Precise measurement of the top quark mass in the dilepton channel at D0

We measure the top quark mass (m(t)) in p ?p collisions at a center of mass energy √s = 1.96 TeV using dilepton t ?t→W(+)bW(-) ?b→?(+)ν(?)b?(-) ?ν(?) ?b events, where ? denotes an electron, a muon, or a tau that decays leptonically. The data correspond to an integrated luminosity of 5.4 fb(-1) collected with the D0 detector at the Fermilab Tevatron Collider. We obtain m(t)=174.0±1.8(stat)±2.4(syst) GeV, which is in agreement with the current world average m(t)=173.3±1.1 GeV. This is currently the most precise measurement of m(t) in the dilepton channel.     

Top-quark mass measurement from dilepton events at CDF II

We report a measurement of the top-quark mass using events collected by the CDF II detector from pp collisions at square root of s = 1.96 TeV at the Fermilab Tevatron. We calculate a likelihood function for the top-quark mass in events that are consistent with tt --> bl(-)nu(l)bl'+ nu'(l) decays. The likelihood is formed as the convolution of the leading-order matrix element and detector resolution functions. The joint likelihood is the product of likelihoods for each of 33 events collected in 340 pb(-1) of integrated luminosity, yielding a top-quark mass M(t) = 165.2 +/- 6.1(stat) +/- 3.4(syst) GeV/c2. This first application of a matrix-element technique to tt --> bl+ nu(l)bl'- nu(l') decays gives the most precise single measurement of M(t) in dilepton events. Combined with other CDF run II measurements using dilepton events, we measure M(t) = 167.9 +/- 5.2(stat) +/- 3.7(syst) GeV/c2.     

Cross-section-constrained top-quark mass measurement from dilepton events at the Tevatron

We report the first top-quark mass measurement that uses a cross-section constraint to improve the mass determination. This measurement is made with a dilepton tt event candidate sample collected with the Collider Detector II at Fermilab. From a data sample corresponding to an integrated luminosity of 1.2 fb(-1), we measure a top-quark mass of 170.7(-3.9)(+4.2)(stat)+/-2.6(syst)+/-2.4(theory) GeV/c(2). The measurement without the cross-section constraint is 169.7(-4.9)(+5.2)(stat)+/-3.1(syst) GeV/c(2).     

Model-independent measurement of the top quark polarisation

We introduce a new asymmetry in the decay t→Wb→?νb$t\to Wb\to ?\nu b$, which is shown to be directly proportional to the polarisation of the top quark along a chosen axis, times a sum of W helicity fractions. The latter have already been precisely measured at the Tevatron and the Large Hadron Collider. Therefore, this new asymmetry can be used to obtain a model-independent measurement of the polarisation of top quarks produced in any process at hadron or lepton colliders.     

Probing vector-like top partner from same-sign dilepton events at the LHC

The standard model is a successful theory but is lacking a mechanism for neutrino mass generation as well as a solution to the naturalness problem. In the models that are proposed to simultaneously solve the two problems, heavy Majorana neutrinos and top partners are usually predicted to lead to a new decay mode of the top partner mediated by the heavy Majorana neutrinos: $T\to b\,{W}^{+}\to b\,{{\ell }}^{+}{{\ell }}^{+}q\bar{q^{\prime} }$. In this paper, we will study the observability of such a new signature via the pair production process of a top partner ${pp}\to T\bar{T}\to 2b+{{\ell }}^{\pm }{{\ell }}^{\pm }+4j$ in a model independent way. By performing Monte Carlo simulations, we present the 2σ exclusion limits of the top partner mass and mixing parameters at the HL-LHC.     

How to extract heavy quark and heavy lepton signals in neutrino-induced dilepton events

Several key tests for neutrino-produced dilepton events are presented which allow one to identify rather clearly new signals arising from heavy quarks or heavy leptons in the presence of a substantial charm background.     

Some thoughts on the mass of the top quark

We propose a new mechanism for up-down symmetry breaking within the context of a technicolor scenario. The experimentally determined ratio $\text{M}{}_{\mathrm{<mtext>W</mtext>}}\text{M}{}_{\mathrm{<mtext>Z</mtext>}}\text{cos}\text{θ}{}_{\mathrm{<mtext>W</mtext>}}\text{? 1}$ is in addition preserved at the technicolor scale. If we assume that the mechanism works at the level of the heaviest generation we find $\text{m}{}_{\mathrm{<mtext>t</mtext>}}\text{= (?}{}^{\mathrm{<mtext>?</mtext><mtext>1</mtext><mtext>2</mtext>}}\text{)38}\text{GeV}$. The parameter ? depends on strong technicolor dynamics and can in principle be determined. Via a crude estimate we find ? to be a number of order 1.     

On measurement of helicity parameters in top quark decay

To enable an evaluation of future measurements of the helicity parameters for decay in regard to violation, this paper considers the effects of an additional pure-imaginary coupling, or , associated with a specific, single additional Lorentz structure, . Sizable violation signatures can occur for low-effective mass scales (), but in most cases can be more simply excluded by precision measurement of the probabilities and . Signatures for excluding the presence of violation associated with the two dynamical phase-type ambiguities are investigated. Received: 10 July 2000 / Published online: 8 September 2000     

Unified spin gauge model and the top quark mass

Spin gauge models use a real Clifford algebraic structure R_p,q associated with a real manifold of dimension p + q to describe the fundamental interactions of elementary particles. This review provides a comparison between those models and the standard model, indicating their similarities and differences. By contrast with the standard model, the spin gauge model based on R_3,8 generates intermediate boson mass terms without the need to use the Higgs-Kibble mechanism and produces a precise prediction for the mass of the top quark. The potential of this model to account for exactly three families of fermions is considered.     

Implications of the top quark mass measurement for the CKM parameters,x s and CP asymmetries

Motivated by the recent determination of the top quark mass by the CDF collaboration,m _t =174±10 _?12 ⁺¹³ GeV, we review and update the constraints on the parameters of the quark flavour mixing matrixV _CKM in the standard model. In performing our fits, we use inputs from the measurements of the following quantities: (i) |∈|, the CP-violating parameter inK decays, (ii) ΔM _d , the mass difference due to the $B_d^0 - \overline {B_d^0 }$ mixing, (iii) the matrix elements |V _cb | and |V _ub |, and (iv)B-hadron lifetimes. We find that the allowed region of the unitarity triangle is very large, mostly due to theoretical uncertainties. (This emphasizes the importance of measurements of CP-violating rate asymmetries in theB system.) Nevertheless, the present data do some-what restrict the allowed values of the coupling constant product $f_{B_d } \sqrt {\hat B_{B_d } }$ and the renormalization-scale-invariant bag constant $\hat B_K$ . With the updated CKM matrix we present the currently-allowed range of the ratio |V _td /V _ts |, as well as the standard model predictions for the $B_s^0 - \overline {B_s^0 }$ mixing parameterx _s and the quantities sin 2α, sin 2β and sin² γ, which characterize the CP-asymmetries inB-decays. The ALEPH collaboration has recently reported a significant improvement on the lower limit on the $B_s^0 - \overline {B_s^0 }$ mass difference, ΔM _s /Δx _d >11.3 (95% C.L.). This has interesting consequences for the CKM parameters which are also worked out.     

Nailing Down the KM matrix and the top quark mass

We present a determination of the KM matrix elements and a prediction of the top quark mass, based on available experimental information and the only theoretical framework, known to us, that is free of experimental and/or theoretical difficulties.     

Independent measurement of the top quark mass and the light- and bottom-jet energy scales at hadron colliders

A method for the simultaneous determination of the energy scales for bottom-quark jets and light jets, the jet energy resolution, and the top quark mass at hadron colliders is presented. The method exploits the unique kinematics of events with top-antitop pair production, where one of the top quarks involves a leptonic and one a hadronic boson decay. The paper shows a feasibility study of how this simultaneous measurement can be performed at the upcoming LHC experiments ATLAS and CMS.     

The top quark mass and flavour mixing in a seesaw model of quark masses

The top quark mass and the flavour mixing are studied in the context of a seesaw model of quark masses based on the gauge group SU(2)_L × SU(2)_R × U(1). Six isosinglet quarks are introduced to give rise to the mass hierarchy of ordinary quarks. In this scheme, we reexamine a mechanism for the generation of the top quark mass. It is shown that, in order to prevent the seesaw mechanism to act for the top quark, the mass parameter of its isosinglet partner must be much smaller than the breaking scale of SU(2)_R. As a result the fourth lightest up quark must have a mass of the order of the breaking scale of SU(2)_R, and a large mixing between the right-handed top quark and its singlet partner occurs. We also show that this mechanism is compatible with the mass spectrum of light quarks and their flavour mixing.     

A study of top quark production at HERA using parton shower simulation of DIS events

A study of the hadronic properties of top quark events at HERA is made in the hope that some characteristics of such events may emerge that distinguish them from light flavour QCD events. Electronproton collisions are simulated using a parton shower Monte Carlo event generator which includes both initial and final gluon radiation and also incorporates certain gluon interference effects. It is found that applying a multiplicity cut of 35 together with considering only charged current events gives a top to back-ground ratio of about 1 and a total of 40 top events at an integrated luminosity of 500 pb^?1 for a top quark mass of 60 GeV. Unfortunately no clear peak in the mass distribution can be obtained as the background and top quark peaks coincide after application of the multiplicity cut. If the top quark mass is 100 GeV there will be only about 6 top quark events under these conditions.     

Properties of the top quark

The top quark was discovered at the CDF and D0 experiments in 1995. As the partner of the bottom quark its properties within the Standard Model are fully defined. Only the mass is a free parameter. The measurement of the top quark mass and the verification of the expected properties have been an important topic of experimental top quark physics since. In this review the recent results on top quark properties obtained by the Tevatron experiments CDF and D0 are summarised. At the advent of the LHC special emphasis is given to the basic measurement methods and the dominating systematic uncertainties.