Improved Measurement of the Form Factors in the Decay ${\Lambda }_c^{+}\to \Lambda e^{+}{\nu }_e$

Using the CLEO detector at the Cornell Electron Storage Ring, we have studied the distribution of kinematic variables in the decay ${\Lambda }_c^{+}\to \Lambda e^{+}{\nu }_e$. By performing a four-dimensional maximum likelihood fit, we determine the form factor ratio, $R=f_2/f_1=-0.31\pm 0.05(\mathrm{stat})\pm 0.04(\mathrm{syst})$, the pole mass, $M_{\mathrm{pole}}=[2.21\pm 0.08(\mathrm{stat})\pm 0.14(\mathrm{syst})]\mathrm{GeV}/c^2$, and the decay asymmetry parameter of the ${\Lambda }_c^{+}$, ${\alpha }_{{\Lambda }_c}=-0.86\pm 0.03(\mathrm{stat})\pm 0.02(\mathrm{syst})$, for $⟨q^2⟩=0.67$ $(\mathrm{GeV}/c^2{)}^2$. We compare the angular distributions of the ${\Lambda }_c^{+}$ and ${\overline{\Lambda }}_c^{-}$ and find no evidence for $CP$ violation: ${\mathcal{A}}_{{\Lambda }_c}=\frac{({\alpha }_{{\Lambda }_c}+{\alpha }_{{\overline{\Lambda }}_c})}{({\alpha }_{{\Lambda }_c}-{\alpha }_{{\overline{\Lambda }}_c})}=0.00\pm 0.03(\mathrm{stat})\pm 0.01(\mathrm{syst})\pm 0.02$, where the third error is from the uncertainty in the world average of the $CP$-violating parameter, ${\mathcal{A}}_{\Lambda }$, for $\Lambda \to p{\pi }^{-}$.

Figure 1

Projections of the data (points with statistical error bars) and the fit (solid histogram) onto $t$, $\cos {\theta }_{\Lambda }$, $\cos {\theta }_W$, and $\chi$. The dashed lines show the sum of the background distributions.

Figure 2

Projections of the data (points with statistical error bars) and the fit (solid histogram) onto $\cos {\theta }_{\Lambda }$, $\cos {\theta }_W$, and $\chi$ for two $t$ regions. The plots labeled (a)–(c) are for $t<0.5$; (d)–(f) are for $t>0.5$. The dashed lines show the sum of the background distributions.