In the older days the world was believed to be made up of four elements combined: water, earth, fire and air.

Since Democritus (400 BC) matter was thought to be composed of indivisible units (atoms). This hypothesis survived until the late 19th century.

The fact that one can classify different types of atoms according to their chemical properties (periodic table of the elements) suggests that they are not fundamental. Moreover, several experiments from the beginning of the 20th century showed that atoms have a structure: a dense nucleus positively charged surrounded by a cloud of negatively charged electrons.

Soon afterwards, nuclei were found to be composed of protons and neutrons. Today we know that protons and neutrons are not indivisible either. They are made up of quarks.

Every particle has its antiparticle. They both have the same mass and opposite charges. When they meet they annihilate each other. The Standard Model particles are the following.

[Units of mass and energy: 1 GeV = 10³ MeV = 10⁶ keV = 10⁹ eV = 1.78 × 10^-27 kg]

The proton, the neutron and many other particles feeling the strong interaction are not elementary but composed of quarks (q) and/or antiquarks (q). In contrast to the rest of particles, quarks can never be isolated and appear in hadrons confined by the strong interaction. Quarks u, c, t have electric charge 2/3 and d, s, b have charge -1/3. [Unit of charge e=1.602×10⁻¹⁹ Coulomb].

Ordinary matter is made up of protons, neutrons and electrons. The rest of particles are produced by cosmic rays or in colliders and decay very rapidly into lighter particles. Outside the nucleus, the neutron lives just 15 minutes in average. Two kinds of hadrons have been observed in nature: baryons (q₁q₂q₃ ó q₁q₂q₃) and mesons (q₁q₂).

In Quantum Field Theories (a combination of Quantum Mechanics and Relativity) the interactions are described by the exchange of force carriers or messenger particles, the gauge bosons. All forces in nature are manifestations of just four fundamental interactions. A quantum theory of gravity has not yet been found.

The Higgs field fills the whole space. It remains even if everything is removed. It forms the vacuum. It is believed that elementary particles acquire their masses due to interaction with the Higgs field. If this mechanism is correct, at least one Higgs boson must exist (an excitation of the Higgs field).

Cosmic rays

In the first half of the 20th century, cosmic rays were the only laboratories for particle physics: a particle from outer space hits a nucleus of the atmosphere starting a cascade of secondary particles. In this way, we discovered positrons (antielectrons), muons, pions and kaons. In cosmic ray experiments we have recorded particles with an energy 100 million times larger than those achieved in our most powerful manmade accelerators.

Accelerators

Accelerators use electric fields to increase the speed and magnetic fields to guide and concentrate particle beams acting as projectiles circulating through high vacuum pipes. In circular accelerators particles gain additional energy in every orbit. The aim is to make them collide head on at one or several interaction points. High energy collisions generate lots of particles. In fact, energy converts into matter according to Einstein's equation E = mc².

Detectors

Detectors record the events taking place at the interaction points. Every detector layer measures different properties of the particles produced in collisions.

Tracking devices reveal the paths of electrically charged particles. A powerful magnet bends the trajectory of these particles allowing their identification. Calorimeters measure the energy of the particles. The event is reconstructed in three dimensions by powerful computers storing all details of the collision.

The LHC started operation in November 2009. It is the largest and most powerful accelerator ever built. Protons are circulated in it. The main aim of the LHC (already achieved) is to discover the Higgs boson, the key piece of the mechanism which provides a mass to elementary particles. Furthermore, the LHC might reveal physics beyond the Standard Model as predicted by extended models (supersymmetry, extra dimensions, etc.), unravel the mystery of dark matter and solve the conundrum of matter-antimatter asymmetry. For one month every year lead nuclei are collided instead of protons. Temperature and matter density are then so high in the interaction points that the conditions of the early universe are reproduced in a brief instant.

The LHC is not only a big microscope exploring length scales about ten times smaller than before but also a fantastic time machine that will show us the Universe at it was a millionth of a second after the Big Bang.

It is an array of 1600 detectors deployed in the Argentinian Pampa distributed over a surface equivalent to 30 times the size of Paris and surrounded by 4 fluorescence detectors. Its mission is the detection of particles produced by cosmic rays of very high energy.

Particle Physics is a basic science, motivated by our curiosity to understand the intimate structure of matter and the fundamental forces of nature. But it also has applications, unexpected sometimes.

Communication and Information Technologies

[WorldWideWeb] Created in 1989 at CERN to share resources among computers through the internet by the hypertext language ("a click of the mouse"). Transfered to society without cost, the WWW has been a big revolution.

[Grid] The next step in computer connectivity. It allows distribution of both information and computing power. Many disciplines will benefit: meteorology, astronomy, geophysics...

Medicine

[Diagnosis] Thanks to detection techniques developed for particle physics new, non-invasive diagnostic tools have been introduced that are now usual in hospitals, such as Magnetic Resonance Imaging (MRI) and Positron Emission Tomography (PET).

[Therapy] Hadron therapy (radiotherapy with protons or carbon ions) has advantages over the conventional X rays, since it acts directly on tumors without damaging healthy tissue.