On estimating the top quark mass from global analyses ofe + e − collision data

A study is undertaken to determine how to best extract the top quark massm _t within the Minimal Standard Model (MSM) using a global fit to a variety of processes (including wide-angle Bhabha scattering) ine ⁺ e ^– collisions near theZ which should be measured in the coming years. Experimental cuts are accommodated as an integral part of the analysis. It is advantageous to use the collinear radiation approximation and to cut data in rapidity, center-of-mass polar angle and minimum final state invariant mass squared,s. This avoids the need for the largest Monte Carlo acceptance correction to the data. Further, high precision cross section, calculations, (which include all one-loop electroweak and QED effects, certain higher-order improvements and perturbative QCD corrections as well as exponentiated soft and collinear photon radiation) then no longer require a Monte Carlo. This results in a speedup factor of at least fifty thousand (>5×10⁴) in EXPOSTAR. The data (corrected only for detector cracks, resolution and small non-collinear radiation effects) can therefore be fit quickly and directly forM _z ,m _t ,m _Higgs and _strong without recourse to unphysical intermediate quantities (weak mixing angles, running couplings, partial widths, ^*, etc.) Determination ofm _t could be as precise as ±15 GeV (and another±20 GeV fromm _Higgs) at the end of LEP running in 1991. Longitudinally polarized beams with very small polarization error could give an error onm _t smaller, by a factor 4 for the same luminosity.     

Measuring the top quark mass in the eµ channel: A study

We describe a simple method to measure the top quark mass in the channel that may be useful in Run II of D? detector. The method is validated by applying it to the Run Ib eμ data.     

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^–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 _?b 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.     

M s −M u quark mass difference and the scalar form factor ofK μ3 0 reaction

We study through QCD sum rules the connection between the invariant quark mass difference \(\hat m_s - \hat m_u\) and the scalar form factor of the reactionK ⁰→π ^? μ ⁺ v _μ in the physical region. We use both theoretical information, (the value off ₊(0) and the Callan-Treiman relation, includingm _π ²/m _k ² corrections) and experimental one (the value ofλ ₀ from a linear fit) to give a lower bound for \(\hat m_s - \hat m_u\) . Taking the world most recent fitted value forλ ₀,λ ₀ = 0.025, which may be reasonably identified with the slope att=0, andf ₊ (0) ≈ 0.98, we obtain \(\hat m_s - \hat m_u\) ≥ 250 MeV for \(\Lambda _{\overline {MS} }\) = 150 MeV. The relevant hypotheses and experimental trends are discussed.     

Isospin invariance,CP violation and B−B̄ mixing

With large B⁰−B̄⁰ mixing, the CP asymmetry of certain B decays become nearly maximal. However, the clear identification of relevant B meson decays required for experimental observation of such effects is difficult. We point out that symmetry relations among various B meson decay channels lead to additional constraints which can be useful in extracting CP violating amplitudes. As an example, we consider decay amplitudes to final states containing both a J/ψ and a K_S, whose interference terms provide information on CP violation. We show that these decays are ΔI=0 transitions and obtain isospin relations between the direct terms in the amplitude and experimental results from corresponding decays involving charged kaons.     

Could the quark electroweak and mass eigenstates coincide?

We conjecture that in the absence of new mixing effects at the weak scale the standard quark mass eigenstates and the quark weak eigenstates would coincide, and that for all three families a simple hierarchy would apply, with m_u^o > m_d^o. We further assume that the observed mass inequality m_d>m_u arises from mixing of d with a heavier quark D. This approach leads to a CKM matrix that is automatically expressed in terms of mass ratios and different from other ones that have been studied; it is consistent with experimental constraints. In addition to requiring the existence of D with a mass on the electroweak scale, in general an unambiguous prediction is that |V_td| = |V_ub|, which is soon testable. The approach can be implemented in E₆-based models.     

Analysis of the fragmentation properties of quark and gluon jets at the CERN SPS pp̄ collider

A sample of two-jet events from the UA1 experiment at the CERN pp̄ Collider has been used to study the fragmentation of high-energy quark and gluon jets into charged hadrons. Compared with lower-energy jets observed in e⁺e⁻ and pp collisions, the fragmentation function measured in the present experiment is softer (i.e. peaked to smaller values of z) and the mean internal transverse momentum is larger, mainly because of the effects of the QCD scaling violations. Using our knowledge of the quark and gluon structure functions in the proton, together with the QCD matrix elements, a statistical separation of quark and gluon jets is achieved within the present experiment. The fragmentation function for the gluon jets is found to be softer, and the angular spread of the fragmentation products larger, than is the case for quark jets.     

Spin-averaged mass formulas and the mass spectrum of low lying Δ-resonances in harmonic-oscillator-like quark models

By spin-averaging the masses of one species of particle over a sufficient number of multiplets we argue that the resulting spin-averaged mass is insensitive to mass operators which are not spin-scalar. This approach may be applied to averaging over quantum numbers other than spin. We discuss the assumptions necessary to restricting the average to only relatively few multiplets and show how mass formulas between averaged masses may be constructed. This approach reduces the number of parameters and predicts patterns of splittings which should be common to many analyses. We apply our results to the low lying Δ-resonances in harmonic-oscillator-like models and examine the comparison of theory with experiment for four different assumptions about which states should be included in particular averages. Using data from experiment we find that the prediction for the spin-averaged mass of the Δ-resonances from (56,1 ₃ ^? ) is, on average, 150 MeV higher than the spin-averaged mass of the experimental Δ states plausibly assigned to the (56,1 ₃ ^? ). This result is insensitive to which of the assumptions is made. Some suggestions are made about rectifying this discrepancy.     

Why should we care about the top quark Yukawa coupling?

Journal of Experimental and Theoretical Physics - In the cosmological context, for the Standard Model to be valid up to the scale of inflation, the top quark Yukawa coupling y t should not exceed...