Evidence for Quarks

Quark Composition of Hadrons

[Secs 15.1 - 15.4 Dunlap]

By 1964 Particle classification became:

Gauge Bosons

FERMION

BOSON

W

W 0Z gi

Not substantiated until 70s and 80s

- -

e

e

Leptons Baryons

Mesons

++ + 0 -

0 +

0

p n

0 -

0

0 0

K K

HADRONS (S+E-W)LEPTONS (E-W)

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:

- + 0 - + 0 0 - 0 0 - 0 + 0, , , , K , K , K , K , , , , , , , Z , p 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

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.

Three Quarks for Muster Mark!

James Joyce Finnegans Wake

Evidence for quark structure[1] Neutral pi meson production:

0e p e p

[2] Neutron and proton magnetic moments are not = the nuclear magneton as predicted by the Dirac equation for point fermions.

Such reactions are difficult to explain if it is assumed that the proton is like the electron a fundamental structureless particle.

Other evidences for quark structureDeep Inelastic Scattering of electrons

The proton has excited states

Particle classification with 3 quarks

Gauge Bosons

FERMION

BOSON

Quarks

W

W 0Z gi

Not substantiated until 70s and 80s

- -

e

e

Leptons Baryons

Mesons

++ + 0 -

0 +

0

p n

0 -

0

0 0

K K

HADRONS (S+E-W)LEPTONS (E-W)

u

d s

Mathematics – SU(2) symmetryspecial unitary group

2 2 3 + 1

Triplet Singlet

Di-Nucleon

Deuteron

Bound

mathematics – SU(3) symmetry

plet

octet octetDecuplet

singlet

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+

PROTON OCTET FAMILY HEAVY PROTON – DECUPLET FAMILY

In reality the strange quark “s” has a heavier intrinsic mass

Discovery of the Omega MinusThe bubble chamber photograph shown was taken in 1964 –

It shows the production of the first observed Omega Minus

0

- 0

0 0

0

1 0 +1 +1 -3

3 -2 0

2

2 -1 0

1 0 0

K p K K

p

A look quick look at all the quarks

u

d s

cQ=+2/3

Q= -1/3 b

t

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 SymbolAnti-particle

MakeupRest massMeV/c2 S C B Lifetime Decay Modes

Pion π+ π- ud 139.6 0 0 02.60x10-8

μ+νμ

Pion π0 Self 135.0 0 0 00.83

x10-16 2γ

Kaon K+ K- us 493.7 +1 0 01.24x10-8

μ+νμ, π+π0

Kaon K0s K0

s 1* 497.7 +1 0 00.89

x10-10 π+π-,2π0

Kaon K0L K0

L 1* 497.7 +1 0 05.2

x10-8π+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 00.4

x10-23 π+π0

Rho ρ0 Self uu, dd 770 0 0 00.4

x10-23 π+π-

Omega ω0 Self uu, dd 782 0 0 00.8

x10-22 π+π-π0

Phi φ Self ss 1020 0 0 020

x10-23 K+K-,K0K0

D D+ D- cd 1869.4 0 +1 010.6

x10-13 K + _, e + _

D D0 D0 cu 1864.6 0 +1 04.2

x10-13 [K,μ,e] + _

D D+s D-s cs 1969 +1 +1 04.7

x10-13 K + _

MESONS

Particle Symbol MakeupRest mass

MeV/c2 Spin B SLifetime

(seconds>

Decay Modes

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 -12.6

x10-10 pπ-, nπ0

Sigma Σ+ uus 1189.4 1/2 +1 -10.8

x10-10 pπ0, nπ+

Sigma Σ0 uds 1192.5 1/2 +1 -1 6x10-20 Λ0γ

Sigma Σ- dds 1197.3 1/2 +1 -11.5

x10-10 nπ-

Delta Δ++ uuu 1232 3/2 +1 00.6

x10-23 pπ+

Delta Δ+ uud 1232 3/2 +1 00.6

x10-23 pπ0

Delta Δ0 udd 1232 3/2 +1 00.6

x10-23 nπ0

Delta Δ- ddd 1232 3/2 +1 00.6

x10-23 nπ-

Xi Cascade Ξ0 uss 1315 1/2 +1 -22.9

x10-10 Λ0π0

Xi Cascade Ξ- dss 1321 1/2 +1 -21.64

x10-10 Λ0π-

Omega minus Ω- sss 1672 3/2 +1 -30.82

x10-10 Ξ0π-, Λ0K-

BARYONS

Adding the charmed quark

Particle classification became

Gauge Bosons

FERMION

BOSON

Quarks

W

W 0Z gi

Not substantiated until 70s and 80s

- -

e

e

Leptons

Baryons

Mesons

++ + 0 -

0 +

0

p n

0 -

0

0 0

K K

HADRONS (S+E-W)LEPTONS (E-W)

u c t

d s b