# What is the measured decay rate of antineutrons? / What is the measured mean lifetime of antineutrons?

Please do not post any "answers" dealing with predicted/theoretical estimates. The question specifically asks for measured / experimental evidence.

According to Fundamental Symmetries, ed. Bloch, P., Pavlopoulos, P., Klapisch, R. 1987, page 82:

The measurement of this lifetime has not yet been attempted as it requires very slow antineutrons. Low-energy antineutrons are created in the antiproton source of antiproton accumulators, and they can be produced in the charge exchange reaction $$\bar{p}p \to \bar{n}n$$, where the momentum of the incoming antiproton is above 1GeV/c. It is not inconceivable that antineutrons could be trapped in a magnetic storage device. In contrast to the neutron, decaying antineutrons can probably be more easily detected owing to the outgoing antiproton and its subsequent annihilation. This could allow the antineutron lifetime to be determined directly from the exponential decay. One would this be free from normalization problems. An accuracy of at least 1% should be achievable.

So unless someone has done the experiment since the answer to your question is that there is no experimental measurement of the antineutron lifetime.

After a search, I found the following review of antineutron studies, from 2003.

The difficulty in obtaining suitable antineutron beams has prevented the direct measurements of the main parameters (mass, magnetic moment, lifetime), assumed to be equal to the neutron’s ones by CPT invariance.

The mass was measured in a bubble chamber study.

In conclusion, apart from the mass, no direct measurement of the antineutron static parameters exist, nor are expected to be performed at forthcoming machines. Their measurement is very hard and not justified by physics arguments.

There was a plan for a SuperLear machine at CERN and part of the objective was measuring the antineutron lifetime, but it was not approved and did not materialize.

There is renewed interest in antineutrons in neutron "antineutron oscillations", (example), so maybe as a side project the lifetime will be in the program. After all if it does not obey CP (and some are even saying CPT) the lifetime will be different and part of the measurement of baryon number violation that such oscillations would induce.

• I would like to comment on how hard it was to find this stuff by searches on google . I guess because early publications ares behind paywalls and no arXiv existed at the time of the first experiments. Nov 15 '18 at 11:37
• thank you anna for looking into this; it is much appreciated. it sounds like there is a possibility of determining the mean-lifetime as a side-project .. if it ever IS determined and is found to be different from that of the neutron, i'd like your opinion on why. Nov 16 '18 at 3:05
• It is the CP violation everybody is looking for. CP violation exists in nature but is not enough to explain why the universe is mostly matter, so it is sought with various particle experiments.hyperphysics.phy-astr.gsu.edu/hbase/Particles/cpt.html . Nov 16 '18 at 5:27
• why we need more CP violation? because we exist in a matter world, not equally matter and antimatter. So the answer to this why is anthropic. Nov 16 '18 at 5:39

Historically it has been very hard to measure the lifetime of the matter neutron. See this historical plot of "best" estimates for a quick glance, or this 2011 review for a more detailed discussion.

If the neutron and antineutron lifetime were not identical, that would be a violation of CP symmetry in low-energy quantum chromodynamics. The usual way of things in the neutron business is that any CP-violating model of the type which would make neutron and antineutron lifetimes different would also generate a nonzero electric dipole moment in the neutron. The anthropic observation that the universe has more matter than antimatter (and the more careful observation of how much more) requires CP violation at a level which should be observable in the current generation of neutron electric-dipole moment experiments.

This state of experimental affairs means that a scientist who, a generation ago, would have tried to spend a decade measuring the antineutron lifetime, is probably today working on one or more of the EDM efforts. If someone were to show up at a conference today with a promising technique for cooling antineutrons enough to make a lifetime measurement, and the community decided there was a scientific justification to pursue the experiment, I wouldn't expect a result before 2025. So your other answers --- that there is no data on the lifetime of the antineutron --- are likely to remain correct for the foreseeable future.

• a LOT of good science can be done with UCN + UCaN dedicated venues, i pray soon someone develops an inexpensive method for generating antineutrons soon so we can get this thing going (i hope to update my profile for my new research agenda / program) but i won't get my hopes up; "the stack" hasn't been very cooperative with me in the past... Oct 22 at 18:24