The theory that accounts for all the experimental results observed so far in particle physics is the Standard Model (SM), a quantum theory in which particles are represented by fields. In this theory, the fundamental forces are transmitted by particles of integer spin (the bosons). Thus, the weak interaction, the force responsible for the β decay of radioactive nuclei, is transmitted by the exchange of W+, W- and Z0 bosons, particles whose mass is about 90 times that of the proton, i.e. approximately 85 GeV/c2 [1]
Within the framework of the SM, the W and Z bosons acquire their mass by a mechanism which requires the existence of a new particle, the Higgs boson, discovered in 2012. The same mechanism is at the origin of the mass quarks and leptons, the fundamental particles of matter (also called fermions).
There are several reasons to think that the Standard Model is not the complete theory of the Universe. For example, it does not describe the gravitational force, the first force discovered by humankind. Moreover, experimental results indicate that matter and energy of unknown nature (called dark matter and dark energy respectively) are present in significant quantities in the Universe. More precisely, the nature of 95% of our Universe is unknown! We also observe that the Universe is mainly made up of matter, whereas the Standard Model predicts a Universe in which antimatter should have been produced in quantities almost equal to those of the matter.
So-called theories of “New Phenomena” are necessary to complete the SM, which would therefore only be an effective theory valid only at energies accessible until now, while at higher energies a new physics (also called Physics at the Beyond the Standard Model) should come forward and provide an answer to unresolved questions. The potential for discovery at the energy of the LHC is significant, but before being able to extract the signals from the Higgs boson or from New Phenomena, it is necessary to master the response of the detectors, which implies finding and measuring with great precision. “standard” physics phenomena already known to physicists.
The group of ATLAS researchers and students at LAPP is involved in all three major research themes: Standard Model physics, the study of the Higgs boson and the search for New Phenomena.