Study of the Dalitz decay ϕ → ηe+e− with the KLOE detector

We have studied the vector to pseudoscalar conversion decay ϕ→ηe+e−ϕ→ηe+e−, with η→π0π0π0η→π0π0π0, with the KLOE detector at DAΦ  NE. The data set of 1.7 fb−11.7 fb−1 of e+e−e+e− collisions at s∼Mϕ contains a clear conversion decay signal of ∼31,000∼31,000 events from which we measured a value of BR(ϕ→ηe+e−)=(1.075±0.007±0.038)×10−4BR(ϕ→ηe+e−)=(1.075±0.007±0.038)×10−4. The same sample is used to determine the transition form factor by a fit to the e+e−e+e− invariant mass spectrum, obtaining bϕη=(1.28±0.10−0.08+0.09) GeV−2, that improves by a factor of five the precision of the previous measurement and is in good agreement with VMD expectations.