CERN Accelerating science

ATLAS further investigates the width of top quark

by Panos Charito

The top quark is the heaviest known elementary particle and completes the quark sector of the standard model (SM). Like other fermions in the standard model (SM), it decays through the electroweak interaction. But unlike b and c quarks, which form long-lived hadrons, the top quark has an extremely short lifetime. Hence, its decay width is the largest of all SM fermions and a challenging measurement for many high-energy experiments.

In view of its large mass (almost as heavy as a gold atom) the top quark is an excellent probe of the mechanism that breaks the electroweak gauge symmetry and should therefore play a key role in clarifying the nature of the force(s)/particle(s) responsible for this phenomenon. The top quark is also good probe for possible new parity-violating and non-Standard Model CP violating interactions which could be induced, for instance, by non-standard Higgs bosons. A precise measurement of the top quark width will allow to better estimate the parameters of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements. A precise measurement is also key for calculating other parameters including the Fermi coupling constant (GF), the strong coupling constant (αs) and even in the mass of the W-boson.

Moreover, departures of this value from the Standard Model prediction could be sign of new physics at play as predicted by many beyond the Standard Model theories. For example the top-quark decay can be modified by direct decays to Higgs boson or via flavour-changing neutral currents or radiative corrections coming from other particles beyond the Standard Model. Therefore, precise measurements of top with are important for mapping the parameter space of many BSM models.

Within the SM, the top quark decays almost exclusively into a W boson and a b quark. The total decay width (Γt) is expected to be dominated by the partial decay width of the above decay channel, which is consistent with the current experimental data. However, direct measurement of Γt that is model-independent can provide useful insights.

The first direct upper bound on the top quark width was set by the CDF collaboration after analysing the invariant mass distribution of tt¯ candidate events using 1 fb−1 of integrated luminosity. A bound on the decay width of 1.10 < Γt < 4.05 GeV for a top-quark mass of 172.5 GeV was set at 68% confidence level. It should be noted that direct measurements are limited by the experimental resolution of the top-quark mass spectrum, and so far are significantly less precise than indirect measurements, but avoid model-dependent assumptions.

Recent results from ATLAS are based on data recorded in 2012 from proton-proton collisions at centre-of-mass energy of √s = 8 TeV corresponding to an integrated luminosity of 20.2 fb−1. The top-quark decay width is extracted using tt¯ events in the lepton+jets channel with t → Wb, where one W boson from the two top quarks decays hadronically into a pair of quarks and the other one decays leptonically into a charged lepton and a neutrino.

In a recent paper (Eur. Phys. J. C 78 (2018) 129), ATLAS measured the decay width for a top-quark mass of 172.5 GeV and found that Γt = 1.76 (+0.86/ −0.76 GeV). This value agrees with the Standard Model prediction of 1.322 GeV. It should be noted, that the new indirect measurement has a total uncertain smaller by a factor of two compared to previous measurements and thus will help to narrow down searches of BSM physics, excluding sets of models where the top width is affected.