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Name: Kevin L.
Status: educator
Age: 30s
Location: N/A
Country: N/A
Date: Thursday, June 06, 2002

How do I explain to a student how a neutron is made up of three quarks (UDD), but can decay into a proton (UUD) upon beta decay? I am not sure how to explain that quarks are fundamental, and so are electrons, but the neutron will emit an electron in its decay, yet still be made up of three quarks?

I do not know of an "easy" or "simple" explanation to your question. This is how nature behaves, so there is no "explanation" it is experimental results. Why is a neutron inside an atomic nucleus indefinitely stable (not counting radioactive fission etc.), but a free neutron has a half life of only 12 min!!!

The "explanation", to the extent that one exists, lies in the domain of quantum electrodynamics (QED), which is the theory of the interaction of photons and electrons. A small book, just over 100 pages, by Richard Feynman entitled, "Q.E.D." is the best treatment of the topic I have seen. It is the transcript of a series of four lectures given by Feynman. It is readable, even for high schoolers, and I do not think it has a single equation in it. Highly recommended reading.

Vince Calder


When a neutron undergoes beta decay, it becomes a set of three particles: proton, electron, anti-neutrino. The number of quarks is still the same: three. The number of leptons is the same: zero. An electron has a lepton number of +1. An anti-neutrino has a lepton number of -1. It is an anti-lepton.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

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