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The cases for a \(17\MeV\) QCD axion and cosmic strings

Today, after some time, I was intrigued by very ambitious interpretations of two experiments. First, there is the delirium over beryllium, a nuclear signal at \(17\MeV\) observed by Krasznahorkay et al. in Hungary (Krasznahorkay is the Hungarian transliteration of Krásna Hôrka, The Beautiful Little Hill [a castle], in Slovak).

In a new preprint,

Signals of the QCD axion with mass of \(17\MeV/c^2\): nuclear transitions and light meson decays
Daniele Alves of Los Alamos proposes a wonderful conclusion: the excess is real and the particle is nothing else than the QCD axion. The particle's mass is comparable to tens of \({\rm MeV}\) which sounds great, decays to electron-positron pairs, and is piophobic, Islamophobic, and homophobic, which a good axion should be. Note that an axion is a light scalar field \(A\) that is coupled to the QCD instanton density \(F\wedge F\) and is driven towards \(A\sim 0\), thus explaining why the coefficient of the instanton density is tiny and close to the CP-invariant value (zero). The axion is something that can solve the strong CP-problem (the problem why the coefficient of the CP-violating instanton density is so tiny), we say.

In fact, Alves finds some additional evidence that this new particle could be real and it could be the QCD axion. In particular, there is a 2-3 sigma deviation from the Standard Model in the measurement of \(\Gamma(\pi^0 \to e^+ e^-)\), the decay rate of the pion to the electron-positron pair, and this deviation also looks compatible with the QCD axion interpretation of the Krasznahorkay bump. Needless to say, it would be cute if the newly discovered elementary particle were some 7,400 times lighter than the previous one (the Higgs boson). Such a counterintuitive chronology would emphasize the possible fact that particles may hide not just by being very heavy but also by their being weakly coupled.

The second paper I want to mention is
Has NANOGrav found first evidence for cosmic strings?
which claims that some stochastic process apparently observed by "The North American Nanohertz Observatory for Gravitational Waves" just a week ago (12.5-year data released) could be due to a network of cosmic strings, huge fundamental filaments, filling the whole Universe. These are really of the "string material" that may be physically identical to fundamental strings inside the elementary particles according to string theory (but "cosmic" in "cosmic strings" means that these pieces of the string were stretched to cosmic lengths); at any rate, they are strings described by the same basic Nambu-Goto mathematics (which is the classical starting point in most introductions to string theory; and the classical limit becomes increasingly important if we consider large i.e. cosmic strings). The tension of such strings \(T=G\mu\) and the loop size \(\alpha\) are the only two important parameters you need to adjust if the stringy explanation of the NANOGrav observations were accurate. All the viable values will be probed by the array of future experiments. So much for the crackpots' claims that everything related to string theory is inaccessible to experiments.

Note that NANOGrav is an experiment that tries to find gravitational waves in a less sensitive way than the lasers in LIGO, namely by measuring some delays coming from pulsars and/or black hole binaries. I don't want to pretend to be an expert in this experiment; I will be satisfied with pretending that I know at least something about NANOGrav. Recall that some 15 years ago, we (including folks like the late Joe Polchinski) were rather excited by a possible observation of a single cosmic string candidate CSL-1. The hope was that some images on the sky are two copies of the exact same image, shifted because of the string's deficit angle. After some time, we saw that it wasn't a cosmic string (because the two patterns weren't quite identical images) but of course the existence of cosmic strings hasn't been falsified in the full generality, it is a possible stunning discovery that has a chance to shock us every day.

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