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Particle physics is the branch of theoretical physics that studies the nature of particles that are the constituents of what is usually referred to as matter and radiation. In this context, particles are excited states of quantum fields, and the most important of these for particle physics are the quarks and leptons, which make up matter, and the gauge bosons, which mediate the fundamental forces. The discovery and understanding of these particles has been a major part of 20th-century physics, involving concepts such as quantum mechanics, relativity, and statistics. It forms the basis for much of current research in theoretical and experimental physics. In the broader context of science and mathematics, particle physics is an essential tool for understanding the universe, from the smallest particles to the largest structures.