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Antimatter
 

Antimatter is composed of antiparticles: antiquarks, anti-electrons, antineutrinos.
We've seen before that all particles are characterised by properties quantified by quantum numbers. For example the electric charge Q and the spin number J, but also other grandiose names as esoteric as the baryon number B, the lepton number L, the isospin T, the strangeness S, the hypercharge Y or the charm C!

It was in 1927 that Paul Dirac imagined (like the mathematical solutions named after him) the existence of  antiparticles. In 1932, the american Carl Anderson detected some anti-electrons (christened positrons as a result of their positive charge Q) amongst the particles produced by the impact of cosmic rays on the atmosphere.
 

What we call a cosmic ray is a very rapid and therefore very energetic flux particle (electrons, protons, ions) which comes from deep space. These flux particles originate from very remote supernovae. A supernova is a massive star which ends its life in a gigantic explosion which emits in several weeks a light as powerful as a galaxie containing many million of stars. An explosion such as this creates a shock wave in the interstellar locality (full of gas and magnetic charge), and plays the role of a natural accelerator of electrically charged particles. The analysis of this cosmic ray at altitude has for a long time been the only means for physicists to study and discover new particles. It was necessary to wait for the 1950s to reproduce this type of very energetic ray in the accelerators.

Lets dream of antiparticles:
 

• The antiproton has the same mass as the proton, but it has a negative charge Q.
• The antineutron has the same null charge Q as the neutron, but the baryonic number of the antineutron is -1 whereas that of the neutron is +1!
• The anti-electron or positron has a positive charge Q.

• Antiquarks u and d are written as(a bar is placed over the name of the particles)

Thus for each particle there is a corresponding symetrical anti-particle.
These anti-particles can assemble themselves; in this way on 4th January 1996, CERN in Geneva announced the fabrication of nine atoms of antihydrogen.
 

Annihilation
 

If a particle meets its antiparticle, their two masses are converted entirely into energy (gamma rays): This is the phenomena of annihilation that physicists can reproduce in colliders.

Moreover it is the only phenomena where the totality of a mass is transformed into energy in accordance with Ensteins celebrated formula E=mc². This colossal energy released by such a particle-antiparticle encounter can rapidly transform itself into other massive particles: this is the inverse phenomena of materialisation of energy.
This explains why there is no antimatter in our matter based environment: all traces of antimatter will be annihilated at the least contact with our world. Furthermore it seems that our entire Universe is composed of nothing but matter.

Why has the Universe opted for one single form of matter? the question is not entirely resolved at this moment in time.
Astrophysicists suppose that 10^-32 seconds after the Big Bang, the entire Universe was nothing but a symmetrical melange (at a temperature of 10²⁶ degrees) of quarks, neutrinos and electrons and of their respective antiparticles. This resulted in an uninterrupted succession of annihilation and of materialisation bathing in a bath of very high energy photons. No material would have been able to emerge from this infernal chaos if a perfect symmetry had been respected between matter and antimatter. Now a minute quantity of matter has survived total annihilation; a particle of matter for a million particle-antiparticle pairs annihilated!
This asymmetry, luckily for us, was not possible but thanks to the "breakage of CP symmetry" discovered in 1967 by Andreï Sakharov, the father of the soviet H bomb.
 

CP symmetry:
 

• C for conjugation of charge: mathematical property of symmetry which transforms the Q charge of a particle. By applying symmetry C to an electron of charge -1, we obtain an anti-electron of charge +1.

          

• P for parity: mirror symmetry which inverses right and left. By applying this symmetry, we change for example the spin of particles.

Conjugation of C and P gives CP symmetry: particles which change, after interacting, their quantum numbers conforming to C and P symmetry, should be able to do so in both senses. But justly this CP symmetry is not respected; which is equivalent to saying that matter and antimatter don't behave exactly the same!
To be more exact, this CP symmetry is violated by one of the 4 universal forces, the most mysterious one called the weak interaction.
 

How to imagine a nucleon? (by Etienne Klein)
 
 

Other families of fermions
 

Our table of  4 fermions + 4 antifermions is it complete?
Not yet...

There exist in fact, two other families of fermions analogous to our family u-d-electron-neutrino:
 
 

• Family 1 suffices to describe our current world. But since the 1940s, we have discovered two other families of particles corresponding to heavier and heavier particles.
• Families 2 and 3 contain more and more massive particles which did not exist in the Universe except right at the beginning, when the density and the temperature was such that these particles could materialise as a result of a sufficient energy density.

Each lepton is associated with a specific neutrino.
The quarks have been christened with poetic names which correspond to their "flavours". Nothing to do with the taste, but the 6 flavours of quarks characterise the influence of the weak interaction upon them.

Family 2:

• Muon: discovered in 1937 by Anderson within cosmic rays. This is a sort of electron but 200 times more massive.
• Quark s or strange: 20 times heavier than quark d
• Quark c or charm: discovered in 1975. 375 times heavier than quark u

• Tau: discovered in 1976. 3500 times heavier than the electron or twice as heavy as the proton.
• Quark b for bottom or sometimes beauty: discovered in 1977. 5 times heavier than the proton.
• Quark t for top: discovered in 1994 at Fermilab. 170 times heavier than the proton! This is the most massive elementary particle known today; it is as heavy as an entire atom of gold.

What it still needed to explain matter?

We are still missing a fundamental family of particles: the bosons that form part of the photon, a quantum grain of light. Bosons are very special particles which do not respect Paulis exclusion principle. Bosons can be superimposed in the same quantum state in contrast to the fermions which are individualised in space.
These bosons are in addition, mediator particles of the 4 fundamental forces of the Universe: the interactions without which our particles of matter couldn't be bound together.

Once these 4 fundamental interactions are defined, we will finally have all of the elements which constitute the present theory of the Standard Model: this is the theory which explains all observable phenomena at the scale of the particles...

crazyflash:Elementary my dear quark!

The ultimate elementary particles are resourceful snobs: In effect, quarks, appear very UP and often  close to the DOWN-Jones index. They adore grouping together into threes to knock it back in the bar(yons), or they get together as couples in mesons. Archeophysicists have discovered the quarks fossilised ancestors: dinoquarks of enormous size with a strange beauty and the dignified charm of top models. For all of their disproportionate development, they still show have a lot of ancestral baggage on tau! Three other species, very difficult to observe, are the neutred rhinos. Physicists on proton safari have great difficulty in capturing them.  Each of these particles has its auntie. Their images in a mirror terrorise them and touching their mirror image annihilates them.