Abstract
The Standard Model of particle physics encapsulates our best current understanding of physics
at the smallest scales. A fundamental axiom of this theory is the universality of the couplings of
the different generations of leptons to the electroweak gauge bosons. The measurement of the
ratio of the decay rate ofπ bosons to π-leptons and muons, π
(π/π), constitutes an important test
of this axiom. Using 139 fbβ1 of protonβproton collisions recorded with the ATLAS detector
at a centre-of-mass energy of 13 TeV, we report a measurement of this quantity from di-leptonic
π‘π‘Β― events where the top quarks decay into a π boson and a bottom quark. We can distinguish
muons originating from π bosons and those originating from an intermediate π-lepton through
the muon transverse impact parameter and differences in the muon transverse momentum
spectra. The measured value of π
(π/π) is 0.992 Β± 0.013 [Β±0.007 (stat) Β± 0.011 (syst)] and
is in agreement with the hypothesis of universal lepton couplings as postulated in the Standard
Model. This is the only such measurement from the Large Hadron Collider, so far, and obtains
twice the precision of previous measurements.