Mirror World

NBC News article (June 2019): “Scientists are searching for a mirror universe. It could be sitting right in front of you“. This article was about the neutron lifetime puzzle.

Natalie Wolchover’s February 2018 article at Quanta “Neutron Lifetime Puzzle Deepens, but no Dark Matter Seen” discussed the same puzzle.

“When physicists strip neutrons from atomic nuclei, put them in a bottle, then count how many remain there after some time, they infer that neutrons radioactively decay in 14 minutes and 39 seconds, on average. But when other physicists generate beams of neutrons and tally the emerging protons — the particles that free neutrons decay into — they peg the average neutron lifetime at around 14 minutes and 48 seconds.” – Natalie Wolchover

It is important to point out that the observed discrepancy is the result of many experiments done independently by several groups. (see the graphics at the end)

Why is it important to know the neutron lifetime precisely?

“Physicists need to know the neutron’s lifetime in order to calculate the relative abundances of hydrogen and helium that would have been produced during the universe’s first few minutes. The faster neutrons decayed to protons in that period, the fewer would have existed later to be incorporated into helium nuclei. …. since galaxies with more hydrogen form more massive, and eventually more explosive, stars. Thus, the neutron lifetime affects predictions of the universe’s far future.” – Natalie Wolchover

Update (July 22, 2022):

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

Long history of hidden sectors and mirror matter

Lev Okun, “Mirror particles and mirror Matter: 50 years of speculation and search“.

Among the particle physicists mirror particles are usually discussed under the heading of “hidden sectors“. In my opinion, the best introduction to this subject can be found in the CERN Physics Briefing Book (Chapter 1 and 9).

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