Particle Physics Tutorial 8 - Classification of Particle (Quarks)








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.


Question 1

What is the key definition of a hadron?




Electrons feel the electromagnetic force but not the strong force.


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:

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.


Question 2

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




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.



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.