PHY 525 Nuclear and Particle Physics
Answer Key for Analysis of Particle Tracks - Revised
Susan Saeli
September 3, 2002
Pair Production
1. The neutral particle exists, but cannot be seen because it is uncharged and only
charged particles can be seen. The two spirals had to come from something conservation of mass-energy. (There is not enough information to determine that
it is a  ray.) Also, momentum is from left to right.
2. There is a magnetic field because the particles are traveling in a curve. There
must be a large velocity since there is a small curvature.
3. The signs of the two electrons are opposite because they are moving in opposite
directions. We know this from the right hand rule or conservation of charge.
4. By conservation of momentum you can deduce the path of the gamma ray.
5. The electrons were traveling slower than the gamma ray or large spiral large
speed. Also, the electrons were traveling faster than the particles moving in the
small spirals.
6. The path of the electrons is a spiral because the particles are continuously
having collisions with the hydrogen, which slow the particle. The particle loses
kinetic energy with each collision.
7. The bottom electron was produced with higher energy, because the bottom spiral
is larger than the top.
Pi-Meson (AKA Pion) Decay
8. The resulting path could not come from the spiral. It is not going to gain energy
from the spiral.
9. Because the electron and pi meson both curve clockwise they must have the
same sign.
10. We know from conservation of momentum. The muon is moving toward the left
even though the initial momentum of the negative pi meson is to the right and up
(kind of clockwise). Something had to move to the right.
11. The radius of curvature is smaller for the electron than the muon meaning its
mass-energy is smaller. Also, decay products must be smaller than what they
came from.
12. The muon must be negative because it also curves clockwise.
13. The pi meson must have a longer half-life.
K-Lambda Associated Production
14. The incoming particle collided with something larger with a positive charge. The
particle it collided with was stationary and, therefore, unseen.
15. The proton tracks are apparently straight even in a strong magnetic field because
they are moving very quickly or have a large mass.
16. The K and the lambda particle are neutral because you cannot see them. Also,
conservation of charge tells that if there was a positive and a negative coming off
the charge on the original must have been zero.
17. We know they exist even though they leave no tracks because the charged
particles that begin shortly after, had to come from something (a decay of the K
and lambda). Conservation of momentum helps determine from where they
came.
18. The proton and the pi-minus meson have equal and opposite charges because
they both curve the same amount in different directions.
19. The K and lambda particles have very short lifetimes. The lambda track is longer
than the K track, therefore the lambda particle has a longer lifetime than the K
particle.