Research explores signal of neutron decay into dark matter
The neutron is one of the particles that make up the atomic nucleus. A free neutron is unstable and decays into other fundamental particles like proton, electron, and anti-electron neutrino. There are two methods of measuring the lifetime of the neutron, namely, the bottle method and beam methods. In the bottle method neutrons are trapped while in the beam method a beam of neutrons is observed to see the decay into a proton and leptons. But the methods of measuring the lifetime of the neutron show different results.
The experiment has been performed by different scientists at different laboratories but neutrons in the beam (885.6 sec approximately) always show a longer lifetime compared to the neutrons in the bottle (877.7 sec approximately) and the difference in the lifetime remains approximately 8 sec, which cannot be explained by the known concepts of physics. This suggests there could be new physics that needs to be investigated to solve the puzzle.
One of hypothesis to solve the neutron lifetime puzzle is that 1% of the time neutrons decay into dark matter and the dark matter effect remains uncounted in the beam method because in the beam method the number of protons is counted, while in the bottle method the dark matter effect is unconsciously included when the number of neutrons left after the decay is counted. Therefore, if we include the effect of proposed decay channel, the puzzle seems to be solved.
To verify the hypothesis, University of Adelaide’s Wasif Husain and Anthony Thomas applied it on the neutron stars, which are very compact stars present in the universe and made of neutrons. So, if neutrons decay into dark matter, then they must decay inside the neutron star also, and that could give us a signal of neutrons’ decay into dark matter. If we could detect a signal, it would not only solve neutrons’ lifetime problem but also the dark matter problem.
They studied two hypotheses for the neutron decay into dark matter particles. In the first, they assumed that neutrons’ decay into dark matter and a massless boson (which leaves the neutron star immediately). Second, they assumed that the neutron decay into three identical dark matter particles. The application of both the hypotheses led to observable signals of the decay. They showed that if neutrons decay into dark matter, then a neutron star must spin up and glow within the first 100,000 seconds after its birth. Specifically, they showed that for both hypotheses, the neutron star must spin up in the range of 5% to 15% and glow with the temperature ranging 2-6 MeV.
However, determining this is a tough ask from the astronomer’s point of view because neutron stars born in supernovae and newly born neutron stars are difficult to observe. Moreover, the time constraint of 100,000 seconds, which is shorter than the blink of an eye on the cosmological scale, makes the observation even harder. But, on the positive side, if the suggested signal is observed, it confirms that neutrons decay into dark matter and opens a new door in the dark matter particle physics field.
Wasif Husain et al JCAP10(2022)028
https://iopscience.iop.org/article/10.1088/1475-7516/2022/10/028
Wasif Husain and Anthony W Thomas 2023 J. Phys. G: Nucl. Part. Phys. 50 015202