To Understand a Special Hadron, Researchers Turn to Supercomputers and Quantum Chromodynamics

Scientists Gain new insights into the nature of the puzzling lambda 1405 hyperon resonance and its controversial partner.

Representation of the magnitude of the elastic πΣ hadron scattering amplitude showing the double-pole structure that suggests the hadron has a pair of resonances at 1380 megaelectronvolts (MeV) and 1405 MeV, not just one at 1405 MeV. — Credit: John Bulava, Barbara Cid-Mora, Andrew D. Hanlon, Ben Hörz, Daniel Mohler, Colin Morningstar, Joseph Moscoso, Amy Nicholson, Fernando Romero-Lopez, Sarah Skinner, and Andre Walker-Loud.

The Science

Hadrons are particles formed by quarks and gluons that are bound together by the strong nuclear force. Nuclear scientists need to understand how hadrons behave and interact with other particles. One type of hadron is the baryon—particles consisting of three quarks. Baryons include protons and neutrons, but they also include particles called hyperons. These hyperons are unusual in part because they contain strange quarks, a kind of quark not found in ordinary matter such as protons and neutrons. They have short-lived high-energy excited states called resonances. One resonance, lambda 1405, was predicted in the late 1950s and early 1960s then confirmed experimentally later in the 1960s. This resonance has unusual properties that make it an important subject for nuclear physics studies. To gain insights on the properties and interactions of the lambda 1405 hyperon resonance and other hadrons, researchers used some of the most powerful computers in the world to run special calculations. This research helps explain the forces that hold hadrons together and how they work inside the matter around us.

The Impact

Scientists are studying the lambda 1405 resonance to understand how it emerges from the underlying quark-gluon interactions. The main question is whether the experimental signature of this resonance corresponds to a single hadron or whether it represents a pair of neighboring resonances: one at lambda 1405, one at lambda 1380. In this study, researchers investigated this question by conducting theoretical calculations of the strong interaction using high-performance computing facilities. The findings conclude that there are indeed two hadrons. This result settles a long-standing question about the properties of hadrons. These properties are an important outstanding issue for the Standard Model of Particle Physics.

Summary

This study presents the first lattice quantum chromodynamics (QCD) computation of the coupled scattering amplitudes of lighter hadrons (πΣ-KN) in the energy region around 1405 megaelectronvolts (MeV), focusing on the controversial nature of the lambda 1404 [Λ (1405)] resonance. This resonance has been the subject of debate, with researchers asking whether it represents a single resonance or two closely spaced poles. These interactions are notoriously challenging to probe experimentally due to the short-lived nature of the involved hadron in the scattering process, making theoretical approaches like lattice QCD indispensable.

Using single-baryon and meson-baryon operators, an international team of nuclear physicists extracted finite-volume stationary-state energies to compute scattering amplitudes at slightly unphysical quark masses, leading to a pion mass of mπ≃200 MeV and a kaon mass of m_K≃487 MeV. The results reveal a virtual bound state below the πΣ threshold in addition to the well-established resonance pole just below the KN threshold. The researchers used several parameterizations of the two-channel K-matrix to fit the lattice QCD data, all supporting the two-pole scenario aligned with predictions from effective field theories. This finding provides new insights into the structure of the Λ (1405) resonance and contributes to resolving a long-standing debate in hadron physics, demonstrating the power of lattice QCD in studying these complex interactions.

Contact

Fernando Romero-Lopez
University of Bern
[email protected]

Funding

Computations were carried out on Frontera at the Texas Advanced Computing Center and at the National Energy Research Scientific Computing Center (NERSC), a Department of Energy (DOE) Office of Science user facility at Lawrence Berkeley National Laboratory. This work was also supported in part by the U.S. National Science Foundation (NSF) and the NSF Faculty Early Career Development Program, the DOE Office of Science Nuclear Physics program, and by the DOE's Graduate Research Fellowship Program. In addition, the research was supported in part by a DOE Scientific Discovery through Advanced Computing award, the Mauricio and Carlota Botton Fellowship, and the Heisenberg Programme of the German Research Foundation.

Publications

Bulava, J., et al. (Baryon Scattering (BaSc) Collaboration), Two-Pole Nature of the Λ (1405) Resonance from Lattice QCD. Physical Review Letters 132, 051901 (2024) (Editors’ Suggestion). [DOI: 10.1103/PhysRevLett.132.051901]

Bulava, J., et al. (Baryon Scattering (BaSc) Collaboration), Lattice QCD study of πΣ-K¯N scattering and the Λ (1405) resonance. Physical Review D 109, 014511 (2024) (Editors’ Suggestion). [DOI: 10.1103/PhysRevD.109.014511]