The helium-four atomic nucleus, containing two protons and two neutrons. Its ground-state spin and isospin are zero. A quantized cubic four-Skyrmion supplies a corresponding nuclear state in the soliton description.
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An alpha particle is a type of subatomic particle that consists of two protons and two neutrons, making it identical to the nucleus of a helium-4 atom. It is emitted during a type of radioactive decay known as alpha decay, which occurs in some heavy unstable atomic nuclei (like uranium or radium) as they seek to become more stable.