Top-quark Charge Asymmetry Research Articles

Measurement of the [formula omitted] charge asymmetry in events with highly Lorentz-boosted top quarks in pp collisions at [formula omitted] TeV

The measurement of the charge asymmetry in top quark pair events with highly Lorentz-boosted top quarks decaying to a single lepton and jets is presented. The analysis is performed using proton-proton collisions at s=13TeV with the CMS detector at the LHC and corresponding to an integrated luminosity of 138 fb−1. The selection is optimized for top quarks produced with large Lorentz boosts, resulting in nonisolated leptons and overlapping jets. The top quark charge asymmetry is measured for events with a tt¯ invariant mass larger than 750 GeV and corrected for detector and acceptance effects using a binned maximum likelihood fit. The measured top quark charge asymmetry of (0.42−0.69+0.64)% is in good agreement with the standard model prediction at next-to-next-to-leading order in quantum chromodynamic perturbation theory with next-to-leading-order electroweak corrections. The result is also presented for two invariant mass ranges, 750–900 and >900GeV.

The top quark charge asymmetry in t{bar{t}}gamma production at the LHC

We consider the top quark charge asymmetry in the process pp rightarrow t{bar{t}}+gamma at the 13 TeV LHC. The genuine tree level asymmetry in the q{bar{q}} channel is large with about -12%. However, the symmetric gg channel, photon radiation off top quark decay products, and higher order corrections wash out the asymmetry and obscure its observability. In this work, we investigate these effects at next-to-leading order QCD and check the robustness of theoretical predictions. We find a sizable perturbative correction and discuss its origins and implications. We also study dedicated cuts for enhancing the asymmetry and show that a measurement is possible with an integrated luminosity of 150,mathrm {fb}^{-1}.

Precise predictions for toverline{t}gamma /toverline{t} cross section ratios at the LHC

With the goal of increasing the precision of NLO QCD predictions for the ppto toverline{t}gamma process in the di-lepton top quark decay channel we present theoretical predictions for the mathrm{mathcal{R}}={sigma}_{toverline{t}gamma }/{sigma}_{toverline{t}} cross section ratio. Results for the latter together with various differential cross section ratios are given for the LHC with the Run II energy of sqrt{s}=13 TeV. Fully realistic NLO computations for toverline{t} and toverline{t}gamma production are employed. They are based on matrix elements for {e}^{+}{nu}_e{mu}^{-}{overline{nu}}_{mu }boverline{b} and {e}^{+}{nu}_e{mu}^{-}{overline{nu}}_{mu }boverline{b}gamma processes and include all resonant and non-resonant diagrams, interferences, and off-shell effects of the top quarks and the W gauge bosons. Various renormalisation and factorisation scale choices and parton density functions are examined to assess their impact on the cross section ratio. Depending on the transverse momentum cut on the hard photon a judicious choice of a dynamical scale allows us to obtain 1%–3% percent precision on mathrm{mathcal{R}} . Moreover, for differential cross section ratios theoretical uncertainties in the range of 1%-6% have been estimated. Until now such high precision predictions have only been reserved for the top quark pair production at NNLO QCD. Thus, mathrm{mathcal{R}} at NLO in QCD represents a very precise observable to be measured at the LHC for example to study the top quark charge asymmetry or to probe the strength and the structure of the t- overline{t} -γ vertex. The latter can shed some light on possible new physics that can reveal itself only once sufficiently precise theoretical predictions are available.

Top-quark charge asymmetry at the LHC and Tevatron through NNLO QCD and NLO EW

We provide the most up-to-date predictions for the top-quark charge asymmetry A_C at the LHC with a centre-of-mass energy of 8 TeV. Our result is accurate at the NNLO level in the strong interactions and at the complete-NLO level in both the strong and electroweak interactions. We present results for the inclusive A_C, several differential asymmetries as well as the P_{T,tt} and M_{tt} cumulative asymmetries. Similarly to the Tevatron forward-backward asymmetry A_FB, both NNLO QCD and NLO EW corrections to A_C are found to be significant. The inclusion of higher-order corrections reduces the residual scale dependence in the predicted A_C. The pattern of higher-order corrections indicates good perturbative control over these observables. We conclude that at present there is nearly uniform agreement between Standard Model predictions and LHC measurements across all A_C-related observables. All previously published differential A_FB predictions at the Tevatron are updated to the same accuracy.

Constraining top quark effective theory in the LHC Run II era

We perform an up-to-date global fit of top quark effective theory to experimental data from the Tevatron, and from LHC Runs I and II. Experimental data includes total cross-sections up to 13 TeV, as well as differential distributions, for both single top and pair production. We also include the top quark width, charge asymmetries, and polarisation information from top decay products. We present bounds on the coefficients of dimension six operators, and examine the interplay between inclusive and differential measurements, and Tevatron / LHC data. All results are currently in good agreement with the Standard Model.

Associated production of a top-quark pair with vector bosons at NLO in QCD: impact on t t ¯ H $$ \mathrm{t}\overline{\mathrm{t}}\mathrm{H} $$ searches at the LHC

We study the production of a top-quark pair in association with one and two vector bosons, $t \bar t V$ and $t \bar t VV$ with $V=\gamma, Z, W^\pm$, at the LHC. We provide predictions at next-to-leading order in QCD for total cross sections and top-quark charge asymmetries as well as for differential distributions. A thorough discussion of the residual theoretical uncertainties related to missing higher orders and to parton distribution functions is presented. As an application, we calculate the total cross sections for this class of processes (together with $t \bar t H$ and $t \bar t t \bar t$ production) at hadron colliders for energies up to 100 TeV. In addition, by matching the NLO calculation to a parton shower, we determine the contribution of $t \bar t V$ and $t \bar t VV$ to final state signatures (two-photon and two-same-sign-, three- and four-lepton) relevant for $t \bar t H$ analyses at the Run II of the LHC.

Application of the principle of maximum conformality to the top-quark charge asymmetry at the LHC

The Principle of Maximum Conformality (PMC) provides a systematic and process-independent method to derive renormalization scheme- and scale- independent fixed-order pQCD predictions. In Ref.\cite{pmc3}, we studied the top-quark charge asymmetry at the Tevatron. By applying the PMC, we have shown that the large discrepancies for the top-quark charge asymmetry between the Standard Model estimate and the CDF and D0 data are greatly reduced. In the present paper, with the help of the Bernreuther-Si program, we present a detailed PMC analysis on the top-quark pair production up to next-to-next-to-leading order level at the LHC. After applying PMC scale setting, the pQCD prediction for the top-quark charge asymmetry at the LHC has very small scale uncertainty; e.g., $A_{\rm C}|_{\rm 7 TeV;PMC} =\left(1.15^{+0.01}_{-0.03}\right)\%$, $A_{\rm C}|_{\rm 8 TeV;PMC} =\left(1.03^{+0.01}_{+0.00}\right)\%$, and $A_{\rm C}|_{\rm 14 TeV;PMC} =\left(0.62^{+0.00}_{-0.02}\right)\%$. The corresponding predictions using conventional scale setting are: $A_{\rm C}|_{\rm 7 TeV;Conv.} =\left(1.23^{+0.14}_{-0.14}\right)\%$, $A_{\rm C}|_{\rm 8 TeV;Conv.} =\left(1.11^{+0.17}_{-0.13}\right)\%$, and $A_{\rm C}|_{\rm 14 TeV;Conv.} =\left(0.67^{+0.05}_{-0.05}\right)\%$. In these predictions, the scale errors are predicted by varying the initial renormalization and factorization scales in the ranges $\mu^{\rm init}_r\in[m_t/2,2m_t]$ and $\mu_f\in[m_t/2,2m_t]$. The PMC predictions are also in better agreement with the available ATLAS and CMS data. In addition, we have calculated the top-quark charge asymmetry assuming several typical cuts on the top-pair invariant mass $M_{t\bar{t}}$. For example, assuming $M_{t\bar{t}}>0.5 ~ {\rm TeV}$ and $\mu_f=\mu^{\rm init}_r =m_t$, we obtain $A_{\rm C}|_{\rm 7 TeV;PMC}=2.67\%$, $A_{\rm C}|_{\rm 8 TeV;PMC}=2.39\%$, and $A_{\rm C}|_{\rm 14 TeV;PMC}=1.28\%$.

Top-quark charge asymmetry and polarization in [formula omitted] production at the LHC

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.

TOP AND HIGGS PHYSICS AT THE HADRON COLLIDERS

This review summarizes the recent results for top quark and Higgs boson measurements from experiments at Tevatron, a proton–antiproton collider at a center-of-mass energy of [Formula: see text], and the Large Hadron Collider, a proton–proton collider at a center-of-mass energy of [Formula: see text]. These results include the discovery of a Higgs-like boson and measurement of its various properties, and measurements in the top quark sector, e.g. top quark mass, spin, charge asymmetry and production of single top quark.

Top Quark Charge Asymmetry Measurement with ATLAS at the LHC

In the Standard Model top-antitop pair production is foreseen to be charge asymmetric in quark-antiquark annihilation at NLO. Although the dominant tt̄ production mechanism in pp collisions at the LHC is gluon-gluon fusion, QCD predicts a small excess of top quarks produced at higher absolute rapidities than antitops. The measurement of the top quark charge asymmetry is therefore an important test of QCD and is also sensitive to new physics. We present measurements of this asymmetry using data corresponding to an integrated luminosity of 1.04 fb−1 and 4.70 fb−1 of pp collisions at = 7 TeV collected by the ATLAS detector in the single lepton and dilepton channels respectively.

Top-quark charge asymmetry goes forward: two new observables for hadron colliders

We propose two new observables to measure the charge asymmetry in hadronic top-quark pair production in association with a hard jet. The incline asymmetry, based on the inclination between the planes of initial- and final-state momenta, probes the charge asymmetry in the quark-antiquark channel. Compared to the hitherto investigated rapidity asymmetries, the incline asymmetry provides improved access to the partonic charge asymmetry at both the Tevatron and the LHC. The energy asymmetry, based on the energy difference between top and antitop quarks, for the first time allows us to probe the charge asymmetry in the quark-gluon channel at the LHC. In quantum chromodynamics, asymmetries of up to -12% at the leading order are achievable with appropriate cuts. Top-pair plus jet production thus has the potential to become the discovery channel of the charge asymmetry in proton-proton collisions.

Transverse-Momentum Resummation for Top-Quark Pairs at Hadron Colliders

We develop a framework for a systematic resummation of the transverse momentum distribution of top-quark pairs produced at hadron colliders based on effective field theory. Compared to Drell-Yan and Higgs production, a novel soft function matrix is required to account for the soft gluon emissions from the final states. We calculate this soft function at the next-to-leading order, and perform the resummation at the next-to-next-to-leading logarithmic accuracy. We compare our results with parton shower programs and with the experimental data at the Tevatron and the LHC. We also discuss the implications for the top quark charge asymmetry.

Top quark pair properties: spin correlation, charge asymmetry and complex final states at LHC in ATLAS

In proton-proton collisions at the LHC, pairs of top and anti-top quarks are expected to be mostly produced through gluon fusion, in contrast to production at the Tevatron, where quark annihilation dominates. Making use of the large number of top quark pairs, we present measurements of the spin correlation between top and anti-top quarks as well as of the top quark charge asymmetry, which constitute important tests of QCD and are sensitive to new physics. We also discuss top production in association of photons and Z bosons.

Top-quark charge asymmetry with a jet handle

Pairs of top and antitop quarks are produced at the LHC to a large extent in association with a hard jet. We investigate the charge asymmetry in $t\overline{t}+j$ production in quantum chromodynamics (QCD) and with additional massive color-octet vector bosons. The total charge asymmetry at the LHC is suppressed by the large charge-symmetric background from gluon-gluon fusion. We show to what extent the asymmetry can be enhanced by suitable phase space cuts and, in particular, elaborate on the kinematics of the hard jet in the $t\overline{t}+j$ final state. We demonstrate that in QCD, the asymmetry amounts to 1.5% for central jets without an excessive reduction of the cross section. By applying additional kinematical cuts, the asymmetry can be enhanced to 4%, but at the cost of a strong reduction of the cross section. Massive color-octet states can generate sizeable effects in $t\overline{t}+j$ production, both on the charge asymmetry and on the cross section. The charge asymmetry probes both vector and axial-vector couplings to quarks. We show that massive color octets can generate asymmetries up to $\ifmmode\pm\else\textpm\fi{}10%$ for moderate and up to $\ifmmode\pm\else\textpm\fi{}30%$ for strong kinematical cuts to be used in experimental analyses at the LHC. Jet kinematics can be used to obtain further information about the nature of the couplings and thereby to discriminate between different models.

Top quark and leptonic charge asymmetries for the Tevatron and LHC

We compute, for $t \bar t$ production at the LHC and at the Tevatron, several charge asymmetries to NLO QCD, including also the electromagnetic and weak-interaction corrections. We calculate these asymmetries both inclusively and with additional kinematic cuts and compare our results, where possible, with recent experimental results and with SM predictions. The $t \bar t$ asymmetries induce also corresponding asymmetries for the charged leptons from semileptonic top-quark decay. Although these asymmetries are, in the SM, smaller than the corresponding ones for top quarks, they are expected to be measurable quite precisely. In fact, measurement of a lepton asymmetry in $\ell$ + jets events was reported by the D$\emptyset$ and CDF experiments. We analyze and compute to NLO in the gauge couplings leptonic charge asymmetries for dileptonic and semileptonic $\ttbar$ events, with and without acceptance cuts, at the Tevatron and the LHC.

Top quark forward-backward asymmetry and charge asymmetry in the left-right twin Higgs model

In order to explain the Tevatron anomaly of the top quark forward-backward asymmetry ${A}_{\mathrm{FB}}^{t}$ in the left-right twin Higgs model, we choose to give up the lightest neutral particle of $\stackrel{^}{h}$ field as a stable dark matter candidate. Then a new Yukawa interaction for $\stackrel{^}{h}$ is allowed, which can be free from the constraint of same-sign top pair production and contribute sizably to ${A}_{\mathrm{FB}}^{t}$. Considering the constraints from the production rates of the top pair ($t\overline{t}$), the top decay rates, and $t\overline{t}$ invariant mass distribution, we find that this model with such new Yukawa interaction can explain ${A}_{\mathrm{FB}}^{t}$ measured at the Tevatron while satisfying the charge asymmetry ${A}_{C}^{t}$ measured at the LHC. Moreover, this model predicts a strong correlation between ${A}_{C}^{t}$ at the LHC and ${A}_{\mathrm{FB}}^{t}$ at the Tevatron, i.e., ${A}_{C}^{t}$ increases as ${A}_{\mathrm{FB}}^{t}$ increases.

Implications of the effective axial-vector coupling of the gluon on top-quark charge asymmetry at the LHC

We study different top quark charge asymmetries and the variation of $t\bar t$ total cross section induced by the effective axial-vector coupling of gluon in the LHC experiments. We show that rapidity cut-dependent asymmetries are more sensitive to the new physics than the independent ones. We also study the dependence of the asymmetries and variations of total $t\bar t$ cross sections on the invariant mass of $t\bar t$ system and show that it would be necessary to measure those quantities as functions of $m_{tt}$ at the LHC. In the context of considered new physics scenario, 7 TeV LHC has enough sensitivity either to confirm the Tevatron top asymmetry anomaly or to rule it out. In the latter case the LHC is able to put stringent constraint on the new physics scale $\Lambda$ in this framework.

Charge asymmetries of top quarks at hadron colliders revisited

A sizeable difference in the differential production cross section of top- compared to antitop-quark production, denoted charge asymmetry, has been observed at the Tevatron. The experimental results seem to exceed the theory predictions based on the Standard Model by a significant amount and have triggered a large number of suggestions for “new physics”. In the present paper the Standard Model predictions for Tevatron and LHC experiments are revisited. This includes a reanalysis of electromagnetic as well as weak corrections, leading to a shift of the asymmetry by roughly a factor 1.1 when compared to the results of the first papers on this subject. The impact of cuts on the transverse momentum of the top-antitop system is studied. Restricting the $ t\overline t $ system to a transverse momentum less than 20 GeV leads to an enhancement of the asymmetries by factors between 1.3 and 1.5, indicating the importance of an improved understanding of the $ t\overline t $ -momentum distribution. Predictions for similar measurements at the LHC are presented, demonstrating the sensitivity of the large rapidity region both to the Standard Model contribution and effects from “new physics”.

Hadronic top-quark pair-production with one jet and parton showering

Abstract We present a calculation of heavy-flavor production in hadronic collisions in association with one jet matched to parton shower Monte Carlo programs at next-to-leading order in perturbative QCD. Top-quark decays are included and spin correlations in the decay products are taken into account. The calculation builds on existing results for the radiative corrections to heavy-quark plus one jet production and uses the POWHEG BOX for the interface to the parton shower programs PYTHIA or HERWIG. A broad phenomenological study for the Large Hadron Collider and the Tevatron is presented. In particular we study — as one important sample application — the impact of the parton shower on the top-quark charge asymmetry.

Top-pair forward-backward asymmetry beyond next-to-leading order

We make use of recent results in effective theory and higher-order perturbative calculations to improve the theoretical predictions of the QCD contribution to the top-quark pair production forward-backward asymmetry at the Tevatron. In particular, we supplement the fixed-order next-to-leading order calculation with higher-order corrections from soft-gluon resummation at next-to next-to-leading order accuracy performed in two different kinematic schemes, which allows us to make improved predictions for the asymmetry in the $p\overline{p}$ and $t\overline{t}$ rest frames as a function of the rapidity and invariant mass of the $t\overline{t}$ pair. Furthermore, we provide binned results which can be compared with the recent measurements of the forward-backward asymmetry in events with a large pair invariant mass or rapidity difference. Finally, we calculate at $\mathrm{NLO}+\mathrm{NNLL}$ order the top-quark charge asymmetry at the LHC as a function of a lower rapidity cutoff for the top and antitop quarks.