Signals of the QCD axion with mass of 17 MeV/c^2: nuclear transitions and light meson decays

Abstract

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 τ a 4 × 10 −14 s; and (iii) it has suppressed isovector couplings, i.e., if it is piophobic. Interestingly, these are precisely the properties required to explain recently observed anomalies in nuclear de-excitations, to wit: the e + e − emission spectra of isoscalar magnetic transitions of 8 Be and 4 He nuclei showed a "bump-like" feature peaked at m e + e − ∼ 17 MeV. In this article, 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 de-excitations. 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 ∼ 2 − 3 σ deviation in the measurement of Γ(π 0 → e + e − ) from the SM theoretical expectation. This article 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 ∆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 η/η and Kaon factories.

Challenges for a QCD Axion at the 10 MeV Scale

“…We consider the effective theory for a variant of the QCD axion which couples mostly to the first generation fermions in the SM. While the low-energy structure of this setup has appeared some time ago [15,31], it has recently been discussed how this scenario remains robust to a variety of constraints [16,17]. From the ultraviolet (UV) perspective, for the axion to couple only to u and d quarks in the infrared (IR), the PQ breaking mechanism must allow for the generation of the appropriate quark and lepton masses as well as the CKM matrix.…”

“…For detailed explanations and results of this procedure we refer to Refs. [16,17]. The same mixing angles can be used to parameterize the effective operator controlling the axion coupling to photons…”

“…Regarding the visible axion, there are efforts [15][16][17] to save it from experimental constraints by making it pion-phobic. In this scenario, the mixing with the neutral pion is suppressed by choosing appropriate PQ charges and an accidental cancellation arising from the low energy quark mass and coupling parameters.…”

“…The experiment looks for high excitation states of nuclei from 8 Be and 4 He, which de-excite into ground states and emit a pair of electron and positron. A peak was observed in the angular correlation of e − e + , which can be interpreted as an intermediate pseudo-scalar particle with mass around 17 MeV produced in the de-excitation and later decay into e − e + [16,17,[20][21][22].…”

“…In general, since m a m π f π /f a , an axion mass in the 10 MeV range implies f a 1 GeV. For this scenario to survive the current experimental constraints, the axion must couple only to first generation quarks and charged leptons with suitable couplings determined by their masses, their QCD charges and f a [16,17].…”