Particles and Radiation: Quarks and Leptons

Question 1

TSR - difference between strong interactions and weak interactions?

Answer 1

Strong force is when hadrons interact, weak force is when a quark changes and there is a decay.

Question 2

How to detect presence of cosmic radiation?

Answer 2

Two Geiger counters can be used to detect presence of cosmic radiation. Present if the two counters click simultaneously.

Question 3

What are cosmic rays?

Answer 3

High-energy particles that travel through space from the stars. Usually fast-moving protons or small nuclei.

Question 4

When they enter the atmosphere they create new short-lived particles and antiparticles (as well as photons):
(3)

Answer 4

• muon (or heavy electron) is -ve particle w rest mass over x200 rest mass of e-)
• pion (or π meson) is +ve, -ve or neutral and has rest mass greater than muon but less than p.
• kaon (or K meson) is +ve, -ve or neutral and has rest mass greater than pion but less than p.

Question 5

Muons decay by what interaction?

Answer 5

Weak interaction

Question 6

When protons moving at high speed crash into nuclei, what is produced in this strong interaction?

Answer 6

Pions or kaons are produced in twos and they each travel far beyond the nucleus in which they originated before they decay.

Question 7

Which decay took longer than expected, and what was concluded?

Answer 7

Decay of kaons took longer than expected and included pions as products ∴ kaons must decay via weak interaction.
(This plus other properties led them to be called strange particles).

Question 8

How else can muons, pions and kaons be created?
How?

Answer 8

Using accelerators in which protons collide head on with other protons at high speed. Ek of protons converted to mass in the creation of these new particles.

Question 9

Can muons, pions and kaons ∴ be studied?

Answer 9

Yes they can e studied using accelerators to create them.

Question 10

Studies into these found 4 things:
1 - kaons can decay into…

Answer 10

Kaons can decay into pions (plural),

(or a muon and an antineutrino, or an antimuon and a neutrino.)

Question 11

2 - pions can decay into…

Don’t acc need to know

Answer 11

A charged pion can decay into a muon and an antineutrino,
or an antimuon and a neutrino.
A π 0 meson can decay into high-energy photons.

Question 12

3 - a muon decays into…

Answer 12

muon decays into a muon neutrino and (to conserve charge a w- is emitted which decays into) an electron and an electron antineutrino

V.v for antimuon

Question 13

4 - decays always obey… (3)

Answer 13

Decays always obey conservation rules for energy, momentum and charge.

Question 14

Muon symbol?
Antimuon symbol?

Answer 14

µ -
µ +

Question 15

Pions and their antipion symbols?

Answer 15

π + π -
π - π +
π 0 π 0

Question 16

What are leptons?
Give 3 examples.

Answer 16

Particles (and antiparticles) that interact through the weak interaction, the gravitational interaction, and through the electromagnetic interaction (if charged).
Eg muons, electrons and neutrinos.

Question 17

What are hadrons?
Give 4 examples.

Answer 17

Hadrons are particles (and antiparticles) that can interact through all 4 fundamental interactions. They interact through the strong interaction and through em interaction if charged.
Eg protons, neutrons, pions, kaons.

Question 18

Apart from the proton…

Answer 18

Apart from the proton, which is stable, hadrons tend to decay through the weak interaction.

Question 19

In the Large Hadron Collider is a ring-shaped accelerator that boosts the Ek of the charged particles in the ring at several places. Fixed magnets bend the path to keep particles in ring. When they collide with other particles:
(think conservation of energy)

Answer 19

Total energy of particles and antiparticles before collision = their rest mass + their Ek.
Total energy of particles and antiparticles after collision = their rest mass + their Ek.

Question 20

Hadrons are split into what?

Answer 20

Baryons and mesons.

Baryons and mesons are composed of smaller particles called quarks and antiquarks.

Question 21

What are baryons?

Answer 21

Baryons are protons and all other hadrons that decay into protons, either directly or indirectly.

Question 22

What are mesons?

Answer 22

Hadrons that do not include protons in their decay products.

In other words, kaons and pions are not baryons.

Question 23

Leptons and antileptons can interact to produce…

Give an eg using e- and e+.

Answer 23

Leptons and antileptons can interact to produce hadrons.

Eg an e- and a e+ annihilation produces a q and an antiquark, which move in opposite directions, producing a shower of hadrons in each direction.

Question 24

Neutrinos and antineutrinos made from muon and antimuon decays can only create…

Answer 24

muons and no electrons when they interact with protons and neutrons,
and v.v. for beta decay.

Question 25

νe, ν ¯e, νµ, ν ¯µ

If there were only one type of neutrino and antineutrino, what would happen?

Answer 25

Equal numbers of electrons and muons would be produced.

Question 26

How do we know leptons are fundamental?

Answer 26

Leptons don’t break down into non-leptons.

Question 27

Leptons can change into other leptons through which interactions?
And can be produced or annihilated in which interactions?

Answer 27

Through weak interactions.

Through particle-antiparticle interactions.

Question 28

In an interaction between a lepton and a hadron, a neutrino or an antineutrino can change into or from a corresponding, charged lepton.

Give an example with neutrino.

Answer 28

ν ¯e + n —-> p + e-

(weak interaction)

Question 29

In muon decay, the muon changes into a muon neutrino. In addition, two leptons are made. What are they and why are they made?

Answer 29

An e- is created to conserve charge, and a corresponding antineutrino is created to conserve lepton number.

  µ- ---> νµ + e- + ν ¯e

C: -1 0 -1 0
L: +1 +1 +1 -1

Question 30

Is this muon decay possible? Why/why not?

µ- —> e- + νe + ν ¯µ

Answer 30

No.
µ can only change to νµ not a
ν ¯µ, and e- can only be created with ν ¯e.

Question 31

The lepton number is 1+ for any lepton and 1- for any antilepton and 0 for any non-lepton.
In light of the previous card, how must we apply this rule?

Answer 31

Apply rule separately to electrons and muons.
So lepton number must balance for e and for µ.

Question 32

What interaction do all strange particles decay through?

Answer 32

All strange particles decay through weak interactions.

Question 33

There are two types of strange particles:
What are kaons?

Answer 33

Strange particles that decay into pions only.

Question 34

There are two types of strange particles:
What’s the other type (give eg and the two properties)?

Answer 34

The others, such as the sigma Σ particle:
- have diff rest masses which are always greater than the proton’s rest mass
- decay either in sequence or directly into protons and pions.

Question 35

When strange particles are created..

Answer 35

they’re created in twos.

Question 36

What makes strange particles different? (3)

Answer 36

Contain strange quark;
longer half life than expected;
decays by weak interaction.

Question 37

When is strangeness conserved? How about when it isn’t? How does it vary when it isn’t conserved?

Answer 37

Strangeness is always conserved in a strong interaction, whereas strangeness can change by 0, +1 or -1 in weak interactions.

(eg K+ has strangeness 1+)

Question 38

Mesons consist of…
A π0 meson is what combo?
ss- aren’t called pions bc they..
There are two unchanged kaons:..

Answer 38

.. a quark and an antiquark.
A uu- or dd- combo.
..decay much faster than pions.
.. K0 meson and K0- meson.

Question 39

All meson combos (9):

Answer 39

π0 = uu- or dd-
π+ = ud-
π- = du-
K+ = us-
K- = su-
K0 = ds-
K0- = sd-

Question 40

Baryons and antibaryons consist of..
Proton is?
Neutron is?
Antiproton is?
Σ is ?

Answer 40

…qqq or q-q-q-

p = udu
n = udd
p- = u-d-u-
Σ is a baryon containing a strange quark.

Question 41

Conservation of energy and conservation of charge applies to..

Answer 41

All interactions and reactions and chemistry in general.

Question 42

Conservation rules used only for particles and antiparticle interactions and decay are? (3)

Answer 42

Conservation of lepton number.
Conservation of total baryon number.
Conservation of strangeness in any strong interaction.

Question 43

Gluon vs pion???

Btw say gluon when they ask for exchange particle of strong force!!

Answer 43

Gluons are the exchange particles that carry the strong force (known as the colour force in this case) between quarks. Pions are involved in the strong force (here called the nuclear force) which holds nucleons (neutrons and protons) together.