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Conservation Rules
Particle Physics Lesson 10
Can these happen?
p p pK
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K  p  n 
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Why/Why not?
0
Hadrons
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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
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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:
Name
Symbol Q B Lifetime (s)
Antipar
ticle
Pion
π0
π+
0
1
0 0.8 × 10-16
0 2.6 × 10-8
Itself
p-
Kaon
K+
K0
1
0
0 1.2 × 10-8
0 8.9 × 10-11
KK0
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Notice how short the lifetimes are of these
mesons.
We should note the following:
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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.
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Here are some more:
Show that this interaction can
proceed:
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π+  μ+ + νm
Answer
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Charge +1 ----> + 1 + 0 (Charge is conserved)
(P)
Baryon 0 ----> 0 + 0 (Baryon number is
conserved) (P)
Therefore it can proceed
Baryons
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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:
Name
Proton
Neutron
Lambda
Sigma
Symbol Q B Lifetime (s) Antiparticle
p
n
Λ0
Σ+
Σ0
ΣOmega Ω-
1
0
0
1
0
-1
-1
1
1
1
1
1
1
1
stable
898
2.6 × 10-10
0.8 × 10-10
7.4 × 10-20
1.5 × 10-10
0.8 × 10-10
p
n
Λ0
Σ+
Σ0
ΣΩ+
Typical Decays
The proton is the only stable baryon.
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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 (Σ+  π+ +
π0) or indirectly (W-  Λ0 + K, then Λ0  π+ + π-).
Mesons decay to photons or leptons.
Show that this decay is possible:
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Λ0  π+ + π-
Answer
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Charge 0  +1 + -1 (Charge is conserved) (P)
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Baryon 1  1 + 0 (Baryon number is
conserved) (P)
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The interaction is possible.