WJEC AS Physics Unit 1.7 - Particles and Nuclear Structure

Lepton

Low mass fundamental particles such as electrons, electron neutrinos and analogous pairs of the so-called second and third generations which exist in isolation

Hadron

• High mass particles consisting of quarks and antiquarks bound together. Only hadrons (and quarks or antiquarks themselves) can ‘feel’ the strong force

• E.g. mesons

Baryon

• A hadron consisting of 3 quarks or 3 antiquarks

• E.g. the nucleons; protons and neutrons

Meson

• A hadron consisting of a quark and antiquark pair

• E.g. pion

Quark

• Elementary particles not found in isolation which combine to form hadrons and mesons

• E.g. up quark/down quark

Antibaryon

• A hadron consisting of 3 antiquarks with the same mass and opposite charge

• E.g. antiproton

What quarks combine to form

Hadrons and baryons

Composition of a hadron

Quarks and/or antiquarks

Composition of a Baryon

3 quarks

Composition of an Antibaryon

3 antiquarks

Composition of a Meson

A quark and antiquark pair

Example of a Baryon

A proton or a neutron

Example of an Antibaryon

Antiproton

Example of a Lepton

Electron or electron neutrino

Matter

• composed of quarks and leptons, of which there are 3 generations (only do calculations with the first)

Fundamental Particle

A particle that cannot be broken down into smaller constituents

Laws which reactions involving subatomic particles must obey

• Conservation of energy

• Conservation of momentum

• Conservation of charge

• Conservation of lepton number

• Conservation of baryon number

The 4 fundamental forces

• Gravitational

• Weak

• Electromagnetic (e-m)

• Strong

Properties to identify the force involved in a reaction

• Strong interactions;

  • Only hadrons involved

  • No change in quark flavour

  • Typically involved in collisions between particles

• Electromagnetic interactions(

  • Particles must be charged or have charged components (neutron is uncharged but made up of charged quarks)

  • No change in quark flavour

  • One or more photons may be emitted

• Weak interactions;

  • Neutral leptons (neutrinos) are involved

  • May be a change in quark flavour

Characteristics of gravitational forces/interactions

Very weak; negligible except between large objects such as planets

What gravitational forces/interactions are experienced by

All matter

Range of gravitational interactions

Infinite

Characteristics of weak forces/interactions

• Only significant when the e-m and strong interactions do not operate

• Within the nucleus

Relative strength of gravitational force

10^-40

What weak forces/interactions are experienced by

All particles; All leptons, all quarks and so all hadrons

Range of weak forces/interactions

Very short

Relative strength of weak forces/interactions

10^-6

Characteristics of electromagnetic forces/interactions

Also experienced by neutral hadrons as these are composed of quarks

What electromagnetic forces/interactions are experienced by

All charged particles

Range of electromagnetic forces/interactions

Infinite

Characteristics of strong forces/interactions

Also affects interactions between hadrons, e.g. binding energy

What strong forces/interactions are experienced by

All quarks, so all hadrons

Range of strong forces/interactions

Short

Relative strength of strong forces/interactions

1

Quark structure of a proton

Two up quarks, one down quark (uud)

Quark structure of a neutron

Two down, one up (ddu)

Pions

• First generation antiquarks combine to form the pions

Antimatter

Matter + antimatter

• Annihilate each other

• Mass of the particles is converted into energy

Annihilation

• Matter + antimatter → annihilate each other

• Mass → energy (total energy of the photons = mass energy + KE of the photons)

• Gamma photons emitted and spread out in opposite directions for momentum to be conserved

• Momentum is conserved

• Energy of The photons depends of the mass of individual particles and the KE they were travelling with

Pair Production

• Opposite of annihilation

• High E photon can produce an electron-positron pair

• Pair travel in opposite directions away from one another

• Mass is conserved

• Some E → mass

• Some E → KE of particles

• Momentum is also conserved

• Gamma photon would need to interact with something, e.g. atomic nucleus

eV and J

eV → J; x 1.6×10^19

J → eV; / 1.6×10^-19

Mass-Energy

Atomic mass unit

Charge of an electron

-1

Charge of an electron neutrino

0

Baryon number

• depends on quark makeup

• Quark; +1/3

• Antiquark; -1/3

Lepton number

• lepton = 1

• Not a lepton = 0

• Antilepton = -1

Building a proton

Determining the particle in a reaction

charge of a proton

+1

Baryon number of a proton

1

charge of a neutron

0

Baryon number of a neutron

1

Delta baryons

Charge of an anti-up quark

Charge of an anti-down quark

Composition of an antineutron

Composition of an antiproton

Charge of an antineutron

Baryon number of an antineutron

Charge of an antiproton

Baryon number of an antiproton

Lepton number of an antineutron

Lepton number of an antiproton

Pion+

Pion-

Pion 0

Meson reactions

Charge of an electron

Lepton number of an electron

Charge of an electron neutrino

Lepton number of an electron neutrino

Charge of a positron

Lepton number of a positron

Charge of an anti electron neutrino

Lepton number of an anti electron neutrino

Baryon number of an electron

Baryon number of an electron neutrino

Baryon number of a positron

Baryon number of an anti electron neutrino

Stability of atoms due to forces

The force responsible for a particular interaction