CERN’s Bold Quest To Discover New Physics Through Higgs Bosons

Since the launch of the Large Hadron Collider, researchers have been studying Higgs bosons and searching for signs of physics beyond the current model of elementary particles. Scientists working with the ATLAS detector have combined these two goals: their latest analysis has not only deepened our understanding of how Higgs bosons interact with each other but also placed stronger limits on potential “new physics” phenomena.

The Large Hadron Collider (LHC) achieved a major success with the discovery of the Higgs boson, the final missing piece of the Standard Model and a key to understanding the origin of mass in elementary particles. However, despite this breakthrough, researchers have yet to find any evidence of physics beyond the Standard Model, which has been a source of ongoing frustration. Scientists at CERN (the European Organization for Nuclear Research) in Geneva are now working to address this by improving the precision of Higgs boson measurements while actively searching for signs of “new physics.”

A recent study, conducted by CERN’s ATLAS experiment team and published in the Journal of High Energy Physics, exemplifies this dual approach. The team focused on observing events that led to the creation of two Higgs bosons, which then decay into multiple particles of the lepton family, primarily electrons and muons.

Exploring Higgs Boson Pair Production

Producing Higgs boson pairs is theoretically possible within the Standard Model, but it is so rare that scientists have not yet observed it in existing data. Some theoretical models beyond the Standard Model, however, suggest that Higgs boson pairs could be produced more frequently. If scientists can identify instances of Higgs boson pair production with current data, it would confirm the existence of a new, previously unknown class of physical phenomena. Consequently, the ATLAS experiment team has made this rare process the focus of their analysis.

“Experimental studies of the interactions of Higgs bosons with each other encounter a fundamental problem. It is this: in proton collisions at the LHC, Higgs bosons appear so infrequently that so far not a single event of Higgs boson pair production has been detected, which at first glance seems absolutely necessary if we want to look at interactions between these particles. How, then, can we study a phenomenon that has not yet been observed?” asks Dr. Bartlomiej Zabinski, a physicist at the Institute of Physics of the Polish Academy of Sciences (IPJ PAN) who coordinated the international team responsible for this analysis.

The Role of Machine Learning in Particle Physics

Within the Standard Model, increasingly precise predictions can be made about the probabilities of various known processes. A rationale for suggesting unexpected properties of Higgs bosons or the existence of new physics would be a discrepancy between theoretical predictions and actual data from the LHC detectors. Operating solely within the framework of the Standard Model, the physicists in the ATLAS experiment therefore simulated (together with the background) the signals that should appear in the detectors in the event of two Higgs boson phenomena, and then normalized the results according to the expected amount of data coming from their detector. The final step was to compare the values thus obtained with those derived from previous observations. The use of machine learning based on decision trees helped in the search for these rare processes.

“Our analysis of double Higgs boson production events in the final state with multiple leptons complements the studies already carried out on other final states. So far, we have not noticed anything in the data from our detectors that disagrees with the Standard Model. However, this result does not rule out the possibility of the existence of ‘new physics’ phenomena, but only informs us that their possible influence on the production of Higgs boson pairs remains too weak to be seen in the data collected so far,” concludes Dr. Zabinski.

Future Prospects at the LHC

In the coming years, the LHC is to undergo a major upgrade. The intensity of the beams will then increase tenfold, resulting in a significant increase in the number of recorded proton collisions. The limitations imposed by the current analysis on the production and parameters describing the interactions of Higgs bosons allow physicists to hope that perhaps already at the beginning of the next decade, it will be possible to select the first events of double Higgs production from more data and to verify today’s predictions in direct observations of the phenomenon.

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DOI: 10.1007/JHEP08(2024)164

On the Polish side, the research was co-financed by the National Science Centre.

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