Abstract:
The ratio of branching ratios of the W boson to muons and electrons, $$R^{\,\mu /e}_W={{\mathcal {B}}(W\rightarrow \mu
u )}$$
R
W
μ
/
e
=
B
(
W
→
μ
ν
)
/$${{\mathcal {B}}(W\rightarrow e
u )}$$
B
(
W
→
e
ν
)
, has been measured using $$140\,\text{ fb}^{-1}\,$$
140
fb
-
1
of pp collision data at $$\sqrt{s}=13$$
s
=
13
$$\text {T}\text {e}\hspace{-1.00006pt}\text {V}$$
Te
V
collected with the ATLAS detector at the LHC, probing the universality of lepton couplings. The ratio is obtained from measurements of the $$t\bar{t}$$
t
t
¯
production cross-section in the ee, $$e\mu $$
e
μ
and $$\mu \mu $$
μ
μ
dilepton final states. To reduce systematic uncertainties, it is normalised by the square root of the corresponding ratio $$R^{\,\mu \mu /ee}_Z$$
R
Z
μ
μ
/
e
e
for the Z boson measured in inclusive $$Z\rightarrow ee$$
Z
→
e
e
and $$Z\rightarrow \mu \mu $$
Z
→
μ
μ
events. By using the precise value of $$R^{\,\mu \mu /ee}_Z$$
R
Z
μ
μ
/
e
e
determined from $$e^+e^-$$
e
+
e
-
colliders, the ratio $$R^{\,\mu /e}_W$$
R
W
μ
/
e
is determined to be $$\begin{aligned} R^{\,\mu /e}_W&= 0.9995\pm 0.0022\,\mathrm {(stat)}\,\pm 0.0036\,\mathrm {(syst)}\ &\quad \pm 0.0014\,\mathrm {(ext)} . \end{aligned}$$
R
W
μ
/
e
=
0.9995
±
0.0022
(
stat
)
±
0.0036
(
syst
)
±
0.0014
(
ext
)
.
The three uncertainties correspond to data statistics, experimental systematics and the external measurement of $$R^{\,\mu \mu /ee}_Z$$
R
Z
μ
μ
/
e
e
, giving a total uncertainty of 0.0045, and confirming the Standard Model assumption of lepton flavour universality in W-boson decays at the 0.5% level.
