Precision electroweak measurements on the Z resonance

Abstract

We report on the final electroweak measurements performed with data taken at the Z resonance by the experiments operating at the electron–positron colliders SLC and LEP. The data consist of 17 million Z decays accumulated by the ALEPH, DELPHI, L3 and OPAL experiments at LEP, and 600 thousand Z decays by the SLD experiment using a polarised beam at SLC. The measurements include cross-sections, forward–backward asymmetries and polarised asymmetries. The mass and width of the Z boson, mZmZ and ΓZΓZ, and its couplings to fermions, for example the ρρ parameter and the effective electroweak mixing angle for leptons, are precisely measured: mZ = 91.1875 ± 0.0021 GeV, Γ Z = 2.4952 ± 0.0023 GeV, ρℓ= 1.0050 ± 0.0010, sin2 θ (sup)lept (sub>eff = 0.23153 ± 0.00016. The number of light neutrino species is determined to be 2.9840 ± 0.0082, in agreement with the three observed generations of fundamental fermions. The results are compared to the predictions of the Standard Model (SM). At the Z-pole, electroweak radiative corrections beyond the running of the QED and QCD coupling constants are observed with a significance of five standard deviations, and in agreement with the Standard Model. Of the many Z-pole measurements, the forward–backward asymmetry in b-quark production shows the largest difference with respect to its SM expectation, at the level of 2.8 standard deviations. Through radiative corrections evaluated in the framework of the Standard Model, the Z-pole data are also used to predict the mass of the top quark, mt = 173+13 −10 GeV, and the mass of theW boson, mW = 80.363 ± 0.032 GeV. These indirect constraints are compared to the direct measurements, providing a stringent test of the SM. Using in addition the direct measurements of mt and and mWmW, the mass of the as yet unobserved SM Higgs boson is predicted with a relative uncertainty of about 50% and found to be less than 285GeV at 95% confidence level

Pinning down electroweak dipole operators of the top quark

“…Similarly, current LHC data only allows constraints on the vector and axial parts of the tt Z vertex with O(100 %) uncertainties [13], while the respective dipole couplings are unconstrained from hadron collider experiments. We note that low-energy observables, such as rare K and B decays [22][23][24], together with electroweak precision data [25][26][27][28] can provide strong constraints on modified tt Z interactions. However, these fairly indirect probes are based on either Z → bb decays or highly off-shell top quarks and Z bosons in b → s Z * /γ * transitions which rely on additional assumptions on the new physics and are prone to hadronic uncertainties.…”

Searches for sterile neutrinos with IceCube

“…The tension between the data of different experiments can be partially resolved by adding new families of neutrinos with a mass difference ∆m 2 ∼ 1 eV 2 . However, these new states cannot interact via standard weak interactions due to constraints from the Z-boson decay width measured at LEP [6] and, therefore, are called sterile.…”

DUNE Sensitivities to the mixing between Sterile and Tau Neutrinos

“…This wealth of observations have established neutrino oscillations as the mechanism behind the observed neutrino flavor transitions. Another important piece of information is the number of active neutrinos, compatible with three, which comes from the measurement of the invisible Z decay width [2]. Neutrino oscillations with three active neutrinos are not only minimal, but more importantly, successfully accounts for most of the observations.…”

“…At the FD, generally planned to observe neutrino oscillations driven by the atmospheric splitting, it is only possible to study the rapid oscillation (or averagedout) regime of the sterile oscillations. Otherwise, it is sensitive to lower values of the sterile mass splitting, below ∼ 0.5eV 2 , making possible to test a complementary ∆m 2 parameter space respect to what SBL experiments can probe. In particular, searches for an sterile oscillations at the far detector of LBL experiments have been performed by several collaborations using neutral-current (NC) events in their analysis [3,4,6,8].…”