Day 8.7
Taming the Particle Zoo
(13.5)
We have come a long way from Dalton and the indivisible atom. First Thomson finds the
electron, Rutherford finds the proton and Chadwick finds the neutron. Then over the next 30
years the list expands to include over 90 different particles. Particle physics of the 50s and 60s is
much like chemistry of the 1880s, there is a tremendous amount of data but no theory to provide
an organizing structure.
1. Take a deck of particle cards and inspect the information on the cards. They provide the
name, mass, spin, charge, Strangeness and the date of discovery. You need to sort the
information into three distinct piles. To do this you should use
A) mass
B) spin
C) charge
D) Strangeness
Explain: Mass is a continuous variable. Strangeness and charge have four possibilities. Only
spin has just three possibilities.
2. Start with the spin =3/2 pile. Use highlighters to colour-code the non-spin information (ie.
all Q=0 are pink). Arrange the cards in a table where the rows match one variable and the
columns match another. There appears to be a particle missing. Predict its characteristics and
write them on the blank card.
Omega Minus
Xi Zero
Eta
3. Repeat with the spin = ½ pile and then the spin =0 pile. There seems to be a particle missing
from each of these piles. Predict their characteristics.
4. How is this activity similar to what Mendeleev did with the periodic table?
Mendeleev also arranged particles by characteristics. He didn’t use mass except as a rough guide
- unlike his predecessors. He arranged them mainly by repeating chemical properties. He left
gaps for missing elements and predicted their properties.
5.
The periodic table puts each element in a specific place. Why does the periodic table suggest
that atoms are made of smaller pieces? What do the three tables that you have built suggest
about these particles - which include the proton and neutron? It suggests that the proton and
neutron are not elementary, but are made of smaller particles.
Murray Gell-Mann won the Nobel Prize in 1969 for his theory that explained all of the known
particles and predicted more (just like you did in this activity). His original theory proposed the
existence of three smaller, more fundamental particles called quarks and their antiparticles.
Quarks have certain characteristics such as charge, spin, and strangeness and they following a set
of simple rules:



Baryons are made of three quarks. (Antibaryons are made of three antiquarks)
Mesons are made of one quark and one antiquark.
Quarks have fractional charge and combine to produce integer charge.
6. Mesons are made of one quark and one antiquark. The spins are opposite for a total spin of
0. List all the combinations and calculate their Q and S values. Use the Q and S values to match
each meson with its quark-antiquark pair.
uu
su
qq
us
du
ds
ss
sd
ud
dd
Q
0
+1
+1
-1
0
0
-1
0
0
S
0
0
+1
0
0
+1
-1
-1
0
0
0


0


0
Spin 0
,
,
, 0






Mesons
What problems do you see in your results. How could you resolve them?
There are two basic problems: First, there are NINE combinations and the students only have
eight. Students who notice this and suggest the possibility of another particle should be given
the Eta Prime (’) card – the missing meson. Second, there are three quark-antiquark pairs that
have the same Q and S values. The 0,  and ’ are MIXED STATES of up-antiup, downantidown and strange-antistrange quarks. This is more information than your students need at
this point - but essentially these mesons are superpositions of possible quantum states.
7. Baryons are made of three quarks and can be arranged with a spin of 3/2 or partially
cancelled with a spin of ½. List all the combinations of quarks and calculate their Q and S
values. Use the Q and S values to determine the quark content for each meson.
qqq
ddd
udd
uud
uuu
dds
uds
uus
dss
uss
sss
Q
-1
0
+1
+2
-1
0
+1
-1
0
-1
S
0
0
0
0
-1
-1
-1
-2
-2
-3
Spin 3/2
Baryons
Spin 1/2
Baryons

0

n
p
 
*
*0

0 , 
*

*

*0
0

What problems do you see in your results. How could you resolve them?
Same basic problems: Missing particles and more than one particle in same spot. Instead of
pointing to missing particles (although that is reasonable and students who suggest that should be
congratulated) the empty boxes are pointing to deeper structural rules. In this activity we have
chosen to only look at the charge and strangeness of the quarks, but quarks also have spin of 1/2.
When spin ½ quarks are combined they will either produce a symmetric spin state of 3/2 or
mixed symmetry state of ½. There are ten quark combinations that will produce a spin of 3/2 but
only eight that will produce a spin of ½. This has to do with the total angular momentum of the
baryon and the possible spin configurations that will produce symmetric or antisymmetric wave
functions. It is similar to the Pauli Exclusion Principle.
p. 709 #4, 5, p. 712 #1, 2, 9, p. 736 #21, p. 744 #51
Q = -1
Q=0

Q = +1
 

0
*
Q = +2

*
*
S =- 2
*0

S = -1

*0
S=0
S =- 3
SPIN 3/2 BARYONS
Q =0
Q =+1
p
n



S =0
Q = +1

0
Q = -1
0

Q=0
0
S =-1

S =-2
0

SPIN 1/2 BARYONS



0
SPIN 0 MESONS

S =+1
S =-1