Quarks and leptons (topic 2)

Question 1

What are cosmic rays?

Answer 1

High-energy particles (protons or small nuclei) travelling through space, from stars

Question 2

What happens when cosmic rays enter the atmosphere?

Answer 2

They collide with gas atoms in the atmosphere, creating photons, and new short-lived particles and anti-particles

Question 3

What are the three short-lived particles created by cosmic rays reaching earth’s atmosphere or protons colliding at high speeds?

Answer 3

Muon, pion, kaon

Question 4

What are the characteristics of the muon? (2)

Answer 4

• negatively charged
• rest mass 200x larger than an electron

Question 5

What are the characteristics of a pion? (2)

Answer 5

• Can have positive, negative or neutral charge
• rest mass greater than muon but less than proton

Question 6

What are the characteristics of a kaon? (3)

Answer 6

• can have positive, negative or neutral charge
• rest mass greater than pion but less than proton

Question 7

What is created by kaon decay? (2)

Answer 7

1) pions

2) muon antineutrino/neutrino antimuon pair

Question 8

What is created by CHARGED pion decay?

Answer 8

muon antineutrino/neutrino antimuon pair

Question 9

What is created by pion(0) decay?

Answer 9

high-energy photons

Question 10

What is creates by muon decay?

Answer 10

electron and antineutrino

Question 11

Hadrons (definition)

Answer 11

Particles/antiparticles that can interact through strong interaction
and are made up of quarks

Question 12

Leptons (definition)

Answer 12

Particles/anitparticles that do NOT interact through the strong interaction and are NOT made up of quarks

Question 13

What is conserved in ALL decays? (5)

Answer 13

Energy, momentum, charge, lepton number, baryon number

Question 14

How do we find the rest energy of products of interactions?

Answer 14

Rest energy of products = total energy before - kinetic energy of products

Question 15

Baryon (definition)

Answer 15

Hadrons that decay into protons (directly or indirectly)

Question 16

Meson (definition)

Answer 16

Hadrons that cannot decay into protons (kaons and pions)

Question 17

What happens when leptons collide?

Answer 17

A quark and its antiqaurk are produced moving in opposite direction, producing a “shower” of hadrons in each direction.

Question 18

How are electron neutrinos and muon neutrinos different?

Answer 18

Muon neutrinos can only create muons. They cannot create electrons when interacting with protons/neutrons.

Question 19

What do current experiments indicate about leptons? How do we come to this conclusion?

Answer 19

Leptons appear to be fundamental, because they cannot break down into non-leptons

Question 20

Why can a neutrino only change into its corresponding lepton, not its corresponding antilepton?

Answer 20

Lepton number must be conserved. Anti leptons have lepton number -1, so must produce a particle with lepton number -1 in a lepton-hadron interaction.

Question 21

What happens in muon decay?

Answer 21

A muon becomes a muon neutrino/antimuon to antineutrino, and a electron/positron is created to conserve the charge + relevant (anti)neutrino. A muon cannot become an antimuon neutrino because lepton number or charge could not be conserved.

Question 22

How does lepton conservation work?

Answer 22

Leptons have lepton number +1, antileptons have lepton number -1. Electrons and muons + their respective neutrinos must be considered respectively as muon neutrinos can’t become electrons and vice versa.

Question 23

How is a kaon different from other strange particles?

Answer 23

It only decays into pions and has a rest mass lower than that of a proton.

Question 24

What is true for all strange particles?

Answer 24

They are produced in twos and decay through the weak interaction

Question 25

How is strangeness conserved? (2)

Answer 25

Always conserved in strong interactions

Can change by +1, -1 or not change during weak interactions

Question 26

Properties of an u quark (3)

Answer 26

Charge = +2/3

Strangeness = 0

Baryon number = +1/3

Question 27

Properties of a d quark (3)

Answer 27

Charge = -1/3

Strangeness = 0

Baryon number = +1/3

Question 28

Properties of an s quark (3)

Answer 28

Charge = -1/3

Strangeness = -1

Baryon number = +1/3

Question 29

Meson quark combinations

Answer 29

A quark-anti-quark pair

Question 30

Baryon quark combinations

Answer 30

Three quarks OR three antiquarks

Question 31

Which baryons are stable?

Answer 31

Only the proton - neutrons outside the nucleus will decay into protons through beta(-)

Question 32

Kaon quark combinations

Answer 32

Kaons must include an (anti)/strange quark. Positive and K(0) have +1S, while anti-positive and anti(K(0)) have -1S.

Question 33

Pion quark combinations

Answer 33

Only contain anti(up/down) quarks. All antimesons are also mesons, so pion+ is the antimeson of pion- and there are two pion(0)s.

Question 34

Which baryon is the only one to include a strange particle?

Answer 34

Sigma

Question 35

Where does beta(-) decay happen? Where does beta(+) decay happen?

Answer 35

B- = in a neutron-rich nucleus
B+ = in a proton-rich nucleus

Question 36

What happens in terms of quarks during beta- decay?

Answer 36

A down quark changes to an up quark

Question 37

What must be conserved in STRONG interactions? (6)

Answer 37

Charge, energy, momentum, lepton number, baryon number, and strangeness

Question 38

Why might number of quarks NOT be conserved in an interaction?

Answer 38

Annihilation or pair production