Experts gather at the LHC Physics Center to discuss multibosons

Earlier this summer, 40 researchers converged at Fermilab for a workshop on multiboson studies at the Large Hadron Collider. Photo: Gabriele Benelli

The W and Z bosons, the force carriers of the electroweak interaction, are produced abundantly in the high-energy proton-proton collisions at the Large Hadron Collider (LHC). The particle physics community has been keenly interested in studies of multiboson events — events with two or more of these heavy gauge bosons — especially after the 2012 discovery of the Higgs boson. The precise measurements of the interaction between the electroweak gauge bosons in these events are expected to provide a deeper insight into the understanding of the electroweak symmetry breaking thought to be caused by the “Higgs mechanism.”

The Fermilab LHC Physics Center and Northwestern University recently hosted about 40 participants – experimentalists at the LHC experiments and theorists — for a two-day workshop titled “Multibosons at the Energy Frontier.” Discussions focused on strategies to best exploit the LHC data in the study of multiboson events.

On the first day, participants discussed the status of theory and LHC measurements. Theorists have made tremendous progress in calculations of multiboson production, while LHC experiments have reached an impressive percent-level precision in their measurements. Speakers talked about measurements of interesting rare multiboson interactions and new physics searches with multibosons. The focus then shifted to the vector boson scattering process and triboson production. These processes are very rare, but they offer a unique way to directly access the interaction between bosons: A key prediction of the Standard Model is that the Higgs boson moderates the behavior of these at high energies, and a different behavior would indicate new physics at work. CMS and ATLAS have conclusively observed heavy vector boson scattering in the 13 TeV LHC data, with more precision and production channels soon becoming available. Triboson production is yet to be observed; projections indicate that LHC data should allow for its observation in the coming years.

Studies in multiboson interactions could lead to interesting news in a few years! This is a slide from a presentation given by R. Sekhar Chivukula of the University of California, San Diego.

On the second day, attendees talked about effective field theories and simulations. The Standard Model effective field theory (SMEFT) is a framework that allows “effective” new couplings to be introduced without relying on a specific new physics model. If a physics process beyond the Standard Model is present, scientists should be able to spot anomalous couplings — deviation from the Standard Model prediction. While the SMEFT is commonly used to interpret multiboson measurements, it can have some issues. Not every effective theory is “unitary” and may lead to meaningless predictions at high energies. There exist solutions to issues like these, which are model-dependent, but then no universal interpretation is possible. This led to some interesting discussions, and the suggestion that ATLAS and CMS collaborators should provide information on their published results in a format that enables phenomenologists to test their theory against previous measurements if and when significant advances are made in the SMEFT or if more self-consistent theory to replace the SMEFT is found.

In the last session of the workshop, researchers discussed the prospects for multiboson physics at future colliders, with particular focus on the High-Luminosity LHC (HL-LHC). With a data set that will be 10 times larger than the nominal LHC data set, the HL-LHC will allow researchers to make precision measurements where electroweak interactions can be filtered out more cleanly than before. The upgrades to the LHC experiments and improved data analysis techniques also benefit the study of interesting multiboson processes, such as the scattering of longitudinal polarized W bosons – a process that is critical to test how the Higgs boson influences the Standard Model, which will be one of the main goals to achieve with the High-Luminosity LHC.

Hannsjörg Weber is a research associate in the Fermilab CMS Department.