Category: Papers

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.

In a paper recently published in Physics Letters B, we analyzed data from a 2018 experiment to search for the possibility of two neutrons forming a short-lived bound state. The experiment was conducted to study how protons move in helium-3 (two protons and one neutron) and tritium (two neutrons and one proton) nuclei. We realized that the data could reveal the presence of a bound di-neutron. Our analysis showed no evidence of any such state.

When a proton is knocked out of helium-3, one proton and one neutron are left behind. These can fly apart, or they can stay bound as a deuterium nucleus, called a deuteron. By contrast, when a proton is knocked out of tritium, two neutrons are left behind. No one has ever observed a “di-neutron” state, and so it is believed that the two neutrons always fly apart. However, if in some small fraction of collisions they could bind, then the signature in the data would look exactly like the deuteron signature in helium-3. The helium-3 data provide a sort of “template” that allowed us to search of a di-neutron without having to model exactly how it would appear in our detectors.

Looking at the data, we saw no evidence for a bound di-neutron state. This allowed us to place exclusion limits on possible di-neutron formation in tritium.

Jefferson Lab’s Positron Working Group advocates for adding a positron source to the CEBAF accelerator, which would allow a whole host of possible new positron-scattering experiments. This past year, the group has produced a white paper which has now been peer reviewed and published as a topical issue of the European Physical Journal A. The GW team contributed to six of the issue’s papers, all relating to using differences between electron scattering and positron scattering to quantify the effects of two-photon exchange. One paper was an all-GW affair: “Target-normal single spin asymmetries measured with positrons” by Gabe, Tyler, and Axel (EPJA 57:213, 2021). In this study, we argue that a positron beam, combined with the new Super-Big-Bite Spectrometer, and a transversely polarized proton target, would allow a first-ever measurement of two photon exchange through a quantity called the “target-normal single spin asymmetry,” labeled by An in the figure below.

This was Gabe’s first publication. He developed and ran the code to determine the theoretical predictions of the GPD model developed by GW’s Andrei Afanasev, and went on to produce all of the paper’s figures, including the one above.

With the Electron Ion Collider being built at Brookhaven, the future of Jefferson Lab is open to a number of possibilities. We see great reasons for that future to include a unique, world-leading positron scattering facility.

In a paper in Physical Review Letters, the CLAS collaboration published some of the first results studying the reaction called Semi-Inclusive Deep Inelastic Scattering (SIDIS), in which a quark is violently ejected from a proton, eventually combining with an anti-quark to form a pi meson, which is detected. This reaction is one of the best ways to learn about the transverse momentum of quarks inside protons.

One of the leaders of the data analysis (and one of the leading authors on the paper) was our group’s own Giovanni Angelini. Giovanni recently defended his PhD thesis, which studied not only at beam spin asymmetries, the subject of this paper, but also pion multiplicities.

One of the strengths of this result is that SIDIS is studied over a multi-dimensional space. The paper presents results mapped in bins of momentum transfer, Q2, longitudinal momentum fraction, xB, pion momentum fraction, z, and pion transverse momentum: essentially the full set of possible dependencies. These results provide valuable constraints on models of how the proton is built from quarks and gluons.

In a recent paper published in the European Physical Journal A, Bill, Igor, and collaborators examine the world’s data on pion photoproduction on deuteron targets in order to learn about photoproduction on neutrons. Studying neutrons directly is not practical, since there is no way to make a stable target on which to perform an experiment. Instead, nuclear physicists must rely on scattering experiments on light nuclei containing neutrons. Among these, deuterium, a nucleus consisting of one proton and one neutron, is the most straightforward to analyze. Nevertheless, neutrons in deuterium are moving, experience binding effects, and any reaction products can re-interact with the nearby proton. This paper describes a theoretical framework for overcoming these challenges and shows what we’ve learned about photon-induced excitations of the neutron.

The EMC Effect is typically observed as a change in quark structure in a heavy nucleus relative to deuterium, one proton weakly bound to one neutron. But that doesn’t mean that the quarks in deuterium also don’t experience some change in their momentum distribution relative to an unbound proton and neutron. In a new paper titled, “Short-range correlations and the nuclear EMC effect in deuterium and helium-3,” Prof. Schmidt and collaborators calculate the size of this change when assuming that this modification depends on nucleon virtuality—roughly how fast the a nucleon is moving. Depending on the assumptions one makes about the structure of the free neutron, or about the dependence of the modification on the Bjorken-x variable, the results can be wildly different. One highly relevant application of the calculation is to make predictions for the BAND and LAD experiments, pictured above.

The paper appears in the April-June issue of journal Physical Review Research, vol. 3.