How to weigh the W boson using hadronsCMS experiment measures the mass of the W boson from hadrons

28 August 2025

Photo: CMS Collaboration

Event Display

Photo: CMS Collaboration

Distribution of the measured mass (mSD) (black markers) compared to predictions from W bosons (orange) and backgrounds (other colors).

Hadrons are particles made of quarks and gluons, and hundreds of them are produced in each particle collision at the Large Hadron Collider (LHC). A team of international scientists at the CMS experiment led by Quantum Universe key researcher Andreas Hinzmann has now successfully extracted the mass of the W boson using hadrons with a machine learning algorithm. By measuring the W boson’s mass in this dense and complicated environment, the scientists showcase the success of theoretical calculations involved in this challenging process from production to detection.

W bosons are big players in the standard model of particle physics, famous for their ability to mediate radioactive decays, for their role in the fate of the universe, their friendship with the Higgs boson, and their participation in physics beyond what’s known. W bosons produced in particle collisions decay instantly. Thus, the only way to identify them is through their decay particles. From their decay to leptons, scientists have made very precise measurements of the properties of W bosons. However, their decay to quarks which in turn give jets (collimated sprays of hadrons) is much more abundant and equally critical when studying rare processes involving W bosons.

Finding W bosons in events containing only hadrons is extremely challenging. Firstly, they decay to quarks which form an undetermined number of hadrons through the strong force is a process that can only be approximated in today’s calculations of the standard model. Secondly, production of hadrons is extremely abundant at the LHC – collisions occur 40 million times per second, and each one, when processed and stored, has a size of around 1 Megabyte. The storage capacity forces researchers to throw away most of them and to keep only those with large overall momentum. Thirdly, the few remaining W bosons recorded are thus extremely fast. That speed creates a spray of hadrons from the decay that end up in a narrow jet around the flight direction. Those particles are close together, which makes them hard to resolve with the granularity of the CMS calorimeter detector. Finally, there is what scientists call pileup—particles from other collisions that swamp each event with hundreds of other hadrons, and those particles also are there amongst the interesting hadrons from the W boson.

Here is where machine learning comes into play. The scientists developed a neural network to disentangle events with W bosons from other stuff by feeding the measured hadrons and their momenta in the form of a graph. After using the neural network to remove a large fraction of the background, the peak of the W boson on top of the background is visible, as shown in the figure above. Steffen Albrecht, who completed his PhD at the University of Hamburg, says: “Investigating how the performance of various detector components affected the precision of the measurement was the most challenging and interesting part. It's exciting to see the precision we achieved in all-hadron events.”

The resulting W boson mass, 80.77 ± 0.57 GeV or in units of daily life 1.4x10–24 kg, is consistent with previous measurements. More importantly, the international group of scientists led by Andreas Hinzmann, key researcher at the Cluster of Excellence Quantum Universe at the University of Hamburg confirmed theoretical models of hadron production from W bosons from production to detection, demonstrating the capability to explore W bosons in all-hadron events in the future. These precise results are also of relevance for the scientific goals at the Cluster of Excellence Quantum Universe, as they enhance particle collision simulations to further study the W, Z and Higgs bosons. At the High-Luminosity LHC, successor of the LHC, scheduled to start operation in 2030, researchers plan to produce collisions at a larger rate. This will allow for further exploration of phenomena involving W bosons in rare, yet unexplored processes.