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Elementary Particles

Particles of Antimatter

W boson, weak interaction, beta decay, weak force, fermions

While the strong force holds the nucleus of an atom together, the weak force can make the nucleus decay, changing some of its particles into other particles. The weak force is so named because it is far weaker than the electromagnetic or strong forces. For example, an interaction involving the weak force is 10 quintillion (10 billion billion) times less likely to occur than an interaction involving the electromagnetic force. Three particles, called vector bosons, carry the weak force. The weak force equivalent to electric charge and color charge is a property called weak hypercharge. Weak hypercharge determines whether the weak force will affect a particle. All fermions possess weak hypercharge, as do the vector bosons that carry the weak force.

All elementary particles, except the force carriers of the other forces and the Higgs boson, interact by means of the weak force. But the effects of the weak force are usually masked by the other, stronger forces. The weak force is not very significant when considering most of the interactions between two quarks. For example, the strong force completely overwhelms the weak force when a quark bounces off another quark. Nor does the weak force significantly affect interactions between two charged particles, such as the interaction between an electron and a proton. The electromagnetic force dominates those interactions.

The weak force becomes significant when an interaction does not involve the strong force or the electromagnetic force. For example, neutrinos have neither electric charge nor color charge, so any interaction involving a neutrino must be due to either the weak force or the gravitational force. The gravitational force is even weaker than the weak force on the scale of elementary particles, so the weak force dominates in neutrino interactions.

One example of a weak interaction is beta decay involving the decay of a neutron. When a neutron decays, it turns into a proton and emits an electron and an electron antineutrino. The neutron and antineutrino are electrically neutral, ruling out the electromagnetic force as a cause. The antineutrino and electron are colorless, so the strong force is not at work. Beta decay is due solely to the weak force.

The weak force is carried by three vector bosons. These bosons are designated the W+, the W-, and the Z0. The W bosons are electrically charged (+1 and –1), so they can feel the electromagnetic force. These two bosons are each other’s antiparticle counterparts, while the Z0 is its own antiparticle. All three vector bosons are colorless. A distinctive feature of the vector bosons is their mass. The weak force is the only force carried by particles that have mass. These massive force carriers cannot travel as far as the massless force carriers of the three long-range forces, so the weak force acts over shorter distances than the other three forces.

When the weak force affects a particle, the particle emits one of the three weak vector bosons—W+, W-, or Z0—and changes into a different particle. The weak vector boson then decays to produce other particles. In interactions that involve the W+ and W-, a particle changes into a particle with a different electric charge. For example, in beta decay, one of the down quarks in a neutron changes into an up quark and the neutron releases a W boson. This change in quark type converts the neutron (two down quarks and an up quark) to a proton (one down quark and two up quarks). The W boson released by the neutron could then decay into an electron and an electron antineutrino. In Z0 interactions, a particle changes into a particle with the same electric charge.

A quark or lepton can change into a different quark or lepton from another generation only by the weak interaction. Thus the weak force is the reason that all stable matter contains only first generation leptons and quarks. The second and third generation leptons and quarks are heavier than their first generation counterparts, so they quickly decay into the lighter first generation leptons and quarks by exchanging W and Z bosons. The first generation particles have no lighter counterparts into which they can decay, so they are stable.

Article key phrases:

W boson, weak interaction, beta decay, weak force, fermions, quintillion, electromagnetic force, Higgs boson, neutrinos, elementary particles, gravitational force, quarks, strong force, electric charge, charged particles, neutron, atom, proton, electron, particles, interaction, interactions, effects, cause, example, means, times, property, reason, work, Z0

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