GWU Jefferson Lab Group

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.

Our group’s latest paper, published in Physical Review C, examines inclusive electron scattering (only the scattered electron is detected) from helium-3 (two protons and one neutron) and tritium (two neutrons and one proton) nuclei. Comparing the scattering cross sections from these two nuclei in large Bjorken-x regime has been suggested as a method for learning about the relative rates of proton-proton, proton-neutron, and neutron-neutron short-range correlations. In our paper, we consider the problem using a theoretical spectral function (a probability distribution for finding a nucleon in the nucleus with a given momentum and separation energy) and find some problems with that approach. One problem, shown in the lower panel of the figure above, is that even at large Bjorken-x (xB>1.5), there is still a large contribution to the cross section from low separation energy (Es) nucleons. This means that we aren’t learning purely about pairs of correlated nucleons, but also about single nucleons and/or triplets. Distinguishing between those scenarios will require looking in a larger nucleus, such as Helium-4.

In a new paper published in Physical Review C, Axel Schmidt and collaborators from MIT explored the onset of the short-range correlated regime for protons inside nuclei. At any given moment, most protons inside nuclei feel strong attraction from many nearby nucleons (other protons and neutrons). However, some protons happen to be very close to a nearby neighbor nucleon, enter a short-range correlated configuration, in which the forces between the paired nucleons are much much stronger than the forces from the rest of the nucleus. These correlations produce states of very high relative momentum, which is typically how we identify them in electron scattering experiments. One such approach is to examine the momentum of electrons after they have scattered from a nucleus. If an electron leaves the collision with significantly more momentum than would be expected, given their scattering angle (high Bjorken-x, xB), then it is likely the electron hit a high-momentum nucleon participating in a short range correlation.

In this paper, we consider a slightly different technique: knocking out a proton from the nucleus. The additional information from the proton momentum vector allowed us to reject collisions with additional undetected particles, which are a background hiding our short-range correlations signal. We showed that this technique works by verifying the scaling in Bjorken-x seen in the carbon-to-deuterium cross section ratio; only we see it over the full range of x, shown in the figure above.

Detecting the knocked out proton has an additional advantage: an extra handle on what the proton’s momentum was prior to the collision, a variable we call missing momentum or pmiss. By selecting clean proton knock-out using missing mass, we could dial in different proton momentum regions using missing momentum. This allowed us to get an estimate of the width of the transition region between the uncorrelated and correlated regimes, shown in the figure above.

Future experiments, such as those in inverse kinematics, or those detecting low-energy recoiling nucleons (such as the recently approved ALERT experiment at Jefferson Lab) will be able to enhance the precision of our understanding of this transition region, and on how short-range correlations form inside the nucleus.