Transcript The quark model

Quarks
M. Cobal, PIF 2006/7
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
 u  c  t 
• Like leptons, quarks occur in 3 generations:  d   s   b 
     
 d   s  b 
• Corresponding antiquarks are:      
u  c   t 
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, C=1 for c-quark, B=-1 for b-quark and T=1 for
t-quark
Since t-quark is a very short living one, there are no hadrons
containg top, i,e, T=0 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:
B=1 for baryons, B=-1 for antibaryons, B=0 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)
e
e’
p
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In the 70’s, deep inelastic
scattering of electrons on p
and bound n show evidence for
the quark model
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, t~10-16- 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 dquarks were discovered in 1947 in cosmic rays, using
photoemulsion to detect particles
 p  n p 
Some reactions induced by cosmic rays:
p p
 p  p p 0
 p  p p  p 
Same reactions can be reproduced in accelerators, with higher
rates (but cosmics may provide higher energies)
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:
ud      
Neutral pions decay mostly by the electro- magnetic interaction,
having shorter lifetime of 0.8x10-16 s
p 0  
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 Κ ,Κ : Kaons,
lifetime of K+ = 1.2x10-8 s
0
0
 baryon L, lifetime of 2.6x10 s
K      
Principal decay modes of strange hadrons: K   p   p 0
( B  0.64)
( B  0.21)
L p  p
( B  0.64)
L p0 n
( B  0.36)
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:
udd   du   (uud )
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: p   p  K 0  L
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More strange particles: S=2
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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. J / Y(3097)  cc
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 C=0
• Shortly after other particle with “naked” charm were discovered:
D  (1869)  cd , D 0 (1865)  cu
D  (1869)  dc , D 0 (1865)  uc
Lc (2285)  udc
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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:
Ds (1 9 6 9)  cs ,
Ds (1 9 6 9)  sc
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
Y (9460)  bb
B  (5279)  ub
B 0 (5279)  db
B  (5279)  bu
B 0 (5279)  bd
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
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