MadSci Network: Physics

Re: Is a proton really just made up of 3 quarks?

Date: Thu Dec 14 22:09:53 2006
Posted By: James Holliday, Grad student, Physics Department, University of California, Davis
Area of science: Physics
ID: 1165400797.Ph


Greetings. Particle physics is tricky, tricky business. The short answer to your question is "no, the proton is not simply made up of three quarks". Let me explain.

The proton has a mass in energy units of 938.256 MeV, and normally we say it is made up of two Up quarks and a Down quark. If protons only had quarks in them it would be simple to calculate the proton masses. Surprisingly, though, physicists believe that these primary quarks account for only a small fraction of the proton's mass. The remaining mass is associated with a "sea" of quark-antiquark pairs that pop out of nowhere (the vacuum), live for a small amount of time, and then annihilate each other. Although individual pairs appear and vanish quickly, they are found with high probability. An often used analogy is the flickering lights in a dense swarm of fireflies. While any one firefly is only occasionally lit up, there is a constant and bright glow. These transient quarks and antiquark pairs (called mesons) can be of the up or down variety (called pions) or of any of the other quarks flavors. Like the fireflies, only handfulls of quarks-antiquark pairs exist at any one time. Also like the fireflies, on average there is a constant presence of quarks-antiquark pairs. By adding the mass of three primary quarks with the mass of all the quarks-antiquark pairs, we can calculate the total mass of the proton. The exact flavor composition of the quark sea, which gives the proton much of its complexity and richness, is the focus of a number of ongoing experiments.

For more information, I'd like to recommend Fermilab's Inquiring Minds web site as a nice jumping off point.

I hope this has helped.


James R Holliday

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