A New Measurement Of The Neutron Mean Lifetime

Background

There are two methods that have been used historically to measure the mean lifetime of a free neutron: the beam method and the storage method.

The beam method measures neutron lifetime by counting the injected neutron and decay product in the beam.

The storage method measures neutron lifetime by storing ultracold neutron in the specific bottle. They count the number of surviving neutrons S(1) and S(2) after distinct storing times t(1) and t(2).

The global average mean lifetime of a free neutron by the beam method is 

888.0 ± 2.0 seconds.

The global average mean lifetime of a free neutron by the storage method is 

879.4 ± 0.6 seconds

There is an 8.6 second (4.1 standard deviation) discrepancy between results from the two measurement methods, which is huge for fundamental physics in both absolute terms and relative to the amount of uncertainty in the respective measurements. 

The New Measurement

A new measurement by a third method has been done at the Japanese J-PARC experiment using pulsed neutron beams. The mean lifetime of a free neutron by this method is:

898 ± 10 (statistical) + 15 -18 (systemic) seconds 

for a combined error of 

898 + 18 - 20.6 seconds 

which dumbed down to a single number is 

898 ± 19.3 seconds

Due to the high uncertainty of the new measurement it is consistent with both the prior beam method and storage method results at the two sigma level, although it tends to favor the less precise beam method result.

Commentary

The neutron was discovered in 1932 and the lifetime of a free neutron was first measured in 1951. Seventy years later, the U.S. National Institution of Standard and Technology is working on the problem and hasn't made much progress.

The neutron is, by far, the longest lived subatomic particle that is not actually stable. Its mean lifetime is a little less than fifteen minutes. The runners up, the fundamental particle known as the muon and the composite meson known as the pion, have mean lifetimes on the order of a microsecond, which is almost a billion times shorter. Of course, embedded in stable atoms, bound neutrons are dynamically stable. In lattice QCD models calibrated to only modestly heavier than physical pion masses, dineutrons without protons are also stable.

Neutrons are not exactly exotic. One 1/7th of the ordinary matter in the universe is made up of them. They are found in Nature and not just ephemerally in particle colliders. Other properties of neutrons have been measured to great precision. We know the mass of a neutron to one part per two billion. And, the only subatomic composite particle whose internal structure is better understood is the proton.

The free neutron mean lifetime is an important experimentally measured physical constant for a variety of theoretical and practical purposes. It contributes to the determination of a couple of standard model of particle physics constants (a CKM matrix parameter and the weak force coupling constant). It is important in nuclear physics. It is important in Big Bang Nucleosynthesis calculations. But those independent indirect measurements of the neutron mean lifetime aren't sufficiently precise to definitively resolve the discrepancy. 

Similarly, a prediction from the Brookhaven National Laboratory of the value based on quantum chromodynamics as of 2018 is still not sufficiently precise to support one over the other.

The scientists have already used the new nucleon axial coupling calculation to derive a purely theoretical prediction of the lifetime of the neutron. Right now, this new value is consistent with the results from both types of experimental measurement, which differ by a mere 9 seconds. “We have a number for the neutron lifetime: 14 minutes and 40 seconds with an error bar of 14 seconds. That is right in the middle of the values measured by the two types of experiments, with an error bar that is big and overlaps both,” [Enrico Rinaldi, a special postdoctoral researcher at the RIKEN BNL Research Center at DOE’s Brookhaven National Laboratory, who was involved in developing simulations essential to the new calculation] said.

Yet, this particular physical constant turns out to be really hard to measure, in part, because neutrons do not have net electromagnetic charge that assists in measuring other particles. 

We can measure the time that it takes an athlete to complete a 100 meter dash with human controlled stopwatches to an order of magnitude or two greater precision in absolute terms. A discrepancy of 1% between two different methods of measuring the constant is awful for a constant that is so commonplace, so macroscopic, and so important.

For all of those reasons, the widely held consensus is that he discrepancy comes from unconsidered systematic error, rather than from beyond the Standard Model physics. A March 2020 pre-print reflects the conventional wisdom on the matter, which favors the more precise storage method measurement, stating in its abstract that:

This article discusses the possible causes of discrepancy in the measurements of the neutron lifetime with beam method experiment. The most probable cause, apparently, is the loss of protons in beam method experiment during storage in a magnetic trap due to charge exchange collisions of protons with the residual gas. The proton becomes neutral and leaves the trap, which leads to a decrease in the number of registered protons, i.e. to a decrease in the probability of neutron decay or to an increase in the measured neutron lifetime.

But the fifteen year old discrepancy has still not been satisfactorily resolved.

The Paper

The paper and its abstract are as follows:

A neutron decays into a proton, an electron, and an anti-neutrino through the beta-decay process. The decay lifetime (∼880 s) is an important parameter in the weak interaction. For example, the neutron lifetime is a parameter used to determine the |Vud| parameter of the CKM quark mixing matrix. The lifetime is also one of the input parameters for the Big Bang Nucleosynthesis, which predicts light element synthesis in the early universe. 

However, experimental measurements of the neutron lifetime today are significantly different (8.4 s or 4.0σ) depending on the methods. One is a bottle method measuring surviving neutron in the neutron storage bottle. The other is a beam method measuring neutron beam flux and neutron decay rate in the detector. There is a discussion that the discrepancy comes from unconsidered systematic error or undetectable decay mode, such as dark decay. 

A new type of beam experiment is performed at the BL05 MLF J-PARC. This experiment measured neutron flux and decay rate simultaneously with a time projection chamber using a pulsed neutron beam. We will present the world situation of neutron lifetime and the latest results at J-PARC.

N. Sumi, et al., "Precise Neutron Lifetime Measurement Using Pulsed Neutron Beams at J-PARC" Proceedings of J-PARC Symposium 2019 arXiv:2102.09758 (February 19, 2021).