Higgs Physics

The Z(ll)γ decay is predicted by the SM, but it could not yet be observed during the LHC Run 1 due to a branching ratio which is more than an order of magnitude smaller compared to H → γγ. The analysis uses very similar techniques as the di-photon decay channel, where a narrow resonance is searched for in a steeply falling background contribution. With the larger luminosity and further sensitivity improvements, this decay might be observable with the LHC Run 2 dataset.

Another area of Higgs physics where the DESY ATLAS Team participates is the production of the Higgs boson in association with a pair of top quarks (ttH). In Run 1 this was mainly in the di-photon decay mode [1]. The ttH production cross section will increase by a factor of 4 when moving from 8 TeV to 13 TeV collisions. The measurement of ttH production and of the top Yukawa coupling are among the highest priorities of the LHC Run 2 physics programme. Though in Run 1 the largest sensitivity was due to H → bb decays, with the increased luminosity the leptonic and di-photon decay modes will strongly increase in the overall contribution to the experimental sensitivity of this production mode. In Run 2 our team will participate in the measurement of the top Yukawa coupling with H → bb and H → γγ decays.

Searching for additional Higgs bosons or Higgs boson production beyond the SM rate is a promising way to find extensions of the SM after the Higgs boson discovery. Our team is searching for new particles or interactions which lead to the production of Higgs bosons in pairs. Although in the SM this process has only a very small cross section, a variety of extensions of the SM predict the production of a pair of Higgs bosons at a higher rate. Experimentally, the highest sensitivity comes from the final state where one of these Higgs bosons decays to a pair of photons and the other one to a pair of b-quarks, HH → γγ bb [1]. While the decay to b-quarks ensures a high event yield due to the large branching ratio, the diphoton decay has a very clean experimental signature and a good detector resolution which are vital to achieve the best background subtraction. An example event display is shown above in this final state (where photons are illustrated in orange and jets in white).

For a direct search of additional Higgs bosons one can look for Higgs decay signatures over a large invariant mass range. It is expected that these processes will occur with a lower rate than the SM Higgs production. However the increased energy of the LHC Run 2 is ideally suited for these searches and the experiments will be able to cover an extended mass range. We participate in the search for additional Higgs decays in the di-photon signature covering a large mass range. The analysis from Run 1 set already exclusion limits from 60 to 600 GeV [1], and this range will be further extended to higher masses during the LHC Run 2.