  In their seminal work J. Lepowsky and R. L. Wilson gave a vertex-operator
theoretic interpretation of Gordon-Andrews-Bressoud's generalization of
Rogers-Ramanujan combinatorial identities, by constructing bases of vacuum
spaces for the principal Heisenberg subalgebra of standard
$\widehat{\mathfrak{sl}}_{2}$-modules, parametrized with partitions satisfying
certain difference 2 conditions. In this paper we define quasi-particles in the
principal picture of $\widehat{\mathfrak{sl}}_{2}$ and construct quasi-particle
monomial bases of standard $\widehat{\mathfrak{sl}}_{2}$-modules for which
principally specialized characters are given as products of sum sides of the
corresponding analytic Rogers-Ramanujan-type identities with the character of
the Fock space for the principal Heisenberg subalgebra.
