Particle Physics Tutorial 9 - Particle Classification (Mesons)


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.



Mesons consist of one quark and one anti-quark.  Mesons can feel the strong force, the weak force, and, if charged, the electromagnetic force.

Mesons do not consist of two quarks (or two anti-quarks).


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

We should note the following:

Here are a few mesons:





Lifetime (s)





0.8 10-16 

Up, anti-up (uu) OR down, anti-down (dd)




2.6 10-8

Up, anti-down (ud)




1.2 10-8

Up, anti-strange (us)




8.9 10-11 

5.2 10-8 

Down, anti-strange (ds)

Strange, anti-down (sd)


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.


Question 1 What is the quark composition of the p- meson?


Question 2 Use the baryon number of the quarks to explain why the baryon number of a meson is zero.


Remember that the meson consists of a quark and an anti-quark.  Therefore the charge on a meson is either +1, 0, or -1.  It is never anything else like +2/3.  If your answer is 2/3, etc, you have forgotten that one quark is an anti-quark.

The picture below shows the complete menagerie of mesons that contain the up, down, and strange quarks:

These are the mesons that can be obtained using the up, down, and strange quarks and their antiparticles.  If the quarks are up and down types (quark and anti-quark), a pi-meson (pion) is formed.  There are three pions. 

If there is a strange quark, then we get a Kaon (K-meson).  Kaons always have strange quarks; pions have no strange quarks.  There are four kaons possible in this diagram.

A considerably bigger particle freak-show can be obtained if the other quarks are involved, but that is not on our syllabus.  The weirder mesons are only found in high-energy particle collisions.

The pion (p meson) is thought to be involved in transmitting the strong force.

It is thought to shuttle backwards and forwards like a rugby football.  The transmission of the strong force is modelled using "colour charge", which is way beyond the syllabus.  At university, you will study quantum chromodynamics.


Meson Interactions

Here is a typical meson interaction:

p0 e- + e+ + g + g

Q: 0 -1 + +1 + 0 + 0

B: 0 0 + 0 + 0 + 0

L: 0 +1 + -1 + 0 + 0

Note how the quantum numbers Q, B, and L are conserved.  The interaction proceeds.

Question 3 Use quantum numbers to show that this interaction can occur.

p+ m+ + nm