Stringent limits on the π^0 -> γX, X -> e+e- decay from neutrino experiments and constraints on new light gauge bosons

Abstract

We report new experimental limits on the π^0 -> γX decay of the neutral pion into a photon and a light boson X followed by the decay X -> e+e-. If this process exist, one would expect a flux of high energy X's produced from π^0's generated by the proton beam in a neutrino target. The X's would then penetrate the downstream shielding and be observed in a neutrino detector via their decays. Using bounds from the NOMAD and PS191 neutrino experiments at CERN that searched for an excess of e+e- pairs from heavy neutrino decays, stringent limits on the branching ratio as small as Br(π^0 -> γX) < 10^{-15} are obtained. These limits are several orders of magnitude smaller than the previous experimental and cosmological bounds. The obtained results are used to constrain models, where the X interacts with quarks and leptons, or it is a new vector boson mixing with photons, that transmits interaction between our world and hidden sectors consisting of SU(3)_C x SU(2)_L x U(1)_Y singlet fields.

Figures (4)

• Figure 1: Schematic illustration of a proton beam dump experiment on search for $\pi^0 \to \gamma X \to e^+e^-$ decay chain: neutral pions generated by the proton beam in the neutrino target (T) produce a flux of high energy $X$'s which penetrate the downstream shielding and decay into $e^+e^-$ pair in a neutrino detector.
• Figure 2: Combined energy spectrum of X bosons with mass M$_{X}$=10 MeV from the SPS neutrino target and from the beam dump region at the NOMAD detector calculated for $Br(\pi^0 \to \gamma X) = 1$.
• Figure 3: The 90$\%~C.L.$ upper limits on the branching ratio $Br(\pi^{0}\to \gamma X) Br(X\to e^+e^-)$ versus $\tau_{X}$ from the NOMAD (solid) and PS191 (dashed, dotted) experiments. The numbers near the curves indicate the corresponding values of $M_X$.
• Figure 4: 90$\%~C.L.$ exclusion regions in the ($M_{V}; \chi$) plane obtained from the results of the NOMAD nomadnuh, PS191 ps191-1, and positronium mitsui experiments. The areas excluded by the electron beam dump experiment E137 e137 and by the KLOE experiment kloe are also shown for comparison.