Bo Cao: A Study of the e+e- → π+π-π0 Process Using Initial State Radiation

Abstract

The anomalous magnetic moment of the muon, aμ≡(g − 2)μ/2, is a cornerstone of precision in particle physics. While Dirac’s theory predicts a g-factor of 2, quantum corrections within the Standard Model (SM) generate a non-zero value. Due to the large muon mass, aμ 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 e+e- → hadrons, 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 e+e- → π+π-π0 process, a significant channel for the LO HVP. The study uses the initial state radiation (ISR) technique on a 1.7 fb−1 data sample collected by the KLOE experiment at the DAΦNE ϕ-factory. From the resulting 3π invariant mass spectrum the fundamental parameters of the ω meson are extracted —its mass Mω, width Γω, and the branching fraction product Bee × B3π:

                     Mω = 782.73 ± 0.04 +0.06−0.07 [MeV/c2],

                      Γω = 8.67 ± 0.12 +0.13−0.17 [MeV],

           Bee × B3π = 5.86 ± 0.06 +0.11−0.08 [10−5].

These results provide an important direct measurement of the 3π channel and a refined characterization of the ω resonance. They demonstrate that the analysis method can yield a highly accurate ω mass parameter and offer crucial input for refining the LO HVP contribution to aμ, thereby helping to clarify the current experimental landscape.