Jetting into a new era of Higgs studies

20 June 2024 | By

The Higgs boson, a central figure in the Standard Model of particle physics, is crucial in explaining the origin of mass for fundamental particles. Understanding its properties and decay modes is pivotal for corroborating the Standard Model's predictions and delving into potential new physics contributions.

Physics,ATLAS — Figure 1: The W/Z boson and Higgs boson recoil against each other and create large jets (red, dark blue). These large jets can be identified through their substructure. (Image: ATLAS Collaboration/CERN)

The ATLAS Collaboration has made a significant advancement in the understanding of the Higgs boson by investigating its production in association with W or Z bosons (known as “VH production”), where the Higgs boson decays to bottom quarks. Their new analysis studies the full LHC Run 2 dataset, collected between 2015 and 2018, totaling an integrated luminosity of 137 fb-1 at 13 TeV.

ATLAS researchers had previously examined VH production in this decay channel, but only in events where the associated W and Z bosons decayed into leptons. Many more W and Z bosons decay hadronically, creating “jets” of particles in the ATLAS detector, than decay leptonically. When a Higgs boson decays to two b-quarks it also creates jets. When it is at low transverse momentum, this results in a multi-jet final state where the b-quarks decay almost back-to-back. Unfortunately for researchers, many other Standard-Model processes leave similar signatures!

In their new analysis, ATLAS researchers instead focused on events where the Higgs boson has a large (or “boosted”) transverse momentum, which is possible when the Higgs boson recoils against the W or Z boson. In these boosted events, instead of back-to-back, the two b-quarks from the Higgs boson are closer together and create a single large jet structure. Similarly, the hadrons originating from the W and Z bosons move close together to form another large jet structure. These jets emerge in opposing directions and contain substructure (see Figure 1).

This is the first time LHC researchers have studied VH production in a fully hadronic final state.

This is the first time LHC researchers have studied VH production in a fully hadronic final state. This success can be attributed to the adoption of multiple innovative techniques. Physicists developed new methods to identify large-radius jets originating from high-momentum W, Z, or Higgs boson decays into hadrons, by studying the jet substructure. This new methodology allowed researchers to examine a significantly larger sample size of VH events than in prior studies, exploring previously unreachable kinematic regions of the Higgs boson.

The ATLAS team measured the VH production cross section inclusively and differentially in several ranges of Higgs boson transverse momentum: 250-450, 450-650, and greater than 650 GeV. The number of VH events found is 1.4 ±1 times higher than the expected Standard-Model value, remaining in full agreement with the Standard Model.

Moreover, this methodology demonstrates great potential for studies involving even larger data samples (expected from LHC Run 3 and beyond). It promises to unveil a kinematic region of the Higgs boson that is highly sensitive to the effects of new physics phenomena.