The anomalous magnetic moment of the muon, ๐๐โก(๐โ2)๐/2, is a cornerstone of precision in particle physics. While Diracโs theory predicts ๐=2, quantum corrections within the Standard Model (SM) generate a non-zero value. Due to the large muon mass, ๐๐ is particularly sensitive to virtual particles from potential new physics, making it a powerful probe for Beyond-the-Standard-Model theories. The long-standing discrepancy between the experimental average and the SM prediction has been a primary hint of such new physics. However, recent lattice QCD calculations suggest that this tension may be resolved.
This situation underscores the critical need to validate all theoretical inputs. The dominant uncertainty in the traditional, data-driven SM prediction stems from the leading-order hadronic vacuum polarization (LO HVP) contribution. Significant experimental tensions in the cross-section data for ๐+๐โโhadrons, most notably the recent CMD-3 result, which contradicts other datasets, must be resolved to consolidate our understanding.
This work presents the first KLOE analysis of the ๐+๐โโ๐+๐โ๐0 process, a significant channel for the LO HVP. The study uses the initial state radiation (ISR) technique on a 1.7 fbโ1 data sample collected by the KLOE experiment at the DA๐ทNE ๐-factory. From the resulting 3๐ invariant mass spectrum the fundamental parameters of the ๐ meson are extracted โits mass ๐๐, width ๐ค๐, and the branching fraction product
โฌ๐๐รโฌ3๐: ๐๐=782.73ยฑ4โ0.07+0.06 [MeV/c2],
๐ค๐=8.67ยฑ12โ0.17+0.13 [MeV],
โฌ๐๐รโฌ3๐=5.86ยฑ6โ0.08+0.11 [10โ5].
These results provide an important direct measurement of the 3๐ channel and a refined characterization of the ๐ resonance. They demonstrate that the analysis method can yield a highly accurate ๐ mass parameter and offer crucial input for refining the LO HVP contribution to ๐๐, thereby helping to clarify the current experimental landscape.
