Axions are a hypothetical new class of particles, whose existence has been theorized as a way of solving two major open problems in physics. The first is the so-called “Strong CP Problem,” the observation that the strong nuclear force seems to be completely symmetric under charge-conjugation and parity transforms. There’s no need for this symmetry and it is an open question as to why the strong force obeys it. The second is the nature of dark matter, which has been repeatedly observed in all manner of astronomical observations, but has never been tangibly manipulated on earth. Dark matter might be made of axions, and these axions might force the strong force to be CP symmetric. It’s an attractive combo, and makes the search for axions an important pursuit.
Our group has recently published a paper in Physics Letters B in which we perform a search for axion-like particles in the data from our recent Short-Range Correlations experiment conducted in 2021 at Jefferson Lab’s Experimental Hall D. This experiment was one of the first to use nuclear targets, rather than hydrogen. This makes the data particularly sensitive to axions produced through the Primakoff process, an electromagnetic process whose rate scales with the square of the nuclear charge, i.e., the production rate from scattering on a carbon nucleus would be 36 times higher than on a hydrogen nucleus.
In our analysis, led by MIT graduate student, Jackson Pybus, we searched our data for scattering events that produced a pair of photons, which may be the result of axion decay. The energies and angles of these photons can be used to reconstruct the mass of the hypothetical progenitor particle. We then searched the mass spectrum of signs of a resonance, a so-called “bump hunt.” Our results were statistically consistent with the background-only hypothesis, hence no discovery. Instead, the data allow us to exclude axions in a certain range of mass and coupling. Our limit is not yet world leading, but this is no surprise for an analysis of an experiment that was not designed around an axion search. We see several targeted improvements that could be made in a future dedicated experiment that would dramatically enhance our sensitivity.
The most significant challenge with our current experiment is material in the path of the beamline downstream from the target. The production of the unstable eta meson (which decays into a photon pair) from collisions with downstream material had the potential to fake an axion like signal. This forced us to perform a background subtraction procedure that hampered our statistical sensitivity. Removing one of the detectors, the Forward Drift Chambers, would be a good start. But adding a downstream bag of helium, which displaces air and reduces the density of downstream material would be a game changer. With these improvements, a search for axions through Primakoff production would become world leading.