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 $\tau_a\lesssim 10^{-13}\,$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 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 $m_{e^+e^-}\sim 17$ 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 $2-3\,\sigma$ deviation in the measurement of $\Gamma(\pi^0\to e^+e^-)$ from the Standard Model 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 $\Delta S=1$ 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 $\eta/\eta^\prime$ and Kaon factories.
Highlights
The past decade has seen a resurgence of interest in the phenomenology of new light particles with feeble interactions with the Standard Model (SM) [1,2,3]
Motivations have been varied, spurred from the growing belief that dark matter might be part of a more complex dark sector with additional matter and force carriers [4,5,6,7,8], and because light dark sectors could be parasitically explored in the broader U.S and worldwide neutrino program [9,10,11,12]
We focus on yet another possibility, whereby the PQ mechanism is realized by new dynamics close to the QCD scale
Summary
The past decade has seen a resurgence of interest in the phenomenology of new light particles with feeble interactions with the Standard Model (SM) [1,2,3]. The third anomaly is related to the persistently high central value observed for the width Γðπ0 → eþe−Þ, whose most recent and precise measurement, performed by the KTeV Collaboration in 2007 [45], showed a discrepancy from the theoretical expectation in the SM at the level of ∼2–3.2σ [48,49,50,51] In combination, these anomalies point to a common BSM origin: a new short-lived boson with mass of ∼16–17 MeV, coupled to light quarks and electrons, and decaying predominantly to eþe− (see [52] for connections with other anomalies). Anomalies are either the result of experimental systematics and/or poorly understood SM effects In our opinion, this illustrates the paradoxical predicament of the light dark sector intensity frontier program: the generic models it seeks to discover or rule out are not strongly motivated, and, at least historically, it has been the case that experimental excesses without theoretically compelling interpretations tend to be received with strong skepticism.
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