Search for pseudoscalar bosons decaying into $e^{+}{e}^{-}$ pairs in the NA64 experiment at the CERN SPS

We report the results of a search for a light pseudoscalar particle $a$ that couples to electrons and decays to $e^{+}{e}^{-}$ performed using the high-energy CERN SPS H4 electron beam. If such light pseudoscalar exists, it could explain the ATOMKI anomaly (an excess of $e^{+}{e}^{-}$ pairs in the nuclear transitions of ${}^8\mathrm{Be}$ and ${}^4\mathrm{He}$ nuclei at the invariant mass $\simeq 17\mathrm{MeV}$ observed by the experiment at the 5 MV Van de Graaff accelerator at ATOMKI, Hungary). We used the NA64 data collected in the “visible mode” configuration with a total statistics corresponding to $8.4\times {10}^{10}$ electrons on target (EOT) in 2017 and 2018. In order to increase sensitivity to small coupling parameter $\epsilon$ we also used the data collected in 2016–2018 in the “invisible mode” configuration of NA64 with a total statistics corresponding to $2.84\times {10}^{11}$ EOT. The background and efficiency estimates for these two configurations were retained from our previous analyses searching for light vector bosons and axionlike particles (ALP) (the latter were assumed to couple predominantly to $\gamma$). In this work we recalculate the signal yields, which are different due to different cross section and lifetime of a pseudoscalar particle $a$, and perform a new statistical analysis. As a result, the region of the two dimensional parameter space $m_a-\epsilon$ in the mass range from 1 to 17.1 MeV is excluded. At the mass of the central value of the ATOMKI anomaly (the first result obtained on the beryllium nucleus, 16.7 MeV) the values of $\epsilon$ in the range $2.1\times {10}^{-4}<\epsilon <3.2\times {10}^{-4}$ are excluded.

Figure 1

The NA64 setup to search for $A^{\prime }(a)\to e^{+}{e}^{-}$ decays of the bremsstrahlung $A^{\prime }(a)$ produced in the reaction $eZ\to eZ{A}^{\prime }(a)$ of the 150 GeV electrons incident on the active WCAL target. The figure is reproduced from Ref. [20].

Figure 2

The 90% C.L. limits on the pseudoscalar particles decaying to $e^{+}{e}^{-}$ pairs. On the right vertical axis we use the standard notation for the pseudoscalar coupling ${\xi }_e=\epsilon (V/m_e)\sqrt{4\pi {\alpha }_{\mathrm{QED}}}$, where $V=246\mathrm{GeV}$ is a vacuum expectation value of the Higgs field [33]. This corresponds to the Lagrangian term $\mathcal{L}\supset -i{\xi }_e\frac{m_e}{V}a{\overline{\psi }}_e{\gamma }_5{\psi }_e$. The red vertical line corresponds to the ATOMKI anomaly at $m_a=16.7\mathrm{MeV}$ (central value of the first result on beryllium). The $\epsilon$ range excluded at this mass is $2.1\times {10}^{-4}<\epsilon <3.2\times {10}^{-4}$. The region excluded using only the data collected with the visible mode geometry is denoted as “NA64 vis.”, the extention of this region obtained using all data is denoted as “NA64 invis.”. The regions excluded by the $(g-2{)}_e$ measurements (Berkley [17] and LKB [18]) are shown. The limits from the electron beam-dump experiments E774 [34] and Orsay [35] are taken from Ref. [33].