${{\mathit n}}$ $\rightarrow$ ${{\mathit p}}{{\mathit e}^{-}}{{\overline{\mathit \nu}}_{{{e}}}}$ DECAY PARAMETERS

See the above “Note on Baryon Decay Parameters.” For discussions of recent results, see the references cited at the beginning of the section on the neutron mean life. For discussions of the values of the weak coupling constants ${\mathit g}_{{{\mathit A}}}$ and ${\mathit g}_{{{\mathit V}}}$ obtained using the neutron lifetime and asymmetry parameter$~\mathit A$, comparisons with other methods of obtaining these constants, and implications for particle physics and for astrophysics, see DUBBERS 1991 and WOOLCOCK 1991. For tests of the $\mathit V−\mathit A$ theory of neutron decay, see EROZOLIMSKII 1991B, MOSTOVOI 1996, NICO 2005, SEVERIJNS 2006, and ABELE 2008.

${{\mathit e}}-{{\overline{\mathit \nu}}_{{{e}}}}$ ANGULAR CORRELATION COEFFICIENT $\mathit a_{0}$

INSPIRE   JSON  (beta) PDGID:
S017BNC
For a review of past measurements of the $\mathit a$-coefficient, see WIETFELDT 2005. The $\mathit a$-coefficient itself is not well defined in any measurement once higher-order radiative and recoil corrections, which are experiment dependent, are included. By contrast, measurements of the ratio $\lambda {}\equiv\mathit g_{A}/\mathit g_{V}$ (see data block above) incorporate such higher-order effects. For this reason, and in order to meaningfully compare results for the angular correlation coefficient from measurements of different observables, we list here the zero-recoil-order $\mathit a$-coefficient, denoted $\mathit a_{0}$, which in the Standard Model and at zero recoil order, is related to $\lambda $ by $\mathit a_{0}$ = (1 $−$ $\lambda {}^{2}$) $/$ (1 + 3$\lambda {}^{2}$); this assumes that $\mathit g_{A}$ and $\mathit g_{V}$ are real. See also the discussion in WIETFELDT 2024.
VALUE DOCUMENT ID TECN  COMMENT
$\bf{ -0.1044 \pm0.0007}$ OUR AVERAGE
$-0.1053$ $\pm0.0018$ 1
WIETFELDT
2024
SPEC Cold ${{\mathit n}}$, unpolarized
$-0.10430$ $\pm0.00084$
BECK
2020
SPEC Proton recoil spectrum
$-0.1054$ $\pm0.0055$
BYRNE
2002
SPEC Proton recoil spectrum
$-0.1017$ $\pm0.0051$
STRATOWA
1978
CNTR Proton recoil spectrum
$-0.091$ $\pm0.039$
GRIGOREV
1968
SPEC Proton recoil spectrum
• • We do not use the following data for averages, fits, limits, etc. • •
$-0.10782$ $\pm0.00124$ $\pm0.00133$ 2
HASSAN
2021
SPEC Cold n, unpolarized
$-0.1090$ $\pm0.0030$ $\pm0.0028$ 3
DARIUS
2017
SPEC Cold ${{\mathit n}}$, unpolarized
$-0.1045$ $\pm0.0014$ 4
MOSTOVOI
2001
CNTR Inferred
1  WIETFELDT 2024 updates HASSAN 2021. Includes radiative and recoil corrections to first order, and is averaged over the full Fermi neutron beta spectrum. Supersedes HASSAN 2021.
2  HASSAN 2021 includes the data of DARIUS 2017. Uses the asymmetry in time-of-flight between the beta electron and recoil proton in delayed coincidence. Supersedes DARIUS 2017.
3  DARIUS 2017 exploits a "wishbone" correlation, where the ${{\mathit p}}$ time of flight is correlated with the momentum of the electron in delayed coincidence. Data is included in HASSAN 2021.
4  MOSTOVOI 2001 calculates this from its measurement of $\lambda =\mathit g_{\mathit A}/\mathit g_{\mathit V}$ above.
References