Abstract
A
bstract
The top-quark mass is measured using top-quark decays producing an isolated lepton and
J
/
ψ
meson reconstructed in its
μ
+
μ
−
decay mode. The data sample was recorded with the ATLAS detector in proton-proton collisions at a centre-of-mass energy of
s
=
13
TeV during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb
−
1
. The measurement is based on the invariant mass
m
(
ℓμ
+
μ
−
) of the system made of the isolated lepton
ℓ
from the
W
boson decay and the non-isolated
μ
+
μ
−
pair from a
J
/
ψ
decay of a
b
-hadron, exploiting its sensitivity to the top-quark mass. An unbinned maximum-likelihood fit to the
m
(
ℓμ
+
μ
−
) distribution is performed to extract the top-quark mass. The top-quark mass is measured to be
m
top
= 172.17 ± 0.80 (stat) ± 0.81 (syst) ± 1.07 (recoil) GeV, with a total uncertainty of 1.56 GeV. The third uncertainty arises from changing the dipole parton shower gluon-recoil scheme used in top-quark decays.