Measurement of the e [superscript +] e [superscript −] → π [superscript +] π [superscript −] π [superscript 0] π [superscript 0] cross section using initial-state radiation at BABAR

The process e þ e − → π þ π − 2 π 0 γ is investigated by means of the initial-state radiation technique, where a photon is emitted from the incoming electron or positron. Using 454 . 3 fb − 1 of data collected around a center-of-mass energy of ffiffiffi s p ¼ 10 . 58 GeV by the BABAR experiment at SLAC, approximately 150000 signal events are obtained. The corresponding nonradiative cross section is measured with a relative uncertainty of 3.6% in the energy region around 1.5 GeV, surpassing all existing measurements in precision. Using this new result, the channel ’ s contribution to the leading order hadronic vacuum polarization contribution to the anomalous magnetic moment of the muon is calculated as ð g π þ π − 2 π 0 μ 9 (cid:2) stat (cid:2) syst × 10 − 10 in the energy range 0 . 85 GeV < E CM < 1 . 8 GeV. In the same energyrange, the impact on the running of the fine-structure constant at the Z 0 -pole is determined as Δ α π þ π − 2 π 0 ð M 2 Z Þ ¼ ð 4 . 44 (cid:2) 0 . 02 stat (cid:2) 0 . 14 syst Þ × 10 − 4 . Furthermore, intermediate resonances are studied and especially the cross section of the process e þ e − → ωπ 0 → π þ π − 2 π 0 is measured.