Author: axelschmidt

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.