Quarks

• Quarks are?fundamental particles?that make up other subatomic particles such as?protons and neutrons
• Protons and neutrons are in a category of particles called?hadrons

Hadrons are defined as any particle made up of quarks

• Fundamental?means that quarks are not made up of any other particles. Another example is electrons
• Quarks have?never?been observed?on their own, they’re either in pairs or groups of three
• There are six flavours (types) of quarks that exist:

The six flavours of quarks

• The charge of a hadron is determined by the sum of the charges of its quarks
• Each flavour of quark has a certain relative charge:

Each flavour of quark has a charge of either +?e or -?e

• For example, a proton is made up of two up quarks and a down quark. Adding up their charges gives the charge of a proton:

+?e + ?e - ?e = +1e

• The equivalent antiparticle of the quark is the anti-quark
• These are identical to quarks except with opposite relative charges

Each flavour of anti-quark has a charge of either -?e or +?e. The quark composition of anti-protons and anti-neutrons changes to anti-quarks

• Quarks have a?baryon number of +1/3
• Anti-quarks having a?baryon number of –1/3
• Strange quarks have?a strangeness of –1
• Anti-strange quarks have?a strangeness of +1
  • This is unique to the strange quark

Leptons

• Leptons?are a group of?fundamental?(elementary) particles
• This means they are not made up of any other particles (no quarks)
• There are six leptons altogether:

The six leptons are all fundamental particles

• The muon and tau particle are very similar to the electron but with slightly larger mass
• Electrons, muon, and tau particles?all?have a charge of -1e and a mass of 0.0005u
• There are three?flavours?(types) of neutrinos?(electron, muon, tau)
• Neutrinos are the most abundant leptons in the universe
  • They have?no charge?and?negligible?mass (almost 0)

• Leptons interact with the weak interaction, electromagnetic and gravitational forces
• However, they do?not?interact with the strong force
• Although quarks are fundamental particles too, they are not classed as leptons
• Leptons do?not?interact with the strong force, whilst quarks do

Lepton Number

• Similar to baryon number, the lepton number,?L?is the number of leptons in an interaction
• L?depends on whether the particle is a lepton, anti-lepton or neither
  • Leptons have a lepton number?L?= +1
  • Anti-leptons have a lepton number?L?= –1
  • Particles that are not leptons have a lepton number?L?= 0

• Lepton number is a quantum number and is conserved in all interactions
• This is helpful for knowing whether an interaction is able to happen

The lepton number depends if the particle is a lepton, anti-lepton or neither

Step 1:?Determine the lepton number of all the particles on both sides of the equation

• • 0 + (–1) = 0 + X

Step 2: Identify the lepton number of X

• • If the lepton number must be conserved, X must also have a lepton number of –1

Step 3: State the particle X

• • Particle X is an anti-neutrino

Protons and Neutrons

• Protons and neutrons are?not?fundamental particles. They are each made up of three quarks
• Protons?are made up of?two up quarks and a down quark
• Neutrons?are made up of?two down quarks and an up quark

Protons and neutrons are made up of three quarks

• You will be expected to remember these quark combinations for exam questions

Protons as Baryons

• The proton is the most stable baryon
• This means it has the?longest half-life?of any baryon and is the particle which other baryons eventually decay to
• It is the most stable baryon because it is also the?lightest baryon
  • Radioactive decay occurs when heavier particles decay into lighter particles
  • A decay of the proton would therefore violate the conservation of baryon number
• It is theorized that the proton has a half-life of around 10³²?years and research experiments are still underway that are designed to detect proton decay

Step 1:?Calculate number of protons:

• • The number of protons is from the proton number = 26 protons

Step 2:?Calculate number of neutrons:

• • The number of neutrons = nucleon number - proton number = 56 - 26 = 30 neutrons

Step 3:?Up quarks in a proton:

• • Protons are made up of?uud?quarks = 2 up quarks

Step 4:?Up quarks in a neutron:

• • Neutrons are made up of?udd?quarks = 1 up quark

Step 5:?Total number of up quarks:

• • 26 protons x 2 up quarks = 52 up quarks
  • 30 neutrons x 1 up quark = 30 up quarks
  • 52 + 30 = 82 up quarks

β– and β+ decay

• Beta decay happens via the?weak interaction
  • This is one of the four fundamental forces and it’s responsible for radioactive decays

Quark Composition: β^-?decay

• Recall that?β^-?decay is when a neutron turns into a proton emitting an electron and anti-electron neutrino
• More specifically, a neutron turns into a proton because a?down quark turns into an up quark

Beta minus decay is when a down quark turns into an up quark

Quark Composition: β⁺?decay

• Recall that?β⁺?decay is when a proton turns into a neutron emitting a positron and an electron neutrino
• More specifically, a proton turns into a neutron because an?up quark turns into a down quark

Beta plus decay is when an up quark turns into a down quark