The CMS experiment has conducted a search for heavy beyond-standard-model particles (referred to as a Z′ boson) that give rise to a top quark-antiquark pair decaying into hadrons. The search excludes the existence of the new Z′ boson with masses below a few TeV, and provides the most stringent constraints to date on its possible production for masses above 3 TeV.
Scientists at the CMS Experiment are looking for signs of a possible new particle, often referred to as the Z′ boson, that could explain the difference in scales between the Planck scale and electroweak scale, the so-called “hierarchy problem”. This manifests itself as an “observational difference”, that is, a measurable discrepancy in energy scale and timing, between the start of the inflationary phase of the early universe after the Big Bang and the electroweak phase transition around 10 picoseconds later. If such a particle exists, it would appear briefly when protons smash together at the Large Hadron Collider (LHC) and then decay into a pair of top quarks.
This scenario is especially intriguing because the top quark is exceptionally heavy, by far the heaviest of the known elementary particles, making it a promising gateway to new physics. In fact, many theoretical models beyond the standard model suggest a Z′ boson could help address some of the biggest mysteries in physics, like the hierarchy problem described above. A new Z′ boson might offer a way to cancel out the huge quantum corrections that would otherwise make the Higgs mass large. You can read more about the Higgs mass here. Some theories even propose that a Z′ boson could be tied to dark matter as a “bridge” that allows dark matter to interact with ordinary matter.
Since the Z′ boson is much heavier than the top quark, any decay into top quarks would likely be very energetic and, therefore, often produce “boosted jets”, highly energetic sprays of particles whose internal structure helps differentiate them from ordinary jets. Sophisticated machine learning algorithms tease out these subtle differences, enabling researchers to spot potential signals in a sea of other collision events. You can read more about machine learning for top-quark boosted jet identification here.
“We look for a peak in the reconstructed mass patterns of top-quark pairs,” explains Dr. Haifa Sfar, a postdoctoral researcher on the team. “If there’s a Z′ out there, it might create a distinct signal, sort of like finding a hidden mountain on a landscape map.” But the Large Hadron Collider is a noisy place, with many collisions happening all at once. This makes it difficult to isolate which jets come from a top-quark pair produced in a potential Z′ boson decay and which are just ordinary background jets.
“Our work refines the limits on the Z′ mass, which will help guide future theoretical models and searches" says Dr. Margaret Morris.
Despite these challenges, the analysis shows no clear sign of a new Z′ boson so far (figure below), meaning that if one exists, it is likely to be much heavier or rarer than our current data can confirm or very weakly interacting. In scientific terms, the experiment places stringent bounds on what the mass or production rate of this hypothetical particle could be. Even in the absence of a direct discovery, these findings significantly refine our understanding of particle physics, revealing where not to look any longer and directing theorists toward the next set of possibilities.
Above: Constraints on the production rate of Z′ bosons in one of the considered scenarios as a function of the mass of the Z′ boson. The solid black line shows the established constraint and the red line indicates the theoretical prediction, implying that the existence of a Z′ boson with a mass of less than 3.9 TeV is excluded by this search.
Read more about these results:
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CMS Physics Analysis Summary (B2G-24-003): "Search for tt resonances in the fully hadronic final state"
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