The complete massless singlet spectrum in the free-field construction of heterotic strings on Calabi--Yau orbifolds

Abstract

We apply the free-field construction of the heterotic string compactified on Berglund--Hübsch Calabi--Yau orbifolds by computing the full spectrum of massless $E_6$ singlets. The contribution of the descendant vertices is obtained by combining the exact content of the irreducible $N=2$ minimal model representations, encoded in the ranks of the Shapovalov matrices, with the twisted sector structure of the orbifold. The method reproduces the known spectrum of the quintic orbifold with Hodge numbers $(17,21)$, namely $17$ generations, $21$ antigenerations and $234$ singlets. For the quintic itself we obtain $330$ singlets. We show that this number, rather than the frequently quoted value $326$, obtained from the geometric description, is the correct one at the Gepner point, in agreement with the Landau--Ginzburg computation of Kachru and Witten. We also provide an explicit construction of the general vertices. We then compute the new spectra of the two remaining quintic orbifolds, $Z_5[0,1,2,3,4]$ with $(21,1,210)$ and $Z_5[0,0,0,1,4]$ with $(49,5,258)$, where the exceptional Hodge number $h^{2,1}=49$ arises from the twisted sectors. All four examples satisfy exact mirror-symmetry checks.