Exotic Matter

Up to this point, we have described all the building blocks needed to explain the matter on Earth: up and down quarks, electrons, neutrinos, and the forces that act between them. But nature turned out to be richer than that. In cosmic rays – and later in particle accelerators – unexpected particles kept appearing that fit no existing scheme. They were not made of the familiar quarks and behaved in strange ways. Research into these exotic particles eventually led to one of the most surprising discoveries in particle physics: the matter we know is only one of three complete families of particles.

Strangeness: a new property

Particles were observed in cosmic rays with a remarkably long lifetime for their mass – as though something in their structure was slowing their decay. Physicists gave this property the name strangeness: a number indicating how much 'strange' content a particle carries. Mesons can carry strangeness of +1 or −1, baryons of −1, −2, or even −3.

The explanation turned out to be simple and elegant: these particles contain a third quark type – the strange quark (s). The strange quark resembles the down quark but is considerably heavier (~100 MeV versus ~5 MeV) and carries strangeness as an additional property. With this discovery, the picture of two quark types was broken for good.

Quark	Charge	Mass	Spin
Strange (s)	−1/3 e	~100 MeV	1/2

Charm: the heavier partner of the up quark

The strange quark is, in a sense, a heavier version of the down quark. That raised an obvious question: does a heavier counterpart of the up quark also exist? In 1974 the answer was found: the charm quark (c), with a charge of +2/3 and a mass of ~1.3 GeV – more than a hundred times heavier than the up quark. Its discovery, simultaneously in two independent experiments, was so spectacular that the community spoke of the 'November revolution' in particle physics.

Quark	Charge	Mass	Spin
Charm (c)	+2/3 e	~1.3 GeV	1/2

Heavier counterparts of the electron

Parallel to the discoveries among quarks, it was established that the electron also had a heavier counterpart. The muon (μ) was discovered in cosmic rays in 1936 and behaves in every respect like an electron, but is ~207 times heavier. The associated muon neutrino ($\nu_\mu$) was confirmed in 1962.

The discovery of the muon was bewildering at the time, because it behaved exactly like an electron yet was ~207× heavier. That shattered the then-prevailing idea that the electron was unique. Nobel laureate I.I. Rabi captured the astonishment with: 'Who ordered that?'

A third generation

In 1973, Kobayashi and Maskawa predicted the existence of a complete third generation of particles, needed to explain a subtle phenomenon: CP violation – the fact that matter and antimatter do not behave in a fully symmetric way. Without this asymmetry, the early universe would have produced equal amounts of matter and antimatter, and the two would have completely annihilated each other. CP violation is a necessary ingredient in explanations of the matter–antimatter asymmetry, but the amount of CP violation in the Standard Model is not sufficient to account for the full observed excess of matter.

The prediction was confirmed step by step in the years that followed:

Discovery	Year
Bottom quark (b)	1977
Tau lepton (τ)	1975
Top quark (t)	1995
Tau neutrino ($\nu_\tau$)	2000

The top quark, with a mass of ~173 GeV – as heavy as a gold atom – is the heaviest fundamental particle currently known.

Three generations: a pattern

With these discoveries, a striking pattern became visible. The fundamental fermions are not scattered randomly, but arranged into three generations, each an exact repetition of the previous one but heavier:

	Generation I	Generation II	Generation III
Quark (charge +2/3)	Up (u)	Charm (c)	Top (t)
Quark (charge −1/3)	Down (d)	Strange (s)	Bottom (b)
Charged lepton	Electron (e)	Muon (μ)	Tau (τ)
Neutrino	$\nu_e$	$\nu_\mu$	$\nu_\tau$

Ordinary matter – everything around us – is built exclusively from the first generation. The second and third generations are unstable and quickly decay back to the first. They existed abundantly in the hot early universe and can be recreated today in particle accelerators. Why there are exactly three generations and not more or fewer is a question the Standard Model describes but does not explain.