Top Quark Decay Research Articles

Higgs characterisation at NLO in QCD: CP properties of the top-quark Yukawa interaction.

At the LHC the CP properties of the top-quark Yukawa interaction can be probed through Higgs production in gluon fusion or in association with top quarks. We consider the possibility for both CP-even and CP-odd couplings to the top quark to be present, and study CP-sensitive observables at next-to-leading order (NLO) in QCD, including parton-shower effects. We show that the inclusion of NLO corrections sizeably reduces the theoretical uncertainties, and confirm that di-jet correlations in H+2 jet production through gluon fusion and correlations of the top-quark decay products in tbar{t}H production can provide sensitive probes of the CP nature of the Higgs interactions.

Searching for new physics with triple-top signal at the LHC

We study the triple-top quark production predicted by the effective operators at the Large Hadron Collider (LHC) with 14 TeV center-of-mass energy. We calculate the production cross section and find that the ratio of signal to Standard Model background can be large. Observation of triple-top signal would give evidence of signature of new physics. We also show that top quarks can be reconstructed using on-shell conditions and MT2 observable in the case that the anti-top quark decays hadronically and two top quarks decay semileptonically. Using the polarization of top quark, we demonstrate that the effective operators that predict different chirality combinations of top quarks in the final state can be distinguished.

Effective field theory approach to top-quark decay at next-to-leading order in QCD

We present analytical results for top-quark decay processes, in an effective field theory beyond the Standard Model, at next-to-leading order in QCD. We parametrize new physics effects using dimension-six operators, and consider all operators that give rise to non-standard interactions of the top quark. We investigate both the flavor-conserving and flavor-changing decay modes, including their two-body and three-body semi-leptonic final states. The QCD mixing among relevant operators are also taken into account. These results provide all information needed for a complete model-independent study of top-quark decay at next-to-leading order accuracy, paving the way to global analyses for new physics effects in an effective field theory approach.

Measurement of the ratio [formula omitted] in pp collisions at [formula omitted

The ratio of the top-quark branching fractions R=B(t→Wb)/B(t→Wq), where the denominator includes the sum over all down-type quarks (q=b,s,d), is measured in the tt¯ dilepton final state with proton–proton collision data at s=8 TeV from an integrated luminosity of 19.7 fb−1, collected with the CMS detector. In order to quantify the purity of the signal sample, the cross section is measured by fitting the observed jet multiplicity, thereby constraining the signal and background contributions. By counting the number of b jets per event, an unconstrained value of R=1.014±0.003(stat.)±0.032(syst.) is measured, in a good agreement with current precision measurements in electroweak and flavour sectors. A lower limit R>0.955 at the 95% confidence level is obtained after requiring R≤1, and a lower limit on the Cabibbo–Kobayashi–Maskawa matrix element |Vtb|>0.975 is set at 95% confidence level. The result is combined with a previous CMS measurement of the t-channel single-top-quark cross section to determine the top-quark total decay width, Γt=1.36±0.02(stat.)−0.11+0.14(syst.) GeV.

Jet substructures of boosted polarized top quarks

We study jet substructures of a boosted polarized top quark, which undergoes the semileptonic decay $t\ensuremath{\rightarrow}b\ensuremath{\ell}\ensuremath{\nu}$, in the perturbative QCD framework. The jet mass distribution (energy profile) is factorized into the convolution of a hard top-quark decay kernel with the bottom-quark jet function (jet energy function). Computing the hard kernel to the leading order in QCD and inputting the latter functions from the resummation formalism, we observe that the jet mass distribution is not sensitive to the helicity of the top quark, but the energy profile is: energy is accumulated faster within a left-hand top jet than within a right-hand one, a feature related to the $V\ensuremath{-}A$ structure of weak interaction. It is pointed out that the energy profile is a simple and useful jet observable for helicity discrimination of a boosted top quark, which helps identification of physics beyond the standard model at the Large Hadron Collider. The extension of our analysis to other jet substructures, including those associated with a hadronically decaying polarized top quark, is proposed.

Search for top quark decays t → qH with H → γγ using the ATLAS detector

A search is performed for flavour-changing neutral currents in the decay of a top quark to an up-type ($c, u$) quark and a Higgs boson, where the Higgs boson decays to two photons. The proton-proton collision data set used corresponds to 4.7 fb$^{-1}$ at $\sqrt{s}$ = 7 \tev\ and 20.3 fb$^{-1}$ at $\sqrt{s}$ = 8 \tev\ collected by the ATLAS experiment at the LHC. Top quark pair events are searched for in which one top quark decays to $qH$ and the other decays to $bW$. Both the hadronic and the leptonic decay modes of the $W$ boson are used. No significant signal is observed and an upper limit is set on the $t\rightarrow qH$ branching ratio of 0.79% at the 95% confidence level. The corresponding limit on the $tqH$ coupling combination $\sqrt{\lambda_{tcH}^{2} + \lambda_{tuH}^{2}}$ is 0.17.

Study of top quark production and decays involving a tau lepton at CDF and limits on a charged Higgs boson contribution

We present an analysis of top-antitop quark production and decay into a tau lepton, tau neutrino, and bottom quark using data from 9 fb−1 of integrated luminosity at the Collider Detector at Fermilab. Dilepton events, where one lepton is an energetic electron or muon and the other a hadronically decaying tau lepton, originating from proton-antiproton collisions at ffiffi s p ¼ 1.96 TeV, are used. A top-antitop quark production cross section of 8.1 2.1 pb is measured, assuming standard-model top quark decays. By separately identifying for the first time the single-tau and the ditau components, we measure the branching fraction of the top quark into the tau lepton, tau neutrino, and bottom quark to be ð9.6 2.8Þ%. The branching fraction of top quark decays into a charged Higgs boson and a bottom quark, which would imply violation of lepton universality, is limited to be less than 5.9% at a 95% confidence level [for BðH− → τν¯Þ ¼ 1].

Search for Flavor-Changing Neutral Currents in Top-Quark Decayst→ZqinppCollisions ats=8 TeV

A search for flavor-changing neutral currents in top-quark decays t to Zq is performed in events produced from the decay chain t t-bar to Zq + Wb, where both vector bosons decay leptonically, producing a final state with three leptons (electrons or muons). A data set collected with the CMS detector at the LHC is used, corresponding to an integrated luminosity of 19.7 inverse femtobarns of proton-proton collisions at a center-of-mass energy of 8 TeV. No excess is seen in the observed number of events relative to the standard model prediction; thus, no evidence for flavor-changing neutral currents in top-quark decays is found. A combination with a previous search at 7 TeV excludes a t to Zq branching fraction greater than 0.05% at the 95% confidence level.

Light top squark in precision top quark sample

The uncertainty of $t\bar{t}$ production cross section measurement at LHC is at a-few-percent level which still allows the stop pair production $\tilde{t}\tilde{t}^{*}$ with identical final states $2b+\ell+nj+{E}_{T}$. In this paper, we attempt to use the existing measurement of $W$-polarization in top quark decay to improve the distinction between stop and top quark states. We apply the ATLAS $\chi^2$ method of $W$-polarization measurement in semi-leptonic $t\bar{t}$ final state to semi-leptonic stop pair samples and study its prediction. We find that the faked top events from stop mostly contribute to the left-handed polarized $W$ due to the reconstruction. The benchmark point with maximal contribution to top events only changes $F_{L}$ by 1%. After comparing with the current experiments, we conclude that the current measurement of $W$-polarization in $t$ decay cannot exclude the light stop scenario.

CP violation in top-quark pair production and decay

CP violation in top-quark pair production and decay

Dark decay of the top quark

We suggest top quark decays as a venue to search for light dark force carriers. The top quark is the heaviest particle in the standard model whose decays are relatively poorly measured, allowing sufficient room for exotic decay modes from new physics. A very light (GeV scale) dark gauge boson (Z') is a recently highlighted hypothetical particle that can address some astrophysical anomalies as well as the 3.6 sigma deviation in the muon g-2 measurement. We present and study a possible scenario that top quark decays as t -> b W + Z's. This is the same as the dominant top quark decay (t -> b W) accompanied by one or multiple dark force carriers. The Z' can be easily boosted, and it can decay into highly collimated leptons (lepton-jet) with large branching ratio. We discuss the implications for the Large Hadron Collider experiments including the analysis based on the lepton-jets.

Probing the anomalous FCNC interactions in a top-Higgs boson final state and the charge ratio approach

We study the anomalous production of a single top quark in association with a Higgs boson at the LHC originating from flavor changing neutral current (FCNC) interactions in $tqg$ and $tqH$ vertices. We derive the discovery potentials and $68\%$ C.L. upper limits considering leptonic decay of the top quark and the Higgs boson decay into a $b\bar{b}$ pair with 10 fb$^{-1}$ integrated luminosity of data in proton-proton collisions at the center-of-mass energy of 14 TeV. We propose a charge ratio for the lepton in top quark decay in terms of lepton $p_{T}$ and $\eta$ as a strong tool to observe the signal. In particular, we show that the charge ratio increases significantly at large $p_{T}$ of the charged lepton. While the main background from $t\bar{t}$ is nearly charge symmetric and $W+jets$ background has much smaller charge ratio with respect to the signal. We show that this feature can also be used in the probe of anomalous single top production with a $Z-$boson or a photon which are under the attention of the experimental collaborations.

Search for baryon number violation in top-quark decays

A search for baryon number violation (BNV) in top-quark decays is performed using pp collisions produced by the LHC at s=8 TeV. The top-quark decay considered in this search results in one light lepton (muon or electron), two jets, but no neutrino in the final state. Data used for the analysis were collected by the CMS detector and correspond to an integrated luminosity of 19.5 fb−1. The event selection is optimized for top quarks produced in pairs, with one undergoing the BNV decay and the other the standard model hadronic decay to three jets. No significant excess of events over the expected yield from standard model processes is observed. The upper limits at 95% confidence level on the branching fraction of the BNV top-quark decay are calculated to be 0.0016 and 0.0017 for the muon and the electron channels, respectively. Assuming lepton universality, an upper limit of 0.0015 results from the combination of the two channels. These limits are the first that have been obtained on a BNV process involving the top quark.

Monotop phenomenology at the Large Hadron Collider

We investigate new physics scenarios where systems comprised of a single top quark accompanied by missing transverse energy, dubbed monotops, can be produced at the LHC. Following a simplified model approach, we describe all possible monotop production modes via an effective theory and estimate the sensitivity of the LHC, assuming 20 fb$^{-1}$ of collisions at a center-of-mass energy of 8 TeV, to the observation of a monotop state. Considering both leptonic and hadronic top quark decays, we show that large fractions of the parameter space are reachable and that new physics particles with masses ranging up to 1.5 TeV can leave hints within the 2012 LHC dataset, assuming moderate new physics coupling strengths.

Top quark physics at the LHC

In 2011, an integrated luminosity of more than 5 fb−1 at 7 TeV has been delivered by the LHC. The measurement of the cross section in top quark pair production and in single top quark production, top quark mass, top quark properties and new physics searches in top quark decays have been performed at the CMS experiment with various integrated luminosities. An overview of the latest results of these measurements and searches by the time of ICFP 2012 conference will be presented.

Monte Carlo simulation of hard radiation in decays in beyond the standard model physics in Herwig++

We use the POWHEG formalism in the Herwig++ event generator to match QCD real-emission matrix elements with the parton shower for a range of decays relevant to Beyond the Standard Model physics searches. Applying this correction affects the shapes of experimental observables and so changes the number of events passing selection criteria. To validate this approach, we study the impact of the correction on Standard Model top quark decays. We then illustrate the effect of the correction on Beyond the Standard Model scenarios by considering the invariant-mass distribution of dijets produced in the decay of the lightest Randall-Sundrum graviton and transverse momentum distributions for decays in Supersymmetry. We consider only the effect of the POWHEG correction on the simulation of the hardest emission in the shower and ignore the normalisation factor required to correct the total widths and branching ratios to next-to-leading order accuracy.

Measurement of the top quark charge in pp collisions at $ \sqrt{s}=7 $ TeV with the ATLAS detector

AbstractA measurement of the top quark electric charge is carried out in the ATLAS experiment at the Large Hadron Collider using 2.05 fb−1of data at a centre-of-mass energy of 7 TeV. In units of the elementary electric charge, the top quark charge is determined to be 0.64 ± 0.02 (stat.) ± 0.08 (syst.) from the charges of the top quark decay products in single lepton$ t\overline{t} $candidate events. This excludes models that propose a heavy quark of electric charge −4/3, instead of the Standard Model top quark, with a significance of more than 8σ.

Observability of heavy charged Higgs through s-channel single top events at LHC

The charged Higgs bosons can be produced as a resonance in s-channel single top events. The light charged Higgs in such events preferably decays to a pair of $\tau \nu$ thus making it difficult to distinguish from the large single W events producing the same final state. However, the heavy charged Higgs decay to a pair of $t \bar{b}$ can be extracted from the SM background events. The final state under consideration in this paper contains the top quark decay to $W^+ b$ followed by $W^+$ decay to electrons or muons. It is shown that this signal can be observed at LHC at a large area of MSSM phase space ($m(H^+), tan\beta$). Finally $5\sigma$ and 95% CL exclusion contours are presented at different integrated luminosities of LHC assuming a nominal center of mass energy of $\sqrt{s}$ = 14 TeV.

Analytic solutions for neutrino momenta in decay of top quarks

We employ a geometric approach to analytically solve equations of constraint on the decay of top quarks involving leptons. The neutrino momentum is found as a function of the 4-vectors of the associated bottom quark and charged lepton, the masses of the top quark and W boson, and a single parameter, which constrains it to an ellipse. We show how the measured imbalance of momenta in the event reduces the solutions for neutrino momenta to a discrete set, in the cases of one or two top quarks decaying to leptons. The algorithms can be implemented concisely with common linear algebra routines.

Measurement of the W-boson helicity in top-quark decays from $ \mathrm{t}\overline{\mathrm{t}} $ production in lepton+jets events in pp collisions at $ \sqrt{s}=7 $ TeV

Abstract The W-boson helicity fractions in top-quark decays are measured with $ \mathrm{t}\overline{\mathrm{t}} $ events in the lepton+jets final state, using proton-proton collisions at a centre-of-mass energy of 7 TeV, collected in 2011 with the CMS detector at the LHC. The data sample corresponds to an integrated luminosity of 5.0 fb−1. The measured fractions of longitudinal, left-, and right-handed helicity are F 0 = 0.682 ± 0.030 (stat.) ± 0.033 (syst.), F L = 0.310 ± 0.022 (stat.) ± 0.022 (syst.), and F R = 0.008 ± 0.012 (stat.) ± 0.014 (syst.), consistent with the standard model predictions. The measured fractions are used to probe the existence of anomalous Wtb couplings. Exclusion limits on the real components of the anomalous couplings g L , g R are also derived.