Precision measurement of the $B^0$ meson lifetime using $B^0\to J\mathrm{/}\psi K^{\ast 0}$ decays with the ATLAS detector

A measurement of the $B^0$ meson lifetime using $B^0\to J\mathrm{/}\psi K^{\ast 0}$ decays in data from 13 TeV proton-proton collisions with an integrated luminosity of 140 fb${}^{-1}$ recorded by the ATLAS detector at the LHC is presented. The measured effective lifetime is $$\tau =1.5053\pm 0.0012(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})\pm 0.0035(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})\mathrm{p}\mathrm{s}\mathrm{.}$$ The average decay width, extracted from the effective lifetime and using a parameter from external sources, is $${\mathrm{\Gamma }}_d=0.6643\pm 0.0005(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})\pm 0.0016(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.}){\mathrm{p}\mathrm{s}}^{-1},$$ where the uncertainties are statistical and systematic. The uncertainty due to external sources is negligible at the given precision. The earlier ATLAS measurement of ${\mathrm{\Gamma }}_s$ in the $B_s^0\to J\mathrm{/}\psi \phi$ decay was used to derive a value for the ratio of the average decay widths ${\mathrm{\Gamma }}_d$ and ${\mathrm{\Gamma }}_s$ for $B^0$ and $B_s^0$ mesons respectively, of $$\frac{{\mathrm{\Gamma }}_d}{{\mathrm{\Gamma }}_s}=0.9910\pm 0.0022(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{.})\pm 0.0036(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t}\mathrm{.})\mathrm{.}$$ The measured lifetime, average decay width and decay width ratio are in agreement with theoretical predictions and with measurements by other experiments. This measurement provides the most precise result of the effective lifetime of the $B^0$ meson to date.