2024 Articles

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

Aad, G.; Aakvaag, E.; Abbott, B.; Abdelhameed, S.; Abeling, K.; Abicht, N. J.; Abidi, S. H.; Aboelela, M.; Aboulhorma, A.; Abramowicz, H.; Abreu, H.; Abulaiti, Y.; Acharya, B. S.; Ackermann, A.; Bourdarios, C. A.; Adamczyk, L.; Addepalli, S. V.; Addison, M. J.; Adelman, J.; Adiguzel, A.; Adye, T.; Affolder, A. A.; Afik, Y.; Agaras, M. N.; Agarwala, J.; Aggarwal, A.; Agheorghiesei, C.; Ahmad, A.; Ahmadov, F.; Ahmed, W. S.; Ahuja, S.; Ai, X.; Aielli, G.; Aikot, A.; Tamlihat, M. A.; Aitbenchikh, B.; Akbiyik, M.; Åkesson, T. P. A.; Akimov, A. V.; Akiyama, D.; Al Khoury, Konie

The ratio of branching ratios of the 𝑊 boson to muons and electrons, 𝑅_𝑊∧𝜇/𝑒 = 𝛽( 𝑊 → 𝜇𝜈 )/𝛽( 𝑊 → 𝑒𝜈) , has been measured using 140 fb⁻¹ of 𝑝𝑝 collision data at √s = 13 TeV collected with the ATLAS detector at the LHC, probing the universality of lepton couplings. The ratio is obtained from measurements of the 𝑡𝑡̄ production cross-section in the 𝑒𝑒, 𝑒𝜇 and 𝜇𝜇 dilepton final states. To reduce systematic uncertainties, it is normalised by the square root of the corresponding ratio 𝑅_𝑍∧𝜇𝜇/𝑒𝑒 for the 𝑍 boson measured in inclusive 𝑍 → 𝑒𝑒 and 𝑍 → 𝜇𝜇 events.

By using the precise value of 𝑅_𝑍∧𝜇𝜇/𝑒𝑒 determined from 𝑒⁺ 𝑒⁻ colliders, the ratio 𝑅_𝑊∧𝜇 /𝑒 is determined to be

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_𝑍∧𝜇𝜇/𝑒𝑒, 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.