More Precise Neutron Lifetime Measurements Are Consistent With Corrected Standard Model Predictions

Experimental indications of new physics from a lack of unitarity in the CKM matrix and disparities in the neutron mean lifetime are both starting to resolve due to a combination of improved Standard Model calculations and better measurements. The exact cause of disparities in the disparate beam based measurements of the neutrino lifetime still aren't clearly understood, however. Thus, another potential indicator of new physics has gone away.

As I noted in a February 22, 2021 blog post:

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.

A 2014 review of the literature summed up the experimental measurements to date as follows:

The narrow uncertainty in a single measurement published in 2013 drives to seemingly low uncertainty of the beam estimate. The Particle Data Group has based its current world average (currently 879.4 ± 0.6 seconds) on "bottle" measurements. It supports this decision, in part, with a published analysis from 2018 (open access pre-print here).

The disparity is particularly striking because it involves a very common particle whose other properties have been measured to exquisite detail with significant practical applications.  It is also important, for example, in Big Bang Nucleosynthesis calculations.

More than eight seconds of disparity is hardly a precision measurement (and even the newly reported measurement by the more precise method is still less precise than the measurement precision of horserace outcomes). 

The free neutrino is the longest lived hadron or fundamental particle that isn't completely stable by a factor of roughly a billion. 

Recent theory work has helped resolve disparity based upon measurements including the neutron mean lifetime that is used to determine the CKM matrix element for weak force mediated up to down quark transitions, which had previously been in a three sigma tension with unitarity. Using the corrected theoretical approach, "the extracted |Vud|mirror=0.9739(10) now is in excellent agreement with both neutron and superallowed 0+→0+ Fermi determinations."

Another recent Standard Model calculation noting theory errors in previous calculations also favors the lower value:

The universal radiative corrections common to neutron and superallowed nuclear beta decays (also known as “inner” corrections) are revisited in light of a recent dispersion relation study that found +2.467(22)%, i.e., about 2.4σ larger than the previous evaluation. For comparison, we consider several alternative computational methods. All employ an updated perturbative QCD four-loop Bjorken sum rule defined QCD coupling supplemented with a nucleon form factor based Born amplitude to estimate axial-vector induced hadronic contributions. In addition, we now include hadronic contributions from low Q2 loop effects based on duality considerations and vector meson resonance interpolators. Our primary result, 2.426(32)%, corresponds to an average of a light-front holographic QCD approach and a three-resonance interpolator fit. It reduces the dispersion relation discrepancy to approximately 1.1σ and thereby provides a consistency check. 

Consequences of our new radiative correction estimate, along with that of the dispersion relation result, for Cabibbo-Kobayashi-Maskawa unitarity are discussed. The neutron lifetime-gA connection is updated and shown to suggest a shorter neutron lifetime less than 879 s. We also find an improved bound on exotic, non–Standard Model, neutron decays or oscillations of the type conjectured as solutions to the neutron lifetime problem, BR(n→exotics)<0.16%.

The "shift reduces the predicted neutron lifetime from 879.5(1.3) s to τn = 878.7(0.6)." This results is consistent with the new measurement to less than two sigma, also very strongly favoring the storage method over the beam method determination. The adjusted result from this paper produces a new value of the CKM matrix element Vud of "0.97414(28), where the increased error is due to an additional nuclear quenching uncertainty. Using it together with Vus=0.2243(9), one finds |VQud|2+|Vus|2+|Vub|2−1=−0.00074(68), so the first CKM row sum is consistent with unity at close to the 1σ level."

The Wikipedia values of the CKM matrix first row which don't reflect this new theoretical calculation are:

 0.97370(14)    0.2245(8)    0.00382(24)

For a sum of squares of 0.99850-1=-0.00150(50).

The likely cause of the discrepancy is unrecognized systemic error in the beam method measurements (such as the large error in the recent J-PARC beam measurement discussed in the linked previous blog post) that causes to beam measurement to miss about 1% of decays that actually occur (or some other modeling error such as beam neutrinos not truly qualifying as "free" or relativistic adjustments being applied incorrectly). As of October 2020, experiments were in the works to identify the source of the discrepancy.

A new measurement from the paper below is in a 2.5 sigma tension with the previous global average of storage based measurement in the direction away from the beam measurements, making the disparity between beam and storage based measurements even greater (a 5.1 sigma discrepancy). But the new measurement is 1.4 sigma from the Standard Model prediction based upon improved calculations that the improved measurements of the CKM matrix elements.

The combined error in the direction of the beam measurement of ± 0.28 seconds, cutting the uncertainty in previous storage based measurements in half, and probably significantly shifting the new world average storage based measurement in the direction of this new measurement (since measurements with less uncertainty are weighted more heavily).

We report an improved measurement of the free neutron lifetime τn using the UCNτ apparatus at the Los Alamos Neutron Science Center. We counted a total of approximately 38 ×10^6 surviving ultracold neutrons (UCN) after storing in UCNτ ’s magneto-gravitational trap over two data acquisition campaigns in 2017 and 2018. We extract τn from three blinded, independent analyses by both pairing long and short storage-time runs to find a set of replicate τn measurements and by performing a global likelihood fit to all data while self-consistently incorporating the β-decay lifetime. Both techniques achieve consistent results and find a value τn = 877.75±0.28 stat+0.22/−0.16 syst s. With this sensitivity, neutron lifetime experiments now directly address the impact of recent refinements in our understanding of the standard model for neutron decay.

From a new preprint.