The search for new bosons beyond Higgs
CMS researchers probed top‑quark pairs for signs of new scalar and pseudoscalar particles The post The search for new bosons beyond Higgs appeared first on Physics World .
In the quest to uncover the mysteries of the universe, particle physicists have long been searching for new fundamental particles that could reveal physics beyond the Standard Model. Among these, scalar and pseudoscalar bosons hold significant promise, as their discovery could help explain phenomena such as dark matter and the existence of the Higgs boson. The Higgs remains the only confirmed scalar boson, while pseudoscalar bosons, though predicted by various theories like those involving axions and axion-like particles, have yet to be observed.
One promising avenue for discovering these elusive particles is by examining their potential decay into a top quark and antiquark pair (tt̄). To this end, researchers from the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) have conducted a thorough analysis of 138 fb⁻¹ of proton-proton collision data. Their goal was to reconstruct the invariant mass of the tt̄ system and utilize angular variables sensitive to its spin and parity to distinguish potential signals from the Standard Model background.
A critical aspect of this analysis was the inclusion of interference between any new boson and the Standard Model tt̄ production. This interference can create peak-dip distortions in the invariant mass of the tt̄ system, rather than a simple bump, which adds complexity to the search. By carefully accounting for these effects, the researchers were able to better understand the data and identify any potential deviations from the Standard Model predictions.
Upon analyzing the data, the observed event yield was found to be consistent with the Standard Model prediction across the majority of the invariant mass spectrum. This consistency allowed the researchers to exclude a contribution from a potential new boson in most regions of the mass spectrum. However, a significant excess was observed near the threshold of tt̄ production, where the energy of colliding particles is just enough to produce top quarks and antiquarks. This excess had a local significance above five standard deviations, indicating a potential anomaly that warranted further investigation.
Interestingly, the kinematics of these excess events were more consistent with a pseudoscalar than a scalar interpretation. Pseudoscalar bosons are predicted by various extensions of the Standard Model, such as those involving axions and axion-like particles, and their discovery would indeed represent a major breakthrough in our understanding of fundamental physics.
Despite the tantalizing hint of a pseudoscalar boson, there is another possible explanation for the observed excess. Researchers have proposed that it could be due to a predicted tt̄ quasi-bound state known as toponium. This state, which forms when a top quark and antiquark are produced in close proximity, could mimic the signature of a new particle without requiring physics beyond the Standard Model.
To further constrain the possibilities, the CMS researchers set upper limits on how strongly new bosons could couple to top quarks across masses from 365 to 1000 GeV and widths from 0.5% to 25%. These constraints exclude couplings down to around 0.3 for pseudoscalars and 0.4 for scalars, significantly narrowing the parameter space for potential new particles.
While the observed excess near the tt̄ production threshold remains intriguing, it is not yet clear whether it signals the discovery of a new boson or is simply a manifestation of the Standard Model's own complexities. As the field of particle physics continues to evolve, further experiments and analyses will be crucial in resolving this mystery and potentially uncovering new fundamental particles that could revolutionize our understanding of the universe.
In the meantime, the work of the CMS researchers serves as a testament to the relentless pursuit of knowledge in particle physics. By pushing the boundaries of what is possible and meticulously analyzing the data from the LHC, they have not only deepened our understanding of the Standard Model but also set the stage for potential discoveries that could reshape our view of the cosmos. As the search for new bosons beyond the Higgs continues, the possibility of uncovering the secrets of the universe remains within reach.
