These elementary particles are excitations of the quantum fields that also govern their interactions. Some particles, such as the photon, are their own antiparticle. These antiparticles can theoretically form a corresponding form of matter called antimatter. The electron has a negative electric charge, the positron has a positive charge. For example, the antiparticle of the electron is the positron. Particles have corresponding antiparticles with the same mass but with opposite electric charges. Mesons are also produced in cyclotrons or other particle accelerators. They occur after collisions between particles made of quarks, such as fast-moving protons and neutrons in cosmic rays. Mesons are unstable and the longest-lived last for only a few hundredths of a microsecond. Two baryons, the proton and the neutron, make up most of the mass of ordinary matter. Hadrons that contain an odd number of quarks are called baryons and those that contain an even number are called mesons. Quarks cannot exist on their own but form hadrons. The three fundamental interactions known to be mediated by bosons are electromagnetism, the weak interaction, and the strong interaction. The first generation consists of up and down quarks which form protons and neutrons, and electrons and electron neutrinos. There are three generations of fermions, although ordinary matter is made only from the first fermion generation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) and bosons (force-carrying particles). Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation.
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