meson mass developed using data from the ATLAS experiment at the LHC.*

A model of the B _s ⁰ meson mass is being developed using data from decays in the channel B_s⁰ →J/ψ φ using information from the ATLAS experiment at the LHC. The data were collected during Runs 2 and 3. Measurement of the B _s ⁰ → J / ψ φ decay probes CP violation due to interference between a direct decay and a decay with B _s ⁰ - B̅ _s ⁰ mixing. Precise fits of peaking and combinatorial backgrounds will be applied. Assessment of the stability of the reconstructed mass over time is an essential contribution to the understanding of the systematic error of the measurement of the branching ratio.