Classification of Particles Flashcards
(14 cards)
What are the two groups of particles in the Standard Model? What are they?
Hadrons - the group of subatomic particles made up of quarks and experience the strong nuclear force.
Leptons - a group of fundamental particles that don’t experience the strong force
What are the classes of hadrons?
Baryons (and anti-baryons), mesons (and anti-mesons)
What are baryons?
Baryons have three quarks and anti-baryons have three antiquarks.
The proton is the only stable baryon - free protons do not decay. This because it is the lightest baryon.
All other baryons are unstable and eventually decay into protons, e.g. free neutrons decay into protons via beta-minus decay.
Baryon number
Baryon number, B, is the number of baryons in an interaction.
Baryons have a baryon number, B = +1. Anti-baryons have B = -1, and non-baryons have B = 0.
Up, down and strange quarks each have a baryon no. of 1/3 each, and their antiparticles have a baryon no. of -1/3
Baryon number is a quatum number and must be conserved in all interactions.
What are mesons?
Mesons have a quark and antiquark pair, and so do anti-mesons but the other way round.
All mesons are unstable and have baryon number = 0.
Two examples are pions and kaons.
What are pions?
Pions are a type of mesons. They are the lightest, making them the least unstable.
They can be positive, negative or neutral.
They contain up and down quarks only.
Originally discovered in cosmic rays and can be observed in a cloud chamber
They act as exchange particles and transmit the strong nuclear force between protons and neutrons - mediate the strong nuclear force.
What are kaons?
Kaons are a type of mesons - heavier and more unstable than pions.
They can be positive, negative or neutral.
Short lifetime and decay into pions, but longer lifetime than other mesons
Unlike the pion, the neutral kaon is not its own antiparticle.
Contain a strange quark paired up with up/down quarks only
Leptons
Group of fundamental particles that don’t feel the strong force
Interact via the weak, gravitational or electromagnetic interactions - not the strong interaction
Most common are: electron, electron neutrino, muon, muon neutrino
The muon is similar to the electron but slightly heavier. They typically decay into electrons, and antimuons typically decay into positrons, through the weak interaction.
Lepton number
The lepton number, L, is the number of leptons in an interaction
There are different lepton numbers (L_e for electrons and L_mu for muons) and they are counted separately
Lepton number must be conserved in all interactions
Quarks
Quarks are the fundamental particles that make up hadrons. The three most common types are up, down and strange.
Each quark has a charge, baryon number and strangeness
The charge of a hadron is determined by by the sum of the charges of the quarks. The same is true for baryon number and strangemness.
Antiquarks are the antiparticles of quarks, identical but with opposite charges, baryon numbers and strangeness.
Quark Combinations
Baryons have three quarks - protons are made of 2 up quarks and a down quark (uud), and neutrons are made of 2 down quarks and an up quark (udd). Antiprotons and antineutrons are the same thing but anti-ups and anti-downs
Mesons have a quark and antiquark pair.
Pions are up and anti-down, anti-pions are anti-up and down, and neutral pions are up and anti-up or down and anti-down. In this way the antiparticle of a neutral pion is itself.
Kaons are up and anti-strange, anti-kaons are anti-up and strange, and neutral kaons are down, anti-strange or anti-down, strange.
Strange Quarks
Strange particles have a strange or anti-strange quark, e.g. kaons.
Strange particlesa re always produced through the strong interaction, decay through the weak interaction, and are produced in quark-antiquark pairs.
Strangeness, S, like baryon number, is a quantum number conserved in all interaactions except the weak interaction - difference between strangeness and other quantum numbers.
Particle Accelerators
Collide particles at very high speeds in order to produce new particles and reveal the inner structure of particles
The particles need to collide at very high energies, meaning they need to travel close to the speed of light. This requires expensive and complicated pieces of equipment.
The Large Hadron Collider
The world’s largest and highest-energy particle accelerator, based in CERN in Geneva
roton beams are accelerated in opposite directions are made to collide at the main detectors: ATLAS, CMS, ALICE, LHCb
Each detector works on its own area of physics.
The ATLAS and CMS detectors both confirmed the Higgs Boson.
