Particles and Interactions

Here is a list of kinds of particle:

Electron(s)

Positron(s)

Photon(s)

Boson(s)

Fermion(s)

Neutrino(s)

Proton(s)

Neutron(s)

Nucleon(s)

Lepton(s)

Antiparticle(s)

Which kind of particle does what?

In each sentence below choose the kind of particle which best fills the empty slot. Try to give a reason why your choice is correct.

  1. The electrical force between an electron and a proton is carried by exchange of virtual [ ].
  2. In particle physics, forces are understood as exchange of virtual [ ].
  3. The antiparticles of electrons are [ ].
  4. Electrons and neutrinos but not protons and neutrons are both [ ].
  5. [ ] are leptons which have zero charge and mass.
  6. Particles which are [ ] all obey the Pauli exclusion principle.
  7. Together, protons and neutrons are called [ ].
  8. In a beta decay, both the [ ] number and the [ ] number are conserved.
  9. In β+ decay, a [ ] as well as a positron is emitted, thus conserving the [ ] number.
  10. Because a neutrino has no electric charge it cannot interact by exchange of [ ].
  11. A photon with enough energy can create a particle–[ ] pair.
  12. Electrons and positrons annihilate to produce [ ].
  13. Electrons, neutrinos, protons and neutrons are all [ ].
  14. The strength of the electromagnetic interaction is indicated by the probability amplitude of interaction of electrons and [ ].
  15. A neutron decays into a(n) [ ], a(n) [ ] and an anti[ ].
  16. Charge would not be conserved if [ ] were not produced at the same time as particles in pair creation.

Putting quarks together: Three quarks for Muster Mark

The American physicist Murray Gell-Mann gave the name ‘quarks’ to the particles he proposed as the basic building bricks of other particles. The name refers to a line in the novel Finnegan’s Wake by James Joyce, who was famed for his word play. The line is: ‘Three quarks for Muster Mark’. His colleague George Zwieg wanted to call the particles ‘aces’, but Gell-Mann’s choice won out. In spite of the rhyme suggested by Joyce’s line, the word ‘quark’ is generally pronounced ‘quork’. The word is also German slang for ‘nonsense’ (and a variety of yoghurt).

These questions ask about how quarks go together to make other particles.

Two kinds of quark

The simplest particles, including all the ones that everyday matter is made of, are built from two kinds (‘flavours’) of quark: ‘up’ and ‘down’. The most peculiar thing about them is that their electric charges come in multiples of 1/3 of the charge on an electron. On a scale where the charge on an electron is –1 e, with e = 1.6 ´ 10–19 C, the charges on the quarks are:

·  Up quark u: charge + 2/3 e.

·  Down quark d: charge – 1/3 e.

Making massive particles

Relatively massive particles like the proton and neutron are made of combinations of three quarks.

1. What is the charge on the combination uuu?

2. What is the charge on the combination uud?

3. What is the charge on the combination udd?

4. What is the charge on the combination ddd?

There are four compound particles here.

5. Which combination has the right charge to be a proton?

6. Which combination has the right charge to be a neutron?

7. There is a particle called the Δ– which has a charge of – 1 e. Which quark combination could be the Δ –?

8. There is a particle called the Δ ++ which has a charge of + 2 e. Which quark combination could be the Δ ++?

9. A neutron can be changed to a proton if one quark changes ‘flavour’. What change is needed? What charge must be carried away if this happens?

Making mesons

Other, lighter ‘middle-weight’ particles called mesons can be made from pairs of quarks. But they have to be made from a special combination: a quark and an antiquark. There are now four particles to play with:

·  Up quark u: charge +2/3 e

·  Down quark d: charge –1/3 e.

·  Antiup quark : charge –2/3 e.

·  Antidown quark : charge + 1 / 3 e.

10. What is the charge on the combination u ?

11. What is the charge on the combination d ?

12. What is the charge on the combination u ?

13. What is the charge on the combination d ?

14. Which combination could be the π+ meson?

15. Which combination could be the π– meson?

16. Which could be the neutral π 0 meson?