Top Quark Decay Research Articles

Effects of Top-quark Decay Modeling in \(t\bar {t}\gamma \) Production at the LHC

We present a systematic comparison of different approaches for the modeling of $t\bar{t}\gamma$ final states with leptonic decays at the LHC. On the one hand, we consider a complete calculation at NLO QCD accuracy which includes all resonant and non-resonant diagrams. On the other hand, we consider predictions in the narrow-width approximation with top quark decays modeled at various accuracies. In this way we quantify the impact of the off-shell effects in $t\bar{t}\gamma$ production. We also discuss the relative importance of double-, single- and non-resonant contributions in the complete calculation. Finally we investigate the fraction of isolated photons from decays of top quarks, which represent a background for measurements of anomalous $t\gamma$ couplings.

Physics Inspired Deep Neural Networks for Top Quark Reconstruction

Deep neural networks (DNNs) have been applied to the fields of computer vision and natural language processing with great success in recent years. The success of these applications has hinged on the development of specialized DNN architectures that take advantage of specific characteristics of the problem to be solved, namely convolutional neural networks for computer vision and recurrent neural networks for natural language processing. This research explores whether a neural network architecture specific to the task of identifying t → Wb decays in particle collision data yields better performance than a generic, fully-connected DNN. Although applied here to resolved top quark decays, this approach is inspired by an DNN technique for tagging boosted top quarks, which consists of defining custom neural network layers known as the combination and Lorentz layers. These layers encode knowledge of relativistic kinematics applied to combinations of particles, and the output of these specialized layers can then be fed into a fully connected neural network to learn tasks such as classification. This research compares the performance of these physics inspired networks to that of a generic, fully-connected DNN, to see if there is any advantage in terms of classification performance, size of the network, or ease of training.

Highlights of top-quark properties measurements at the ATLAS experiment

The top quark is the heaviest known fundamental particle. As it is the only quark that decays before it hadronizes, this gives a unique opportunity to probe the properties of pseudo-bare quarks. This proceeding focuses on a few recent precision measurements of top-quark properties in top-quark pair events by the ATLAS Collaboration using LHC proton-proton collision data. Among these is the measurement of color flow and measurement of spin correlations in tt‾ pair production. Measurements of the top-quark mass and searches for rare top-quark decays are also presented.

Determination of Ds+ meson fragmentation functions through two different approaches

The hadronization mechanism is described by the fragmentation functions (FFs) which are universal and process-independent quantities. They can be generally determined theoretically or phenomenologically. In the phenomenological approach which is based on the data analysis, we present the FFs of ${D}_{s}^{+}$ both at leading-order (LO) and next-to-leading order (NLO) and, for the first time, at next-to-next-to-leading order in the minimal subtraction factorization scheme with five massless quarks. These functions are determined by fitting all available experimental data of inclusive single ${D}_{s}^{+}$-meson production in electron-positron annihilation, from the OPAL Collaboration at CERN LEP1. We shall also estimate the uncertainties in these FFs as well as in the corresponding observables. In the theoretical approach, we apply the perturbative FFs formalism based on the Suzuki's model and present our analytical results at LO and NLO perturbative QCD framework. For the first time, a comparison between both approaches will be presented in this work. We also apply our new FFs to generate theoretical predictions for the energy distribution of ${D}_{s}^{+}$ mesons produced through the decay of unpolarized top quarks, to be measured at the CERN LHC.

NNLO charmed-meson fragmentation functions and their uncertainties in the presence of meson mass corrections

The main aim of this paper is to present new sets of non-perturbative fragmentation functions (FFs) for D^0 and D^+ mesons at next-to-leading (NLO) and, for the first time, at next-to-next-to-leading order (NNLO) in the overline{mathrm {MS}} factorization scheme with five massless quark flavors. This new determination of FFs is based on the QCD fit to the OPAL experimental data for hadron production in the electron-positron single-inclusive annihilation (SIA). We discuss in detail the novel aspects of the methodology used in our analysis and the validity of obtained FFs by comparing with previous works in literature which have been carried out up to NLO accuracy. We will also incorporate the effect of charmed meson mass corrections into our QCD analysis and discuss the improvements upon inclusion of these effects. The uncertainties in the extracted FFs as well as in the corresponding observables are estimated using the “Hessian” approach. For a typical application, we use our new FFs to make theoretical predictions for the energy distributions of charmed mesons inclusively produced through the decay of unpolarized top quarks, to be measured at the CERN LHC. As a result of this analysis, suggestions are discussed for possible future studies on the current topic to consider any theory improvements and other available experimental observables.

Measurements of top-quark pair differential and double-differential cross-sections in the ell +jets channel with pp collisions at sqrt{s}=13 TeV using the ATLAS detector

Single- and double-differential cross-section measurements are presented for the production of top-quark pairs, in the lepton + jets channel at particle and parton level. Two topologies, resolved and boosted, are considered and the results are presented as a function of several kinematic variables characterising the top and tbar{t} system and jet multiplicities. The study was performed using data from pp collisions at centre-of-mass energy of 13 TeV collected in 2015 and 2016 by the ATLAS detector at the CERN Large Hadron Collider (LHC), corresponding to an integrated luminosity of 36~mathrm {fb}^{-1}. Due to the large tbar{t} cross-section at the LHC, such measurements allow a detailed study of the properties of top-quark production and decay, enabling precision tests of several Monte Carlo generators and fixed-order Standard Model predictions. Overall, there is good agreement between the theoretical predictions and the data.

Parton showers and matching uncertainties in top quark pair production with Herwig 7

We evaluate the theoretical uncertainties in next-to-leading order plus parton shower predictions for top quark pair production and decay in hadronic collisions. Our work is carried out using the Herwig 7 event generator and presents an in-depth study of variations in matching schemes with two systematically different shower algorithms, the traditional angular-ordered and alternative dipole shower. We also present all of the required extensions of the Herwig dipole shower algorithm to properly take into account quark mass effects, as well as its ability to perform top quark decays. The predictions are compared at parton level as well as to Large Hadron Collider data, including in the boosted regime. We find that the regions where predictions with a non-top-quark-specific tune differ drastically from data are plagued by large uncertainties which are consistent between our two shower and matching algorithms.

Top-quark physics at the CLIC electron-positron linear collider

The Compact Linear Collider (CLIC) is a proposed future high-luminosity linear electron-positron collider operating at three energy stages, with nominal centre-of-mass energies sqrt{s} = 380 GeV, 1.5 TeV, and 3 TeV. Its aim is to explore the energy frontier, providing sensitivity to physics beyond the Standard Model (BSM) and precision measurements of Standard Model processes with an emphasis on Higgs boson and top-quark physics. The opportunities for top-quark physics at CLIC are discussed in this paper. The initial stage of operation focuses on top-quark pair production measurements, as well as the search for rare flavour-changing neutral current (FCNC) top-quark decays. It also includes a top-quark pair production threshold scan around 350 GeV which provides a precise measurement of the top-quark mass in a well-defined theoretical framework. At the higher-energy stages, studies are made of top-quark pairs produced in association with other particles. A study of t̄tH production including the extraction of the top Yukawa coupling is presented as well as a study of vector boson fusion (VBF) production, which gives direct access to high-energy electroweak interactions. Operation above 1 TeV leads to more highly collimated jet environments where dedicated methods are used to analyse the jet constituents. These techniques enable studies of the top-quark pair production, and hence the sensitivity to BSM physics, to be extended to higher energies. This paper also includes phenomenological interpretations that may be performed using the results from the extensive top-quark physics programme at CLIC.

(g − 2)μ versus flavor changing neutral current induced by the light (B − L)μτ boson

We propose the local (B − L)μτ model, which minimally retains the local B − L extension for the sake of neutrino phenomenologies, and at the same time presents an invisible gauge boson Z′ with mass ∼ \U0001d4aa (10) MeV to account for the discrepancy of the muon anomalous magnetic moment. However such a scenario is challenged by flavor physics. To accommodate the correct pattern of Cabibbo-Kobayashi-Maskawa matrix, we have to introduce either a SU(2)L doublet flavon or vector-like quarks plus a singlet flavon. In either case Z′ induces flavor changing neutral current (FCNC) in the quark sector at tree-level. We find that the former scheme cannot naturally suppress the FCNC from the down-type quark sector and thus requires a large fine-tuning to avoid the stringent K → πν overline{v} bound. Whereas the latter scheme, in which FCNC merely arises in the up-type quark sector, is still free of strong constraint. In particular, it opens a new window to test our scenario by searching for flavor-changing top quark decay mode t → u/c+(invisible), and the typical branching ratio ∼ \U0001d4aa (10−4).

Top quark decay at next-to-leading order in the standard model effective field theory

We consider top quark decay in the Standard Model Effective Field Theory (SMEFT). We present a calculation of the total decay width and the $W$-boson helicity fractions at next-to-leading order (NLO) in SMEFT. Our result includes the complete set of contributing four-fermion operators in addition to QCD dipole operators and bottom-mass suppressed effects. We show that operators that first appear at NLO in the SMEFT can be bounded by the current data as well as future data from both a high-luminosity LHC and a potential $e^+e^-$ collider, demonstrating the importance of going beyond leading order when studying the SMEFT. We discuss technical aspects of our calculation that we believe will be useful in future higher-order studies of the SMEFT, in particular the treatment of $\gamma_5$ in loop diagrams.

Reports of my demise are greatly exaggerated: $N$-subjettiness taggers take on jet images

We compare the performance of a convolutional neural network (CNN) trained on jet images with dense neural networks (DNNs) trained on nn-subjettiness variables to study the distinguishing power of these two separate techniques applied to top quark decays. We find that they perform almost identically and are highly correlated once jet mass information is included, which suggests they are accessing the same underlying information which can be intuitively understood as being contained in 4-, 5-, 6-, and 8-body kinematic phase spaces depending on the sample. This suggests both of these methods are highly useful for heavy object tagging and provides a tentative answer to the question of what the image network is actually learning.

Search for a Light Charged Higgs Boson Decaying to a W Boson and a CP-Odd Higgs Boson in Final States with eμμ or μμμ in Proton-Proton Collisions at sqrt[s]=13 TeV.

A search for a light charged Higgs boson (H^{+}) decaying to a W boson and a CP-odd Higgs boson (A) in final states with eμμ or μμμ is performed using data from pp collisions at sqrt[s]=13 TeV, recorded by the CMS detector at the LHC and corresponding to an integrated luminosity of 35.9 fb^{-1}. In this search, it is assumed that the H^{+} boson is produced in decays of top quarks, and the A boson decays to two oppositely charged muons. The presence of signals for H^{+} boson masses between 100 and 160GeV and A boson masses between 15 and 75GeV is investigated. No evidence for the production of the H^{+} boson is found. Upper limits at 95%confidence level are obtained on the combined branching fraction for the decay chain, t→bH^{+}→bW^{+}A→bW^{+}μ^{+}μ^{-}, of 1.9×10^{-6} to 8.6×10^{-6}, depending on the masses of the H^{+} and A bosons. These are the first limits for these decay modes of the H^{+} and A bosons.

Enhancing CP measurement of the Yukawa interactions of top-quark at e−e+ collider

It is well known that CP-violating coupling of the Yukawa interaction of top-quark is a promising candidate of a new source of CP violation effect. Precisely measurement of its CP properties is crucial for understanding new physics above the electroweak scale. In this paper, we introduce a complete analysis method for probing CP violation effects in the associated production of top-quark pair and Higgs boson at e−e+ collider. Reconstructions of the top-quarks are not needed in our strategy. The observables are defined based on spin correlation effects of the leptons emerging from decays of top-quarks, and their formal expressions are universal at any reference frames. Two reference frames, the rest frame of the top-quark pair and the rest frame of Higgs boson, are examined. Importantly, large enhancement effects for both CP-odd and CP-even observables are observed in the rest frame of Higgs boson. This can be essential for probing CP violation effects at future e−e+ collider.

Quark decay top to two bodies

Is investigated the top quark decay (t) two bodies using the spectator model. Is analyzed the decay (t→bW⁺) calculated theoretically the width of decay and decay fraction of the particle whose result is compared with the experimental data of the table Particle Data group. The model prediction on the decay process can be considered good as a first approach to this class of decay. This type of analysis is particularly important because it allows one hand, familiarize students with the decays of particles and on the other, open a space to introduce topics of particle physics in the introductory courses at university level.

Search for top-quark decays t \u2192 Hq with 36 fb\u22121 of pp collision data at sqrt{s} = 13 TeV with the ATLAS detector

A search for flavour-changing neutral current decays of a top quark into an up-type quark (q = u, c) and the Standard Model Higgs boson, t → Hq, is presented. The search is based on a dataset of pp collisions at sqrt{s} = 13 TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider and corresponding to an integrated luminosity of 36.1 fb−1. Two complementary analyses are performed to search for top-quark pair events in which one top quark decays into Wb and the other top quark decays into Hq, and target the Hto boverline{b} and H → τ+τ− decay modes, respectively. The high multiplicity of b-quark jets, or the presence of hadronically decaying τ-leptons, is exploited in the two analyses respectively. Multivariate techniques are used to separate the signal from the background, which is dominated by top-quark pair production. No significant excess of events above the background expectation is found, and 95% CL upper limits on the t → Hq branching ratios are derived. The combination of these searches with ATLAS searches in diphoton and multilepton final states yields observed (expected) 95% CL upper limits on the t → Hc and t → Hu branching ratios of 1.1 × 10−3 (8.3 × 10−4) and 1.2 × 10−3 (8.3 × 10−4), respectively. The corresponding combined observed (expected) upper limits on the |λtcH| and |λtuH| couplings are 0.064 (0.055) and 0.066 (0.055), respectively.

Search for a heavy resonance decaying to a top quark and a vector-like top quark in the lepton + jets final state in pp collisions at sqrt{s} = 13,text {TeV}

A search is presented for a heavy spin-1 resonance mathrm {Z}' decaying to a top quark and a vector-like top quark partner text {T} in the lepton + jets final state. The search is performed using a data set of mathrm {p}mathrm {p} collisions at a centre-of-mass energy of 13,text {TeV} corresponding to an integrated luminosity of 35.9{,text {fb}^{-1}} as recorded by the CMS experiment at the CERN LHC in the year 2016. The analysis is optimised for final states arising from the text {T} decay modes to a top quark and a Higgs or mathrm {Z} boson (text {T} rightarrow mathrm {H} mathrm {t}, mathrm {Z} mathrm {t}). The event selection makes use of resolved and merged top quark decay products, as well as decays of boosted Higgs bosons and mathrm {Z} and mathrm {W} bosons using jet substructure techniques. No significant deviation from the standard model background expectation is observed. Exclusion limits on the product of the cross section and branching fraction for mathrm {Z}'rightarrow mathrm {t}text {T}, text {T} rightarrow mathrm {H} mathrm {t}, mathrm {Z} mathrm {t}, mathrm {W}mathrm {b} are presented for various combinations of the mathrm {Z}' resonance mass and the vector-like text {T} quark mass. These results represent the most stringent limits to date for the decay mode mathrm {Z}'rightarrow mathrm {t}text {T} rightarrow mathrm {t}mathrm {H} mathrm {t}. In a benchmark model with extra dimensions, invoking a heavy spin-1 resonance mathrm {G}^* , masses of the mathrm {G}^* between 1.5 and 2.3,text {TeV} and between 2.0 and 2.4,text {TeV} are excluded for text {T} masses of 1.2 and 1.5,text {TeV}, respectively.

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}.

Unveiling the yoctosecond structure of the QGP with top quarks

Top quarks have recently been measured for the first time in nuclear collisions. With most of the integrated luminosity of the LHC PbPb programme still to be recorded and promising projections for the future HL-LHC, HE-LHC or FCC, top quark observables will be measured with good precision and become an excellent probe of the QGP. We argue here that the unique properties of the top quark provide a new way to study differentially the space-time evolution of the medium created in heavy ion collisions. Top quarks decay almost exclusively into a W boson and a b quark. The finite lifetimes of the top and W particles and the time-delay in the interaction of the (colour-singlet) W-boson's decay products with the medium add up to a total time during which the top-decay system is unaffected by the QGP. The three times are correlated with the kinematics of the top quark, allowing the approximate determination of the time at which the interaction with the QGP begins. We carry out a simple Monte Carlo feasibility study and find that the LHC has the potential to bring first, limited information on the time structure of the QGP. More extensive studies will require larger luminosities (e.g. with ions lighter than lead), and/or the higher energies of a future HE-LHC or FCC.

MSSM scenarios with a light CP-odd Higgs boson

The possibility of the presence of a light pseudoscalar with the mass of 28 GeV in the MSSM Higgs sector extended by dimension-six operators is analyzed. The perturbative unitarity and vacuum stability constraints are discussed. Tree-level production cross sections of a light pseudoscalar in the process gg → µµbb−, and cross sections of a light pseudoscalar production in the topquark decay are evaluated for suitable benchmark points.

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.