The Laws of Particle Physics
In physics class, one of the fundamental laws that a student learns is called
the law of conservation of energy. The law states that the total energy of an
isolated system does not change. This law applies universally to all physical
processes. No violation of this law has ever been discovered.
A similar law occurs in chemistry and it is called the conservation of matter.
This law states that during a chemical reaction, atoms may be separated and
rearranged, but not created or destroyed. Of course, at the turn of the 20th
century, Albert Einstein presented his now very famous and important
equation, E = mc2. This equation essentially states that matter and energy
are the same thing and must be conserved in total. It is this equation that has
proven to be very significant when it comes to particle physics research.
To demonstrate how a conservation law works, let us look at the reaction
between hydrogen and oxygen. Remember, matter may not be created or
destroyed, so the same number of each type atom must appear on both sides
of the equation.
H2
+
O2
→
H2O
# of H atoms
2
0
2
# of O atoms
0
2
1
This equation is not balanced. To balance the equation, one must do as
follows:
2H2 +
O2
→
2H2O
# of H atoms
4
0
4
# of O atoms
0
2
2
The most common type of observation in particle physics is called an event.
An event is similar to the chemical reactions above, because one set of
particles is formed from another. As particle physicists discovered, the
particle interactions and decays are also governed by certain conservation
laws, some of which had never been seen before. These laws explain why
certain interactions or events occur but not others.
Purpose: To determine conservation laws for particle interactions based on
upon both observed and unobserved events.
Procedure:
1. Examine the “Events Observed” and “Events Not Observed” on the last
page. What do the observed events have in common? What is different
between the observed events and the unobserved events? Think about the
example used in the introduction. Can you use the example as a guide to
help you? Does the particle type matter? Can you determine the rules for
the ways that particles can interact? Make a list of your conservation laws.
NOTE: Antimatter particles are denoted either with a bar over the
symbol (for baryons and neutrinos) or by using the same symbol with
opposite charge (for mesons and leptons).
YOUR LIST:
___________________________
___________________________
___________________________
___________________________
___________________________
___________________________
2. In groups of three or four, combine your laws into one master list that fits
“Events Observed” and “Events Not Observed”.
GROUP LIST:
___________________________
___________________________
___________________________
___________________________
___________________________
___________________________
3. On the lines below, explain which conservation law(s) is/are violated in
the “Events Not Observed” section.
1.
2.
3.
4.
5.
6.
___________________
___________________
___________________
___________________
___________________
___________________
7.
8.
9.
10.
11.
12.
____________________
____________________
____________________
____________________
____________________
____________________
Table of Symbols
Baryons:
p, n, 0 ,  , 0 ,  ,   ,  , 0 ,   , 0 ,   , 
Mesons:
  ,  0 , K  , K 0 , 0 ,   , 0 , D , D0 , J / 0
Leptons:
e ,   ,  , e, ,
Energy:
γ
Events Observed
1. n  p  e  e
10.   p  

0
2.   n  p  



11.   p  n    
0


3.   e  e  




12. e  e  D  D
4. K    0    
13.

0



0
5. K    
K0      0


14. e  e    

0
6.   p  D  D  n



0
15. K  p    K  K




7. D  K    
16.

0

8.     
17.
18.
0  0    
   e      e
     
1. n  p  p  p
7.
p    0


2. p    
8.   n  K  K
9.
   e   e
Events Not Observed
3. n  p  e

0


4.   n    
5.
   e  
6.
   e   


0
0  K   K 
0
0
10.   n     e

11. n  p  e 


12.   e   e
9.