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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2011

Top quark mass measurement in the dilepton channel during the D0 experiment at the Tevatron. Mesure de la masse du quark top dans les canaux di-leptoniques auprès de l’expérience D0 au Tevatron

The top quark is the heaviest standard model quark. Discovered in 1995 by the two Tevatron experiments it has atypical properties. In particular its time life is so short that it decays before hadronizing, so the top quark mass could be measured with a high precision. Data collected by the DØ experiment between 2002 and 2009, which represent an integrated luminosity of 5.4 fb⁻¹, are used to measure the top quark mass by using the matrix element method in the three dilepton channels: dielectron, electron--muon and dimuon. The measured mass, 174.0 ± 1.8 (stat.) ± 2.4 (syst.) GeV, is in a good agreement with other measurements and limited by the systematic uncertainties for the first time in these channels. In this thesis different approaches have been studied to improve the accuracy of this measurement: the use of b-quark jet identification in order to optimize the selection of top--anti-top events and a better determination of the main systematic uncertainties. A special attention has been paid to the Monte-Carlo simulation of muons in D0: the improved smearing procedure for the simulated muons, discussed in this thesis, will be used to increase the accuracy of the top properties measurements as well as the precision of many other D0 measurements.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Measurement of the inclusive cross-sections of single top-quark and top-antiquark t-channel production in pp collisions at $ \sqrt{s}=13 $ TeV with the ATLAS detector

A measurement of the t-channel single-top-quark and single-top-antiquark production cross-sections in the lepton+jets channel is presented, using 3.2 fb –1 of proton-proton collision data at a centre-of-mass energy of 13 TeV, recorded with the ATLAS detector at the LHC in 2015. Events are selected by requiring one charged lepton (electron or muon), missing transverse momentum, and two jets with high transverse momentum, exactly one of which is required to be b-tagged. Using a binned maximum-likelihood fit to the discriminant distribution of a neural network, the cross-sections are determined to be σ(tq) = 156 ± 5 (stat.) ± 27 (syst.) ± 3 (lumi.) pb for single top-quark production and σ(t¯q)=91±4 (stat.) ± 18 (syst.) ± 2 (lumi.) pb for single top-antiquark production, assuming a top-quark mass of 172.5 GeV. The cross-section ratio is measured to be R t = σ(tq)/σ(t¯q)=1.72±0.09 (stat.) ± 0.18 (syst.). All results are in agreement with Standard Model predictions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Top Quark Forward-Backward Asymmetry and Same-Sign Top Quark Pairs

The top quark forward-backward asymmetry measured at the Tevatron collider shows a large deviation from standard model expectations. Among possible interpretations, a nonuniversal Z{sup '} model is of particular interest as it naturally predicts a top quark in the forward region of large rapidity. To reproduce the size of the asymmetry, the couplings of the Z{sup '} to standard model quarks must be large, inevitably leading to copious production of same-sign top quark pairs at the energies of the Large Hadron Collider (LHC). We explore the discovery potential for tt and ttj production in early LHC experiments at 7-8 TeV and conclude that if no tt signal is observed with 1 fb{sup -1} of integrated luminosity, then a nonuniversal Z{sup '} alone cannot explain the Tevatron forward-backward asymmetry.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

Search for dark matter produced in association with a single top quark or a top quark pair in proton-proton collisions at $ \sqrt{s}=13 $ TeV

A search for dark matter produced in association with top quarks in proton-proton collisions at a center-of-mass energy of 13 TeV is presented. The data set used corresponds to an integrated luminosity of 35.9 fb$^{-1}$ recorded with the CMS detector at the LHC. Whereas previous searches for neutral scalar or pseudoscalar mediators considered dark matter production in association with a top quark pair only, this analysis also includes production modes with a single top quark. The results are derived from the combination of multiple selection categories that are defined to target either the single top quark or the top quark pair signature. No significant deviations with respect to the standard model predictions are observed. The results are interpreted in the context of a simplified model in which a scalar or pseudoscalar mediator particle couples to a top quark and subsequently decays into dark matter particles. Scalar and pseudoscalar mediator particles with masses below 290 and 300 GeV, respectively, are excluded at 95% confidence level, assuming a dark matter particle mass of 1 GeV and mediator couplings to fermions and dark matter particles equal to unity.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Measurement of the top quark mass using single top quark events in proton-proton collisions at $\sqrt{s}= 8$ TeV

A measurement of the top quark mass is reported in events containing a single top quark produced via the electroweak t channel. The analysis is performed using data from proton-proton collisions collected with the CMS detector at the LHC at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.7 fb$^{-1}$ . Top quark candidates are reconstructed from their decay to a $\mathrm {W}$ boson and a b quark, with the $\mathrm {W}$ boson decaying leptonically to a muon and a neutrino. The final state signature and kinematic properties of single top quark events in the t channel are used to enhance the purity of the sample, suppressing the contribution from top quark pair production. A fit to the invariant mass distribution of reconstructed top quark candidates yields a value of the top quark mass of $172.95 \pm 0.77\,\text {(stat)} ^{+0.97-0.93}\,\text {(syst)} \,\text {GeV} $ . This result is in agreement with the current world average, and represents the first measurement of the top quark mass in event topologies not dominated by top quark pair production, therefore contributing to future averages with partially uncorrelated systematic uncertainties and a largely uncorrelated statistical uncertainty.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Top quark pair production and top quark properties at CDF

We present the most recent measurements of top quark pairs production and top quark properties in proton-antiproton collisions with center-of-mass energy of 1.96 TeV using CDF II detector at the Tevatron. The combination of top pair production cross section measurements and the direct measurement of top quark width are reported. The test of Standard Model predictions for top quark decaying into $b$-quarks, performed by measuring the ratio $R$ between the top quark branching fraction to $b$-quark and the branching fraction to any type of down quark is shown. The extraction of the CKM matrix element $|V_{tb}|$ from the ratio $R$ is discussed. We also present the latest measurements on the forward-backward asymmetry ($A_{FB}$) in top anti-top quark production. With the full CDF Run II data set, the measurements are performed in top anti-top decaying to final states that contain one or two charged leptons (electrons or muons). In addition, we combine the results of the leptonic forward-backward asymmetry in $t\bar t$ system between the two final states. All the results show deviations from the next-to-leading order (NLO) standard model (SM) calculation.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Search for pair production of up-type vector-like quarks and for four-top-quark events in final states with multiple b-jets with the ATLAS detector

A search for pair production of up-type vector-like quarks ( T ) with a significant branching ratio into a top quark and either a Standard Model Higgs boson or a Z boson is presented. The same analysis is also used to search for four-top-quark production in several new physics scenarios. 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 corresponds to an integrated luminosity of 36.1 fb -1 . Data are analysed in the lepton+jets final state, characterised by an isolated electron or muon with high transverse momentum, large missing transverse momentum and multiple jets, as well as the jets + E T miss final state, characterised by multiple jets and large missing transverse momentum. The search exploits the high multiplicity of jets identified as originating from b -quarks, and the presence of boosted, hadronically decaying top quarks and Higgs bosons reconstructed as large-radius jets, characteristic of signal events. No significant excess above the Standard Model expectation is observed, and 95% CL upper limits are set on the production cross sections for the different signal processes considered. These cross-section limits are used to derive lower limits on the mass of a vector-like T quark under several branching ratio hypotheses assuming contributions from T → W b , Zt , Ht decays. The 95% CL observed lower limits on the T quark mass range between 0 . 99 TeV and 1 . 43 TeV for all possible values of the branching ratios into the three decay modes considered, significantly extending the reach beyond that of previous searches. Additionally, upper limits on anomalous four-top-quark production are set in the context of an effective field theory model, as well as in an universal extra dimensions model.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Dark decay of the top quark

We suggest top quark decays as a venue to search for light dark force carriers. 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 σ 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. In addition, we discuss the implications for the Large Hadron Collider experiments including the analysis based on the lepton-jets.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Measurement of the top quark mass in dileptonic top quark pair decays with √s = 7 TeV ATLAS data

The top quark mass in dileptonic top quark pair decays was measured using 4.7 fb{sup –1} of √s = 7 TeV proton-proton (pp) collision data recorded by the ATLAS experiment at the LHC in 2011. The event topology is characterized by the presence of two charged leptons, at least two neutrinos and several jets, two of which originate from bottom quarks. Using the template method and the m{sub ℓb} observable, defined as the average invariant mass of the two charged lepton plus b-jet pairs in each event, the top quark mass is measured to be 173.09 ± 0.64(stat) ± 1.50(syst) GeV. This proceeding is based on a preliminary result, which has been superseded meanwhile.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Top Quark Amplitudes with an Anomolous Magnetic Moment

The anomalous magnetic moment of the top quark may be measured during the first run of the LHC at 7 TeV. For these measurements, it will be useful to have available tree amplitudes with t{bar t} and arbitrarily many photons and gluons, including both QED and color anomalous magnetic moments. In this paper, we present a method for computing these amplitudes using the Britto-Cachazo-Feng-Witten recursion formula. Because we deal with an effective theory with higher-dimension couplings, there are roadblocks to a direct computation with the Britto-Cachazo-Feng-Witten method. We evade these by using an auxiliary scalar theory to compute a subset of the amplitudes.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Tevatron Top-Quark Combinations and World Top-Quark Mass Combination

Almost 20 years after its discovery, the top quark is still an interesting particle, undergoing precise investigation of its properties. For many years, the Tevatron proton antiproton collider at Fermilab was the only place to study top quarks in detail, while with the recent start of the LHC proton proton collider a top quark factory has opened. An important ingredient for the full understanding of the top quark is the combination of measurements from the individual experiments. In particular, the Tevaton combinations of single top-quark cross sections, the ttbar production cross section, the W helicity in top-quark decays as well as the Tevatron and the world combination of the top-quark mass are discussed.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2016

Determinacion de la Masa Del Quark Top Usando Variables Leptonicas en el Experimento CMS del LHC

A novel technique for measuring the top quark mass using only leptonic observables is discussed. Top quark pair events with one electron and one muon and at least one jet in the final state are selected in proton-proton collision data collected by the CMS experiment at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.7 fb$^{-1}$. Several observables are studied and the transverse momentum distribution of the charged lepton pair originating from the decay of the top quark pair is chosen to extract the top quark mass. After the calibration with simulated events a value of mt=171.7 1.1 (stat.)0.5 (exp.)-3.1+2.5 (th.)+0.8 (p(t)) GeV is measured, where the dominant systematic uncertainties stem from signal modeling.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2022

Measurements of differential cross-sections in top-quark pair events with a high transverse momentum top quark and limits on beyond the Standard Model contributions to top-quark pair production with the ATLAS detector at $ \sqrt{s} $ = 13 TeV

Cross-section measurements of top-quark pair production where the hadronically decaying top quark has transverse momentum greater than 355 GeV and the other top quark decays into ℓνb are presented using 139 fb –1 of data collected by the ATLAS experiment during proton-proton collisions at the LHC. The fiducial cross-section at √s = 13 TeV is measured to be σ = 1.267 ± 0.005 ± 0.053 pb, where the uncertainties reflect the limited number of data events and the systematic uncertainties, giving a total uncertainty of 4.2%. The cross-section is measured differentially as a function of variables characterizing the tt¯ system and additional radiation in the events. The results are compared with various Monte Carlo generators, including comparisons where the generators are reweighted to match a parton-level calculation at next-to-next-to-leading order. The reweighting improves the agreement between data and theory. The measured distribution of the top-quark transverse momentum is used to search for new physics in the context of the effective field theory framework. No significant deviation from the Standard Model is observed and limits are set on the Wilson coefficients of the dimension-six operators O tG and $O$ $^{(8)}_{tq}$, where the limits on the latter are the most stringent to date.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗