Quarks

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Quarks
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Quarks

• Quarks are s ½ fermions, subject to all kind
  of interactions.
• They have fractional electric charges
• Quarks and their bound states are the only
  particles which
• interact strongly
• Like leptons, quarks occur in 3 generations
• Corresponding antiquarks are

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The quark model Baryons and antibaryons are
bound states of 3 quarks Mesons are bound states
of a quark and an anti-quark Barions and Mesons
are Hadrons
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Hadrons
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Quantum Numbers and flavours
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Strangeness is defined so that S-1 for s-quark
and S 1 for the anti s-quark. Further, C1 for
c-quark, B-1 for b-quark and T1 for
t-quark Since t-quark is a very short living
one, there are no hadrons containg top, i,e, T0
for all Quark numbers for up and down quarks
have no name, but just like any other flavour,
they are conserved in strong and em
interactions Baryons are assigned own quantum
number B B1 for baryons, B-1 for
antibaryons, B0 for mesons
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• Strange, charmed, bottom and top quarks each
  have an additional
• quantum number
• strageness S, charm C, beauty B and truth T
  respectively.
• In strong interactions the flavour quantum
  number is conserved
• Quarks can change flavours in weak interactions
  (DS ?1, DC ?1)

Theory postulated in 1964 (Gell-Mann)
In the 70s, deep inelastic scattering of
electrons on p and bound n show evidence for the
quark model
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Particles and Interactions
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Hadrons and lifetime
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• Majority of hadrons are unstable and tend to
  decay by the strong
• interaction to the state with the lowest possible
  mass (t 10-23 s)
• Hadrons with the lowest possible mass for each
  quark number
• (S, C, etc.) may live much more before decaying
  weekly
• (t 10-7- 10-13 s) or electromagnetically
  (mesons, t10-16- 10-21 s)
• Such hadrons are called stable particles

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Brief history of hadron discoveries

• First known hadrons were proton and neutron
• The lightest are pions p. There are charged
  pions p, p-, with mass of 0.140 GeV/c2, and
  neutral ones p0, with mass of 0.135 GeV/c2
• Pions and nucleons are the lightest particles
  containing u- and d-quarks were discovered in
  1947 in cosmic rays, using
• photoemulsion to detect particles
• Some reactions induced by cosmic rays
• Same reactions can be reproduced in accelerators,
  with higher rates (but cosmics may provide higher
  energies)

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Positron Discovery

• Nobel Prize 1936 C. D. Andersson (Berkeley)
• for his discovery of the positron

Plate of steel
B?
positron
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Pion discovery
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Charged pions decay mainly to the muon-neutrino
pair (BR 99.99) having lifetimes of 2.6x10-8
s. In quark terms Neutral pions decay
mostly by the electro- magnetic interaction,
having shorter lifetime of 0.8x10-16 s At the
beginning discovered pions were believed to be
responsible for the nuclear forces However, at
ranges comparable with the size of nucleons this
description fails.
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Strange mesons and baryons

• Were called so because, being produced in strong
  interactions, had quite long lifetimes and
  decayed weakly rather than strongly
• The most light particles containing s-quark
• mesons K, K- and Kaons,
• lifetime of K 1.2x10-8 s
• baryon L, lifetime of 2.6x10 s
• Principal decay modes of strange hadrons

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Strange particles kaon discovery
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Problem While the first decay in the list is
clearly a weak one, decays of L can be very well
described as strong ones, if not the long
lifetime However, this decay should have t
10-23 s. Thus, L cannot be another sort of
neutron.... Solution To invent a new quark,
bearing a new quark number strangeness- which
does not have to be conserved in weak
interactions In strong interactions, strange
particles have to be produced in pairs to save
strangeness
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More strange particles S2
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(No Transcript)
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The charm quark
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• Bubble chambers turned out to be a great tool
  for particle
• discovery. Numerous hadrons, all fitting the
  u-d-s quark scheme
• until 1974
• In 1974 a new particle was discovered, which
  demanded a new
• flavour to be introduced.
• Since it was detected simultaneously by two
  rival groups in
• Brookhaven (BNL) and Stanford (SLAC), it
  received a double name
• The new quark was called charmed and the
  corresponding quark
• number is charm, C. J/Y itself has C0
• Shortly after other particle with naked charm
  were discovered

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J/Y
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J/Y Width
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• Even heavier charmed mesons were found
• which contained strange quark as well
• Lifetimes of the lightest charmed particles are
  of 10-13 s, well in
• the expected range of weak decays
• Discovery of charmed particles was a triumph
  for electroweak
• theory, which demanded number of quarks and
  leptons to be equal
• In 1977, beautiful mesons
• were discovered
• And the lightest b-baryon L0b(5461) udb
• This is the limit top quark is too unstable to
• form observable hadrons

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J/Y Decay
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J/Y Decay
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Other Y tests