Quarks and Leptons

Quarks and Leptons

1. Particle Zoo

   • Cosmic rays are high-energy particles
     • When they enter the Earth's atmosphere, they create new short-lived particles and antiparticles
     • Most cosmic rays are fast-moving protons or small nuclei
     • They collide with gas atoms in the atmosphere creating particles and antiparticles which can be detected
     • The rays have a greater ionising effect at a greater altitude
   • The muon is a negatively charged particle with a rest mass over 200x that of the electron
     • A muon decays into an electron and an antineutrino
     • An antimuon decays into a positron and a neutrino
   • The pion or pi meson can be positively/negatively charged or neutral, rest mass heavier than muon but less than proton
     • A charged pion can decay into a muon and an antineutrino, or an antimuon and a neutrino
     • A neutral pion decays into high-energy photons
   • the kaon or K meson can also have any charge, it is heavier than a pion but lighter than a proton
     • kaons are produced in twos through strong interaction
       • Produced when protons moving at high speed crash into nuclei and each travel far away from the nucleus in which they originated
     • kaons decay through the weak interaction
       • A kaon can decay into pions, or a muon and an antineutrino, or an antimuon and a neutrino
       • they are called strange particles
   • Pions, kaons and muons can also all be created by colliding protons together head-on at high speed
     • KE is converted into mass in the creation of new particles
   • Decays always observe the conservation rules for energy, momentum and charge
   • A TV tube in a traditional TV is an accelerator, it accelerates electrons through a p.d. of around 5000V
     • Electrons form a beam that hits the inside of the TV screen
       • Magnetic fields produced by electromagnets deflect the beam so it scans the screen to create an image

2. Particle Sorting

   • Particles are divided into two groups, hadrons and leptons, according to whether or not they interact through the strong interaction
     • Hadrons are particles and antiparticles that can interact through the strong interaction (protons, neutrons, pions and kaons
     • Leptons are particles and antiparticles that do not interact through the strong interaction (electrons, muons and neutrinos
   • Leptons interact through the weak, gravitational and, if charged, electromagnetic interaction
   • Hadrons interact through all four fundamental interactions
     • Apart from the proton, which is stable, hadrons tend to decay through the weak interaction
   • Total energy of particles and antiparticles before = rest energy + kinetic energy
     • Total energy of new particles and antiparticles = rest energy + kinetic energy
   • rest energy of products = total energy before - EK of products
   • 1 GeV = 1000 MeV
   • When kaons are created, short-lived particles with greater rest masses than protons may also be produced
     • They are created through the strong interaction so must be hadrons
       • However, in comparison with kaons, they decay into protonsas well as into pions
         • Consequently, hadrons are divided into two groups
           • Baryons are protons and all other hadrons that decay into protons either directly or indirectly
           • Mesons are hadrons that do not include protonsin their decay products, ie pions and kaons
           • Baryons and mesons are composed of smaller particles called quarks and antiquarks

3. Leptons at work

   • Leptons and antileptons can interact to produce hadrons
     • eg. when electrons and positrons annihilate hadrons will be produced
     • This also produces a quark and a corresponding antiquark, which move in opposite directions, producing a shower of hadrons in each direction
   • Neutrinos travel just below the speed of light and interact with very little
     • Neutrinos and antineutrinos produced by in beta decays are different to those produced by muon decays
       • therefore, we have muon-flavoured neutrinos and electron-flavoured neutrinos
     • ie. neutrinos and antineutrinos from muon and antimuon decays create only muons and no electrons when they interact with protons and neutrons
   • Leptons can change into other leptons through the weak interaction and can be produced or annihilated in particle-antiparticle interactions
     • In an interaction between a lepton and a hadron, a neutrino or antineutrino can change into a charged lepton
       • An electron neutrino can interact with a neutron to produce a proton and an electron
         • However, even though charge is conserved, an electron neutrino and a neutron could not change into an antiproton and a positron
           • This is because lepton number has not been conserved in the second equation
     • In muon decay the muon changes into a muon neutrino
       • An electron is created to conserve charge and a corresponding antineutrino is created to conserve lepton number
         • However, a muon cannot decay into a muon antineutrino, an electron and an electron antineutrino as lepton number would not be conserved
   • Leptons appear to be fundamental particles
   • A muon can only change into a muon neutrino, not a muon antineutrino
     • Similarly, an electron can only be created with an electron antineutrino

4. Quarks and antiquarks

   • strange particles that decay into pions only were referred as kaons
     • other strange particles, eg. the sigma particle, were found to all have greater rest masses than the proton
       • strange particles are created in twos
   • hadrons are composed of quarks and antiquarks
   • mesons are hadrons, each consisting of a quark and an antiquark
     • a neutral pion can be any quark-antiquark pair
       • each pair of charged mesons is a particle-antiparticle pair
         • there are two uncharged kaons
           • antiparticle of any meson is a quark-antiquark pair, therefore another meson
   • baryons and antibaryons are hadrons that consist of three quarks for a baryon or three antiquarks for an antibaryon
     • neutron is the udd combination
       • proton is the uud combination
         • antiproton is the equivalent antiquarks
         • the proton is the only stable baryon
       • a free neutron decays into a proton, releasing an electron and an electron atineutrino, as in B- decay
   • in B- decay, a neutron in a neutron-rich nucleus changes into a proton and other products
     • in quark terms, a down quark changes to an up quark
   • in B+ decay, a proton in a proton-rich nucleus changes into a neutron, releasing a positron and an electron neutrino
     • In quark terms, an up quark changes to a down quark

5. Conservation rules

   • conservation of energy and conservation of charge apply to all changes in science
     • Conservation of energy includes the rest energy of the particles involved
   • conservation of lepton numbers
     • in any change the total lepton number for each lepton branch before the change is equal to the total lepton number for that branch after the change
     • eg. when an antimuon decays, it changesinto a positron, an electron neutrino, and a muon antneutrino
   • conservation of strangeness
     • strangeness is always conserved during the strong interaction
     • eg. a neutral kaon and a strange baryon called the neutral lambda particle are produced when a negative pion collides with a proton
     • however, strangeness is not necessarily conserved when the weak interaction is involved
       • in weak interactions, strangeness can be changed by +/- 1
   • In any reaction, the total baryon number is conserved

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