Exploring how hadrons and nuclei are built from quarks and gluons
The CaFe Experiment, studying short range correlations in Calcium (Ca) and Iron (Fe) is underway in Hall C at Jefferson Lab. We use the High Momentum Spectrometer (HMS) and the Super High Momentum Spectrometer (SHMS) to detect protons and electrons, respectively, emerging from collisions with the target nuclei.
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.
A few of our group’s projects extend beyond Jefferson Lab. One such project is the MUSE Experiment, being conducted at the Paul Scherrer Institute (PSI) in Switzerland. The Experiment is lead by GW’s Prof. Downie, and will attempt to resolve the proton radius puzzle by measuring the proton radius twice; once with electrons and once with unstable muon particles.
Bill and Peter have contributed to a recent paper, published in the journal Physical Review C, documenting studies of the MUSE beam-line. The incoming particles in MUSE are actually a secondary beam, produced by primary collisions of protons on a production target. The MUSE beam-line transports muons, pions, electrons and positrons to the MUSE proton target. It is crucial for the experiment that the muon and electron beams have identical properties.
Our group’s new analysis on nucleon sum rules has now been published in the journal Physical Review C. The Gerasimov-Drell-Hearn (GDH) sum rule, the Baldin sum rule, and the Gell-Mann–Goldberger–Thirring (GGT) sum rule are important relationships between photo-production cross sections and fundamental properties of protons and neutrons: their anomalous magnetic moments, their electromagnetic polarizabilities, and forward spin polarizability, respectively. While these sum rules cover the probabilities for all possible combinations of particles produced in photon scattering, in principle the most important contribution comes from the reaction in which one pion is produced.
To study the single-pion contribution, we looked at predictions of SAID, a global partial wave analysis tool developed at GW. One of the striking findings is that the convergence of the GDH sum rule is very different for protons and neutrons. Whereas the GDH sum rule is nearly entirely satisfied by single-pion production for protons, on the neutron, single pion production only satisfies about 60% of the sum rule.
Still to tackle is the Schwinger sum rule, which involves a particular contribution to the electro-production cross section called the LT interference. We plan to address the convergence of this sum rule in a future paper.
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.
Check out this profile of Gabe and his research, by GW’s Columbian College of Arts and Sciences.
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.
On Nov. 23rd, Chan Kim successfully defended his PhD thesis, titled “Measurement of the Helicity Asymmetry E for the γp → π0p reaction in the Resonance Region.” Chan analyzed data from the “FROST” experiment at Jefferson Lab, which used a polarized proton target in the form of frozen butanol beads. A polarized photon beam was scattered from the target, and Chan was interested in collisions that produced a single pi0 meson. In his analysis, Chan determined the slight difference in scattering rates when the photons and protons had their spins aligned versus anti-aligned. This can help reveal excited baryon resonances and in turn help us better understand the different ways quarks can bind together.
Data is coming in on two of our group’s new experiments at Jefferson Lab investigating how protons and neutrons form short-range correlations within nuclei. In experimental hall B, we are colliding electrons accelerated to an energy of 6 Giga-electron Volts (GeV) on, hydrogen, deuterium, helium, carbon, argon, calcium, and tin nuclei, with the goal of knocking out protons and neutrons and detecting the emerging particles in the CLAS12 spectrometer. At the same time in experimental hall D, we are colliding photons with energies in the range of 8 to 8.5 GeV on deuterium, helium, and carbon, hoping to cause reactions that produce short-lived mesons that we can identify with the GlueX spectrometer. We are still in the early stages of calibrating the instruments, and teams are working around the clock to make sure the data is looking good, but we are already starting to see the reactions we expect!
