CONSTITUENTS OF MATTER

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• CONSTITUENTS OF MATTER
• Hadrons (baryons and mesons)
• Leptons (muons, electrons and neutrinos)
• Exchange (messenger) particles (will discuss
  later)

In the 60s Gell-Mann and Zweig working
independently showed that properties of Baryons
(charge, strangeness) could be established by
adopting arrangements with 3 constituent
particles, Gell Mann called quarks . A certain
combination of quarks inside a Baryon could
describe both its charge and strangeness.
e.g. a proton (belongs to the Baryon group) is
made up of three quarks 2 up quarks and one down
quark.
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QUARKS Proton u 2/3e , u 2/3e , d
-1/3e Proton (uud 1e) Neutron (udd 0e) If
was also found that some Baryons could have the
same quark combination. However Paulis Exclusion
Principle suggests that no two different
particles can have the same quantum properties.
This problem was overcome by introducing a quark
colour. Red , Blue and Green. There are now 6
quarks with either charge of 2/3 or 1/3 and
Baryon Number 1/3 and colour either red, blue or
green. Up (lightest) Charmed Down Bottom Strang
e Top (heaviest)
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Quarks were experimentally verified both at
Stanford and at CERN in Geneva. At SLAC, highly
energetic electrons were fired at protons and
there scattering angles suggested the existence
of 2 ups and one down quark.
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• Fundamental Particle summary
• Leptons
• electron, electron neutrino
• muon, muon neutrino
• tau, tau neutrino
• Quarks
• Up, down, strange, charmed, bottom, top
• Mesons (made up of quark / antiquark pairs.
  Colour neutral)
• Baryons (made up of 3 quarks. Also Colour
  neutral)

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• FIRST GENERATION (stable particles)
• Up and Down quarks, electron and electron
  neutrino
• SECOND GENERATION (unstable)
• Charmed and Strange quarks, muon and muon
  neutrino
• THIRD GENERATION (more unstable)
• Bottom, Top quarks, Tau and Tau neutrino

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THE FUNDAMENTAL INTERACTIONS

• Gravity
• Strong
• Weak
• Electromagnetic

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• ELECTROMAGNETIC INTERACTION
• Occurs between charged particles.
• Classically explained by electric and magnetic
  fields
• Recent explanation (1940s) through QED. Quantum
  Electrodynamics or Quantum Field Theory
• Charged particles interact with each other via
  the EXCHANGE of a messenger particle called a
  PHOTON. The photon carries the energy and
  momentum characterized classically by the
  electric field.
• The messenger particle is virtual, meaning that
  it cannot be detected.

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Consider two electrons
Electrons repel because they are exchanging
virtual messenger particles (virtual photons).
Note It would appear that the creation of a
virtual photon from an electron would defy the
conservation of energy (extra energy in the form
h.f). Heisenbergs Uncertainty Principle claims
that energy can be created (DE) as long as it
destroyed before (Dt). i.e. Dt h / DE This
also accounts for why the strength of the e/m
interaction is long range and gets weaker with
distance. A greater distance separating charges
would take a longer time and therefore a smaller
DE exchanged.
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• WEAK INTERACTION
• A similar process is used to describe the weak
  interaction.
• The weak force is extremely short range (less
  than the diameter of a proton) and comparatively
  weak in comparison to the strong force.
• The short range nature of the Weak Force force
  suggests messenger particles with mass, unlike
  the photon in the case of the e/m interaction.
• The messenger particle for the weak force
  initially the W and W-
• Both of which have mass (energy) and therefore
  limited in time and range as a messenger
  particle. (Dt h / DE )
• A new theory was established in the 1970s
  unifying both the e/m and the weak interaction
  called the ELECTROWEAK interaction .There are now
  4 messenger particles
• W, W- , Zo, g (photon)

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Examples of Weak Interactions
ne
p
Beta Decay A neutron emits a W- and transforms
itself into a proton.
e-
W-
n
Proton-Neutrino Scattering The Zo is the
mediating particle between the proton and neutrino
ne
p
Zo
ne
p
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• STRONG INTERACTION
• Describes the interactions between quarks
• The messenger particle is the gluon
• The gluon has the ability to transform the colour
  of a quark but not its flavour or type.
• Quarks are held together inside hadrons by the
  exchange of gluons.

Up (blue)
Down (red)
g
Quark Encounter
Up (red)
Down (blue)
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• Residual Strong Force
• The gluons that bind quarks together inside
  hadrons are also responsible for the interaction
  of hadrons themselves.
• Protons and Protons are held together inside the
  nucleus because of a residual messenger particle
  called a pion

Strong force interactions do not result in a net
change in quark type or flavour. Eg. There are
still the same number of us and ds after the
interaction.
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Quark Confinement

• Quarks have never been isolated from eachother
• Confinement theory suggests that Quarks cannot
  exist individually because the color force
  increases as they are pulled apart. It becomes
  more energetically favorable to convert the input
  energy into the mass of new quark anti quark
  pairs.

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• Weak Interaction Revised
• Weak interactions can alter the flavour (type) of
  a quark without altering its colour

u
u
d
W-
A down quark from the neutron is converted into
an up quark through the emission of a W-
particle. The W- then produces an electron
u
d
d
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• GRAVITATION
• The graviton is the messenger particle for the
  gravitational force.
• Similar to the photon it has zero rest mass which
  explains its long range capabilities.
• It is by far the weakest of all interactions and
  offers the greatest challenge to unify with the
  other 3 fundamental interactions

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• Summary
• There are 18 types of fundamental particles
• 6 leptons
• 6 quarks (make up mesons and baryons)
• 6 messenger or mediator particles (determine
  strength and range of interaction)