What particles are closed strings? In String Theory, I understand that gravitons are particles that are closed strings. Are there other particles that are manifested from closed strings? 
 A: The heterotic group decomposes as $E_8~\rightarrow~SU(3)\times E_6$, The $\bf 248$ of the $E_8$ decomposes as
$$
{\bf 248}~\rightarrow~(\bf 8,~\bf 1) + (\bf 1,~\bf 78) + (\bf 3,~\bf 27) + (\bf\bar 3,~\bf\bar{27})
$$
We have here the $(\bf 8,~\bf 1)$ of $SU(3)$ which is identical in form to the irreducible representation used for gluons, or the old nonet theory of hadrons. However, this is not directly QCD in the standard sense. The $(\bf 1,~\bf 78)$ representation of the $E_6$ is one of the triplets transforming under $SU(3)$ with the complex $\bf 27$ dimensional representation of $E_6$. The adjoint representation of $E_8$ contains the adjoints of these subgroups so the 162 generators of the $(\bf 3,~\bf 27) + (\bf\bar 3,~\bf\bar{27})$ of $E_6$ transforms as triplets of $SU(3)$
Now let us look at the decomposition of the $E_6~\rightarrow SO(10)\times U(1)$ The two irreps of the $E_6$ decompose as
$$
{\bf 78}~\rightarrow~{\bf 45}_0 + {\bf 16}_{-3} + {\bf\bar{16}}_3 + {\bf 1}_0
$$
$$
{\bf 27}~\rightarrow~{\bf 16}_1 + {\bf 10}_{-2} + {\bf 1}_1.
$$
Further decompositions are then possible down to $SU(3)\times SU(2)\times U(1)$ The upshot is that all of the particles in the standard model can embed in the $E_6$, and ultimately into the $E_8$. The $(\bf 8,~\bf 1)$ connect with the graviton and the weights of the $E_8$ group.
The GUT model $SU(5)$ as well as $SO(10)$ have complicated $X^{1/3},~Y^{4/3}$ charged bosons that interchange quarks and leptons. The $E_6$ has much the same with more elaborate structure; it is considerably more involved than $SO(10)$. So as a result there are additional particles or states in these more complex gauge theories with exceptional groups.
