It is possible that the renowned Higgs boson, which plays a crucial role in imparting mass to elementary particles, interacts with a new realm of physics that scientists have been searching for over the years. This intriguing scenario suggests that the Higgs boson could decay in a distinctive manner involving exotic particles. Researchers at the Institute of Nuclear Physics of the Polish Academy of Sciences in Cracow have indicated that if such decays indeed occur, they could be observed in future particle accelerators that are being designed as successors to the Large Hadron Collider (LHC).
In the realm of high-energy physics, the term “hidden valley” refers to specific models that expand the existing set of known elementary particles. In these models, known as Hidden Valley models, the particles we are familiar with from the Standard Model belong to the low-energy group, while exotic particles reside in the high-energy region.
Theoretical considerations propose the possibility of the Higgs boson decaying into these exotic particles, which has not been observed thus far at the LHC despite extensive searches. However, scientists at the Institute of Nuclear Physics argue that if the Hidden Valley models accurately reflect reality, the decay of the Higgs boson into exotic particles should already be observable in future particle accelerators that succeed the LHC.
According to the Hidden Valley models, there are two distinct groups of particles separated by an energy barrier. The theory suggests the existence of massive exotic particles that could traverse this barrier under specific circumstances. Particles like the Higgs boson or a hypothetical Z’ boson would serve as messengers between the particles in both worlds. The Higgs boson, being one of the most massive particles in the Standard Model, is an excellent candidate for such a messenger.
Once the Higgs boson crosses into the low-energy region, it would decay into two relatively massive exotic particles. Each of these particles would then rapidly decay, within picoseconds (trillionths of a second), into two other particles with even smaller masses, which would be consistent with the particles described by the Standard Model.
So, what would be the expected signals in the detectors of future accelerators? The Higgs boson itself would go unnoticed, as would the two Hidden Valley particles. However, the exotic particles would gradually separate and eventually decay, typically producing jets of particles—specifically, quark-antiquark beauty pairs—that can be observed in modern detectors as particle sprays deviating from the axis of the lepton beam.
According to Mateusz Goncerz, a co-author of the paper, the search for Higgs boson decays would involve identifying the jets of particles produced by quark-antiquark pairs. The tracks of these particles would then need to be reconstructed to locate the potential decay vertices of the exotic particles. These decay vertices, characterized by a distinctive displacement from the beam axis, would appear in pairs. The magnitude of these displacements depends on factors such as the masses and average lifetimes of the exotic particles involved in the Higgs decay.
The current energy of proton collisions at the LHC, the largest particle accelerator in the world, reaches several teraelectronvolts, which theoretically allows for the production of Higgs bosons capable of crossing the energy barrier to the Hidden Valley. However, protons are composite particles made up of three valence quarks bound by strong interactions, resulting in a complex internal structure that generates a significant background of secondary particles. This background makes it extremely challenging to isolate the particles associated with the exotic Higgs boson decays being sought.
The situation is expected to improve significantly with the development of future accelerators designed as successors to the LHC, such as the Compact Linear Collider (CLIC) and the Future Circular Collider (FCC). These accelerators will enable collisions between electrons and positrons, which lack internal structure, thereby reducing the background for exotic Higgs boson decays. The researchers at the Institute of Nuclear Physics took into account the key parameters of the CLIC and FCC accelerators and calculated the probability of exotic Higgs decays resulting in final states with four beauty quarks and antiquarks. To encompass a broader range of models, they considered a wide range of values for the masses and mean lifetimes of the exotic particles.
The findings of their research are remarkably positive, suggesting that future electron-positron colliders could significantly reduce the background for exotic Higgs decays by several orders of magnitude and potentially even render it negligible in some cases.
It is important to note that the existence of particle-communicators is not exclusive to Hidden Valley models but is also a possibility in other extensions of the Standard Model. Therefore, if future accelerator detectors observe signatures corresponding to the Higgs decays studied by the scientists in Cracow, it would mark just the initial step towards comprehending the realm of new physics. The subsequent steps would involve collecting a sufficient number of events and determining the primary decay parameters for comparison with the predictions of theoretical models.
In conclusion, the main practical outcome of the study is that the researchers have demonstrated that the phenomenon of Higgs boson decays into exotic particles, as predicted by the Hidden Valley model employed in their research, should be observable in forthcoming electron-positron colliders. While the specific model used may not be definitive, it is representative of various proposals for new physics, providing valuable insights for future experimental endeavors.
Source: Polish Academy of Sciences