Neutrons Search for Time-Reversal Symmetry Violation express
Deep inside the accelerator, neutrons are sped up and fly toward a target like a swarm of tiny tops. Each top has an axis of rotation: its spin. If all the tops are spun in the same direction, the beam is called polarized. Such a beam is exactly what's needed to catch the elusive symmetry violation: why our world has almost no antimatter left, even though the Big Bang should have produced equal amounts of matter and antimatter.
To get polarized neutrons, physicists use helium-3 gas. Atoms of this gas, like a sponge, absorb neutrons with one spin direction and let through those with the other. This produces a beam where the spins are aligned like compass needles in a magnetic field. The beam was directed at a lanthanum-139 nucleus. When a neutron hits a resonance with the nucleus, the absorption probability depends on the relative orientation of the spins. By measuring the difference in neutron counts at two opposite settings, the scientists obtained an asymmetry of about 7.8%.
This result matched previous measurements, meaning the new polarized neutron source works correctly. Now it can be used for more delicate experiments: searching for time-reversal symmetry violation. If such a violation is found, it would explain why matter triumphed over antimatter, and why we exist.
Original abstract
as written by the authors arXiv:2602.17742
The dynamic enhancement of symmetry-breaking effects in neutron-nucleus resonances provides a sensitive testing ground for Time-Reversal Invariance Violation (TRIV). Exploiting this mechanism, the Neutron Optics Parity and Time Reversal Experiment (NOPTREX) seeks to elucidate the origin of the universe's baryon asymmetry. Critical to this effort is the precise measurement of Parity Violation (PV) asymmetries, which is essential to calibrate the nuclear parameters required for future TRIV experiments. To facilitate these studies, we developed an eV polarized neutron at the Back-n white neutron beamline of the China Spallation Neutron Source (CSNS). Neutron polarization is generated by an in-situ Spin-Exchange Optical Pumping (SEOP) $$^3$$He filter. Spin manipulation is performed by an adiabatic spin flipper, while spin polarization is preserved over the flight path by a vacuum transport system equipped with a solenoidal guide field. Experiments successfully measured an asymmetry of approximately $$7.8 \pm 2.4$$ (stat.) $$\pm 0.3$$ (sys.) % at the 0.747 eV p-wave resonance of $$^{139}$$La. These results are in agreement with previous results on this resonance and validate the system's capability for PV measurements.
Helium-3, used for polarizing neutrons, is rare on Earth: it is extracted from natural gas and also produced by the decay of tritium. In the future, it is planned to be mined on the Moon, where it has been accumulated by the solar wind.
In Greg Egan's novel 'The Clockwork Rocket', characters travel through a universe where matter and antimatter are separated, and they seek a way to restore symmetry — physicists pondering neutron experiments are thinking about roughly the same puzzle.