The ATLAS experiment

LHC Physics: The Higgs particle

We don't really know why particles have different mass, or why they have a mass at all. We can understand that a lead atom is heavier than a helium atom, since the lead atom contains many protons and neutrons while a helium atom only contains two of each.

The elementary particles, quarks and electrons, come in 6 different species each. The top quark, the heaviest of the particles, is believed to be just as fundamental and "pointlike" as the electron. Pointlike means that no size or structure can be observed with the present resolution of 10-17 m. The top quark, however, is 350000 times heavier than the electron.
Particle chart.
Elementary particles.

With the ATLAS detector it will be possible to look inside to the mechanism of how masses are 'created'. The most convincing theory describes the mass through what is called the Higgs field. According to this theory, invented by the Scottish physicist Peter Higgs, the vacuum is not really empty, but filled with something called the Higgs field. When particles move in this Higgs field they accumulate mass. The stronger the interaction between the particles and the Higgs field is, the more mass the particles get. The Higgs field should also manifest itself as a one particle, the Higgs boson, that we will be able to discover in ATLAS, if it exists at all.

The graph on the right shows the status of the Higgs searches as of 2009. The yellow mass region is excluded, and the blue band shows the probability to find a Higgs with a certain mass, as predicted by the Standard Model of particle physics. The lower the y-axis value, the higher the probability.
Higgs searches status in 2009.
Higgs searches status in 2009.

If you want to know more about the Higgs particles, you can look at The Waldegrave Higgs Challenge. The physics tour will bring you through the existing physics to be revealed with ATLAS.