Level-one intercept bound in covariant string quantization
= Level-one intercept bound in covariant string quantization
{title2=$a\leq1$}
A polarization obeys $p\cdot\epsilon=0$ and $p^2=(a-1)/\alpha'$. If $a>1$, spacelike momentum allows a negative-norm timelike polarization. Thus ghost absence requires $a\leq1$. At $a=1$ the <null string state> identification $\epsilon\sim\epsilon+\lambda p$ leaves $D-2$ polarizations.