"Curiosity is what sets humanity apart from other life; it is what has always delivered us the knowledge to advance and the technology to improve our lives. Basic research is the key to turning curiosity into innovation." Rolf Heuer, CERN Director-General, 5 Jan 2009.
High Energy Physics
Physicists at CERN study what happens when very energetic particles (usually protons or electrons) collide: their total energy (in their center of mass reference) becomes entirely available for a number of interesting processes.
For example, at low energies (compared to their rest mass, times the square of the light speed: mc2) a common process is the elastic scattering, but when their energy becomes higher and higher it can happen that new particles are created. The original particles can even disappear, and the number of possible products can be very high.
What can we learn from this kind of experiments? The emerging picture of the Nature at very high energies is quite simple: all matter is made of atoms whose mass is approximatively an integer multiple of the "nucleon mass", m = A mu; each atom has a nucleus made of Z protons and A-Z neutrons, surrounded by a "cloud" of Z electrons. Today we know that protons and neutrons are composite objects: they are made of "quarks". Electrons appear to be really "atomic" (i.e. not divisible) things instead.
Will this process continue indefinitely? Actually it seems that it will not: quarks seem to be elementary particles. Thus we can construct a table of elementary particles:
This table shows that there exist 6 different types of quarks, called "up", "down", "strange", "charm", "top" and "bottom", and 6 types of "leptons": the electron and its "neutrino", the muon with its neutrino and the tau lepton with the corresponding neutrino. [well, this is a simplified picture ;-P]
The particle above are all fermions. You can not put two identical fermions in the same place: they must occupy some space, hence will constitute an extended system [again, this is a simplified picture...]. This is the fundamental reason why matter takes space and can not be arbitrarily compressed using any finite amount of energy.
However, other particles exist that are not subject to this limitation: two (or any number of) identical bosons can be put in the same quantum state. It comes out that this kind of particles is responsible of transmitting interactions. For example, the photons are the bosons that carry on the electromagnetic interaction.
Cosmic evolution
Particle accelerators are probing Nature at higher and higher energies, allowing scientists to test their models about cosmic evolution, and suggesting scenarios for the different epochs of the Universe life.
In fact, energies as high as those obtained using colliders can be found only in the very early Universe, or in "exotic" astronomic objects, like supernovae (SN), pulsars or active galactic nuclei (AGN), thought to be the sources of cosmic rays.
Thus, high energy physics and high energy astrophysics, even though they deal with microscopic scale process and macroscopic systems respectively, are quite close to each other.
Let's consider the present picture of the whole Universe. It's experimentally known that it is expanding, then in the past it had to be smaller. If you went back in the time, you would see the temperature to increase and the distances between objects to shorten: you would see the collapse of the whole visible universe.
At a given temperature, the mean speed of the particles is known, and their mean energy is proportional to the temperature [well, in the assumption of a gas in equilibrium]. Thus during our back travel along time we would see an increasing mean energy of matter.
Sooner or later the mean energy would overcome the chemical energies range, and no stable molecule could survive. Next it would be greater then the ionization energies range, so that electrons and nuclei would decouple (we arrived to an energy at the level of few tens of keV [= 1000 eV = 1.6 10-16 Joule]). After few tens of MeV [= 1000 keV], the order of magnitude of the nuclei binding energy, protons and neutrons are free. At that time the Universe is a hot gas made of protons, neutrons, electrons and photons.
It is expected that for higher energies even the protons and the neutrons have to decouple into a free quarks plasma, but we weren't able to reach this energy level in laboratory, where the highest energies today are around 1-10 TeV [= 1012 eV].
Cosmic Rays Physics
The Cosmic Rays (usually abbreviated in CR) are particles coming to the Earth from every direction. They have a very large energy range, from few MeV to 1014 MeV (where 1 MeV = 1.6 10-13 Joule). In addition, their composition is very interesting: at 90% level CR are protons, followed by Helium nuclei, electrons and all the other nuclei, but their quantities are different from those measured in the Solar system.
CR are supposed to be created by SN explosions: this is the reason why the abundances of the elements heavier than Fe are present among CR in percentage greater than that observed in the Solar system. SN can also accelerate charged particles to maximum energies of about 10 TeV; even more powerful "engines" are pulsars and AGNs.
CR travel for about 10 million years before reaching Earth, and the different abundances of primary elements (produced by stars nuclear reactions) and secondary ones (consumed in the stars reactions), with respect to the local abundances, can give us informations about the propagation of CR in our Galaxy.
The most energetic CR come probably from other galaxies, providing us with important informations about cosmic structure and evolution.
One of the most exciting fields of contemporary fundamental physics is centered around the antimatter. High energy physicists know that matter and antimatter are always created in the same amount: for each electrons, also a positron (i.e. anti-electron) is produced. But the visible universe appears to be quite homogeneus: systems up to the galaxy clusters have to be entirely made of matter (or antimatter). The search for antimatter of cosmic origin is the main goal of the AMS experiment, even if there are other important topics that can be covered by AMS.
Links and further readings
J. García-Bellido's lectures on "Astrophysics and Cosmology" can be found on arXiv:hep-ph/0004188.
M. Jacob, "Fundamental physics from space and in space", Adv. Space Res. Vol. 32, No. 7. pp. 1197-1202, (doi:10.1016/S0273-1177(03)90318-8).
W. Bernreuther's lectures on "CP violation and baryogenesis" can be found on arXiv:hep-ph/0205279.
My chapter on cosmic antimatter of "Frontiers in Cosmic Ray Research", can be also found on arXiv:astro-ph/0405417.