Double the Higgs for double the difficulty

11 July 2019 | By

Plots or Distributions,Physics,ATLAS — Upper limit at 95% confidence level on the cross-section for the non-resonant Higgs boson pair production as a function of κ2V. The arrow indicates the Standard Model expected value. The red line corresponds to the theoretical prediction, while the black lines are the experimental limits. The regions where the latter are below the red line are excluded. (Image: ATLAS Collaboration/CERN)

Seven years after its discovery by the ATLAS and CMS collaborations, the Higgs boson remains the most mysterious of the known particles. As an excitation of the “Higgs field” and the only known spin-zero particle, understanding its unique properties and interaction is essential to understanding the Standard Model of particle physics.

A key interaction not yet observed by LHC experiments is the production of “double Higgs”. The Standard Model predicts that the Higgs field can interact with itself to create a Higgs boson pair. The rate with which this happens is critical, as it allows physicists to directly probe the potential energy of the Higgs field, which is responsible for mass of particles. Deviations from the expectation would be a strong hint of new physics.

While previous searches for Higgs boson pairs utilised the most abundant production process (the fusion of two gluons), a new ATLAS search studies, for the first time, the rarer process where two vector bosons instead fuse to form a pair of Higgs bosons. The results of this new search were presented at the European Physical Society Conference on High-Energy Physics (EPS-HEP) in Ghent, Belgium.

In a new result, the ATLAS collaboration studies – for the first time – the rare process where two vector bosons fuse to form a pair of Higgs bosons.

This process, dubbed vector-boson fusion, leaves a tell-tale remnant in the detector. The vector bosons themselves radiate off quarks emerging from the colliding protons, and those quarks, emitted preferentially with a very large angular separation, can be measured in addition to the decaying Higgs boson pair to identify this unique signature.

Higgs boson pair production is expected to be extremely rare in proton-proton collisions, with a production rate roughly a thousand times smaller than that of single production. The production via vector-boson fusion is an even rarer process, but is uniquely sensitive to a thus-far untested relationship between two Higgs bosons and two vector-bosons. Deviations from the expected strength of this relationship could be signs that the Higgs boson has a different role in physics than we assume in the Standard Model, or a hint of new physics entering the process. New physics could also appear in the form of a new massive resonance, which would appear as a localized excess in the mass spanned by the Higgs boson pair over a smoothly falling background.

Physicists analysed data collected in 2016-2018 with the ATLAS detector, studying the Higgs boson’s most abundant (although challenging) decay to a pair of b-quarks. In the detector, this leaves a signature of four particle “jets” (sprays of hadrons) originating from the b-quarks, in addition to the two jets originating from the vector-boson-fusion quarks. Physicists found no hints of a signal in the new search; they performed statistical interpretations to exclude at 95% confidence level the production of new physics and to set a limit on the production rate.

No hints of deviations from the Standard Model or new particles were observed, but the new ATLAS result probes a yet-untested property of the Higgs boson’s interaction with weak bosons. It provides the first constraints on the relationship between two vector-bosons and two Higgs bosons (a parameter called κ_2V) to be between –0.43 to 2.56 times the value predicted by the Standard Model. Future larger datasets, as well as searches using other decays of the Higgs boson, will help ATLAS to sharpen its sensitivity to this unique property.

This briefing, originally published 11 Jul 2019, was updated on 19 Apr 2021 to reflect the latest results published in JHEP 07 (2020) 108.