Measurement of the Transverse Polarization of Electrons Emitted in Free-Neutron Decay

Both components of the transverse polarization of electrons (${\sigma }_{T_1}$, ${\sigma }_{T_2}$) emitted in the $\beta$-decay of polarized, free neutrons have been measured. The $T$-odd, $P$-odd correlation coefficient quantifying ${\sigma }_{T_2}$, perpendicular to the neutron polarization and electron momentum, was found to be $R=0.008\pm 0.015\pm 0.005$. This value is consistent with time reversal invariance and significantly improves limits on the relative strength of imaginary scalar couplings in the weak interaction. The value obtained for the correlation coefficient associated with ${\sigma }_{T_1}$, $N=0.056\pm 0.011\pm 0.005$, agrees with the Standard Model expectation, providing an important sensitivity test of the experimental setup.

Figure 1

Schematic top view of the experimental setup. A sample projection of an electron V-track event is indicated.

Figure 2

Background-corrected experimental energy distributions (shaded areas) of (a) the single-track and (b) V-track events compared with simulations. (c) Background contribution (shaded) to vertex $x$-coordinate distribution of V-track events. The arrow indicates the Mott-foil position.

Figure 3

Left: experimental asymmetries $\mathcal{A}$ corrected for the $P\overline{\beta }A\overline{\mathcal{F}}$ term for the 2007 data set as a function of ($\alpha$ (defined in text). The solid line illustrates the two-parameter ($N$, $R$) least-square fit to the data. Indicated errors are of statistical nature. Right: geometrical factors $\overline{\mathcal{F}}(\alpha )$, $\overline{\mathcal{G}}(\alpha )$, and $\overline{\mathcal{H}}(\alpha )$ for the same data set. Dotted lines are to guide the eye only.

Figure 4

Experimental bounds on the scalar vs tensor normalized couplings (upper) and leptoquark exchange helicity projection amplitudes (lower panels). The grey areas represent the information as defined in Ref. 18, while the lines represent the limits resulting from the present experiment. Solid, dashed, and dotted lines correspond to $1/\sigma$, $2/\sigma$, and $3/\sigma$ confidence levels, respectively, in analogy to decreasing intensity of the grey areas.