Elementary Particles

So far we have met or mentioned several "elementary" particles: electrons, protons (which are really not elementary; that is why they can change to neutrons in Beta decay), positrons, neutrinos and antineutrinos. The names inspire a sense of science fiction. While the remainder of this chapter presents some of what we know (to a very impressive degree of accuracy), this "Standard Model" of particle physics is almost certainly another (albiet successful) model of things which hold more mysteries the closer we examine them.

We begin by describing the particles themselves. So far as we have observed, all elementary particles are "fermions". These are particles which have spin 1/2 or - 1/2, and which obey the Pauli Principle. Why the spin should be linked to the exclusion principle is the subject of a very advanced "spin - statistics theorem" which we will take as an axiom for now. There appear to be five major groups of elementary particles:

  1. "left handed" "quarks" (whose momentum is parallel to their magnetic moment)

    ParticleCharge(e)Mass(MeV / c 2)
    u (up)2/3400
    d (down)-1/3700
    s (strange)-1/3150
    c (charmed)2/31500
    t (top)2/3158,000 - 194,000
    b (bottom)-1/34700

    (only a few dozen tops have been observed to date)
    The quarks are the constituent particles in composites like the proton and the neutron (see section F below). That their electric charge comes in fractions of e seems to contradict our earlier (Chapter 4) statement that all charge was quantized in units of e. We get around this by saying that quarks only exist in bound groups of two or three with a total integer electric charge. The mass of the top quark is not accurately known, since only a handful have been detected to date. The other strange thing about quarks is that they serve not only as charges for the electric force, but also as charges for the strong force which binds quarks and nucleons together. This strong charge has been called "color", with values red, green and blue, since there are three different kinds of strong charge. Any quark can be any color.
  2. "right handed" quarks (momentum is antiparallel to magnetic moment)

    also come in u, d, s, c, t and b

    We shall see below why we distinguish the left and right handed quarks as separate groups.

  3. left handed "leptons"

    ParticleCharge(e)Mass(MeV / c 2)
    e-1.511
    m-1105.66
    t-11784.2
    ne0<. 000016
    nm0< .3
    nt0< 40

    These include our friend the electron, its heavier brothers and sisters, and the very light (perhaps massless) neutrinos. Neutrinos do not interact electrically, strongly or (very possibly) gravitationally. In fact, there are millions passing through you 24 hours a day and you don't even feel them! They only feel the "weak" force, which we will see in the next section is responsible for radioactive decay. Their masses are given as limits since making them exactly massless has profound consequences for the Standard Model. Therefore, we do not want to assume anything, even though the evidence for massless neutrinos is not bad. Note that there is one neutrino brother for each of the more massive leptons. We will see in the next section that they can exchange identities within those pairs.

  4. right handed leptons

    ParticleCharge(e)Mass(MeV / c 2)
    e-1.511
    m-1105.66
    t-11784.2

    That leptons should come in left and right handed subspecies is not surprising. What is mysterious is that there are no right handed neutrinos! Notice these particles are otherwise identical to their left handed cousins.

  5. the "antiparticles"

    These are predicted by Dirac's Theory. It says that for every particle mentioned above, there also exists an antiparticle with the same mass, opposite charge and "helicity" (handedness). And we've seen (or detected) most of them.

We combine these particles into "generations" of "doublets":

ust
dcb
emt
nenmnt

together with their right handed "singlet" brothers:

ust
dcb
emt

(there are of course antiparticle generations and singlets as well). There is excellent evidence that there are only three generations, but the case is not closed. If so, then we can take the seven particles in the left generation above to be a sort of template for all elementary particles: we simply make successively more massive copies for the next two generations, and antiparticles of all of them. It is possible (as was reported in early 1996) that the quarks themselves may be made up of smaller particles ("partons"), in which case we have a lot more work to do!

The next section is about particle interactions.



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1996, Kenneth R. Koehler. All Rights Reserved. This document may be freely reproduced provided that this copyright notice is included.

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