A Study of the Λ(1405) in Lattice QCD

Abstract

The lowest-lying odd-parity state of the Λ baryon lies surprisingly low in mass. Even though it contains a heavier valence strange quark and has odd parity, with an energy of 1405 MeV it lies lower than any other excited spin-1/2 baryon. In the almost 50 years since its discovery there has been significant effort expended to describe the structure of the Λ(1405), however there has been no convincing resolution. It is very difficult to reconcile this low mass with the quark model interpretation for this state, and lattice QCD studies have consistently failed to reproduce it. In this work, we use the PACS-CS (2+1)-flavour full-QCD ensembles together with a variational analysis using source and sink smearing in an attempt to isolate this otherwiseelusive state using lattice QCD. For the first time, we report masses for the lowest-lying states consistent with those in nature – including the Λ(1405). We then build on this result by investigating their electromagnetic properties, and show that the strange quark contribution to the magnetic form factor for the Λ(1405) near-vanishes in the physical limit. Together with a Hamiltonian effective field theory model analysis of the lattice QCD energy levels, this strongly suggests that the structure of the Λ(1405) is dominated by a molecule-like, bound antikaon-nucleon component. That is, we report evidence for the existence of molecular meson-baryon bound states in QCD.