| Summary: | A measurement has been made of the angular correlation of the neutrino
with a specific nuclear de-excitation γ-ray following exclusive muon capture on ²⁸Si ,
in order to determine the size of the induced-pseudoscalar coupling constant gp of
the weak hadronic current. The correlation is observed via the Doppler-broadened
line shape of the 1229 keV ²⁸ Al γ-ray, which is measured using a pair of Comptonsuppressed
intrinsic Ge detectors. Significant background suppression is achieved
through the use of a coincidence technique in which the 1229 keV γ-ray of interest
is 'tagged' by the subsequent 942 keV γ-ray in the cascade, which is detected in an
array of 24 NaI(Tl) scintillators.
Through a detailed attention to the detectors' response functions as well as
the use of background-subtracted spectra, the γ — v correlation coefficient a is
found to be 0.360±0.059 i n good agreement with a recent measurement at Dubna.
The result obtained yields the value of gp/gA = 0.0±3.2 when compared to the
latest calculation of the γ — v angular correlation, suggesting a massive quenching
of the induced-pseudoscalar coupling constant in 2 8 S i in comparison with the
value expected for a free nucleon. However other available calculations give the
values gp/gA — 5.3±2.0 and gp/gA = 4.2±2.5, but the model-dependence of these
intriguing results has yet to be assessed fully.
A measurement of the correlation coefficient a of the 2171 keV ( 1⁺ → 2⁺)
γ-ray has solved the enigma of the unphysical result that was found by a previous
experiment. In addition, the lifetime of the 2201 keV ²⁸ Al level has been measured
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