Past exam of the mathematics course of the University of Cambridge / 2022 / iii / Paper 306 / 3 / b

At closed-string level $N=\tilde{N}$, the bosonic string mass spectrum isThe state on the leading Regge trajectory uses only level-one oscillators and has maximal spin $J=N+\tilde{N}=2N$. Eliminating $N$ givesBecause $N$ is unbounded, the spectrum contains infinitely many particles of increasing mass and spin. The intercept $J=2$ at $M^2=0$ includes the graviton.