assuming that the unknown ggrightarrow VV background K-factor is equal to the signal K-factor, this translates into an observed (expected) 95 % CL upper limit on the Higgs boson total width of 22.7 (33.0) MeV.

HAWK 2.0: A Monte Carlo program for Higgs production in vector-boson fusion and Higgs strahlung at hadron colliders

The Monte Carlo integrator HAWK provides precision predictions for Higgs production at hadron colliders in vector-boson fusion and Higgs strahlung, i.e. in production processes where the Higgs boson is Attached to WeaK bosons. The fully differential predictions include the full QCD and electroweak next-to-leading-order corrections. Results are computed as integrated cross sections and as binned distributions for important hadron-collider observables. Program summaryProgram title: HAWK, version 2.0Catalogue identifier: AEWT_v1_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/AEWT_v1_0.htmlProgram obtainable from: CPC Program Library, Queen’s University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 132878No. of bytes in distributed program, including test data, etc.: 1400686Distribution format: tar.gzProgramming language: Fortran 77, Fortran 90.Computer: Any computer with a Fortran 90 compiler.Operating system: Linux, Mac OS.RAM: Less than 1 GBClassification: 4.4, 11.1, 11.2.External routines: LHAPDF (https://www.hepforge.org/downloads/lhapdf).Nature of problem:Precision calculation of cross sections and differential distributions for Higgs-boson production in vector-boson fusion and Higgs strahlung at the LHC as described in Refs. [1–3].Solution method:Multi-channel Monte Carlo integration of perturbative matrix elements including higher-order QCD and electroweak corrections which are based on a Feynman-diagrammatic calculation.Restrictions:For vector-boson fusion, only the Higgs-boson decay into a pair of massless singlets is supported. For Higgs strahlung, decay products are not supported, while the Higgs boson can be off shell.Running time:Meaningful results can be obtained within a few hours on a single core. The statistical uncertainty of the results typically improves with the square root of the run time.

The m W − m Z interdependence in the Standard Model: a new scrutiny

The m W − m Z interdependence in the Standard Model is studied in the $$ \overline{M\ S} $$ scheme at the two-loop level, including the known higher-order contributions. The relevant radiative parameters, $$ \varDelta \widehat{\alpha}\left(\mu \right) $$ , $$ \varDelta {\widehat{r}}_W $$ , $$ \widehat{\rho} $$ are computed at O(α 2) taking into account higherorder QCD corrections and the resummation of the reducible contributions. We obtain m W = 80.357 ± 0.009 ± 0.003 GeV where the errors refer to the parametric and theoretical uncertainties, respectively. A comparison with the known result in the On-Shell scheme gives a difference of ≈ 6 MeV. As a byproduct of our calculation we also obtain the $$ \overline{M\ S} $$ electromagnetic coupling and the weak mixing angle at the top mass scale, $$ \widehat{\alpha}\left({M}_t\right)={(127.73)}^{-1}\pm 0.0000003 $$ and $$ { \sin}^2{\widehat{\theta}}_W\left({M}_t\right)=0.23462\pm 0.00012 $$ .

Higher-order QCD corrections to supersymmetric particle production and decay at the LHC

We review recent results on higher-order calculations to squark and gluino production and decay at the LHC, as obtained within the Collaborative Research Centre / Transregio 9 “Computational Particle Physics”. In particular, we discuss inclusive cross sections, including the summation of threshold corrections, higher-order calculations for specific squark production channels and for top squark decays, and next-to-leading order calculations for exclusive observables matched to parton showers.

Pseudo-observables in Higgs decays

We define a set of pseudo-observables characterizing the properties of Higgs decays in generic extensions of the Standard Model with no new particles below the Higgs mass. The pseudo-observables can be determined from experimental data, providing a systematic generalization of the "kappa-framework" so far adopted by the LHC experiments. The pseudo-observables are defined from on-shell decay amplitudes, allow for a systematic inclusion of higher-order QED and QCD corrections, and can be computed in any Effective Field Theory (EFT) approach to Higgs physics. We analyze the reduction of the number of independent pseudo-observables following from the hypotheses of lepton-universality, CP invariance, custodial symmetry, and linearly realized electroweak symmetry breaking. We outline the importance of kinematical studies of $h\to 4\ell$ decays for the extraction of such parameters and present their predictions in the linear EFT framework.