quarks
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Particle PhysicsParticle Physics
QuarksQuarks
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QuarksQuarks
• 100’s of hadrons (baryons and mesons) are
known
• Classified in terms of their properties
(mass, spin, parity, baryon number,
strangeness, isospin ….)
• Development of a schematic classification
scheme allowed one to predict the
properties of particles
• (a Periodic Table for particles)
• Eventually showed that all hadrons can be
constructed from fundamental, point-like
particles called quarks (and anti-quarks)
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• Particles having the same spin-parity (Jπ)
tend to have similar masses
HadronsHadrons
Rohlf 17.7
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• 1961: Gell-Mann, Ne’eman et al.
• Plot the known particles on Strangeness vs
Isospin plots
The Eightfold WayThe Eightfold Way
Krane 18.11
Same charge
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The Eightfold WayThe Eightfold Way
Krane 18.12 & 13
A building block ?
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The Eightfold WayThe Eightfold Way
Successful in predicting particles
Are quarks real or just nice maths ?
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• All quarks are spin-1/2 fermions
• Fractional charge
• Baryon number =
• Intrinsic parity is +1
• Initially, 3 quark “flavours”
QuarksQuarks
eore31
32 −+
31+
− 1/3Stranges
−1/3Downd
+2/3Upu
Charge (e)NameFlavour
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• A quark-antiquark pair
• Mesons have B = 0.
• 9 bound states (mesons) from the 3 basic
quarks:
• (The are in fact admixtures
of pairs)
• “Pseudoscalar Mesons”
MesonsMesons
031
31 =−+
ηπππη ′−−++ 000KKKK
ssdsussdddudsuduuu
ηηπ ′,,0
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• These 9 mesons form an Octet (8) and a
Singlet (1 – the η′).
• Interchange quarks e.g.
MesonsMesons
sdsd ↔↔ &
),,( ssdduu⇒′η
sdKsdK =→= 00
ηη ′→′
Same particle
Different particles
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• Lowest energy state has the quark and anti-
quark spins with
• The lowest energy state of a meson has
• Excited energy state has the quark and
anti-quark spins with
MesonsMesons
−= 0πJ
0=l
1)1()()()(
)()(
−=−⋅⋅=∴
−=l
qqqq
πππ
ππ fermions
0=l
↑↓
↑↑−=1π
J
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• A quark triplet
• Baryons have B = +1.
• Antiparticles have B = −1
BaryonsBaryons
sduuds
uddnuudp
=Λ=Λ
==00 ,
,
qqq
qqq
+1/2−1/2+1/2+1/2T3
+1/2+1/2+1/2+1/2T
+11/31/31/3B
+1/2+1/2+1/2+1/2Spin
+1−1/3+2/3+2/3Q
uudduuProton
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• Lowest energy state has the quark spins with
• The lowest energy state of a baryon has
• Excited energy state has the quark spins with
BaryonsBaryons
+=21π
J
0=l
fermions
0=l
↑↓↑
+=23π
J
1)()(
1)()(
)()(
−==
+==∴
−=
qqqq
qqqq
ππ
ππ
ππ
↑↑↑
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• 19** Discovery of the
• Strangeness:
• Spin = 3/2 so
• 3 ‘s’ quarks, in the same quantum state –violates Exclusion Principle.
• Introduce another property “Colour” or “Colour Charge”
• NOTHING to do with visual colour
• So, we have 3 quarks with the same ‘flavour’ i.e. ‘s’ but different ‘colours’.
• Red, Green & Blue so when they’re combined, the resulting baryon is White i.e. ‘Colour Neutral’.
• Mesons are also ‘Colour Neutral’ e.g. Red & Anti-Red
ColourColour
−Ω
sssS ⇒−= 3
↑↑↑
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• 1974 Stanford and Brookhaven:
Discovery of the meson
• Cannot be constructed from the u, d, and s
quarks and anti-quarks.
• Must be a fourth quark --- “Charm (c)”.
• Yet another introduced property, u, d and s
quarks have charm = 0. The c quark has
charm = +1
CharmCharm
ψ/J
eq32+=
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• 1977 Fermilab: Discovery of the
• Cannot be constructed from the u, d, s and
c quarks and anti-quarks.
• Must be a fifth quark --- “Bottom (b) or
Beauty”.
Bottom (“Beauty”)Bottom (“Beauty”)
Υ
eq31−=
• Reason to suspect there are 3 families of
quark pairs, just like the 3 generations of
leptons
• 1997 Fermilab: Evidence for the top quark
Top (“Truth”)Top (“Truth”)
eq32+=
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QuarksQuarks
+2/3Topt
−1/3Bottomb
+2/3Charmc
−1/3Stranges
−1/3Downd
+2/3Upu
Charge (e)NameFlavour