Type II superstring theory

Type II superstring theory is a ten-dimensional closed oriented superstring theory with spacetime supersymmetry and no tachyon. Its massive closed-string levels have squared masses separated by $4/{\alpha }^{\prime }$.

The four-point tree amplitude exposes poles at $M_n^2=4n/{\alpha }^{\prime }$ for $n=0,1,2,\dots$. The $n=0$ level is massless, and positive $n$ form an infinite tower of massive string excitations.