Stringent limits on the ${\pi }^0\to \gamma X$, $X\to e^{+}{e}^{-}$ decay from neutrino experiments and constraints on new light gauge bosons

We report new limits on the ${\pi }^0\to \gamma X$ decay of the neutral pion into a photon and a new gauge boson $X$ followed by the decay $X\to e^{+}{e}^{-}$. If this process exists, one would expect a flux of high energy $X$’s produced from ${\pi }^0$’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 $\mathrm{Br}({\pi }^0\to \gamma X)\lesssim {10}^{-15}$ 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 $SU(3{)}_C\times SU(2{)}_L\times U(1{)}_Y$ singlet fields.

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 an $e^{+}{e}^{-}$ pair in a neutrino detector.

Figure 2

Combined energy spectrum of $X$ bosons with mass $M_X=10\mathrm{MeV}$ from the SPS neutrino target and from the beam dump region at the NOMAD detector calculated for $\mathrm{Br}({\pi }^0\to \gamma X)=1$.

Figure 3

The 90% C.L. upper limits on the branching ratio $\mathrm{Br}({\pi }^0\to \gamma X)\mathrm{Br}(X\to e^{+}{e}^{-})$ vs ${\tau }_X$ from the NOMAD (solid line) and PS191 (dashed, dotted lines) 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 [22], PS191 [20], and positronium [30] experiments. The areas excluded by the electron beam dump experiment E137 [28] and by the KLOE experiment [31] are also shown for comparison.