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Matter Today 1

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Quarks and neutrinos

Up until 1964, it was believed that there only existed three elementary particles making up the atom: the electron, the proton and the neutron.
However, numerous unstable particles (with a lifetime of the order of 10^-23 seconds) have been detected, either in cosmic rays, or in the high energy impacts created in particle accelerators constructed after the 39-45 war.
 

Particle accelerators
 

These accelerators, also called colliders, are enormous and very expensive machines which can reach several tens of kilometers in length. The aim is to accelerate charged particles (electrons, protons, ions) to speeds close to that of light!
The energy attained by these particles is enormous (ie their speed) and probe particles are then projected into target particles: this permits the consequences of the impacts provoked in this way to be studied. The enormous energy of these impacts can be converted into new particles. The higher the energy of the collision, the more massive are these newly created particles and the more information they give physicists on the ultimate constituants of matter.

Accelerators can currently be of two types:

• synchrotrons or circular accelarators of which the largest is the LEP (Large Electron Positron ring) at CERN (Centre d'Etude et de Recherche Nucléaire) in Geneva. Why not visit their web site.
• Linacs or linear accelerators of which the most famous is the SLC at Stanford in California. You can visit them too.

For more info on accelerators, go to CERN by clicking on their logo:

Quarks
 

Discovery:

It had become clear, at the start of the 60s, that the very large number of particles (more than 300) detected in the colliders had little chance of being elementary.

Set out for the first time in 1964 by Murray Gell-Mann and independantly by George Zweig, the theory of quarks progressively established its pedigree and won aclaim as advances corroborated the theory by experiment; It was not until 1975 that quarks were detected experimentaly.

The strange name of Quark comes from a roman phrase of James Joyce in "Finnegans Wake": Three Quarks for Muster Mark!:

Quarks are the constituents of nucleons. There exist two types in ordinary matter.
 

Why are these Q charges fractional in the way that they are? Because nucleons are always made up of 3 quarks:
 

The size of a quark is theoretically point sized, but in reality is less than 10^-18 m; that is at least a thousand times smaller than the size of a nucleon which is 10^-15 m.

Quarks, like nucleons, are fermions of spin J = 1/2 and are joined by a force called the strong interaction (the same as binds together nucleons).
 

Quarks have a unique property: they are incapables of existing alone, unaccompanied!
It is absolutely impossible to observe a quark in isolation; quarks cannot aggregate except in two different manners and only therefore give rise to two families of composite particles:

• 
• baryons which are always formed from a triplet of quarks. Nucleons are the most common baryons. There exist other triplets formed from more exotic and heavier quarks (see the standard model); from where we get the origin of the word baryon (from the greek "barus" = heavy). These baryons which are heavier than nucleons are designated by the greek letters (particles lambda, sigma, delta etc..) and are grouped into families of hyperons.

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• mesons which are formed of pairs of quarks and antiquarks (see antimatter). These very unstable particles are also designated by greek letters (pions, kaons, Êta...).

We thus approach the current understanding of the structure of the nucleus: 2 sorts of quark forming protons and neutrons and 1 electron. In reality, it lacks one other particle of matter (or fermion) fundamental to our game of Lego:
 

The neutrino
 

This fundamental particle was "invented" in 1930 by Wolfgang Pauli (him again!) to explain the mechanism of Beta radioactivity: a neutron transforms into a proton by emitting an electron (beta ray) and another mysterious particle that the italian physicist Enrico Fermi named in 1933 "neutrino"; which in italian means "little neutron".

It was not until 1956, after a conclusive experiment that Frederick Reines and Clydes Cowan showed evidence for the neutrino. These two scientists installed a neutrino detector close to a nuclear reactor at Savannah River, in South Carolina.

The neutrino has a Q charge of null and it is 50,000 times smaller than an electron.
Its extremely small mass has only ever been detected on the 5 June 1998 in Japan! The measurement of this mass could be of capital importance for the destiny of the Universe: According to the current cosmological model, the universe was born from a gigantic explosion, the Big Bang. It will from here, continue to dilute indefinitely, unless it has enough mass to re-contract into a  Big Crunch. This missing mass of the Universe could be found in neutrinos which are very numerous in the Cosmos.
Such is the way that an infinitesimaly small particle can have an enormous power over the destiny of the infinitely large!
This discrete particle does not interact with matter very much at all.: One would need a sheet of lead a light year thick to stop half of the neutrinos which attempt to traverse it!

Neutrinos are produced abundantly by stars such as our Sun: each second, hundreds of  millions of solar neutrinos can pass through the earth and our bodies without suffering the least impact! Hence the extreme difficulty in detecting them...

The neutrino and the electron (light weight particles), are grouped in the familly of leptons (from the greek "leptos" = light).
The combination of two leptons and two quarks u and d  are thus the building blocks of our world.
 

Is that all ?
No !
There exists a mirror universe where matter is transformed into anti-matter...