ATLAS Experiment - Physics Program

Physics Program

ATLAS is intended to investigate many different types of physics that might become detectable in the energetic collisions of the LHC. Some of these are confirmations or improved measurements of the Standard Model, while many others are possible clues for new physical theories.

One of the most important goals of ATLAS is to investigate a missing piece of the Standard Model, the Higgs boson. The Higgs mechanism, which includes the Higgs boson, is hypothesized to give mass to elementary particles, giving rise to the differences between the weak force and electromagnetism by giving the W and Z bosons mass while leaving the photon massless. On July 4, 2012, ATLAS (together with CMS - its sister experiment at the LHC) reported evidence for the existence of a particle consistent with the Higgs boson at the level of five sigma, with a mass around 125 GeV or 125 times the proton mass. This new "Higgs-like" particle was detected by its decay into two photons, and four leptons. Many further studies are needed in order to confirm whether or not the new particle is indeed the sought after Standard Model Higgs boson.

The asymmetry between the behavior of matter and antimatter, known as CP violation, is also being investigated. Current CP violation experiments, such as BaBar and Belle, have not yet detected sufficient CP violation in the Standard Model to explain the lack of detectable antimatter in the universe. It is possible that new models of physics will introduce additional CP violation, shedding light on this problem. Evidence supporting these models might either be detected directly by the production of new particles, or indirectly by measurements of the properties of B-mesons. (LHCb, an LHC experiment dedicated to B-mesons, is likely to be better suited to the latter).

The properties of the top quark, discovered at Fermilab in 1995, have so far only been measured approximately. With much greater energy and greater collision rates, the LHC produces a tremendous number of top quarks, allowing ATLAS to make much more precise measurements of its mass and interactions with other particles. These measurements will provide indirect information on the details of the Standard Model, perhaps revealing inconsistencies that point to new physics. Similar precision measurements will be made of other known particles; for example, ATLAS may eventually measure the mass of the W boson twice as accurately as has previously been achieved.

Perhaps the most exciting lines of investigation are those searching directly for new models of physics. One theory that is the subject of much current research is broken supersymmetry. The theory is popular because it could potentially solve a number of problems in theoretical physics and is present in almost all models of string theory. Models of supersymmetry involve new, highly massive particles. In many cases these decay into high-energy quarks and stable heavy particles that are very unlikely to interact with ordinary matter. The stable particles would escape the detector, leaving as a signal one or more high-energy quark jets and a large amount of "missing" momentum. Other hypothetical massive particles, like those in the Kaluza-Klein theory, might leave a similar signature, but their discovery would certainly indicate that there was some kind of physics beyond the Standard Model.

One remote possibility (if the universe contains large extra dimensions) is that microscopic black holes might be produced by the LHC. These would decay immediately by means of Hawking radiation, producing all particles in the Standard Model in equal numbers and leaving an unequivocal signature in the ATLAS detector. If this occurs, the primary studies of Higgs bosons and top quarks would in fact be looking at those produced by the black holes.