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

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:

• Zero baryon number

• Short lifetimes.  The longest lived has a life-time of 10-8 s (not very long).

• Antiparticles

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 are a few mesons:

 Name Symbol Q Lifetime (s) Quarks Pion p0 0 0.8 × 10-16 Up, anti-up (uu) OR down, anti-down (dd) p+ 1 2.6 × 10-8 Up, anti-down (ud) Kaon K+ 1 1.2 × 10-8 Up, anti-strange (us) K0 0 8.9 × 10-11  5.2 × 10-8 Down, anti-strange (ds) Strange, anti-down (sd) 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.

 What is the quark composition of the p- meson? Use the baryon number of the quarks to explain why the baryon number of a meson is zero.

Quantum Numbers

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 baryon number = +1/3 + -1/3 = 0.

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 between baryons.  The gluons transmit the strong force between the quarks.

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 Yes/No Q 0 ® -1 + +1 + 0 + 0 Y B 0 ® 0 + 0 + 0 + 0 Y Le 0 ® +1 + -1 + 0 + 0 Y

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

 Use quantum numbers to show that this interaction can occur. p+ ® m+ + nm