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Q & A: Beta decay

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Q:
I understand beta decay in terms of what particles are created but i dont understand why an electron is created when a down quark decays to an up quark.
- Daniel (age 17)
King Henry VIII School, Coventry, England
A:
Well, there are some rules that these particles have to play by. One is the conservation of electric charge. A down quark has a charge of -1/3 (in units of a proton's charge), and an up quark has a charge of +2/3. Two up quarks and a down quark give you a total charge of +1 (a proton) and two down quarks and an up quark give you a total charge of zero (a neutron).

In neutron decay, the end products are a proton, an electron, and an electron antineutrino. To keep the charge adding up the same before and after the decay, the neutron has to emit something negatively charged. Seen on the quark level, one of the down quarks emits the electron and the antineutrino in order to turn into an up quark.

The antineutrino is there to conserve the total number of electrons in the interaction. It has an "electron-number" of -1 while the electron has electron-number +1.

The whole interaction is mediated by exchanging a W- boson (charge: -1). The down quark emits the W- boson and turns into the up quark, and the W- boson immediately produces the electron and the antineutrino. W bosons, the carriers of the weak nuclear force, have masses of 80.5 GeV or so, about 80 times that of a proton. So they cannot lead honest lives, but must hide under Heisenberg's uncertainty principle for energy and time (they can only exist for a very short amount of time, do their interaction, and disappear).

Heavier versions of the down quark also decay in this way, but they can produce heavier versions of electrons, as well as the electrons themselves. The heavier down-type quarks are the strange and bottom quarks, and the heavier cousins of the electron are the muon and tau. Neutrons can only emit electrons and neutrinos because the amount of available energy (given by the mass differnece between the proton and the neutron) is not enough to produce any of the heavier particles.

The W boson was postulated in the 1960's as a way to explain neuclear beta decay in a way that makes theoretical sense, and it was discovered at CERN in 1982 in high-energy proton-antiproton collisions.

Tom

(republished on 07/20/06)

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