From: baez@galaxy.ucr.edu (john baez)
Subject: Re: This Week's Finds in Mathematical Physics (Week 131)
Date: 8 Mar 1999 22:21:56 GMT
Newsgroups: sci.physics.research,sci.physics,sci.math
Keywords: hierarchy of elementary particles

In article <36G42008.63DB29B1@dcwi.com>, Ralph Frost  <refrost@dcwi.com> wrote:
>John Baez wrote:
> 
>> [...] bigger theory where all 15 particles of each generation [...]
>
>Is there some real simple list you folks have of the different
>generations of particles and the number of particles in each generation?

Yeah.

The first generation looks like this:

electron        electron neutrino         down quark        up quark

Now that may not look like 15 particles - and indeed, it's often good
to say there are only 4 particles per generation - but if you count all
the states you get a total of 15:

left-handed electron
right-handed electron
left-handed neutrino
left-handed down quark: red, green and blue
right-handed down quark: red, green and blue
left-handed up quark: red, green and blue  
right-handed down quark: red, green and blue

In other words, each particle *except the neutrino* comes in
left-handed and right-handed forms, and besides that, every 
sort of quark comes in three colors.  

The second and third generations work *exactly the same way* except 
that the names are different:

muon            muon neutrino             strange quark     charm quark
tau             tau neutrino              bottom quark      top quark

There are only 3 generations.

These are not all the particles in the Standard Model.  These
are the "fermions".  There are also "gauge bosons" carrying the 
electromagnetic, weak and strong forces.  The electromagnetic
field is carried by the photon, the weak force is carried by the
W+, W- and Z, and the strong force is carried by gluons, which 
come in 8 colors.  

There is also an extra boson of a different sort, the Higgs boson,
which has not yet been seen.  

In short the Standard Model looks roughly like this:

------------------------------------------------------------------------

                            GAUGE BOSONS
                           
    ELECTROMAGNETIC FORCE          WEAK FORCE         STRONG FORCE
        
         photon                     W+                  gluon
                                    W-                (8 colors)
                                    Z  


                            FERMIONS

        LEPTONS                                     QUARKS

electron        electron neutrino         down quark        up quark
muon            muon neutrino             strange quark     charm quark
tau             tau neutrino              bottom quark      top quark

(right&left)    (only left)               (right&left)      (right&left)
                                          (3 colors)        (3 colors)


                        HIGGS BOSON (not yet seen)

-----------------------------------------------------------------------

Of course I am leaving out the most important thing of all: the equations 
that actually say what happens.  It's the incredible quantitative success
of these equations that makes this theory deserve the name "Standard Model".  

Included in these equations are the masses of the various particles
above.  The masses of the particles are nicely listed at:

http://www.cern.ch/Public/SCIENCE/massmystery.html

As you can see, one of the most irritating asymmetries of the Standard 
Model is the absence of right-handed neutrinos.  Nobody has ever 
detected a right-handed neutrino, but the recent experiments I was 
describing in "week130" indirectly hint that right-handed neutrinos
may exist - which would make me very happy.