5/20/2026 Bill Bell for Illinois Physics
Illinois Physics Professor Mark Neubauer and his team observed semi-leptonic vector boson scattering, an essential part of the standard model.
Written by Bill Bell for Illinois Physics
Illinois Physics Professor Mark Neubauer and his team observed semi-leptonic vector boson scattering, an essential part of the standard model.
When the Higgs boson was first observed experimentally using CERN’s Large Hadron Collider (LHC) in 2012, Illinois Physics Professor Mark Neubauer and his research team had been on the hunt for it for more than a decade. They were instrumental in the development of key aspects of the ATLAS particle detector—helping to design the trigger system, which analyzes the results of particle collisions in real time and determines which collisions and what data about those collisions get stored for further analysis. The team also led development of computing resources used to process, store, and share those enormous amounts of data.
More than a dozen years later, myriad questions about the nature of the Higgs boson remain. Neubauer—and the physics world at large—continue to benefit from this seminal work. The LHC is the highest-energy particle accelerator ever built. It’s still crashing together beams of protons at 13.6 tera-electron volts of energy. And it’s still producing more than 15 petabytes of data per year for physicists to plumb.
In late March, an international team including Neubauer and his recently graduated doctoral student JianCong Zeng published a preprint of their most recent efforts. They observed, for the first time, vector boson scattering in what is known as its semi-leptonic decay channel. Applying powerful machine learning techniques to data gathered between 2015 and 2018, the findings confirm long-held theories about the origin of mass and open a new window to study electroweak symmetry breaking. This research is now published in the European Physical Journal C.
“This is a very rare, subtle process, which is intimately tied to how fundamental particles get their mass,” Neubauer says. “We observed it with a significance greater than five standard deviations.”
“We have this threshold as physicists. When we want to say we discovered or observed something, we have a certain criterion, which is arbitrary but standard in our field, which is five standard deviations. Five standard deviations means there is about a 0.00003 percent likelihood of observing such a result by chance alone, assuming the distribution is normal.”
Never more than always
In the standard model (SM) of particle physics, the Higgs boson plays the central role in the electroweak interaction, which provides a unified description of the electromagnetic force (think of electric charge and magnets) and the weak force (think of radioactivity). At very high energy, such as was present in the early universe, the electromagnetic and weak interactions are the same. As the universe cooled and expanded, this electroweak symmetry was broken into the distinct forces we commonly observe today.
“The symmetry-breaking process is like balancing a pencil on its tip. There’s symmetry in the gravitational field around the pencil, but if you let go, it will eventually fall over to a ground state—lying on the table in some (seemingly) random direction. This stable state is not symmetric with respect to gravity,” Neubauer explains.
Through the electroweak symmetry process, interactions of massless particles with the Higgs field impart mass to the weak force-carrying particles (W and Z bosons), while the carrier of the electromagnetic force (the photon) remains massless. The Higgs boson is the quantum excitation of the Higgs field, and its observation in 2012 at the LHC is consistent with this “Higgs mechanism” described in the SM.
The SM predicts that weak bosons can scatter off themselves and each other like billiard balls in a game of pool. A problem with this scattering occurs at the very high energies that are accessible at the LHC: the calculation of this scattering process alone makes no sense.
“Without the Higgs boson, vector boson scattering at high energy is predicted to occur more than 100 percent of the time, which is nonsensical since something either always happens, or happens less than always,” Neubauer explains. This nonsensical scenario is referred to as a unitarity violation and indicates an incomplete theory.
When exchange of a Higgs boson between the scattered weak bosons is included in the calculation, the unitarity violation at high energy disappears because these two processes destructively interfere with one another.
“Weak boson pool without the Higgs boson is pandemonium,” Neubauer says. “It’s the subtle interplay between weak and Higgs bosons that makes the game playable.
“This delicate interplay means that even small deviations in the Higgs boson properties from those predicted by the SM can lead to large effects in vector boson scattering, which are observable at high energy. This provides a sensitive probe of new physics and an excellent opportunity for further discovery.”
The vector boson scattering process varies collision by collision, and effects from new physics are most pronounced in the most energetic scattering events. The scattered weak bosons are not observed directly because they are unstable and decay into combinations of charged leptons (electrons, for example), neutral leptons (neutrinos), or sprays of stable particles called jets. The process takes less than one-tenth of one septillionth of a second.
The vector boson scattering process was first observed by the CMS collaboration in 2018 by studying the production of two W bosons having the same electric charge that both decay to leptons. This fully leptonic final-state signature has low backgrounds from other SM processes but occurs at a low rate and does not allow for measurement of the total scattering energy in a given event.
Neubauer’s group instead studied a signature where one W boson decays to a pair of leptons and the other boson decays to a pair of jets. This semi-leptonic final state has two advantages over the fully leptonic final state.
First, in fully leptonic events, two neutrinos leave the detector without being measured because they are so weakly interacting. As such, the vector boson scattering energy cannot be measured in fully leptonic events, as it can be measured in semi-leptonic decay. Second, weak bosons decay significantly more often to jets than leptons. That, combined with the ability to measure the scattering energy, means that there is more data at high energy to search for new physics effects in semi-leptonic final states.
“The semi-leptonic decay signature provides a very powerful means to search for new physics at the LHC since it makes the highest-energy scattering of vector bosons experimentally accessible for a detailed study,” notes Neubauer.
“This is what makes the Ph.D. thesis work of JianCong Zeng and the work of my ATLAS collaborators such a tour de force in the use of machine learning and other state-of-the-art experimental methods to observe, for the first time, semi-leptonic vector boson scattering with an enormous impact on the field of high-energy physics.”
A New Era
Future measurements of vector boson scattering and the Higgs boson may still hold surprises for our understanding of fundamental physics. With sufficient experiments at the highest-scattering energies, sophisticated AI analysis methods, and powerful theoretical tools used to interpret the results, it is even possible to observe deviations from the SM for new physics that lie beyond the direct energy reach of the LHC using semi-leptonic vector boson scattering.
What if there are other forms of quartic gauge coupling? Interactions that produce some other combination of particles? It took nearly 10 years to see vector boson scattering for the first time and 17 years to observe it in its semi-leptonic decay. What else might be found with further study at the LHC or a future, higher-energy collider?
The Illinois-led team’s results also offer insights into those possibilities. Says Neubauer, “Now we’re entering an era of precision measurement of vector boson scattering at the highest available energies, which provides a powerful new tool to explore electroweak symmetry breaking and search for physics beyond the SM.”
Should physicists start to see deviations from the SM in future research into phenomena like vector boson scattering, they could begin to incorporate these findings as new particles or interactions in pursuit of a more fundamental theory of nature.
“This is a new playing field we’ve opened up,” Neubauer says. “It allows us to look for physics beyond the standard model in novel ways, which I find very exciting. I am looking forward to advancing future research in this area.”