Precision measurement of the $B^{0}$ meson lifetime using $B^{0} \rightarrow J/ψK^{*0}$ decays with the ATLAS detector

TL;DR

The ATLAS collaboration reports a high-precision measurement of the B^0 meson lifetime via B^0 → J/ψ K^{*0} decays using 140 fb^{-1} of pp data at √s = 13 TeV. A two-dimensional unbinned maximum-likelihood fit to mass and proper decay time, with detailed signal and background models and time-efficiency corrections, yields the effective lifetime τ_{B^0} = 1.5053 ± 0.0012(stat) ± 0.0035(syst) ps, and from this Γ_d = 0.6643 ± 0.0005(stat) ± 0.0016(syst) ps^{-1}. The ratio Γ_d/Γ_s is determined as 0.9910 ± stat ± syst, using ATLAS’s prior Γ_s measurement and external input y to relate τ_{B^0} to Γ_d. These results are consistent with HQE and lattice predictions and constitute the most precise measurement of the B^0 effective lifetime to date, strengthening tests of heavy-quark dynamics in the b-quark sector.

Abstract

A measurement of the $B^0$ meson lifetime using $B^0 \to J/ψK^{*0}$ 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 $$ τ= 1.5053 \pm 0.0012 ~\mathrm{(stat.)} \pm 0.0035 ~\mathrm{(syst.)~ps}. $$ The average decay width, extracted from the effective lifetime and using a parameter from external sources, is $$ Γ_d = 0.6643 \pm 0.0005 ~\mathrm{(stat.)} \pm 0.0016 ~\mathrm{(syst.)}~\mathrm{ps}^{-1}, $$ where the uncertainties are statistical and systematic. The uncertainty due to external sources is negligible at the given precision. The earlier ATLAS measurement of $Γ_s$ in the $B^0_s \to J/ψφ$ decay was used to derive a value for the ratio of the average decay widths $Γ_d$ and $Γ_s$ for $B^0$ and $B^0_s$ mesons respectively, of $$ \frac{Γ_d}{Γ_s} = 0.9910 \pm 0.0022 ~\mathrm{(stat.)} \pm 0.0036 ~\mathrm{(syst.)}. $$ 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.

Figures (3)

• Figure 1: (a) Invariant mass fit projection and proper decay time fit projection for the $B^{0} \ra J/\psi K^{*0}$ sample shown in two different proper decay time ranges: (b) $t\xspace \in (-0.5;1.0)$ ps and (c) $t\xspace \in (1;20)$ ps. The solid blue line shows the total fit while the short-dashed red line shows the signal. The sum of prompt and combinatorial backgrounds in the mass fit projection is represented by the dash-dotted grey line. The lifetime projections show the prompt background as a dash-dotted grey line and the combinatorial background as a long-dashed black line. The lower panel of each figure shows the ratio of each data point to the total fitted value. The green band in the ratio plot represents the envelope of model variations included in the systematic uncertainty, while the bars on the data points indicate statistical uncertainties. In subfigure (c), the model variation band is too small to be visible.
• Figure 2: The fitted values of the $B^{0}$ lifetime, measured with $B^{0} \ra J/\psi K^{*0}$ decays, for the 2015+2016, 2017 and 2018 subsamples compared to the value for the whole sample. The $B^{0}$ lifetime value for each subsample is shown by a black point, with the error bar indicating the statistical uncertainty.
• Figure 3: A comparison of the current ATLAS result for the $B^{0}$ lifetime with the previous ATLAS result BPHY-2012-03 in the $B^{0} \ra \PJgy \PKzS$ channel, and with those from other experiments. For the LHC, there are two measurements by LHCb LHCb:2014qsd in $B^{0} \ra J/\psi K^{*0}$ and $B^{0} \ra \PJgy \PKzS$ decays, another LHCb result LHCb:2014bqh in the $B^{0} \ra K^+ \pi^-$ channel, and the CMS CMS-BPH-13-008 combined result for $B^{0} \ra J/\psi K^{*0}$ and $B^{0} \ra \PJgy \PKzS$ decays. Other contributions come from the Tevatron experiments: from D0 PhysRevLett.114.062001 in the $B^{0} \ra D^{-}\mu^{+} \nu_{\mu}$ channel, and from CDF PhysRevLett.106.121804 with a combined result for $B^{0} \ra J/\psi K^{*0}$ and $B^{0} \ra \PJgy \PKzS$. From $e^+e^-$ colliders, the leading result from Belle II Abudinn2023 in the $B^{0} \ra D^{(*)-} K^+ / \pi^+$ channel and the result from BaBar PhysRevD.73.012004 in the $B^{0} \ra D^{*-}\ell^{+}\nu_{\ell}$ channel are shown. The last measurement presented is a combination of multiple channels, performed by the Belle experiment Belle:2004hwe. This combination includes measurements from the following decays: $B^{0} \ra D^{*-} \ell^+ \nu$, $B^{0} \ra D^{*-} \pi^+$, $B^{0} \ra D^- \pi^+$, $B^{0} \ra D^{*-} \rho^+$, $B^{0} \ra J/\psi K^{*0}$, $B^{0} \ra \PJgy \PKzS$.