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Quarks

Quarks (pronounced “quork” as in pork) are the fundamental particles from which hadrons are made.  They cannot exist on their own.  In baryons they are found as triplets.  In mesons, they are always in a quark-antiquark pair.

There are three main quarks, up, down, and strange.  The names have no real significance beyond the imagination of the physicist that dubbed them such.  They have corresponding antiquarks.  There are three others with even odder names, top (sometimes called "truth"), bottom ("beauty"[!]), and charm, which we won’t worry about here.

Notice how they go in matching pairs.

There are three quantum numbers associated with quarks:

• Charge, expressed as the fraction of the electronic charge.  1/3 e = 5.33 ´ 10^-20 C

• Baryon number

• Strangeness number, when there are strange quarks.

• Each antiquark has equal and opposite values of charge, baryon number and strangeness.

• Baryons are made of three quarks; antibaryons of three antiquarks.

• Mesons are made up of one quark and one antiquark.

• Gluons bind quarks together; they are subject to the strong interaction.

Some mesons such as the neutral Kaon and the neutral Pion (Pi-meson) can be made up in different ways.

Hadrons

There are a very large number of particles that are classified as hadrons, which are subdivided into two further classifications, the mesons, and the baryons. 

• Hadrons interact by the strong, weak, and electromagnetic force.    

• They are not fundamental particles but have a structure.

• They have non-zero rest masses, about 1 GeV/c²

• They have an associated value of charge, Q,and baryon number B.

Hadrons with zero baryon number are called mesons; those with baryon number of 1 are called baryons. 

Mesons

These particles have a smaller rest mass than the baryons (and a lower rest mass than the tau lepton).  They have:

• Zero baryon number. 

• Short lifetimes.

• Antiparticles

Here are a few mesons:

 Notice how short the lifetimes are of these mesons.

We should note the following:

• Mesons have TWO quantum numbers that must be conserved in interactions.  The charge is denoted by Q, the baryon number by B.  Mesons have a baryon number of 0.

• Mesons have a lepton number of 0.  This must be conserved in any interactions with leptons.

Here is a typical decay:

Notice the conservation of charge and baryon number.

Here are some more:

Baryons

These are the heavyweights of particle physics, and include the familiar proton and neutron.

• They are made up of three quarks

• They have quantum numbers such as charge and baryon number, which must be conserved in interactions.

  Let us look at the properties of the baryons:

Typical Decay

The proton is the only stable baryon.  All the others spontaneously decay, although the neutron within a nucleus is stable, apart from beta decay.  The decay times are incredibly short, except the isolated neutron which takes about 8 to 10 minutes.  Baryons decay to protons, either directly (S+ --> p + p⁰) or indirectly (W- --> L⁰ + K, then L⁰ -->  p +  p-).  Mesons decay to photons or leptons.

As in radioactivity, the decay of particles is random.  The values quoted are the mean lifetimes, not half-lives.

 It is believed that gluons are shuttled backward and forward between the quarks like rugby footballs. 

Mesons bind the baryons together with the strong force.   

The proton is the only baryon that is stable in isolation.  The neutron on its own decays to a proton by beta minus decay after about 14 minutes.  The decay is as a result of the weak interaction that occurs within nucleons.