Signals of the QCD axion with mass of 17 MeV/c2: Nuclear transitions and light meson decays

The QCD axion remains experimentally viable in the mass range of O(10 MeV) if (i) it couples predominantly to the first generation of SM fermions; (ii) it decays to e(+)e(-) with a short lifetime tau(a) less than or similar to 4 x 10(-14) s; and (iii) it has suppressed isovector couplings, i.e., if it is piophobic. Remarkably, these are precisely the properties required to explain recently observed anomalies in nuclear deexcitations, to wit: the e(+)e(-) emission spectra of isoscalar magnetic transitions of Be-8 and He-4 nuclei showed a bumplike feature peaked at m(e)S(+)e(-)- similar to 17 MeV. In this paper, we argue that on-shell emission of the QCD axion (with the aforementioned properties) provides an extremely well-motivated, compatible explanation for the observed excesses in these nuclear deexcitations. The absence of anomalous features in other measured transitions is also naturally explained: piophobic axion emission is strongly suppressed in isovector magnetic transitions and forbidden in electric transitions. This QCD axion hypothesis is further corroborated by an independent observation: a similar to 2-3 sigma deviation in the measurement of Gamma (pi(0)-> e(+)e(-)) from the Standard Model theoretical expectation. This paper also includes detailed estimations of various axionic signatures in rare light meson decays, which take into account contributions from low-lying QCD resonance exchange, and, in the case of rare kaon decays, the possible effective implementations of Delta S = 1 octet enhancement in chiral perturbation theory. These inherent uncertainties of the effective description of the strong interactions at low energies result in large variations in the predictions for hadronic signals of the QCD axion; in spite of this, the estimated ranges for rare meson decay rates obtained here can be probed in the near future in eta/eta' and kaon factories.

Automated summary

The QCD axion remains experimentally viable in a specific mass range with certain properties, which can explain anomalies observed in nuclear deexcitations. These properties also naturally explain the absence of anomalous features in other measured transitions.