Probing Dark Matter With the Higgs Boson
Visible matter – everything from pollen to stars and galaxies – makes up about 15% of the total mass of the Universe. The remaining 85% is made up of something completely different from things we can touch and see: dark matter. Despite overwhelming evidence of the observation of gravitational effects, the nature of dark matter and its composition remain unknown.
How can physicists study dark matter beyond gravitational effects when it is practically invisible? Three different approaches are pursued: indirect detection with astronomical observers, searching for the decay products of dark matter destruction in galactic centers; direct detection with highly sensitive experiments with low background, looking for dark matter dispersing nuclei; and by creating dark matter in the controlled laboratory environment of the Large Hadron Collider (LHC) at CERN.
Although successful in describing elementary particles and their low energy interactions, the Standard Model of Particle Physics does not include viable particles of dark matter. The only possible candidates, neutrinos, do not have the proper properties to explain the observed dark matter. To solve this problem, a simple theoretical extension of the Standard Model suggests that existing particles, such as the Higgs boson, act as a ‘portal’ between known particles and dark dust particles. Since the Higgs boson couples to mass, massive dark matter particles must interact with it. The Higgs boson still has large uncertainties associated with the strength of its interaction with Standard Model particles; up to 30% of Higgs boson decay can be potentially invisible, according to the latest ATLAS combined Higgs boson measurements.
Could some of the Higgs bosons decay in dark matter? Because dark matter does not interact directly with the ATLAS detector, physicists look for signs of “invisible particles”, triggered by momentary protection of the proton-proton collision products. According to the Standard Model, the fraction of Higgs bosons that decay to an invisible definitive state (four neutrinos!) Earns just 0.1% and is therefore negligible. If such events were observed, it would be a direct indication of new physics and potential evidence that Higgs bosons have decayed into particles of dark matter.
Could the Higgs boson decay in dark matter? The ATLAS collaboration sought the complete dataset of LHC Run 2 to set the strongest boundaries on the Higgs boson that decay to today invisible dark dust particles.
At the LHC, the most sensitive channel for direct decay of the Higgs boson to search for invisible particles is through the so-called vector Boson Fusion (VBF) production of the Higgs boson. VBF Higgs boson production results in two sprays of particles (called “jets”) pointing in a more forward direction into the ATLAS detector. This, combined with a large lack of momentum in a perpendicular direction (‘transverse’) to the beams of the invisible particles of dark matter, provides a unique signature that ATLAS physicists can search for.
Figure 1: Mass of the leading two jets (x-axis) in the search area with all background processes stacked and compared with data. A hypothetical Higgs boson signal that decays to invisible definitive states is shown in red. Credit: ATLAS Collaboration / CERN
In recently presented results, the ATLAS Collaboration studied the complete LHC Run 2 dataset, collected by the ATLAS detector in 2015–2018 to search for decay of Higgs boson to dark matter particles in VBF events. No significant excess of events over the expected background of known standard model processes was found in the analysis. ATLAS derived, at a 95% confidence level, an exclusion bound from the Higgs boson decay to invisible particles of 13%. This analysis included roughly 75% more data than the previous ATLAS search, and the team implemented several improvements, including:
- Faster filtering algorithms to generate more simulated collisions with equivalent computing power. Lack of simulated events was the leading uncertainty in the first 13 TeV version of this analysis.
- Optimized collision selection to accept ~ 50% more Higgs boson events on the same dataset.
- Refine categorization of events to result in a higher signal-to-background ratio in the search areas. This can be seen in Figure 1 as the red curve in the lower panel increases with higher invariant mass of the two leading jets (mjj).
- Improved acceptance for collisions enriches in background processes, allowing analysts to improve the background process models.
Figure 2: Upper limit on WIMP nucleon cross-section at 90% confidence level decreases in this analysis compared to direct detection experiments. Credit: ATLAS Collaboration / CERN
This observed exclusion is consistent with no signs of the Higgs boson decaying to dark matter. The new results promote the search for weak interactive solid particles (WIMPs), a popular candidate for dark matter. ATLAS sets additional exclusion limits for lower WIMP masses, which are compared to other direct detection experiments in Figure 2. These limits are competitive with the best direct detection experiments for WIMP masses up to half the Higgs boson. -mass, assuming that the Higgs boson interacts directly with dark matter.
This new analysis places the strongest existing limits on the Higgs boson that has decayed to invisible particles to date. As the search continues, physicists will continue to increase the sensitivity to this fundamental probe of dark matter.
