PROBING ELECTROWEAK BARYOGENESIS WITH THE TWO-HIGGS-DOUBLET MODEL
Motivation
The observed asymmetry in the abundance of matter and antimatter in the universe remains one of the most important unsolved puzzles in particle physics. While the Standard Model (SM) provides a mechanism for violating charge-parity conservation, the level of matter-antimatter asymmetry it would have generated in the early universe is expected to be many orders of magnitude weaker than what is observed. One particularly simple solution, known as electroweak baryogengesis, requires the Higgs potential to have undergone a strong first-order phase transition in the early universe. Such a phase transition, though, can only happen in the SM when the mass of the Higgs boson, h, is ≲ 80 GeV, contrary to its measured value of 125 GeV.
However, a strong first-order phase transition can still occur with mh = 125 GeV if nature contains two Higgs doublets. In this two-higgs-doublet model (2HDM), there exist five physical Higgs bosons, denoted A, H±, H, and h. A phenomenological scan over the 2HDM parameter space has identified scenarios in which the Higgs potential would have undergone a strong first-order phase transition in the early universe. At the alignment limit, the preferred parameter space has 230 GeV < mA < 800 GeV and 130 GeV < mH < 700 GeV, with A → ZH being the dominant decay mode.
ATLAS search for A → ZH
A search for the A → ZH process was performed using √s = 13 TeV data recorded by the ATLAS detector, corresponding to 36.1 fb-1. The final state considered is one in which the Z boson decays to a pair of charged leptons and the H boson decays to a pair of bottom-quarks. Motivated by electroweak baryogenesis, the mass of the A boson is assumed to be between 230 GeV and 800 GeV, while the mass of the H boson runs from 130 GeV to 700 GeV to ensure that the Z boson is on-shell. Two production mechanisms are considered: gluon-gluon fusion and b-associated production, whose cross-sections depend on tanβ and the type of 2HDM. Using the invariant mass of the ℓℓbb system as the final discriminating variable, no significant excess over the SM prediction was observed and the data was used to set limits on the masses of the A and H bosons in each 2HDM.
