NLO QCD Research Articles

Resonant top pair production at NLO in QCD

We compute the contribution of a scalar H or pseudo-scalar A resonance in top-quark pair production at NLO accuracy in QCD including the interference between pure QCD SM diagrams and the resonant p p → A H → t t process which drastically modifies the lineshape of the signal from a simple peak to peak-dip structure. We assume a point-like coupling between scalar and the gluons in a consistent effective field theory framework and improve the results with reweighting techniques in the case of resolved fermion loops in the production. The computation is set in an automatic framework and can be used to produce showered unweighted events. The detailed study is presented in Ref [1].

TWH associated production at the LHC

We study Higgs boson production in association with a top quark and a W boson at the LHC. At NLO in QCD, tWH interferes with tbar{t} H and a procedure to meaningfully separate the two processes needs to be employed. In order to define tWH production for both total rates and differential distributions, we consider the diagram removal and diagram subtraction techniques that have been previously proposed for treating intermediate resonances at NLO, in particular in the context of tW production. These techniques feature approximations that need to be carefully taken into account when theoretical predictions are compared to experimental measurements. To this aim, we first critically revisit the tW process, for which an extensive literature exists and where an analogous interference with t bar{t} production takes place. We then provide robust results for total and differential cross sections for tW and tWH at 13 TeV, also matching short-distance events to a parton shower. We formulate a reliable prescription to estimate the theoretical uncertainties, including those associated to the very definition of the process at NLO. Finally, we study the sensitivity to a non-Standard-Model relative phase between the Higgs couplings to the top quark and to the W boson in tWH production.

\textsf {Fuchsia} and Master Integrals for Energy--Energy Correlations at NLO in QCD

In this talk we discuss some aspects of the analytical calculation of energy correlations in electron-positron annihilation at a next-to-leading order in QCD. Our primary focus is on the most difficult task: the calculation of master integrals for real-emission contributions, which are functions of two dimensionless variables and the dimensional regulator. We use a method of differential equations and their so-called epsilon-form which is constructed with the help of the Fuchsia program based on Lee's algorithm.

Full mass dependence in Higgs boson production in association with jets at the LHC and FCC

The first computation of Higgs production in association with three jets at NLO in QCD has recently been performed using the effective theory, where the top quark is treated as an infinitely heavy particle and integrated out. This approach is restricted to the regions in phase space where the typical scales are not larger than the top quark mass. Here we investigate this statement at a quantitative level by calculating the leading-order contributions to the production of a Standard Model Higgs boson in association with up to three jets taking full top-quark and bottom-quark mass dependence into account. We find that the transverse momentum of the hardest particle or jet plays a key role in the breakdown of the effective theory predictions, and that discrepancies can easily reach an order of magnitude for transverse momenta of about 1 TeV. The impact of bottom-quark loops is found to be visible in the small transverse momentum region, leading to corrections of up to 5 percent. We further study the impact of mass corrections when VBF selection cuts are applied and when the center-of-mass energy is increased to 100 TeV.

Colored Particle Production in New Physics at NLO QCD and Its Matching to Parton Showers

In this talk, I show the automated Monte Carlo simulations at next-to-leading order in QCD as well as its matching to parton showers are already feasible within the framework of \MG5aMC. I briefly overview the recent activities and take the colored particle production at the LHC as examples. The tools and the models are ready for using by both phenomenologists and experimentalists.

Measurements of [formula omitted] using the ATLAS detector

The large centre-of-mass energy available at the LHC proton-proton collider allows for copious production of top-quark pairs in association with other final state particles at high transverse momentum. Results on the top-quark pair production in association with W and Z bosons at both 8 and 13 TeV are presented as well as the measurement of the cross section for production with an associated isolated photon at 7 TeV. The ATLAS experiment has measured several final state observables that are sensitive to additional radiation in top anti-top quark final states. Analyses probing the top-quark pair production with additional QCD radiation include the multiplicity of jets for various transverse momentum thresholds in the 13 TeV data. These measurements are compared to some of the most recent Monte Carlo event generators based on NLO QCD or LO multi-leg matrix element calculations.

Off-shell $t\bar {t}j$ Production and Top-quark Mass Studies at the LHC

Precision studies of the properties of the top quark represent a cornerstone of the LHC physics program. In this contribution we focus on the production of $t\bar{t}$ pairs in association with one hard jet and in particular on its connection with precision measurements of the top quark mass at the LHC. We report a summary of a full calculation of the process $pp \to e^+\nu_e\mu^-\bar{\nu}_\mu b \bar{b}j$ at NLO QCD accuracy, which describes $t\bar{t}j$ production with leptonic decays beyond the Narrow Width Approximation (NWA), and discuss the impact of the off-shell effects through comparisons with NWA. Finally we explore the sensitivity of $t\bar{t}j$ in the context of top-quark mass extraction with the template method, considering two benchmark observables as case studies.

COMPASS results on proton longitudinal spin structure function g1and quark fragmentation functions

We present a few recent highlights from the COMPASS experiment at CERN, related to the nucleon spin and structure: the proton spin structure function g1 p (x) measured using 200 GeV polarized muons, together with a QCD fit of g1 world data; pion and kaon multiplicities in DIS in view of constraining quark fragmentation functions.The statistical precision on g1 p is improved by a factor of ~2 at low x. A reevaluation of the Bjorken sum rule based on COMPASS data alone confirms its validation within 9% accuracy. An NLO QCD fit to g1 world data leads to a value of the quark spin contribution to the nucleon spin ΔΣ, between 0.26 and 0.36 at Q2 = 3 (GeV/c)2 , the size of the uncertainty being driven by the unknown shape of the polarized parton distributions.The results on pion and kaon multiplicities produced in DIS of muons off an isoscalar target constitute an impressive data set of more than 400 points in π and 400 in K, covering a large kinematical domain in (x, z, Q2 ), which will be used in future NLO QCD fits to extract quark fragmentation functions into π and K. A LO fit is performed to extract the favoured and unfavoured quark fragmentation functions into pions.

On the maximal use of Monte Carlo samples: re-weighting events at NLO accuracy.

Accurate Monte Carlo simulations for high-energy events at CERN’s Large Hadron Collider, are very expensive, both from the computing and storage points of view. We describe a method that allows to consistently re-use parton-level samples accurate up to NLO in QCD under different theoretical hypotheses. We implement it in MadGraph5_aMC@NLO and show its validation by applying it to several cases of practical interest for the search of new physics at the LHC.

An NLO+PS generator for $${\varvec{t \bar{t}}}$$ t t ¯ and $${\varvec{W t}}$$ W t production and decay including non-resonant and interference effects

We present a Monte Carlo generator that implements significant theoretical improvements in the simulation of top-quark pair production and decay at the LHC. Spin correlations and off-shell effects in top-decay chains are described in terms of exact matrix elements for $p p \to \ell^+\nu_{\scriptscriptstyle\ell}\, l^-\bar{\nu}_{\scriptscriptstyle l} b \bar{b}$ at NLO QCD, where the leptons $\ell$ and $l$ belong to different families, and $b$ quarks are massive. Thus, the contributions from $t\bar{t}$ and $Wt$ single-top production as well as their quantum interference are fully included. Matrix elements are matched to the Pythia8 parton shower using a recently proposed method that allows for a consistent treatment of resonances in the POWHEG framework. These theoretical improvements are especially important for the interpretation of precision measurements of the top-quark mass, for single-top analyses in the $Wt$ channel, and for $t\bar{t}$ and $Wt$ backgrounds in the presence of jet vetoes or cuts that enhance off-shell effects. The new generator is based on a process-independent interface of the OpenLoops amplitude generator with the POWHEGBOX framework.

Next-to-leading order QCD corrections to Higgs boson decay to quarkonium plus a photon **Supported by National Natural Science Foundation of China (11305001, 11105083, 11205003)

In this paper, we investigate the decay of the Higgs boson to J/ψ(ϒ) plus a photon based on NRQCD factorization. For the direct process, we calculate the decay width up to QCD NLO. We find that the decay width for process H → J/ψ(ϒ) + γ direct production at the LO is significantly reduced by the NLO QCD corrections. For the indirect process, we calculate the H → γ*γ with virtual γ substantially decaying to J/ψ(ϒ), including all the SM Feynman diagrams. The decay width of indirect production is much larger than the direct decay width. Since it is very clean in experiment, the H → J/ψ(ϒ) + γ decay could be observable at a 14 TeV LHC and it also offers a new way to probe the Yukawa coupling and New Physics at the LHC.

Parton distribution functions in Monte Carlo factorisation scheme

A next step in development of the KrkNLO method of including complete NLO QCD corrections to hard processes in a LO parton-shower Monte Carlo (PSMC) is presented. It consists of generalisation of the method, previously used for the Drell-Yan process, to Higgs-boson production. This extension is accompanied with the complete description of parton distribution functions (PDFs) in a dedicated, Monte Carlo (MC) factorisation scheme, applicable to any process of production of one or more colour-neutral particles in hadron-hadron collisions.

Top quark mass determination from the energy peaks of b-jets and B-hadrons at NLO QCD

We analyze the energy spectra of $single$ b-jets and B-hadrons resulting from the production and decay of top quarks within the SM at the LHC at the NLO QCD. For both hadrons and jets, we calculate the correlation of the peak of the spectrum with the top quark mass, considering the "energy-peak" as an observable to determine the top quark mass. Such a method is motivated by our previous work where we argued that this approach can have reduced sensitivity to the details of the production mechanism of the top quark, whether it is higher-order QCD effects or new physics contributions. As part of the NLO improvement over the original proposal, we assess the residual sensitivity of the extracted top quark mass to perturbative effects both in top quark production and decay. For a 1% jet energy scale uncertainty (and assuming negligible statistical error), the top quark mass can then be extracted using the energy-peak of b-jets with an error +- (1.2 (exp) + 0.6(th)) GeV. We note that recently the CMS collaboration reported a top quark mass measurement based on the original proposal (with b-jets) so that our result contributes to a precise evaluation of the associated theory uncertainty. In view of the dominant jet energy scale uncertainty in the measurement using b-jets, we also investigate the extraction of the top quark mass from the energy-peak of the corresponding B-hadrons which, in principle, can be measured without this uncertainty. The calculation of the B-hadron energy spectrum is carried out using fragmentation functions at NLO. The dependence on the fragmentation scale turns out to be the largest theoretical uncertainty in this extraction of top quark mass. Future improvement of the treatment of bottom quark hadronization can reduce this uncertainty, rendering methods based on the B-hadron energy-peak competitive for the top quark mass measurement.

Off-shell top quarks with one jet at the LHC: a comprehensive analysis at NLO QCD

We present a comprehensive study of the production of top quark pairs in association with one hard jet in the di-lepton decay channel at the LHC. Our predictions, accurate at NLO in QCD, focus on the LHC Run II with a center-of-mass energy of 13 TeV. All resonant and non-resonant contributions at the perturbative order ${\cal O}(\alpha_s^4 \alpha^4)$ are taken into account, including irreducible backgrounds to $t\bar{t}j$ production, interferences and off-shell effects of the top quark and the $W$ gauge boson. We extensively investigate the dependence of our results upon variation of renormalisation and factorisation scales and parton distribution functions in the quest for an accurate estimate of the theoretical uncertainties. Additionally, we explore a few possibilities for a dynamical scale choice with the goal of stabilizing the perturbative convergence of the differential cross sections far away from the $t\bar{t}$ threshold. Results presented here are particularly relevant for searches of new physics as well as for precise measurements of the top-quark fiducial cross sections and top-quark properties at the LHC.

NLO QCD + EW corrections to ZZZ production with subsequent leptonic decays at the LHC

In this paper we present the NLO QCD + NLO EW corrections to the ZZZ production with subsequent Z-boson leptonic decays at the LHC by adopting the improved narrow width approximation, which takes into account off-shell contributions and spin correlations. Integrated cross sections at 13, 14, 33 and hadron colliders and various kinematic distributions are presented. Our numerical results show that the jet emission correction accounts for a large part of the total QCD correction, especially in the high energy region. By applying a proper cut to the jet transverse momentum, e.g., , the jet emission correction can be reduced and the cross section is less dependent on the factorization/renormalization scale. This work reveals that both the NLO QCD and the NLO EW corrections are significant. For example, the NLO QCD, NLO EW and NLO QCD + EW relative corrections in the inclusive event selection scheme at the LHC can reach , and , respectively, which means that neither the NLO QCD nor the NLO EW correction is negligible in precision calculations.

Higgs production in association with a top-antitop pair in the Standard Model Effective Field Theory at NLO in QCD

We present the results of the computation of the next-to-leading order QCD corrections to the production cross section of a Higgs boson in association with a top-antitop pair at the LHC, including the three relevant dimension-six operators ($O_{t \varphi }, O_{\varphi G}, O_{tG}$) of the standard model effective field theory. These operators also contribute to the production of Higgs bosons in loop-induced processes at the LHC, such as inclusive Higgs, $Hj$ and $HH$ production, and modify the Higgs decay branching ratios for which we also provide predictions. We perform a detailed study of the cross sections and their uncertainties at the total as well as differential level and of the structure of the effective field theory at NLO including renormalisation group effects. Finally, we show how the combination of information coming from measurements of these production processes will allow to constrain the three operators at the current and future LHC runs. Our results lead to a significant improvement of the accuracy and precision of the deviations expected from higher-dimensional operators in the SM in both the top-quark and the Higgs-boson sectors and provide a necessary ingredient for performing a global EFT fit to the LHC data at NLO accuracy.

Signal background interference effects in heavy scalar production and decay to a top-anti-top pair

We analyse the production of a top quark pair through a heavy scalar at the LHC. We first review the main features of the signal as well as the interference with the top-anti-top background at leading order in QCD. We then study higher order QCD effects. While the background and the signal can be obtained at NNLO and NLO in QCD respectively, that is not the case for their interference, which is currently known only at LO. In order to improve the accuracy of the prediction for the interference term, we consider the effects of extra QCD radiation, i.e. the $2 \to 3$ (loop-induced) processes and obtain an estimate of the NLO corrections. As a result, we find that the contribution of the interference is important both at the total cross-section level and, most importantly, for the line-shape of the heavy scalar. In particular for resonances with widths larger than a couple of percent of the resonance mass, the interference term distorts the invariant mass distribution and generically leads to a non-trivial peak-dip structure. We study this process in a simplified model involving an additional scalar or pseudoscalar resonance as well as in the Two-Higgs-Doublet-Model for a set of representative benchmarks. We present the constraints on simplified models featuring an extra scalar as set by the LHC searches for top-anti-top resonances, and the implications of the 750 GeV diphoton excess recently reported by CMS and ATLAS for the top pair production assuming a scalar or a pseudoscalar resonance.

Erratum to: NLO QCD + NLO EW corrections to WZZ productions with leptonic decays at the LHC

Erratum to: NLO QCD + NLO EW corrections to WZZ productions with leptonic decays at the LHC

Higgs Boson production in association with jets in gluon-gluon fusion

We present the computation of the NLO QCD corrections to the production of a Standard Model Higgs boson in association with up to three jets through gluon-gluon fusion. The computation is performed in the approximation of an infinitely heavy top quark. Before studying some phenomenologically important observables, we describe some technical detail about the computation.

Two-loop QED corrections to the Altarelli-Parisi splitting functions

We compute the two-loop QED corrections to the Altarelli-Parisi (AP) splitting functions by using a deconstructive algorithmic Abelianization of the well-known NLO QCD corrections. We present explicit results for the full set of splitting kernels in a basis that includes the leptonic distribution functions that, starting from this order in the QED coupling, couple to the partonic densities. Finally, we perform a phenomenological analysis of the impact of these corrections in the splitting functions.