Quantum Spectrum of BPS Instanton States in $ 5d $ Guage Theories
We construct $ 1/2 $ BPS 1-instanton states in the Hilbert space of $ 5d $ gauge theories. The states are local with the scale size $ \rho=0 $. With the degeneracy from the gauge orientation taken into account, the BPS spectrum consists of an infinite number of states in the definite representations of the gauge group and the $ (J,0) $ representation of the $ SO(4)\cong SU(2)_{L} \times SU(2)_{R} $ rotation group. In the brane webs realization of the $ 5d $ $ \mathcal{N}=1 $ gauge theory, BPS states are string webs inside the 5-brane webs. For a theory with the gauge group $ SU(N) $ and the Chern-Simons level $ \kappa $, we give a classification of string webs in the Coulomb branch and show that for each web, one can always find an instanton state carrying the same spin and the same electric charge. The action of supercharges on instanton states gives the instanton supermultiplets. For the $ \mathcal{N} =1$ $ SU(2) $ gauge theory with $ N_{f} \leq 4$, we construct the instanton vector and hyper multiplets, which, together with the original multiplets of the theory, furnish the complete representation of the $ E_{N_{f}+1} $ group at UV.