Precise test of lepton flavour universality in W-boson decays into muons and electrons in pp collisions at s=13TeV with the ATLAS detector

Abstract

AbstractThe ratio of branching ratios of the W boson to muons and electrons, $$R^{,\mu /e}_W={{\mathcal {B}}(W\rightarrow \mu \nu )}$$ R W

μ / e

=

B ( W → μ ν )

/$${{\mathcal {B}}(W\rightarrow e\nu )}$$

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.