To measure the lifetime of a free neutron, physicists take two approaches that should arrive at the same answer: one traps neutrons in a magnetic bottle and counts their disappearance; the other counts protons appearing in a beam as neutrons decay. It turns out neutrons appear to live nine seconds longer in a beam than in a bottle. To solve this unexplained discrepancy between ‘beam’ and ‘bottle’ measurements, a team of U.S. physicists entertained a wild but testable theory positing the existence of a right-handed version of our left-handed Universe. They designed a mind-bending experiment to try to detect the so-called dark-matter mirror neutrons.
Broussard et al. proposed that conversions of neutrons into mirror neutrons can increase the apparent neutron lifetime by 1%. Image credit: Tomislav Jakupec.
“The neutron lifetime is an important parameter in the Standard Model because it is used as an input for calculating the quark mixing matrix, which describes quark decay rates,” said Dr. Frank Gonzalez, a physicist at the Oak Ridge National Laboratory.
“If the quarks don’t mix as we expect them to, that hints at new physics beyond the Standard Model.”
Over the years, perplexed physicists have considered many reasons for the discrepancy between ‘beam’ and ‘bottle’ measurements of the neutron lifetime.
One theory is that the neutron transforms from one state to another and back again.
“Oscillation is a quantum mechanical phenomenon. If a neutron can exist as either a regular or a mirror neutron, then you can get this sort of oscillation, a rocking back and forth between the two states, as long as that transition isn’t forbidden,” said Dr. Leah Broussard, also from the Oak Ridge National Laboratory.
The authors performed the first search for neutrons oscillating into dark-matter mirror neutrons using a novel disappearance and regeneration technique.
The neutrons were made at the Spallation Neutron Source (SNS), a DOE Office of Science user facility. A beam of neutrons was guided to SNS’s magnetism reflectometer.
The team used the instrument to apply a strong magnetic field to enhance oscillations between neutron states. Then the beam impinged on a ‘wall’ made of boron carbide, which is a strong neutron absorber.
If the neutron does in fact oscillate between regular and mirror states, when the neutron state hits the wall, it will interact with atomic nuclei and get absorbed into the wall.
If it is in its theorized mirror neutron state, however, it is dark matter that will not interact.
So only mirror neutrons would make it through the wall to the other side.
It would be as if the neutrons had gone through a ‘portal’ to some dark sector, a figurative concept used in the physics community.
“The dynamics are the same on the other side of the wall, where we try to induce what are presumably mirror neutrons to turn back into regular neutrons,” said Dr. Yuri Kamyshkov, a physicist at the University of Tennessee.
“If we see any regenerated neutrons, that could be a signal that we’ve seen something really exotic.”
“The discovery of the particle nature of dark matter would have tremendous implications.”
The team’s conclusion: no evidence of neutron regeneration was seen.
“100% of the neutrons stopped; 0% passed through the wall,” Dr. Broussard said.
Regardless, the result is still important to the advancement of knowledge in this field.
With one particular mirror-matter theory debunked, the scientists turn to others to try to solve the neutron lifetime puzzle.
“We’re going to keep looking for the reason for the discrepancy,” Dr. Broussard said.
The findings were published in the journal Physical Review Letters.
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L.J. Broussard et al. 2022. Experimental Search for Neutron to Mirror Neutron Oscillations as an Explanation of the Neutron Lifetime Anomaly. Phys. Rev. Lett 128 (21): 212503; doi: 10.1103/PhysRevLett.128.212503
