For almost 50 years the structure of the Λ(1405) resonance has been a mystery. Even though it contains a heavy strange quark and has odd parity, its mass is lower than any other excited spin-1/2 baryon. Dalitz and co-workers speculated that it might be a molecular state of an antikaon bound to a nucleon. However, a standard quark-model structure is also admissible. Although the intervening years have seen considerable effort, there has been no convincing resolution. Here we present a new lattice QCD simulation showing that the strange magnetic form factor of the Λ(1405) vanishes, signaling the formation of an antikaon-nucleon molecule. Together with a Hamiltonian effective-field-theory model analysis of the lattice QCD energy levels, this strongly suggests that the structure is dominated by a bound antikaon-nucleon component. This result clarifies that not all states occurring in nature can be described within a simple quark model framework and points to the existence of exotic molecular meson-nucleon bound states.The spectrum of hadronic excitations observed at accelerator facilities around the world manifests the fundamental interactions of elementary quarks and gluons, governed by the quantum field theory of quantum chromodynamics (QCD). Understanding the complex emergent phenomena of this field theory has captivated the attention of theoretical physicists for more than four decades.Of particular interest is the unusual nature of the lowestlying excitation of the Lambda baryon [1-8] the "Lambda 1405," Λ(1405). The Lambda baryon is a neutral particle, like the neutron, composed of the familiar up (u) and down (d) quarks together with a strange quark (s).For almost 50 years the structure of the Λ(1405) resonance has been a mystery. Even though it contains a relatively massive strange quark and has odd parity, both of which should increase its mass, it is, in fact, lighter than any other excited spin-1/2 baryon. Identifying the explanation for this observation has challenged theorists since its discovery in the 1960s through kaon-proton [1] and pion-proton production [2] experiments.While the quantum numbers of the Λ(1405) can be described by three quarks, (uds), its totally unexpected position in the spectrum has rendered its structure quite mysterious [9]. Before the quark model had been established, Dalitz and co-workers [10,11] speculated that it might be a molecular state of an antikaon, K, bound to a nucleon, N . Whereas the πΣ energy threshold is well below the Λ(1405) resonance position the KN energy threshold is only slightly above. A molecular KN bound state with a small amount of binding energy presents an interesting candidate for the structure of the Λ(1405). Although the intervening years have seen enormous effort devoted to this resonance [8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24], there has been no convincing resolution.Herein, we present the very first lattice QCD calculation of the electromagnetic form factors of the Λ(1405). This calculation reveals the vanishing of the...

The odd-parity ground state of the Λ baryon lies surprisingly low in mass. At 1405 MeV, it lies lower than the odd-parity ground-state nucleon, even though it has a valence strange quark. Using the PACS-CS (2+1)-flavor full-QCD ensembles, we employ a variational analysis using source and sink smearing to isolate this elusive state. For the first time we reproduce the correct level ordering with respect to nearby scattering thresholds. With a partially quenched strange quark to produce the appropriate kaon mass, we find a low-lying, odd-parity mass trend consistent with the experimental value.

The ability for most hadrons to decay via strong interactions prevents the direct measurement of their electromagnetic properties. However, a detailed understanding of how these resonant states feature in scattering processes can allow one to disentangle such information from photo production processes. In particular, there has been increasing interest in the determination of magnetic dipole moments using such methods. In a recent study [1], Gudiño et al. provide the first experimental determination of the magnetic dipole moment of the rho meson. To facilitate a comparison with this experimental determination, we present a calculation of the rho meson and pion electromagnetic form factors calculated in the framework of Lattice QCD. Using the PACS-CS 2+1 flavour full QCD gauge field configurations, we are able to access low Q 2 values at near-physical quark masses. Through the use of variational techniques, we control excited state systematics in the matrix elements of the lowest-lying states and gain access to the matrix elements of the first excited state. Our determination of the rho meson g-factor gρ = 2.21(8) is in excellent agreement with this experimental determination, but with a significantly smaller uncertainty.