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Abstract:
We study the embedding of Kac–Moody algebras into Borcherds (or generalized Kac–Moody) algebras which can be explicitly realized as Lie algebras of physical states of some completely compactified bosonic string. The extra “missing states” can be decomposed into irreducible highest or lowest weight “missing modules” w.r.t. the relevant Kac–Moody subalgebra; the corresponding lowest weights are associated with imaginary simple roots whose multiplicities can be simply understood in terms of certain polarization states of the associated string.We analyse in detail two examples where the momentum lattice of the string is given by the unique even unimodular Lorentzian lattice II1;1 or II9;1, respectively. The former leads to the Borcherds algebra gII1;1 , which we call “gnome Lie algebra”, with maximal Kac–Moody subalgebra A1. By the use of the denominator formula a complete set of imaginary simple roots can be exhibited, while the DDF construction provides an explicit Lie algebra basis in terms of purely longitudinal states of the compactified string in two dimensions. The second example is the Borcherds algebra gII9;1 , whose maximal Kac–Moody subalgebra is the hyperbolic algebra E10. The imaginary simple roots at level 1, which give rise to irreducible lowest weight modules for E10, can be completely characterized; furthermore, our explicit analysis of two nontrivial level2 root spaces leads us to conjecture that these are in fact the only imaginary simple roots for gII9;1.