Measurement of the Helicity ofWBosons in Top Quark Decays

We use the transverse momentum spectrum of leptons in the decay chain $t\to \mathrm{bW}$ with $W\to l\nu$ to measure the helicity of the $W$ bosons in the top quark rest frame. Our measurement uses a $t\overline{t}$ sample isolated in $106\pm 4{\mathrm{pb}}^{-1\mathrm{}}$ of data collected in $p\overline{p}$ collisions at $\sqrt{s}=1.8\mathrm{TeV}$ with the CDF detector at the Fermilab Tevatron. Assuming a standard $V-A$ weak decay, we find that the fraction of $W$'s with zero helicity in the top rest frame is $F_0=0.91\mathrm{}\pm 0.37\left(\mathrm{stat}\right)\pm 0.13\left(\mathrm{syst}\right)$, consistent with the standard model prediction of $F_0=0.70\mathrm{}$ for a top mass of $175\mathrm{GeV}{/c}^2$.