Particle Physics Tutorial 8 - Classification of Particle (Quarks)

 Contents

We can show the way the particles are classified as a tree:

We are going to look at the hadrons, which are subdivided into mesons and baryons.

The key definition of a hadron is that it feels the strong force.  No other particles feel the strong force.

 What is the key definition of a hadron?

Baryons

• A baryon is defined as a particle made of quarks and that feels the strong force.

• A proton is a hadron that feels the electromagnetic force and the strong force.

• A neutron does not feel the electromagnetic force, but does feel the strong force.

Electrons feel the electromagnetic force but not the strong force.

Quarks

Quarks (pronounced “quork” as in pork) are the fundamental particles from which hadrons are made.  They cannot exist on their own.  Quarks make up the hadrons.

In baryons (the “heavy weights”) they are found as triplets.  In mesons (the “middle weights”), 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.

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.

 Quark Charge (Q) Baryon number  (B) Strangeness (S) Down (d) -1/3 1/3 0 Up (u) +2/3 1/3 0 Strange (s) -1/3 1/3 -1 Anti-down (d) +1/3 -1/3 0 Anti-up (u) -2/3 -1/3 0 Anti-strange (s) +1/3 -1/3 +1 NOTE: In many books you will see the anti-particles with a bar over the symbol, for example, ū, ("u-bar") for anti-up.  However it is not easy to produce these in this particular web editor.  Therefore I will represent antiparticles by using a white letter on a black background.  So d is to be read as "d-bar", meaning anti-down.

• 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.

The strange quark has a quantum number of its own, the strangeness number.

The diagram shows the three generations of quarks:

Physicists see patterns in the quarks.  They call this symmetry.  The up quark matches the down, the strange is balanced by the charm (so named because it worked like a charm to support the symmetry).  The top quark matches the bottom quark.

The quark with the lowest mass is the up.  The down quark has a slightly higher mass, therefore slightly higher energy.  Remember that mass and energy at this level are interchangeable.  One of the down quarks in a neutron decays by beta minus decay to an up quark.  The hadrons made up from other quarks have a very short lifetimes.

 It is wrong to say that the up quark is the most stable, as it implies that quarks can be found singly.  Single quarks have never been observed.

 What is the charge in Coulomb (C) of an up-quark?

Strangeness

The strange quark is an oddity.  When the particle zoo was first described in the earliest particle physics experiments, most particles were found to have an average lifetime of about 10-23 s (not very long).  These particles decayed through the strong interaction.

However, there were some particles whose behaviour was strange, in that they lasted about 10-10 s (still not that long, but a lifetime compared with the others).  The first of these particles was discovered in 1947, but the existence of the strange quark was predicted in 1964.

Strange quarks have a quantum number called strangenessFor matter, the value of strangeness is strange itself; the strangeness number for the strange quark is -1.  For the strange anti-quark, it is +1.  All other quantum numbers for matter are positive, while all other quantum numbers for antimatter are negative.

To work out the strangeness of a particle, we use this simple relationship:

Strangeness = (number of strange quarks + number of strange anti-quarks)

For a K+ (anti-strange and up):

Strangeness = 0 + 1 = +1

It is possible to have a strangeness of 2, or even 3 in a very strange baryon.

Pentaquarks

There has been some recent interpretations of results to suggest that groups of 5 quarks are possible.  The data are very uncertain, and many physicists are not convinced.  The pentaquark forms a group of 4 quarks and 1 antiquark, giving a baryon number of 1.