The limit is for $\Gamma\mathrm {( {{\mathit X}^{0}} \rightarrow {{\mathit e}^{+}} {{\mathit e}^{-}} )}\cdot{}\Gamma\mathrm {( {{\mathit X}^{0}} \rightarrow {{\mathit \gamma}} {{\mathit \gamma}} {{\mathit \gamma}} )}/\Gamma _{{\mathrm {total}}}$. $\mathit C$ invariance forbids spin-0 ${{\mathit X}^{0}}$ coupling to both ${{\mathit e}^{+}}{{\mathit e}^{-}}$ and ${{\mathit \gamma}}{{\mathit \gamma}}{{\mathit \gamma}}$ .

VALUE ($ 10^{-3} $ eV) CL% DOCUMENT ID TECN  COMMENT • • We do not use the following data for averages, fits, limits, etc. • • $<0.2$ 95 1 VO 1994 CNTR ${\mathit m}_{{{\mathit X}^{0}}}=1.1 - 1.9$ MeV $<1.0$ 95 2 VO 1994 CNTR ${\mathit m}_{{{\mathit X}^{0}}}=1.1$ MeV $<2.5$ 95 2 VO 1994 CNTR ${\mathit m}_{{{\mathit X}^{0}}}=1.4$ MeV $<120$ 95 2 VO 1994 CNTR ${\mathit m}_{{{\mathit X}^{0}}}=1.7$ MeV $<3.8$ 95 3 SKALSEY 1992 CNTR ${\mathit m}_{{{\mathit X}^{0}}}$= 1.5 MeV 1  VO 1994 looked for ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}{{\mathit \gamma}}$ decaying at rest. The precise limits depend on ${\mathit m}_{{{\mathit X}^{0}}}$. See Fig.$~$2(b) in paper. 2  VO 1994 looked for ${{\mathit X}^{0}}$ $\rightarrow$ ${{\mathit \gamma}}{{\mathit \gamma}}{{\mathit \gamma}}$ decaying in flight. 3  SKALSEY 1992 also give limits $4.3$ for ${\mathit m}_{{{\mathit X}^{0}}}$ = $1.54$ and $7.5$ for $1.64$ MeV. The spin of ${{\mathit X}^{0}}$ is assumed to be one.

References:

VO 1994 PR C49 1551 Search for Resonance in Multiphoton Final States from Low Energy ${{\mathit e}^{+}}$ ${{\mathit e}^{-}}$ Scattering SKALSEY 1992 PRL 68 456 Search for Three Photon Final State Resonances in Low-Energy ${{\mathit e}^{+}}$ ${{\mathit e}^{-}}$ Scattering