After the discovery of the long‐sought Higgs boson at a mass of 125 GeV, a major question in particle physics is whether the electroweak symmetry breaking sector is indeed as simple as the one implemented in the Standard Model (SM), or whether there are additional Higgs bosons. Additional Higgs bosons occur in many theories, for example, the well‐known minimal super‐symmetric extension of the SM (MSSM), which is required by string theory. The discovery of additional Higgs bosons could therefore be a gateway to new symmetries in nature.

ATLAS has recently released results of a search for heavy Higgs bosons decaying into a pair of tau leptons using the complete LHC Run 2 dataset (139 fb^–1 of 13 TeV proton–proton data) with a strong contribution from a Warwick Ph.D. student. The new analysis provides a considerable increase in sensitivity to MSSM scenarios compared to previous results.

The MSSM features a second Higgs doublet, giving four more Higgs bosons, and the observed Higgs boson is generally lighter than all of them. The couplings of the heavy Higgs bosons to down‐type leptons and quarks, such as the tau lepton and bottom quark, are enhanced for large values of tan β–the ratio of the vacuum expectation values of the two Higgs doublets, and one of the key parameters of the model. The heavy neutral Higgs bosons A (CP odd) and H (CP even) are produced mainly via gluon–gluon interactions or in association with bottom quarks. Their branching fractions to tau leptons can reach sizeable values across a large part of the model‐parameter space, making this channel particularly sensitive to a wide range of MSSM scenarios.

The new ATLAS search requires the presence of two oppositely charged tau‐lepton candidates, one of which is identified as a hadronic tau decay, and the other as either a hadronic or a leptonic decay. To profit from the enhancement of the production of signal events in association with bottom quarks at large tan β values (for example when the heavy Higgs boson is radiated by a b‐quark produced in the collision of two gluons), the data are further categorised based on the presence or absence of additional b‐jets. One of the challenges of the analysis, where Warwick's role was particularly important, is the misidentification of backgrounds with hadronic jets as tau candidates. These backgrounds are estimated from data by measuring the misidentification probabilities and applying them to events in control regions representative of the event selection.

The data agree with the prediction assuming no additional Higgs bosons, despite a small, non‐significant excess around a putative signal mass value of 400 GeV. The measurement places limits on the production cross section that can be translated into constraints on MSSM parameters. One realisation of the MSSM is the hMSSM scenario, in which the knowledge of the observed Higgs‐boson mass is used to reduce the number of parameters. The A/H → ττ exclusion limit dominates over large parts of the parameter space (see figure), but still leaves room for possible discoveries at masses above the top‐anti‐top quark production threshold. ATLAS continues to refine this and conduct further searches for heavy Higgs bosons in various final states.

This article is adapted from one which first appeared in the May/June 2020 edition of CERN Courier.

Further reading : ATLAS Collab. 2020 arXiv:2002.12223.