Most precise measurement of electrically neutral beauty meson lifetime achieved

The ATLAS collaboration at the Large Hadron Collider (LHC) has released a new high-precision measurement of the lifetime of the electrically neutral beauty (B⁰) meson—a hadron composed of a bottom antiquark and a down quark.

Beauty (B) mesons are made up of two quarks, one of which is a bottom quark. Over the past decades, by studying B mesons, physicists have been able to examine rare and precisely predicted phenomena to gain insights into interactions mediated by the weak force and into the dynamics of heavy-quark bound states. The precise measurement of the B⁰ meson lifetime—the average time it exists before decaying into other particles—is of critical importance in this context.

The new ATLAS study of the B⁰ meson looked for the particle’s decay into an excited neutral kaon (K^*0) and a J/ψ meson. The J/ψ meson subsequently decays into a pair of muons while the K^*0 meson is studied through its decay into a charged pion and a charged kaon. The analysis is based on proton–proton collision data collected by the ATLAS detector during Run 2 of the LHC (2015–2018), amounting to an impressive data set of 140 inverse femtobarns (1 inverse femtobarn corresponds to approximately 100 trillion proton–proton collisions).

The ATLAS researchers measured the B⁰ meson lifetime to be 1.5053 picoseconds (1 picosecond [ps] is a trillionth (10^-12) of a second), with a statistical uncertainty of 0.0012 ps and a systematic uncertainty of 0.0035 ps. This result is the most precise to date and significantly improves upon previous measurements, including a previous ATLAS result.

To achieve such precision, many experimental challenges had to be overcome, including minimizing systematic uncertainties, performing precise modeling and improving detector alignment.

In addition to measuring the lifetime of the B⁰ meson, the ATLAS team calculated its decay “width.” Width is a fundamental parameter of any unstable particle with a finite lifetime. The shorter the lifetime, the broader the decay width—a direct consequence of Heisenberg’s uncertainty relation in quantum mechanics. The decay width of the B⁰ meson was measured to be 0.664 inverse picoseconds (ps^-1), with a total uncertainty of 0.004 ps^-1.

The researchers then compared this result to a previous measurement of the decay width of the B_s⁰ meson (composed of a bottom quark and a strange quark). They found that the ratio of the decay widths was consistent with unity, reflecting the close values of the measured widths. These results are in line with heavy-quark model predictions and can be used to further tune these predictions.

The new ATLAS precision measurements enhance the understanding of weak-force-mediated decays in the Standard Model and provide valuable data for future theoretical developments.