21 August 2026

Photo: CERN/Maximilien Brice, Michael Hoch, Joseph Gobin

A look inside the CMS Detector at the Large Hadron Collider (LHC).

The CMS experiment at CERN, with key contributions from the Cluster of Excellence Quantum Universe at the University of Hamburg, sets the world's most precise limits yet on how force-carrying particles interact — narrowing down where new, undiscovered physics could be hiding.

Physics has a very successful theory called the Standard Model of physics, which describes all the known fundamental particles and forces. One of its key predictions is exactly how the W and Z bosons — the particles that carry the weak nuclear force responsible for effects like radioactive decay — interact with each other. If experiments ever found even a tiny deviation from predictions in the Standard Model, it would be a sign of new, unknown physics.

To test this, physicists at CMS, one of the large particle detectors at CERN's Large Hadron Collider (LHC), collide protons at extremeley high energies and look for pairs of W and Z bosons produced in the collisions. Because these bosons decay almost instantly, the team has to reconstruct them from their decay products: one boson turning into an electron or muon plus a neutrino, and the other into a spray of particles called a "jet". By carefully measuring these particles, researchers could check whether the bosons' interactions matched the Standard Model's predictions — especially at high energies, where any new physics effects would be most visible.

Narrowing the hiding places for new physics

The result: the data shows no signs of anything unexpected. While that might sound unremarkable, it is a valuable result: it lets physicists rule out entire categories of theories that predicted new particles or forces. Using a mathematical tool called an Effective Field Theory (EFT), the team calculated the tightest constraints yet on how "different" these boson interactions could possibly be, beating all previous measurements.

"By setting strict limits on the parameters governing anomalous couplings with gauge bosons, we've effectively narrowed the space where new physics could still be hiding," explains Ankita Mehta, now a postdoc at Ghent University, who previously worked on this research at CERN and the University of Hamburg.

Hamburg's role

This research was initiated and led by Ankita Mehta, former postdoc at the University of Hamburg, and Andreas Hinzmann who led a DFG Emmy Noether junior group at the University of Hamburg during the time this research was conducted and now works at DESY. Their work is part of a broader effort to understand how particles interact at the smallest, most fundamental level which is one of the central questions driving Hamburg's particle physics research.

Even though no new physics turned up this time, the search continues: as the LHC undergoes upgrades to produce even more collision data in the next decade, physicists will be able to test these interactions with even greater precision in the years ahead.