Particle Collisions
Particle collisions are used to study the smallest building blocks in Nature - the quarks and the leptons. Some of these build up the world we see around us, some existed naturally only at the beginning of time, the Big Bang, but are now produced in high energy collisions at a few large physics laboratories in the world.

Accelerators
Accelerators are big machines that accelerate beams of charged particles. In accelerators the speed and energy of particles is increased to very high values. The speed of light is almost reached, but we prefer to talk about the energy of the particles rather than the speed. It is the high energy that is exploited in the collisions and is used to explore the interior of matter and to produce particles that were abundant at the early times of the Universe. For more details see Accelerators.

Quarks and Leptons
There are six types of quarks and six types of leptons. In addition there are the corresponding antiquarks and antileptons. The proton and the neutron, which build the matter we are surrounded by, are composed of quarks. The electron, which orbits the nucleus, is one of the leptons. More about quarks and leptons is found in the Standard Model. We explore the quarks and leptons with "Hands on CERN".

New particles
In the high energy particle collisions part of the energy can be made into new particles. Mass and energy are equivalent as Einstein said long ago (E=mc2) which is the reason why the kinetic energy of the colliding particles can be transformed into new particles. One of the hypothetical particles that the scientists are searching for is the Higgs particle (see the Standard Model.

Forces
The processes we observe around us are governed by only four fundamental forces. In the modern description of these forces, the force mediator plays the important role. Particles feel the different types of forces by emitting or absorbing the mediators of the forces. These mediators, important actors in the interior of matter, are fundamental particles, which can be studied in modern particle physics experiments. Examples of mediators are the massive W and Z particles and the massless foton and gluon (see the Standard Model.) These particles can be studied with the particle collisions of Hands on CERN.

Detectors and Experiments
The detectors are the physicist's tool to detect the particles that either build up the matter we see around us or the particles that once took part in the formation of the Universe. Each detector is very specialised. By combining many of them into a complete experiment it is possible to determine what happened in the particle collision (see Particle Detectors.)

Exploring Particle Collisions
For an untrained eye it will take a while to understand what happened in the particle collision, or event as we call it, just by looking at it. Sometimes very few particles are produced in the collision, sometimes there are sprays of particles containing 10-30 particles. Particles like the electron can be identified as a charged particle giving away all its energy in one of the energy detectors and the muon is the charged particle that goes through all the detectors almost without being affected at all. The mysterious quarks and gluons that seem impossible to observe directly, give rise to sprays of particles.

W and Z particles
At LEP (the Large Electron Positron collider) W and Z particles can be produced. At LEP1, the first phase of the accelerator, single Z particles are produced, while at LEP2, the second phase of the accelerator that will run until year 2000, pairs of Z or W particles can be produced.

The DELPHI Experiment
The DELPHI experiment is composed of a large number of particle detectors (see Particle Detectors) and is situated at the LEP accelerator (see Accelerator) at CERN, the European particle physics laboratory outside Geneva, Switzerland. Around 500 physicists from all over the world take part in this truly international project.