Search for a light exotic particle in $J/\psi$ radiative decays

Abstract

Using a data sample containing $1.06×10^8 ψ^′$ events collected with the BESIII detector at the BEPCII electron-positron collider, we search for a light exotic particle X in the process $ψ′→π^+π^−J/ψ, J/ψ→γX, X→μ^+μ^−$. This light particle $X$ could be a Higgs-like boson $A^0$, a spin-1 $U$ boson, or a pseudoscalar sgoldstino particle. In this analysis, we find no evidence for any $μ^+μ^−$ mass peak between the mass threshold and $3.0 \text{GeV}/c^2$. We set 90%-confidence-level upper limits on the product-branching fractions for $J/ψ→γA^0, A^0→μ^+μ^−$ which range from $4×10^{−7} $to $2.1×10^{−5}$, depending on the mass of $A^0$, for $M(A^0)<3.0 \text{GeV}/c^2$. Only one event is seen in the mass region below $255 \text{MeV}/c^2$, and this has a $μ^+μ^−$ mass of $213.3 \text{MeV}/c^2$ and the product-branching-fraction upper limit $5×10^{−7}$.