Insights into the neutron lifetime crisis: A new recommended UCN lifetime average

Neutron lifetime is a critical parameter in the Standard Model. Its measurements using the ultracold neutron (UCN) storage and beam-line techniques reveals serious tension. Measurements of $(891\pm9)~$s [Ris\"o and Kurchatov], $(889.2\pm4.8)~$s [Sussex-ILL], and $(887.7\pm2.2)~$s [NIST], using the beam-line technique and counting the proton, average to $(888.1\pm2.0)~\text{s}~(\chi^2/ndf=0.09)$, whist the measurements of %$(887.6\pm3.0)~$s [Mambo], $(885.40\pm0.98)$ [ILL-Kurchatov], $(878.50\pm0.76)~$s [ILL-PNPI: nested cyl.], $(880.7\pm1.8)~$s [Mambo-II], $(882.5\pm2.1)~$s [Mambo], $(880.2\pm1.2)~$s [ILL-Kurchatov], $(881.5\pm0.9)~$s [ILL-PNPI: gravi. trap], $(878.3\pm1.9)~$s [ILL-PNPI: solenoid-gravi. trap], $(877.75\pm0.36)$s [LANL], using the UCN storage technique, average to $(878.56\pm0.29)~$s ($\chi^2/ndf=3.24$), and disagree by $4.6\sigma$. When UCNs are stored in material bottles, they can be lost to various processes: $\beta-$decay, absorption, and most importantly up-scattering on material walls. We recently studied various systematic effects that contributes to modeling UCN interaction with material surfaces, chief of which was identified as the uncertainty on the UCN energy spectra. In combination with the uncertainty on the loss-per-bounce parameter, uncertainty on the UCN energy spectra, dramatically increases the ultimate uncertainty on the measured neutron lifetime. Therefore, we recommend a UCN lifetime average of $(877.96\pm0.34)~$s ($\chi^2/ndf=1.72$), by dropping the four measurements involving interaction of UCNs with material walls. This improves the $\chi^2$ but also increases the tension to $5.0\sigma$. Furthermore, this insight into the neutron lifetime will be presented in light of the measurements of $\lambda=g_A/g_V$, the ratio of axial to vector coupling of weak decay, and $V_{ud}$, a CKM matrix element.