1. What makes a particle “elementary” ? • A particle is elementary if it has no inner

structure (i.e not “made” of some even smaller entities).

2. Which particles were considered elementary throughout History?

• Antiquity : Four elements. Unsuccessful attempt at an atomistic theory during the 5th century BC (Democritus).

• 18th century : Lavoisier and Dalton verify experimentally the validity of the atomic structure.

• 1868 : Mendeleev proposes his chart of elements, containing the 63 atoms known at the time. The “empty cases” he left were soon filed. By 1896, 77 atoms have been discovered, and are considered elementary.

• 1897 : Discovery of the first subatomic particle by J.J Thompson : the electron. The search for its positive counterpart begins, until…

• 1911 : Rutherford discovers the nucleus. Transmutation reactions showed that the hydrogen nucleus played a specific role (4

2He + 147N --> 18

9F --> 178O + 1

1p) . Rutherford named it proton (protos = first)

• 1932 : Chadwick discovers the neutron, which is not stable when isolated, and decays as follows : n p + e- (+ ¯νe) . The proton, electron and neutron account for all the atoms of all the elements in the Universe.

This was the “simplest” elementary particle set ever described. A small number of particles, a small number of interactions. LEPTON (leptos = light) : e-

BARYONS (baryos = heavy) : p , n

However, some problems were already present.1. The photon : Photoelectric effect ; Compton scattering.2. Antiparticles : Discovery of the positron by Anderson (1932), studying cosmic rays. Many more particles would be discovered in cosmic rays…

3. Mesons : These particles were first postulated by Yukawa (1935) to explain the force that binds the nucleus together. Being of intermediate masses, they were called mesons (mesos = middle).4. Neutrinos : Necessary to preserve E conservation in β decay

• From the particle garden to the jungle :In 1937, Anderson discovered the muon μ. The μ proved to be some sort

of heavier electron (lepton).

I.I Rabi, Nobel 1944

Who ordered THAT ?

The muon decays into through β decay:

μ νμ + e- +¯νe

In 1947, pions (mesons) were detected in cosmic rays. They were thought of as Yukawa’s mediator particle for the strong interaction. The Universe was in order again, except for the muon, which played no visible role.

In December 1947, new mesons were found : the kaons. The place got crowded again…

With the use of particle accelerators in the 50’s, many new particles were discovered. Some of them were « strange » because they were produced by the strong force but decayed through the weak force.

• Moreover, some rules seemed to be missing to predict if a decay could occur or not :

• Why is π- + p+ K+ + Σ- possible ,• When π- + p+ K0 + n is impossible ?

• In 1953, Gell-Mann and Nishijima came with a simple and elegant idea. Each particle was to be assigned a «strangeness », and the overall strangeness had to be conserved during a collision (not through decay).

• There were then THREE laws of conservations for reactions :

• Charge• Baryonic number (proton like particles)• Strangeness

• Still, there were dozens of “elementary” particles by 1960, either pion like (mesons) or proton like (baryons). Mesons do not feel the strong interaction, whereas baryons do. Either type can be strange or non-strange. In 1955, Willis Lamb started his Nobel Prize acceptance speech by saying that “maybe physicists discovering a new particle ought to be fined 10 000$”

• There was a strong need for simplification, which Gell Mann provided in 1961. He acted like Mendeleev had done a century before for chemistry. His Periodic Table was known as...

Fine them !

The Baryon Octet

n p

Σ+

Ξ0Ξ-

Σ-Σ0 ; Λ

S=0

S=-1

S=-2

Q=0

Q=1

Q=-1

The Meson Octet

K0 K+

π+

¯K0K-

Π-π0 ; η

S=1

S= 0

S= 1

Q=0

Q=1

Q=-1

The Quark Model (1964)

ud

¯u ¯d

s

¯s

S=0

S=-1

S=1

Q=2/3Q=-1/3

Q=-2/3

Q=-1/3

Quark Hypothesis

• Mesons are bound states of quark-antiquark : π+ is u ¯d.

• Baryons are bound states of three quarks : p is uud.

The quarks as a model were confirmed by the discovery of the Ω- sss baryon of strangeness -3 in 1964.

• The existence of the quarks as particles was confirmed experimentally by Rutherford-like experiments at SLAC in 1968 (Friedman, Kendall, Taylor). They are today’s «elementary» particles, with the leptons and the mediator particles.

3. New particles again, but some symmetry and order gained...

• Quark dynamics was understood later, and brought 8 photon like mediator particles : gluons.

• After a few years of quiet, the November Revolution (1974) brought a new quark (charm quark) through the discovery of the J/ψ meson (c ¯c).

• In 1975, the Τ lepton was discovered.• In 1977, the Υ meson (b ¯b) was

discovered, introducing the bottom quark.

• In 1983, the mediators for the weak interaction were discovered at CERN : W+- and Z0

• The symmetry of six quarks and six leptons was completed with the top quark in 1995.

Top quark discovery (Fermilab 1995)

Elementary particles today

Orders of magnitude for distances

4. Tomorrow• There are no theoretical

reasons for the quarks to be the final elementary particles.

• The electron is still being probed, in search of an internal structure.

• New accelerators (LHC) will provide higher energies to explore yet uncharted territories (“small Big Bangs”) and maybe discover new particles (Higgs Boson). The Higgs is predicted by the Standard Model.