ATLAS Experiment at CERN

Why Build ATLAS? Before the LHC there was LEP (large electron positron collider) the experiments at LEP had observed the W and Z boson which carry the weak force. At the upper end of the LEP energies it was hypothesised that the Higgs Boson would be observed. The Higgs boson is only one particle left in the standard model that has yet to be observed. It is unknown why fundamental particles have such varying masses and it is believed that the discovery of the higgs boson could help to answer some of these questions.

Although the higgs boson was the main motivation for building ATLAS it is also hoped that it will answer other questions left open by the standard model such as: 1-Why are there three types of quarks and leptons of each charge? 2-Are there more types of particles and forces to be discovered? 3-Are the quarks and leptons really fundamental, or do they, too, have substructure? 4-What particles form the dark matter in the universe? 5-How can the gravitational interactions be included in the standard model?

Why can ATLAS answer these questions? The LHC will provide colliding beams of much greater energy than available at LEP. The maximum energy of the protons in the beam will be 7TeV. From Einstein’s famous equation it is known that mass and energy are equivalent. The higher energy beam means that in a collision a more massive particle can be produced. This will be needed if the Higgs boson is to be observed. Higher energy beams may also answer some of the questions left from the standard model.

The ATLAS Detector A diagram of the ATLAS detector which is the size of a five story building

The Higgs Boson The higgs boson is the last of the particles predicted by the standard model yet to be detected directly. The Higgs is believed to exist in a field which is present in all of space. An analogy to both of these is given below. To understand the Higgs mechanism, imagine that a room full of physicists chattering quietly is like space filled with the Higgs field a well-known scientist walks in, creating a disturbance as he moves across the room and attracting a cluster of admirers with each step this increases his resistance to movement, in other words, he acquires mass, just like a particle moving through the Higgs field

... if a rumor crosses the room, it creates the same kind of clustering, but this time among the scientists themselves. In this analogy, these clusters are the Higgs particles.

What if the Higgs boson is not Found? If the Higgs is not found then there are plenty of other things that scientists at ATLAS can search for. GUTS- Grand Unified Theory, where all the forces will act together as one -already started with Maxwell ’ s theory of electromagnetism. -electroweak theory by Glashow, Salam, and Wienberg. Other possible searches include looking for supersymmetry and dark matter.