2.1 The Particle Zoo

Question 1

What are cosmic rays

Answer 1

High energy particles that travel through space from the stars including the Sun.

Question 2

What happens when cosmic rays enter the Earth’s atmosphere

Answer 2

They create new short-lived particles and antiparticles as well as photons.

Question 3

What did most physicists think when cosmic rays were first discovered

Answer 3

That they were from terrestrial radioactive substances

Question 4

New types of short-lived particles and antiparticles were discovered:

Answer 4

Muon
Pion
Kaon

Question 5

What is the muon

Answer 5

Symbol μ
Also known as a heavy electron
A negatively charged particle with a rest mass over 200 times the rest mass of the electron

Question 6

What is a pion

Answer 6

Also known as π meson
Can be positively charged π +
Can be negatively charged π -
Can be neutral π 0
Has rest mass greater than muon but less than proton

Question 7

What is a kaon

Answer 7

Also known as K meson
Can be positively charged K+
Can be negatively charged K-
Can be neutral K0
Has rest mass greater than pion but still less than proton

Question 8

Who predicted the existence of exchange particles for the strong nuclear force between nucleons

Answer 8

Hideki Yukawa

Called them mesons

Question 9

What did the cloud chamber photograph obtained by Carl Anderson show

Answer 9

An unusual track that could’ve been produced by an exchange particle
Through further experiments, particle shown to be a heavy electron (muon)

Question 10

Who discovered Yukawa’s meson

Answer 10

British physicist Cecil Powell
From tiny microscopic tracks found in photographic emulsion exposed to cosmic rays at high altitude
Called them π mesons or pions
Yukawa’s prediction was correct

Question 11

What did further cloud chamber photographs (less than a year after Powell’s discovery) reveal

Answer 11

The existence of short-lived particles now known as kaons

Question 12

How do kaons decay

Answer 12

Via the weak interaction

Therefore were led to being called ‘strange particles’

Question 13

The kinetic energy of the protons is converted into mass in the creation of…

Answer 13

…the new particles (mesons)

Question 14

What can a kaon decay into

Answer 14

Pions, or a muon and an antineutron, or an antimuon and a neutrino

Question 15

What can a charged pion decay into

Answer 15

A muon and an antineutrino, or an antimuon and a neutrino.

Question 16

What does a π 0 meson decay into

Answer 16

High-energy photons

Question 17

What does a muon decay into

Answer 17

An electron and an antineutrino

Question 18

What does an antimuon decay into

Answer 18

A positron and a neutrino

Question 19

What do decays always obey

Answer 19

The conservation rules for energy, momentum and charge

Question 20

Why is a TV tube in a traditional TV an accelerator

How did traditional TVs work

Answer 20

Because it accelerates electrons inside the tube through a potential difference of about 5000V
Electrons form beam that hits the inside of TV screen
Magnetic fields produced by electromagnets deflect beam so scans screen to create an image

Question 21

What is the longest accelerator in the world

Answer 21

The Stanford linear accelerator in California
3km through p.d. of 50000 million volts
Electrons collide with target to form particle-antiparticle pairs

Question 22

What is the biggest accelerator in the world

Answer 22

The large hadron collider at CERN near Geneva
Accelerate charged particles to energies of more than 7000 GeV
27km circumference ring constructed in circular tunnel

Question 23

Where and when was the Higgs boson discovered

Answer 23

At CERN in 2012

Question 24

What is the Higgs boson

Answer 24

An uncharged particle

Was predicted in 1964 as the cause of the rest mass of W bosons