The discovery of the Higgs boson by the ATLAS and CMS experiments at the CERN LHC in 2012 represents the culmination of a 50-year theoretical and experimental endeavour to complete the “Standard Model” of particle physics. However, the Standard Model does not provide the particle physics basis to explain all of our current observations of the universe. For example, it does not explain the existence of “Dark Matter”, the observed preponderance of matter over antimatter in the universe, or that neutrinos have mass.
Candidate theories of physics “beyond the Standard Model” (BSM) predict the existence of new, as yet undiscovered, fundamental particles, such as Higgs bosons. As well as additional neutral Higgs bosons, it is possible that BSM Higgs bosons carrying electric charge, H±, might exist. A particularly intriguing possibility is that a Higgs boson carrying a double electric charge, H±±, might also exist. This would be unique amongst the elementary particles. This PhD involves the search with the ATLAS experiment at the CERN LHC for, H±± bosons. Higgs bosons decay predominantly to the highest mass elementary particles. The tau is the highest mass lepton, which means that it is much more likely to be produced in Higgs boson decays than are the lighter leptons (muons and electrons). We shall be searching, therefore, for pairs of tau leptons that have the same sign electric charge.
Searching at the LHC for “weakly interacting” particles, such as Higgs bosons, is difficult because the very rare events containing such particles are in danger of being buried beneath the large strong interaction backgrounds. One possible “trick” to improve the chances to find BSM Higgs bosons at the LHC is to exploit a very special kind of interaction in which the two colliding protons both emit a W boson. The two W bosons “fuse” to produce the searched for final particle(s). The special characteristics of such “Vector Boson Fusion” (VBF) events allows better discrimination between the weakly interacting particles of interest and the strong interaction backgrounds.
In summary, the successful candidate will search for events produced in the fusion of two vector bosons at the LHC that contain same-sign pairs of tau leptons that are clustered in mass. This is a very challenging project, but it will proceed in stages, starting with the observation and measurement of the much more frequent, but still challenging, production of opposite-sign tau pairs in VBF. The project will be underpinned by technical work in identifying tau leptons and in identifying VBF events, and will be supported by, and build upon, the considerable expertise we have built up in Manchester and Melbourne in these areas. In addition to the experimental measurements, interpretation of the results in terms of BSM physics models will involve collaboration with relevant theoretical particle physicists.
The successful candidate will divide their time between CERN, Manchester and Melbourne. The student plus supervisory/supporting teams in CERN/Manchester/Melbourne will have frequent video meetings, and occasional face-to-face meetings (likely at CERN).