The particle zoo and particle sorting

Question 1

By using cloud chambers and other detectors, new types of short-lived particles and antiparticles were discovered, including:

Answer 1

• The muon or heavy electron, a negatively charged particle with a rest mass over 200x the rest mass of the electron.
• The pion or π meson, a particle which can be positively charged, negatively charged or neutral and has a rest mass > a muon but < a proton.
• The kaon or K meson, which can also be positively charged, negatively charged or neutral and has a rest mass > a pion but < a proton.

Question 2

Who predicted the existence of exchange particles?

Answer 2

Japanese physicist Hideki Yukawa predicted the existence of exchange particles for the strong nuclear force between nucleons. He thought they would have a range of no more than about 10^-15 m and he calculated that their mass would be between the electron and proton mass. Because of this he called them mesons.

Question 3

A cloud chamber photograph obtained by Carl Anderson showed an unusual track that could have been produced by Yukawa’s particle.

Answer 3

However the track length of 40 mm indicated that it lasted much longer than a strongly interacting particles should.Further investigations showed this unexpected particle to be a heavy election, now referred to a muon but not classed as a meson. It decays through the weak interactions

Question 4

Yukawa’s meson was discovered:

Answer 4

By the British Physicist Cecil Powell from tiny microscopic tracks found in photographic emulsions exposed to cosmic rays at high altitude. Powell called these particles π mesons or pions.

Question 5

Like pions.

Answer 5

Kaons are produced in twos through the strong interaction, when protons moving at high speed crash into nuclei and they each travel far beyond the nucleus in which they originate before they decay. However, the decay took longer than expected and includes pions as the product. This means kaons decay via the weak interaction. These and other factories of kaons led to them being called strange particles.

Question 6

Decay mode: Kaon

Answer 6

1. Pions.
2. A muon and antineutrino.
3. An antimuon and neutrino.

Question 7

Decay mode: π^{+ or -}

Answer 7

• A muon and antineutrino.
• An antimuon and neutrino.

Question 8

Decay Mode: π⁰ meson

Answer 8

High energy photons

Question 9

Decay Mode: Muon

Answer 9

• An electron and an antineutrino.
• A positron and a neutrino.

Question 10

What are hadrons?

Answer 10

Particles and antiparticles that can interact through the strong interaction.

Question 11

What are leptons?

Answer 11

Particles and antiparticles that do not interact through the strong interaction.

Question 12

Leptons interact through:

Answer 12

The weak interaction, the gravitational interaction, and the electromagnetic interaction if charged.

Question 13

Hadrons can interact through:

Answer 13

All four fundamental interactions. They interact through the strong interaction and through the electromagnetic force if charged. Apart from the proton, which is stable, hadrons decay through weak interactions.

Question 14

The rest energy of the products =

Answer 14

total energy before - the kinetic energy of the products.

Question 15

Total energy of the particles and antiparticles before the collision =

Answer 15

their rest energy + their kinetic energy

Question 16

Total energy of the particles and antiparticles after the collision =

Answer 16

their rest energy + their kinetic energy,

Question 17

1 GeV

Answer 17

= 1000 MeV

Question 18

Baryons are:

Answer 18

Protons and all other hadrons (including neutrons) that decay into protons, either directly or indirectly.

Question 19

Mesons are:

Answer 19

Hadrons that do not include protons in their decay products. In other words, kaons and pions are not baryons.