• When particles interact, they must obey a set of laws associated to the type of particles invovled
  • These laws are governed by numbers called?quantum numbers

• Quantum numbers that are?always conserved?(i.e., they are the same before and after an interaction) are:
  • Charge,?Q
  • Baryon number,?B
  • Lepton number,?L

• Using these quantum numbers, physicists are able to determine whether certain interactions are?possible?or not
  • In other words, an interaction that does?not?conserve charge, baryon or lepton number is?not allowed?by the laws of physics

Conservation of Charge

• The charge of a particle?Q,?is the charge carried by that particle
  • Protons have a charge?Q?= +1
  • Electrons have a charge?Q?= –1
  • Up quarks have a charge?Q?= +2/3
  • Neutral particles, like photons and neutrinos, have a charge?Q?= 0

   

Conservation of Baryon Number

• The baryon number,?B, is the number of baryons in an interaction
• B?depends on whether the particle is a baryon, anti-baryon or neither
  • Baryons have a baryon number?B?= +1
  • Anti-baryons have a baryon number?B?= –1
  • Particles that are not baryons have a baryon number?B?= 0

   

The baryon number of a particle depends if it is a baryon, anti–baryon or neither

• The up (u), down (d) and strange (s) quark have a baryon number of 1/3 each
• This means that the anti–up, anti–down and anti–strange quarks have a baryon number of –1/3 each
• Note:?The baryon number of each quark is provided on the datasheet
• The implication of this is that baryons are made up of all quarks and anti-baryons are made up of all anti-quarks
• There are no baryons (yet) that have a combination of quarks and anti-quarks e.g. up, anti-down, down
• The reason being that this would equate to a baryon number that is not a whole number (integer)

Conservation of 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 on 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 identity of particle X

• • Particle X is an anti-neutrino