Top Quark Decay Products Research Articles

Measurements of jet observables sensitive to b -quark fragmentation in tt¯ events at the LHC with the ATLAS detector

Several observables sensitive to the fragmentation of $b$-quarks into $b$-hadrons are measured using 36 fb${}^{-1}$ of $\sqrt s = 13$ TeV proton-proton collision data collected with the ATLAS detector at the LHC. Jets containing $b$-hadrons are obtained from a sample of dileptonic $t\bar{t}$ events, and the associated set of charged-particle tracks is separated into those from the primary $pp$ interaction vertex and those from the displaced $b$-decay secondary vertex. This division is used to construct observables that characterize the longitudinal and transverse momentum distributions of the $b$-hadron within the jet. The measurements have been corrected for detector effects and provide a test of heavy-quark-fragmentation modeling at the LHC in a system where the top-quark decay products are color-connected to the proton beam remnants. The unfolded distributions are compared with the predictions of several modern Monte Carlo parton-shower generators and generator tunes, and a wide range of agreement with the data is observed, with $p$-values varying from $5 \times 10^{-4}$ to 0.98. These measurements complement similar measurements from $e^+e^-$ collider experiments in which the $b$-quarks originate from a color-singlet $Z/\gamma^*$.

Measurement of differential tt¯ production cross sections in the full kinematic range using lepton+jets events from proton-proton collisions at s=13 TeV

Measurements of differential and double-differential cross sections of top quark pair ($\text{t}\overline{\text{t}}$) production are presented in the lepton+jets channels with a single electron or muon and jets in the final state. The analysis combines for the first time signatures of top quarks with low transverse momentum $p_\text{T}$, where the top quark decay products can be identified as separated jets and isolated leptons, and with high $p_\text{T}$, where the decay products are collimated and overlap. The measurements are based on proton-proton collision data at $\sqrt{s} = $ 13 TeV collected by the CMS experiment at the LHC, corresponding to an integrated luminosity of 137 fb$^{-1}$. The cross sections are presented at the parton and particle levels, where the latter minimizes extrapolations based on theoretical assumptions. Most of the measured differential cross sections are well described by standard model predictions with the exception of some double-differential distributions. The inclusive $\text{t}\overline{\text{t}}$ production cross section is measured to be $\sigma_{\text{t}\overline{\text{t}}} = $ 791 $\pm$ 25 pb, which constitutes the most precise measurement in the lepton+jets channel to date.

Evidence for top quark production in nucleus-nucleus collisions

Droplets of quark-gluon plasma (QGP), an exotic state of strongly interacting quantum chromodynamics (QCD) matter, are routinely produced in heavy nuclei high-energy collisions. Although the experimental signatures marked a paradigm shift away from expectations of a weakly coupled QGP, a challenge remains as to how the locally deconfined state with a lifetime of a few fm can be resolved. The only colored particle that decays mostly within the QGP is the top quark. Here we demonstrate, for the first time, that top quark decay products are identified, irrespective of whether interacting with the medium (bottom quarks) or not (leptonically decaying W bosons). Using 1.7±0.1 nb−1 of lead-lead (A = 208) collision data recorded by the CMS experiment at a nucleon-nucleon center-of-mass energy of 5.02 TeV, we report evidence of top quark pair (tt¯) production. Dilepton final states are selected, and the cross section (σtt¯) is measured from a likelihood fit to a multivariate discriminator using lepton kinematic variables. The σtt¯ measurement is additionally performed considering the jets originating from the hadronization of bottom quarks, which improve the sensitivity to the tt¯ signal process. After background subtraction and analysis corrections, the measured σtt¯ is 2.56 ± 0.82(tot) and 2.02 ± 0.69(tot)μb in the two cases, respectively, consistent with predictions from perturbative QCD.

What Is the Top Quark Mass?

This review provides an overview of the conceptual issues regarding the interpretation of so-called direct top quark mass measurements, which are based on the kinematic reconstruction of top quark decay products at the Large Hadron Collider (LHC). These measurements quote the top mass parameter [Formula: see text] of Monte Carlo event generators with current uncertainties of around 0.5 GeV. The problem of finding a rigorous relation between [Formula: see text] and top mass renormalization schemes defined in field theory is unresolved to date and touches perturbative as well as nonperturbative aspects and the limitations of state-of-the-art Monte Carlo event generators. I review the status of LHC top mass measurements, illustrate how conceptual limitations enter the picture, and explain a controversy that has permeated the community in the context of the interpretation problem related to [Formula: see text]. I then summarize recent advances in acquiring first principles insights and outline what else has to be understood to fully resolve the issue. I conclude with recommendations on how to deal with the interpretation problem for the time being when making top mass–dependent theoretical predictions.

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}.

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.

Measurements of top quark properties in top pair production and decay at the LHC using the CMS detector

Measurements are presented of the properties of top quarks in pair production and decay from proton-proton collisions at the LHC. The data were collected at centre-of-mass energies of 7 and 8 TeV by the CMS experiment during the years 2011 and 2012. The top quark-antiquark charge asymmetry is measured using the difference of the absolute rapidities of the reconstructed top and anti-top kinematics, as well as from distributions of the top quark decay products. The measurements are performed in the decay channels of the tt‾ pair into both one and two leptons in the final state. The polarization of top quarks and top pair spin correlations are measured from the angular distributions of top quark decay products. The W-boson helicity fractions and angular asymmetries are extracted and limits on anomalous contributions to the Wtb vertex are determined. The flavor content in top-quark pair events is measured using the fraction of top quarks decaying into a W-boson and a b-quark relative to all top quark decays, R=B(t→Wb)/B(t→Wq), and the result is used to determine the CKM matrix element Vtb as well as the width of the top quark resonance. All of the results are found to be in good agreement with standard model predictions.

Single top quark production in heavy ion collisions at energies available at the CERN Large Hadron Collider

The paper presents analysis of the single top quark production in PbPb collisions at the Large Hadron Collider at center-of-mass energy 5.5 TeV per nucleon pair. The analysis is performed with CompHEP and PYQUEN event generators. The neutron and proton content in the nuclei is taken into account. NLO precision has been implemented to simulate kinematic properties and rate of single top production. The modification of different characteristics of single top quark decay products due to interactions of jet partons in quark-gluon medium, and the specific charge asymmetry of top/anti-top quark yields due to the isospin effect are evaluated.

Precise heavy-quark masses

We report on determinations of the running masses for charm- and bottom-quarks from deep-inelastic scattering reactions and for the top-quark from hadro-production of top-quark pairs. The running masses in the MS¯ scheme can be extracted with good precision at next-to-next-to-leading order in QCD. In the global fits to data the full correlations of the extracted mass parameters with the parton distributions in the proton and with the strong coupling constant αs are kept. For charm- and bottom-quarks the method provides complementary information on these fundamental parameters from hadronic processes with space-like kinematics. The measured top-quark mass is confronted with the Monte Carlo top-quark mass parameter determined from a comparison to events with top-quark decay products. The Monte Carlo mass is not identical with the pole mass. Its translation to the pole mass scheme introduces an additional uncertainty of the order of 1 GeV.

QCD NLO and EW NLO corrections to [formula omitted] production with top quark decays at hadron collider

Higgs boson production associated with a top quark pair is an important process in studying the nature of the newly discovered Higgs boson at the LHC. In this letter, we report on our calculations including the next-to-leading order (NLO) QCD and NLO electroweak corrections to the pp→tt¯H process in the standard model. We present the integrated cross sections at the 14 TeV LHC and even at the future proton–proton colliders with s=33 and 100TeV. Our calculation includes the top quark subsequent decays by adopting the narrow width approximation. The kinematic distributions of Higgs boson and top quark decay products at the LHC are provided. We find that the O(αs2αew2) corrections are quantitatively comparable with the O(αs3αew) corrections in some kinematic region.

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.

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.

Top quark mass measurements at and above threshold at CLIC

We present a study of the expected precision of the top quark mass determination, measured at a linear e+ e- collider based on CLIC technology. GEANT4-based detector simulation and full event reconstruction including realistic physics and beam-induced background levels are used. Two different techniques to measure the top mass are studied: The direct reconstruction of the invariant mass of the top quark decay products and the measurement of the mass together with the strong coupling constant in a threshold scan, in both cases including first studies of expected systematic uncertainties. For the direct reconstruction, experimental uncertainties around 100 MeV are achieved, which are at present not matched by a theoretical understanding on a similar level. With a threshold scan, total uncertainties of around 100 MeV are achieved, including theoretical uncertainties in a well-defined top mass scheme. For the threshold scan, the precision at ILC is also studied to provide a comparison of the two linear collider technologies.

Top quark angular distributions at the LHC

Results from the CMS and ATLAS Collaborations are presented on angular distributions in top quark decays. The studies presented here are based on proton-proton collisions at a center of mass energy of 7 TeV collected by the two experiments at the LHC in 2011. The angular distributions of the top quark decay products are used to probe several fundamental properties of the top quark and its couplings. The polarization of produced top quarks is investigated as well as the correlation of the spin of pair-produced top and anti-top quarks. The polarization states of the W-boson originating from the decay of the top quark are determined and used to constrain possible contributions of new physics processes to the W tb vertex.

On the difference between the pole and the [formula omitted] masses of the top quark at the electroweak scale

We argue that for a Higgs boson mass MH∼125 GeV, as suggested by recent Higgs searches at the LHC, the inclusion of electroweak radiative corrections in the relationship between the pole and MS¯ masses of the top quark reduces the difference to about 1 GeV. This is relevant for the scheme dependence of electroweak observables, such as the ρ parameter, as well as for the extraction of the top quark mass from experimental data. In fact, the value currently extracted by reconstructing the invariant mass of the top quark decay products is expected to be close to the pole mass, while the analysis of the total cross section of top quark pair production yields a clean determination of the MS¯ mass.

Searching for charged Higgs boson in polarized top quark

The charged Higgs boson is quite common in many new physics models. In this study we examine the potential of observing a heavy charged Higgs boson in its decay mode of top-quark and bottom-quark in the Type-II Two-Higgs-Doublet-Model. In this model, the chirality structure of the coupling of charged Higgs boson to the top- and bottom-quark is very sensitive to the value of $\tan\beta$. As the polarization of the top-quark can be measured experimentally from the top-quark decay products, one could make use of the top-quark polarization to determine the value of $\tan\beta$. We preform a detailed analysis of measuring top-quark polarization in the production channels $gb\to tH^-$ and $g\bar{b}\to \bar{t}H^+$. We calculate the helicity amplitudes of the charged Higgs boson production and decay.Our calculation shows that the top-quark from the charged Higgs boson decay provides a good probe for measuring $\tan\beta$, especially for the intermediate $\tan\beta$ region. On the contrary, the top-quark produced in association with the charged Higgs boson cannot be used to measure $\tan\beta$ because its polarization is highly contaminated by the $t$-channel kinematics.

Boosted Top Quarks, Top Pair Resonances, and Top Partner Searches at the LHC

The increased center-of-mass energy of proton-proton collisions at the LHC allows for higher and higher mass scales to be probed in the search for new physics. These searches for new physics can often involve the reconstruction of top quark decays. In this high-mass regime, event topologies can change and top quark decay products become highly boosted, requiring the use of special identification algorithms. This report summarizes the latest techniques used in the reconstruction of boosted top quarks, and highlights several new physics searches from ATLAS and CMS where they are utilized. Such analyses include the search for top quark pair resonances as is expected from Z’ decays, as well as top quark partner searches, such as fourth generation vector-like quarks, excited top quarks, and top quark partners with non-standard charges

Top mass and properties at the LHC and Tevatron

An overview of the properties of the top quark measured with the ATLAS and CMS detectors at the LHC and with the CDF and D0 detectors at the Tevatron is presented. The latest and most precise measurements are shown, giving special emphasis to the top quark mass determinations. Measurements of the b -jet content in the top quark decay products and the W polarisation, as well as a search for flavour changing neutral current processes in top quark decays are also reviewed.

Top quark pair production in association with a jet: QCD corrections and jet radiation in top quark decays

We consider top quark pair production in association with a hard jet through next-to-leading order in perturbative QCD. Top quark decays are treated in the narrow width approximation and spin correlations are retained throughout the computation. We include hard jet radiation by top quark decay products and explore their importance for basic kinematic distributions at the Tevatron and the LHC. Our results suggest that QCD corrections and jet radiation in decays can lead to significant changes in shapes of basic distributions and, therefore, need to be included for the description of ttbar+jet production. We compare the shape of the transverse momentum distribution of a top quark pair recently measured by the D0 collaboration with the result of our computation and find reasonable agreement.

Azimuthal correlations in top pair decays and the effects of new heavy scalars

We present the azimuthal correlations in the decay products of the top quark pair created in the process $gg\ensuremath{\rightarrow}t\overline{t}\ensuremath{\rightarrow}b\overline{b}{W}^{+}{W}^{\ensuremath{-}}$ calculated in leading-order QCD. We then discuss deviations in this distribution that could arise from new physics contributions, focusing, in particular, on the case of a new heavy scalar coupling to the top quark and produced via the gluon fusion mechanism. Within this scenario, we consider two cases: one in which the top quark is the only particle with a significant coupling to the scalar and another in which there are additional heavy fermions that modify the effective coupling of the scalar to gluons. We discuss the effects on the overall cross section and azimuthal correlations of the top quark decay products, with particular emphasis on the effect of interference between the QCD and new physics contributions and on the existence of $CP$-nonconserving couplings.