Check for Conservation Laws as a Particle Physicist Would

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Conservation Laws: Read - Watch - Answer

Concepts Addressed

Classroom Use

Conservation of energy and conservation of charge are two concepts already covered by many physics and chemistry curricula. The decay page provides a convenient focus for another lesson in such a unit.

Students can test the decays and observe that the net charge of all the products does in fact equal the charge on the original particle. They can also confirm, with the information given in the table, that they do not see particles produced with a greater total rest mass (or energy) than that of the original particle. Mass does not seem to be conserved until you conclude that the seemingly lost mass is converted into kinetic energy of the resulting particles (i.e., they each move apart from the original center of mass at some nonzero speed). The equivalence of mass and energy, with the same units of MeV (1.000 mega-electron volt/c2 = 1.602 x 10-13 joule = 1.783 x 10-30 kilogram), hopefully becomes more evident.

What Happens on the Decay Page

The decay page shows a clickable menu of particles. When you click on the symbol for a particle, a decay is shown near the bottom of the page. You can clear the result, or you can click any symbol any number of times to observe more decays.

Background Information

Particle notation nearly matches the versions put forth by the Particle Data Group at Berkeley Lab. Many antiparticles have a bar over their symbol. (Antiparticles are equal in mass and opposite in charge to their respective particles.) For example, a "p" with a bar over it represents the antiproton, which has the same mass as the proton, but has a charge of -1. Some particle-antiparticle pairs, however, are only distinguished by their signs. For example, the positive and negative pions are a particle-antiparticle pair, just like the proton and antiproton are.

The result can be confusing. Just because a particle has a partner that is oppositely charged does not make it an antiparticle. For example, one kind of particle, the sigma, has six manifestations. There is a "positive" sigma, which has an equal-massed antiparticle that is negative. There is the "neutral" sigma, which has an equal-massed antiparticle that is neutral. Then there is the "negative" sigma, which has an equal-massed antiparticle that is positive. What is inconvenient is that the "positive" sigma and the anti-"negative" sigma do not have the same mass, and therefore are not antiparticles to each other. The same mismatch applies to the "negative" sigma and the anti-"positive" sigma.

What remains though is the basic symmetry between particles and antiparticles. They are still equal in mass and opposite in charge if they are true antiparticles. Indeed a particle is the antiparticle to its own antiparticle.

However, there are some complications. Some uncharged particles, like the eta, the neutral pion, and the gamma, are their own antiparticles. On the other hand, we observe that other uncharged particles, like the neutron, do have an antiparticle. Thus, the pattern seems muddled until we suppose that there are elementary particles that make up the hadrons. These elementary particles are called quarks.

For your preparation, the particles names, their charges, and their rest masses in megaelectron volts per speed-of-light-squared (MeV/c2) are shown on the table.

Actually, there are many more hadrons than are shown on these pages. However, these hadrons were all discovered by the 1960s, and it was their existence that first prompted scientists to develop the quark model.

Since then, more hadrons have been discovered. Now, there are six known quarks, but only three (up, down, and strange) form the hadrons you see on these pages.

Also, though there are gauge bosons for the strong interaction (gluons) and weak interaction (W+, W-, and Z0), only the boson for the electromagnetic interaction, the gamma, is shown here.