1
Breit-Wigner mass. May also be the ${{\mathit f}_{{0}}{(1500)}}$.
References:
UEHARA
2010A
PR D82 114031
Measurement of ${{\mathit \eta}}{{\mathit \eta}}$ Production in Two-Photon Collisions
AMSLER
1992
PL B291 347
Proton Antiproton Annihilation into ${{\mathit \eta}}{{\mathit \eta}}{{\mathit \pi}}$: Observation of a Scalar Resonance Decaying into ${{\mathit \eta}}{{\mathit \eta}}$
ALDE
1986D
NP B269 485
Production of ${{\mathit G}{(1590)}}$ and other Mesons Decaying into ${{\mathit \eta}}$ Pairs by 100 ${\mathrm {GeV/}}\mathit c$ ${{\mathit \pi}^{-}}$ on Protons