June 2022 – GWU Jefferson Lab Group

The “missing energy” spectrum measured for electrons knocking out protons from Helium-3 (red) and Tritium (i.e., Hydrogen-3, black) as measured in a 2018 experiment. The helium spectrum shows a peak at 5.5 MeV, corresponding to collisions leaving behind a bound deuteron. A similar peak in tritium would imply a bound di-neutron state.

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.

The exclusion limits placed by our search. The search becomes less sensitive for smaller hypothetical dineutron binding energies.

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 schematic showing the bending and focusing magnets of the MUSE beamline

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.

Month: June 2022

Novel search for a bound di-neutron comes up empty

MUSE Beamline Paper published in Phys. Rev. C