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

  • Not felt by leptons

  • Total quark number conserved

  • Occur in times of the order 10^-23 s

  • No change in quark flavour

  • Typically involved in collisions between particles

• Electromagnetic interactions;

  • absorption of emission of photons

  • Typically take 10^-16 s

  • The total quark number is conserved

  • 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;

  • typically take 10^-8 s

  • 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

Have the same mass, equal and opposite charge of their matter particles

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

E=mc²

m=E/c²

Mass-energy (MeV per c²)

Mass= MeVperc² x (1.6×10^-13) / 9×10^16

Atomic mass unit

• 1u=1.66×10^-27 kg

• kg → MeV/c²=xc² and divide by 1.6×10^-3

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

• p→ u + u + d

• Q= 2/3+2/3+(-1/3)=1

• B=1/3+1/3+1/3=1

• Q=+1 (+e)

• B=1 (conserved quantity)

Determining the particle in a reaction

• Conservation of momentum

• Conservation of mass-energy

• Conservation of charge

• Conversation of Baryon number (in any interaction between particles in a system, the total baryon number in the system must not change)

• Conservation of lepton number (in any interaction between particles in a system, the total lepton number in the system must not change)

• Determine these values in order to determine the particle

charge of a proton

+1

Baryon number of a proton

1

charge of a neutron

0

Baryon number of a neutron

1

Delta baryons

• delta++; uuu, Q=2

• Delta+; uud, Q=1

• Delta0; udd, Q=0

• Delta-; ddd, Q=-1

• + and 0 have same Q as proton or neutron. Delta are heavier. Not protons or neutrons despite the same composition

• Decay → other particle combinations with mass or photon with energy

• Are in an excited state so more mass so less h-l than proton or neutron

Charge of an anti-up quark

-2/3e

Charge of an anti-down quark

+1/3e

Composition of an antineutron

anti up, anti down, anti down.

Q=-2/3+1/3+1/3=0

B=-1/3-1/3-1/3=-1

Composition of an antiproton

Anti up, anti up, anti down

Q=-2/3+-2/3+1/3=-1

B=-1/3+-1/3+-1/3=-1

Charge of an antineutron

0

Baryon number of an antineutron

-1

Charge of an antiproton

-1

Baryon number of an antiproton

-1

Lepton number of an antineutron

0

Lepton number of an antiproton

0

Pion+

• Up and anti down

• Q=1

• B=0

Pion-

• down and anti up

• Q=-1

• B=0

Pion 0

• up and anti up OR down and anti down

• Q=0

• B=0

Meson reactions

• charge conserved

• Baryon number conserved

• Quark flavour conserved so can determine force responsible as well as particles

Charge of an electron

-1 or -e

Lepton number of an electron

1

Charge of an electron neutrino

0

Lepton number of an electron neutrino

1

Charge of a positron

+1

Lepton number of a positron

-1

Charge of an anti electron neutrino

-1

Lepton number of an anti electron neutrino

-1

Baryon number of an electron

0

Baryon number of an electron neutrino

0

Baryon number of a positron

0

Baryon number of an anti electron neutrino

0

Stability of atoms due to forces

• atoms are stable due to the 3 non-gravitational forces

• e-s bound to protons (e-m)

• Strong nuclear force holds protons and neutrons together (opposes e-m repulsion)

• Weak nuclear force responsible for B- decay (B- or B+)

The force responsible for a particular interaction

The force responsible for a particular interaction is the strongest one felt by all particles on both sides