High-Energy Particle Anomalies

Описание: Resolved and Fading Anomalies
• W Boson Mass: The crisis sparked by the 2022 CDF II measurement, which reported a mass 7σ higher than the Standard Model (SM) prediction, is considered resolved. In September 2024, the CMS experiment released a new, highly precise measurement (80,360.2±9.9 MeV) that aligns perfectly with the SM prediction (80,357±6 MeV) and results from ATLAS and LHCb. This suggests the CDF II result was likely an outlier due to unrecognized systematic errors.
• RK​ and RK∗​: Previous hints of Lepton Flavour Universality violation in b→sℓ+ℓ− decays (where muons appeared to be produced less frequently than electrons) have disappeared. Updated LHCb analyses using the full 9 fb−1 dataset show these ratios are consistent with the SM. The earlier discrepancies are now attributed to underestimated systematics in electron identification.
Persistent and New Anomalies
• Muon g-2: Fermilab released its final result in June 2025, achieving a precision of 127 ppb (aμ​(exp)=116592070.5±14.8×10−11), confirming previous measurements. However, the status of the anomaly is ambiguous due to shifts in the theoretical prediction. While "data-driven" SM calculations historically showed a ∼5σ discrepancy, newer Lattice QCD calculations and 2024 data from the CMD-3 experiment yield SM predictions much closer to the experimental result, significantly reducing the tension.
• Semi-Tauonic Decays (RD​,RD∗​): Tensions in b→cτν transitions remain robust. Measurements from LHCb, Belle, and BaBar consistently show branching ratios for decays involving tau leptons that are higher than SM predictions, with a combined significance of 3.2σ to 4.4σ. This persists as a strong hint of New Physics, potentially involving leptoquarks.
• B→Kννˉ: A new anomaly has emerged from the Belle II experiment, which reported a branching ratio for this rare flavour-changing neutral current decay that is 2.7σ higher than the SM prediction. This excess could indicate the presence of light dark matter or new mediators.
Summary While the Standard Model has proven remarkably resilient, resolving the W boson and RK​ crises, the "flavour sector" remains the primary source of tension. The persistence of anomalies in semi-tauonic B decays and the new excess at Belle II suggest that New Physics may preferentially interact with the third generation of fermions (taus and bottom quark