Search

We present a new Monte Carlo event generator for the hadronic production of four top quarks in the POWHEG BOX framework. Besides the dominant next-to-leading order QCD corrections at $\mathcal{O}(\alpha_s^5)$ we also include all subleading electroweak productions channels at leading-order accuracy. We validate our theoretical predictions by comparing to parton-shower matched predictions obtained within the MC@NLO framework for stable top quarks. Furthermore, we investigate in detail the various sources of theoretical uncertainties. Finally, we investigate a single lepton plus jets signature to study for the first time the impact of the electroweak production modes as well as spin-correlation effects at the fiducial level., Comment: 14 pages, 8 figures, 1 table, matches published version

We present a new calculation of next-to-leading order QCD corrections to electroweak top-quark pair hadroproduction in extensions of the Standard Model (SM) with extra heavy neutral and charged spin-1 resonances which considerably extends and improves an earlier calculation performed by some of us. In particular, we allow for flavor-non-diagonal ${Z}^{\ensuremath{'}}$ couplings and take into account nonresonant production in the SM and beyond including the contributions with $t$-channel $W$ and ${W}^{\ensuremath{'}}$ bosons. As a result, models with a more complicated flavor structure which have been proposed to explain the flavor anomalies in $B$ decays can now be accommodated in our code. Moreover, the new $t$-channel contributions lead to improved cross sections predictions at higher energies beyond the LHC reach. All amplitudes are generated using the recola2 package. As in our previous work, we include next-to-leading order QCD corrections and consistently match to parton showers with the POWHEG method fully taking into account the interference effects between the SM and new physics amplitudes. We consider the sequential Standard Model, the topcolor model, as well as the third family hypercharge model featuring non-flavor-diagonal ${Z}^{\ensuremath{'}}$ couplings and present numerical results for $t\overline{t}$ cross sections at hadron colliders with a center-of-mass energy up to 100 TeV.

Measurements of $$t \bar{t} H$$ tt¯H production in the $$H\rightarrow b \bar{b} $$ H→bb¯ channel depend in a critical way on the theoretical uncertainty associated with the irreducible $$t \bar{t} +b$$ tt¯+b -jet background. In this paper, analysing the various topologies that account for b-jet production in association with a $$t \bar{t} $$ tt¯ pair, we demonstrate that the process at hand is largely driven by final-state $$g\rightarrow b \bar{b} $$ g→bb¯ splittings. We also show that in five-flavour simulations based on $$t \bar{t} +$$ tt¯+ multi-jet merging, b-jet production is mostly driven by the parton shower, while matrix elements play only a marginal role in the description of $$g\rightarrow b \bar{b} $$ g→bb¯ splittings. Based on these observations we advocate the use of NLOPS simulations of $$pp\rightarrow t \bar{t} b\bar{b} $$ pp→tt¯bb¯ in the four-flavour scheme, and we present a new Powheg generator of this kind. Predictions and uncertainties for $$t \bar{t} +b$$ tt¯+b -jet observables at the 13 TeV LHC are presented both for the case of stable top quarks and with spin-correlated top decays. Besides QCD scale variations we consider also theoretical uncertainties related to the Powheg matching method and to the parton shower modelling, with emphasis on $$g\rightarrow b \bar{b} $$ g→bb¯ splittings. In general, matching and shower uncertainties turn out to be remarkably small. This is confirmed also by a consistent comparison against Sherpa+OpenLoops.

Measurements of $$t \bar{t} H$$ production in the $$H\rightarrow b \bar{b} $$ channel depend in a critical way on the theoretical uncertainty associated with the irreducible $$t \bar{t} +b$$ -jet background. In this paper, analysing the various topologies that account for b-jet production in association with a $$t \bar{t} $$ pair, we demonstrate that the process at hand is largely driven by final-state $$g\rightarrow b \bar{b} $$ splittings. We also show that in five-flavour simulations based on $$t \bar{t} +$$ multi-jet merging, b-jet production is mostly driven by the parton shower, while matrix elements play only a marginal role in the description of $$g\rightarrow b \bar{b} $$ splittings. Based on these observations we advocate the use of NLOPS simulations of $$pp\rightarrow t \bar{t} b\bar{b} $$ in the four-flavour scheme, and we present a new Powheg generator of this kind. Predictions and uncertainties for $$t \bar{t} +b$$ -jet observables at the 13 TeV LHC are presented both for the case of stable top quarks and with spin-correlated top decays. Besides QCD scale variations we consider also theoretical uncertainties related to the Powheg matching method and to the parton shower modelling, with emphasis on $$g\rightarrow b \bar{b} $$ splittings. In general, matching and shower uncertainties turn out to be remarkably small. This is confirmed also by a consistent comparison against Sherpa+OpenLoops.

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.

We present a calculation of direct photon production at next-to-leading order of QCD and a matching of this calculation with parton showers using POWHEG BOX. Based on simulations with POWHEG+PYTHIA, we perform a detailed phenomenological analysis of PHENIX data on prompt photon production and photon-hadron jet correlations in pp collisions at RHIC, considerably improving the description of these data with respect to previous calculations, and we suggest additional interesting analyses., 25 pages, 8 figures

In this work we present a new subtraction method for next-to-leading order calculations that is particularly convenient even when narrow resonances are present. The method is particularly suitable for the implementation of next-to-leading order calculations matched to parton shower generators. It allows at the same time for the inclusion of all finite width effects, including interferences, and for a consistent treatment of resonances in the shower approach, preserving the mass of resonances near their peak. We implement our method, in a fully general and automatic way, within the POWHEG BOX framework, and illustrate it using as a test case the process of $p p \to \mu^+ \nu_\mu j_b j$, that is dominated by $t$-channel single top production., Comment: 48 pages, 21 figures

QCD resummation predictions for the production of charged (W') and neutral (Z') heavy gauge bosons decaying leptonically are presented. The results of our resummation code at next-to-leading order and next-to-leading logarithmic (NLO+NLL) accuracy are compared to Monte Carlo predictions obtained with PYTHIA at leading order (LO) supplemented with parton showers (PS) and FEWZ at NLO and next-to-next-to-leading order (NNLO) for the $p_T$-differential and total cross sections in the Sequential Standard Model (SSM) and general SU(2)xSU(2)xU(1) models. The LO+PS Monte Carlo and NNLO fixed-order predictions are shown to agree approximately with those at NLO+NLL at small and intermediate $p_T$, respectively, and the importance of resummation for total cross sections is shown to increase with the gauge boson mass. The theoretical uncertainties are estimated by variations of the renormalisation/factorisation scales and of the parton densities, the former being significantly reduced by the resummation procedure. New limits at NLO+NLL on W' and Z' boson masses are obtained by reinterpreting the latest ATLAS and CMS results in general extensions of the Standard Model., Comment: 26 pages, 10 figures, 6 tables

General $\mathrm{SU}(2)\ifmmode\times\else\texttimes\fi{}\mathrm{SU}(2)\ifmmode\times\else\texttimes\fi{}\mathrm{U}(1)$ models represent a well-motivated intermediate step towards the unification of the standard model gauge groups. Based on a recent global analysis of low-energy and LEP constraints of these models, we perform numerical scans of their various signals at the LHC. We show that total cross sections for lepton and third-generation quark pairs, while experimentally easily accessible, provide individually only partial information about the model realized in nature. In contrast, correlations of these cross sections in the neutral and charged current channels may well lead to a unique identification.

We analyze the properties of the Aivazis-Collins-Olness-Tung (ACOT) scheme for heavy quark production and make use of the $\overline{\mathrm{MS}}$ massless results at next-to-next-to-leading order and ${\mathrm{N}}^{3}\mathrm{LO}$ for the structure functions ${F}_{2}$ and ${F}_{L}$ in neutral current deep-inelastic scattering to estimate the higher order corrections. For this purpose we decouple the heavy quark mass entering the phase space from the one entering the dynamics of the short distance cross section. We show numerically that the phase space mass is generally more important. Therefore, the dominant heavy quark mass effects at higher orders can be taken into account using the massless Wilson coefficients together with an appropriate slow-rescaling prescription implementing the phase space constraints. Combining the exact ACOT scheme at next-to-leading order with these expressions should provide a good approximation to the missing full calculation in the ACOT scheme at next-to-next-to-leading order and ${\mathrm{N}}^{3}\mathrm{LO}$.

Global analyses of parton distribution functions (PDFs) have provided incisive constraints on the up and down quark components of the proton, but constraining the other flavor degrees of freedom is more challenging. Higher-order theory predictions and new data sets have contributed to recent improvements. Despite these efforts, the strange quark parton distribution function has a sizable uncertainty, particularly in the small $x$ region. We examine the constraints from experiment and theory, and investigate the impact of this uncertainty on LHC observables. In particular, we study $W/Z$ production to see how the $s$ quark uncertainty propagates to these observables, and examine the extent to which precise measurements at the LHC can provide additional information on the proton flavor structure.