Physics U1 Quarks and Leptons

1) The Particles

2) Particle Sorting

3) Leptons at work

4) Quarks and antiquarks

5) Conservation Rules

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Different Particles

Investigations show that most cosmic rays were fast moving protons or small nuclei. They collide with gas atoms in the atmosphere, and create particles and antiparticles that can be detected on ground level. Some new types that were discovered were:

1) Muon: or heavy electron, negatively charged with a rest mass over 200 times the rest mass of an electron

2)  Pion: or pi meson, positively or negatively charged or neutral and has a rest mass greater than a muon but less than a proton

3) Kaon: or K meson, positively or negatively charged or neutral and has a rest mass greater than a pion but still less than a proton

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Decaying Particles

K mesons decay into muons and antineutrinos or antimuons and neutrinos

Charged pi mesons decay into muons and antineutrinos or anti muons and neutrinos

Muons and antimuons decay into electrons and antineutrinos or positrons and neutrinos

The decay always ibeys the conservation rules for energy and for charge.

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Classifying particles and antiparticles

Particle    Charge     Antiparticle    Charge    Rest energy    Interaction 

proton      +1            Antiproton      -1             938                strong, electromagnetic

neutron     0             antineutron     0              939                strong

electron     -1           positron           +1           0.511             weak, electromagnetic

neutrino     0           antineutrino      0              0                    weak

pi meso   +1,0,-1     ............      -1,0,+1     140;135;140       strong, electromagnetic 

muon          -1          antimuon         +1        106                   weak, electromagnetic

K meson  +1,0,-1     .............     -1,0,+1    494;498;494       strong, electromagnetic

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Hadrons and Leptons

Hadrons are particles and antiparticles that can interact through the strong interaction

Leptons are particles and antiparticles that do not interact through the strong interaction

Leptons interact through the weak interaction and through the electromagnetic interaction if charged

Hadrons interact through the strong interaction and through the electromagnetic interaction if charged. Apart from the proton which is stable, hadrons decay  through the weak interaction

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Energy Matters

The large hadron collider is a ring shaped accelerator that boosts the kinetic energy of the charged particles in the ring at several places round it. When they collide with other particles:

1) the total energy of the particles amd antiparticles before the collision= their rest energy + their kinetic energy

2) the total energy of the new particles and antiparticles after the collision = their rest energy + their kinetic energy

Using conservation of energy

the rest energy of the products=total energy before- the kinetic energy of the products

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Baryons and Mesons

Baryons are protons and all other hadrons that decay into protons, either directly or indirectly

Mesons are hadrons that do not include protons in their decay products. In other words K mesons and pi mesons are not Baryons.

                                          Matter and antimatter

             Hadrons                                                                  Leptons

Baryons            Mesons

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Lepton Collisions and Neutrino Types

Leptons and antileptons can interact to produce hadrons. An electron-positron annihilation event can produce a quark and an antiquark whu=ich move away in opposite directions, producing lots of hadrons in each direction.

Muon and Electron neutrinos

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Lepton Rules

Leptons can change into other leptons through the weak interaction and can be produced or annihilated in particle- antiparticle interactions.


1) in an interaction between a lepton and a hadron, an neutrino or antineutrino can change into or from a corresponding charged lepton.

2) in muon decay, the muon changes into a muon neutrino.In addition, an electron and an electron neutrino are created to conserve charge.

In any change, the total lepton number before the change is equal to the total lepton number after the change. In other words, the lepton number is conserved in any change.

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Quarks and antiquarks - Strangeness

To explain why certain reactions were not observed, a strangeness number was introduced for each particle and antiparticle so that strangeness is always conserved in strong interactions. Non strange particles ( proton , neutron, pi meson, leptons) were assigned zero strangeness

Strangeness is always conserved in a strong interaction.

There are 3 different types of quarks and their corrosponding antiquarks. Up, Down and Strange.

                   Up        Down       Strange      Antiup      Antidown      Antistrange           Charge        +2/3      -1/3          -1/3           -2/3         +1/3            +1/3                  

Strangeness  0        0                -1             0               0             +1

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Quark Combinations and beta decay

Mesons are hadrons and consist of one quark and one antiquark.

Baryons and antibaryons are hadrons that consist of 3 quarks for a baryon, and 3 antiquarks for an antibaryon.

proton- uud

neutron- udd

In beta- decay a down quark changes into an up quark

In beta+ decay an up quark changes into a down quark

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Conservation Rules

1) Conservation of energy and conservation of charge apply to all changes in science

2) Conservation rules used only for particle and antiparticle interaction and decays are particle counting rules, based on what reactions are observed:

a) Conservation of lepton numbers- in any change the total lepton is the same before and after the change

b) Conservation of strangeness- in any strong interaction, strangeness is always conserved

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