LHCb Measures Rare Anomalous Decays of Eta Mesons
The LHCb experiment reports new branching fraction measurements for rare anomalous decays of the η and η' mesons, including the first search for η'→μ⁺μ⁻
The LHCb collaboration has released new measurements for the branching fractions of several rare anomalous decays involving the η and η'(958) mesons. The results, based on proton-proton collision data at a center-of-mass energy of 13 TeV, provide stringent tests of the Standard Model of particle physics.
These decays are classified as 'anomalous' because they proceed through quantum loops and are highly suppressed, making them sensitive probes for new physics. The analysis used a dataset corresponding to an integrated luminosity of 5.4 inverse femtobarns collected by the LHCb detector at CERN's Large Hadron Collider.
Key Branching Fraction Results
The team measured precise values for two decays and set upper limits for two others. The reported branching fractions and their uncertainties are detailed below.
| Decay Channel | Branching Fraction | Notes |
|---|---|---|
| η → μ⁺ μ⁻ | (5.02 ± 0.14 ± 0.18 ± 0.26) × 10⁻⁶ | Most precise measurement to date. |
| η'(958) → μ⁺ μ⁻ | < 1.9 × 10⁻⁷ | Upper limit at 90% confidence level (CL); first experimental search. |
| η → π⁺ π⁻ μ⁺ μ⁻ | < 5.4 × 10⁻⁷ | Upper limit at 90% CL. |
| η'(958) → π⁺ π⁻ μ⁺ μ⁻ | (2.73 ± 0.14 ± 0.13 ± 0.14) × 10⁻⁵ | Measured value. |
The uncertainties listed for the measured values are, in order, statistical, systematic, and due to the external branching fraction of the φ(1020)→μ⁺μ⁻ decay used for normalization.
Significance of the Measurements
The measurement of the η meson decaying into two muons is now the most precise ever achieved. This process is a key benchmark for theoretical calculations involving quantum chromodynamics and electromagnetic interactions at low energies. Any significant deviation from the predicted Standard Model value could hint at the influence of new particles or forces.
Conversely, the decay of the η' meson into two muons had never been experimentally searched for prior to this analysis. The LHCb result establishes a new upper limit for its occurrence. The non-observation is consistent with theoretical expectations, which predict an even smaller branching fraction than for the η meson due to the η'’s different quark structure and larger mass.
The Four-Body Decay Channels
The study also investigated decays where the mesons produce two pions in addition to two muons. For the η meson, only an upper limit could be set. For the η' meson, however, the team made a clear observation and measurement. The four-body final state provides a different window into the dynamics of the decay, involving both strong and electromagnetic interactions.
The analysis, described in a paper submitted to the arXiv preprint server, marks a significant advance in the study of light meson decays. The LHCb collaboration states that the results will help constrain theoretical models and improve the understanding of fundamental symmetries in particle physics. The work was led by Miguel Fernández Gómez and colleagues.
Future analyses with larger datasets from the upgraded LHCb detector are expected to further improve the precision of these measurements and potentially probe even rarer decay modes.
