Update on neutron lifetime (mirror neutron hypothesis falsified)

Bound to the atomic nucleus neutron is stable. Free from the nucleus neutron decays into (proton + electron + electron type antineutrino) in about 15 minutes. 

Free neutrons inside a bottle decay in 877.8 ± 0.3 seconds. [1]

Free neutrons in the form of a beam decay in 888 seconds.

The difference has been puzzling physicists. There were two major hypotheses (both involving dark matter) that could explain the discrepancy.

Hypothesis 1: some of the neutrons might be decaying into a dark-matter particle plus photon in the case of bottle experiments resulting in shorter lifetime on average.

Experimental result (February 5, 2018): hypothesis 1 is falsified 

Hypothesis 2: neutrons may be oscillating between two states: ordinary neutron and its dark matter twin (the so-called “mirror neutron”). This would be happening more often in a bottle resulting in shorter lifetime on average.

Experimental result (June 30, 2022): hypothesis 2 is falsified

Keep looking

Physicists will keep looking for an explanation of the discrepancy.


Neutron bottle: There is an excellent review by Paul Huffman titled “Overview of Neutron Lifetime Magnetic Trapping Experiments“.

Neutron beam: What is a neutron beam? Neutron beam applications


[1] Improved Neutron Lifetime Measurement with UCNτ

[2] Physicists confront the neutron lifetime puzzle

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