Evidence for Quarks
Quark Composition
of Hadrons
[Secs 15.1 - 15.4 Dunlap]
By 1964 Particle classification became:
LEPTONS (E-W)
HADRONS (S+E-W)
Leptons

e
e
Baryons
 
  
-
p
-
n
 0  + 
K

0

W
W
FERMION
 0

Gauge
Bosons
 ++  +  0  -
Z0
Mesons
0
K
-
BOSON
0
 0


gi
Not substantiated until 70s and 80s
The 1950s particle Explosion
Young man, if I could
remember the names of all
these particles, I would have
been a botanist
Enrico Fermi
In th 1950s the race was on to build larger and larger accelerators. Many physicists
such as Fermi were alarmed to say the least. By the end of the 50s we had the:
p,  - ,  + ,  0 , K- , K + , K0 , K0 ,   ,  - ,  0 ,  0 , - , 0 , Z+ , 0
Then in 1964 the cascades were discovered
  , 0
The situation was getting out of hand – some solution had to be found
The quark or “straton” model or hadrons
Three Quarks for Muster Mark!
James Joyce
Finnegans Wake
1964. Murray Gell-Mann and George Zweig tentatively put
forth the idea of quarks. They suggested that mesons and
baryons are composites of three quarks or antiquarks, called
up, down, or strange (u, d, s) with spin 1/2 and fractional
electric charge.
u
d
s
charges 2/3, -1/3, -1/3
Since the charges had never been observed, the introduction of
quarks was treated more as a mathematical explanation. Later
theoretical and experimental developments allow us to now
regard the quarks as real physical objects, even though they
cannot be isolated.
Evidence for quark structure
[1] Neutral pi meson production:
e  p  e  p 


0
Such reactions are difficult to explain if it is assumed
that the proton is like the electron a fundamental
structureless particle.
[2] Neutron and proton magnetic moments are not = the
nuclear magneton as predicted by the Dirac equation for
point fermions.
Other evidences for quark structure
Deep Inelastic Scattering of electrons
The proton has excited
states
Particle classification with 3 quarks
LEPTONS (E-W)
HADRONS (S+E-W)
u
d
Leptons

e
e
Baryons
 
  
-
-
p
n

0

Z0
FERMION
 0
K
W
W
 ++  +  0  -
 0  + 

Gauge
Bosons
Quarks
s
Mesons
0
K
-
BOSON
0
 0


gi
Not substantiated until 70s and 80s
Mathematics – SU(2) symmetry
special unitary group
Singlet
Triplet
2
2
3
+
1
Di-Nucleon
Deuteron
Bound
mathematics – SU(3) symmetry
plet
singlet
octet
octet
Decuplet
The strange quark is seen as being the identical
particle to the up and down quark – just in a
different quantum state. Rotations in quark
space do not change the strong interaction
SU3 found in experiment
PROTON
FAMILY
=
OCTET
Jπ = 1/2+
HEAVY
PROTON
FAMILY
=
dicuplet
Jπ =3/2+
In reality the strange quark “s” has a heavier intrinsic mass
PROTON OCTET FAMILY
HEAVY PROTON – DECUPLET FAMILY
Discovery of the Omega Minus
The bubble chamber photograph shown
was taken in 1964 –
It shows the production of the first
observed Omega Minus  
K   p  K 0  K   
1 0
+1
+1
-3
-   0   
3
-2
0
 0   0  2
2
-1
0
0  p   
1
0
0
A look quick look at all the quarks
Q=+2/3
u
c
t
Q= -1/3
d
s
b
1st Gen
2nd Gen
3rd Gen
SU3 symmetry for q-q system
The Pi- Meson Family (of nine)
The energies of the Pi-meson family
The higher mass of the K- mesons and Eta
meson results from the larger mass of the
strange quark
Particle
MESONS
Symbol
Antiparticle
Makeup
Rest mass
MeV/c2
S
C
B
Lifetime
Decay Modes
139.6
0
0
0
2.60
x10-8
μ+νμ
135.0
0
0
0
0.83
x10-16
2γ
us
493.7
+1
0
0
1.24
x10-8
μ+νμ, π+π0
K0s
1*
497.7
+1
0
0
0.89
x10-10
π+π-,2π0
K 0L
K0L
1*
497.7
+1
0
0
5.2
x10-8
π+e-νe
Eta
η0
Self
2*
548.8
0
0
0
<10-18
2γ, 3μ
Eta prime
η0'
Self
2*
958
0
0
0
...
π+π-η
Rho
ρ+
ρ-
ud
770
0
0
0
0.4
x10-23
π+π0
Rho
ρ0
Self
uu, dd
770
0
0
0
0.4
x10-23
π+π-
Omega
ω0
Self
uu, dd
782
0
0
0
0.8
x10-22
π+π-π0
Phi
φ
Self
ss
1020
0
0
0
20
x10-23
K+K-,K0K0
D
D+
D-
cd
1869.4
0
+1
0
10.6
x10-13
K + _, e + _
D
D0
D0
cu
1864.6
0
+1
0
4.2
x10-13
[K,μ,e] + _
D
D+s
D-s
cs
1969
+1
+1
0
4.7
x10-13
K+_
Pion
π+
π-
Pion
π0
Self
Kaon
K+
K-
Kaon
K 0s
Kaon
ud
BARYONS
Particle
Rest mass
MeV/c2
Spin
B
Lifetime
(secon
ds>
Symbol
Makeup
S
Proton
p
uud
938.3
1/2
+1
0
Stable
...
Neutron
n
ddu
939.6
1/2
+1
0
920
pe-νe
Lambda
Λ0
uds
1115.6
1/2
+1
-1
2.6
x10-10
pπ-, nπ0
Sigma
Σ+
uus
1189.4
1/2
+1
-1
0.8
x10-10
pπ0, nπ+
Sigma
Σ0
uds
1192.5
1/2
+1
-1
Sigma
Σ-
dds
1197.3
1/2
+1
-1
1.5
x10-10
nπ-
Delta
Δ++
uuu
1232
3/2
+1
0
0.6
x10-23
pπ+
Delta
Δ+
uud
1232
3/2
+1
0
0.6
x10-23
pπ0
Delta
Δ0
udd
1232
3/2
+1
0
0.6
x10-23
nπ0
Delta
Δ-
ddd
1232
3/2
+1
0
0.6
x10-23
nπ-
Xi Cascade
Ξ0
uss
1315
1/2
+1
-2
2.9
x10-10
Λ0π0
Xi Cascade
Ξ-
dss
1321
1/2
+1
-2
1.64
x10-10
Λ0π-
Omega minus
Ω-
sss
1672
3/2
+1
-3
0.82
x10-10
Ξ0π-, Λ0K-
6x10-20
Decay Modes
Λ0γ
Adding the charmed quark
Particle classification became
LEPTONS (E-W)
Leptons
e
e

HADRONS (S+E-W)

u
d
c
s
t
b
Quarks
Baryons
p
n
 ++  +  0  -
 0  + 
FERMION
 0

K

0
Gauge
Bosons

W
W
Z0
Mesons
0
K
-
BOSON
0
 0


gi
Not substantiated until 70s and 80s