Weak interaction

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Weak interaction (often called the weak force or sometimes the weak nuclear force) is one of the four fundamental interactions of nature, along with strong interaction, electromagnetic force, and gravitation. In the Standard Model of particle physics, it is due to the exchange of the heavy W and Z bosons. Its most familiar effect is beta decay (or the emission of electrons by neutrons or positrons by protons in atomic nuclei) and the associated radioactivity. It is a non-contact force, called weak because the typical field strength is 10−11 times the strength of the electromagnetic force and some 10−13 times that of the strong force, when forces are compared between particles interacting in more than one way.

The weak force was originally (in the 1930s) described by Fermi's theory of a contact four-fermion interaction, that is to say, a "force" without range.



The weak interaction affects all left-handed fermions and right-handed anti-fermions. Other than gravity, it is the only force affecting neutrinos. The weak interaction is unique in a number of respects:

Due to the large mass of the weak interaction's carrier particles (about 90 GeV/c2), their mean life is about 3×10−25 seconds.

Since the weak interaction is both very weak and very short range, its most noticeable effect is due to its other unique feature: flavor changing. Consider a neutron (quark content: udd, or one up quark and two down quarks). Although the neutron is heavier than its sister nucleon, the proton (quark content uud), it cannot decay into a proton without changing the flavor of one of its down quarks. Neither the strong interaction nor electromagnetism allow flavour changing, so this must proceed by weak decay. In this process, a down quark in the neutron changes into an up quark by emitting a W boson, which then decays into a high-energy electron and an electron antineutrino. Since high-energy electrons are beta radiation, this is called a beta decay.

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