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.
Details
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
References
Suresh Emre
Renaissance Universal 