The CMS experiment studies the Higgs boson as a potential window into dark matter, and sets constraints on this mechanism, which gives rise to isotropic sprays of low-energy particles.
Quantum chromodynamics (QCD) describes the strong interaction in our world; the force that binds quarks together in particles such as protons and neutrons. In the shadows beyond our visible world, there may exist a dark sector, inhabited by a unique ecosystem of dark matter particles governed by a new dark QCD force. One class of theories that hypothesize the existence of dark sectors is referred to colloquially as “Hidden Valley Models”. Similar to how the mountainous peaks surrounding Mordor – in J. R. R. Tolkien's fiction Lords of the Rings – make it difficult to enter from the outside, at the LHC, these mountains are akin to enormous energy barriers which we must overcome before we can access the dark sector. One such way to breach these barriers is via the Higgs boson, which could provide an alternative passage into the dark sector occupying the hidden valley. At the LHC, the Higgs boson can be produced with sufficient energy to overcome the energetic barriers and access the hidden valley.
Once inside the hidden valley, the Higgs boson decays into a cascade of dark particles formed through the influence of the dark QCD force. In hidden valley models where the dark QCD force is very strong, the dark particles are radiated spherically, making them look like fireworks of particles. This is in stark contrast to what we observe in standard QCD, where particle radiation tends to be collinear. The dark particles eventually decay back outside of the hidden valley where we can observe their unique signature - a large number of spherically distributed, low-energy particles - which we call a “soft unclustered energy pattern” (SUEP). Scientists of the CMS experiment, including graduate student G. de Castro, analyzed data recorded in the years 2016–2018 to look for the unique SUEP signature that could point towards the existence of a dark sector.
Above: A schematic (Feynman diagram) showing hypothesized SUEP production via a Higgs boson produced along with a Z boson.
The SUEP is a unique signature that is characterized by its large number of low-energy particles which tend to mimic the background of secondary particles created in the collisions at the LHC. To separate the SUEP from this background, we exploit its spherical nature to create a large radius cluster of particles. Additionally, we focus on SUEPs originating from Higgs boson associated production with a Z boson. This allows us to use the decay products of the Z boson as a handle to filter out the events of interest.
We define different SUEP models by varying the mass and temperature (which are proxies for the average energy) of the dark particles. For a few different combinations of masses and temperatures, the level at which we can exclude their production is shown below. There is little evidence at low masses and temperatures that would indicate the production of SUEPs at the LHC. We expect greater sensitivity to the SUEP signal with the larger samples being collected now during Run 3 of the LHC and down the line with the high-luminosity upgrade HL-LHC.
Above: Current limits on the production of SUEPs at the LHC as a function of the temperature and mass of the dark particles.
Read more about these results:
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CMS Physics Analysis Summary (EXO-23-003): "Search for soft unclustered energy patterns in association with a Z boson in proton-proton collisions at 13 TeV"
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Display of collision events: CERN CDS
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