It is both a great success and a minor disappointment: after five years of work, physicists at the CERN research center were able to present a high-precision measurement of the mass of the W boson to an international audience of experts on Tuesday. For a long time, the scientific community assumed that such measurements at the Large Hadron Collider (LHC), CERN’s large particle accelerator, would only be possible with great difficulty. The success of this high-precision measurement is only marred by the small fly in the ointment that the result so far provides no instructions for new physics that goes beyond the standard model of Particle Physics.

The W boson is an elementary particle that was discovered at CERN in 1983 and is responsible for mediating the so-called weak interaction. The exact clause of its mass allows indirect conclusions to be drawn about the properties of the Higgs boson, which was also discovered at CERN in 2012. The Higgs particle was considered to be the last missing piece of the puzzle in the zoo of elementary particles according to the current standard model. If the mass of the W boson were to deviate significantly from the predicted values, this model would be decisively called into question and scientists would receive instructions on as yet unknown physics.

The measurement now presented achieves a precision of 0.2 percent, making it one of the most accurate single measurements ever obtained. The measurement was published by the ATLAS Collaboration, an association of more than 3,000 scientists from all over the world who constructed the ATLAS detector at the LHC and have been operating it since 2008. Only a measurement at the Tevatron accelerator in the USA achieves a similar accuracy. “After many years of work and many unexpected problems, we are all very proud of this result. With a measurement like this, you have to be able to rely on the expertise and cooperation of the entire collaboration,” said Professor Matthias Schott of Johannes Gutenberg University Mainz (JGU) about the results. Particle Physics Professor Schott and his work group played a key role in the measurement. The research work was supported by both the Volkswagen Foundation and the German Research Foundation (DFG).

The measurement is considered one of the most complex programs of study at the Large Hadron Collider, the world’s largest particle accelerator. For this purpose, two particle beams were brought to collision with protons at 7 teraelectronvolts (TeV). On the one hand, the 45 meter long and 22 meter high ATLAS detector has to be calibrated to an accuracy of a few thousandths of a millimeter for the measurement. Secondly, the physicists must have a very good theoretical and experimental understanding of the internal structure of the colliding protons and the mechanisms that lead to the formation of W bosons in order to achieve the planned accuracy of the mass measurement.

“The mass of the W boson that we measured confirms the consistency of the Standard Model of Particle Physics. But perhaps we would have been much happier if we had discovered a deviation and could have found signs of something new,” notes Matthias Schott. “However, in recent years we have only analyzed the collisions from 2011 and so I am optimistic that we can still improve the accuracy considerably. After all, we have already recorded more than ten times as much data from W boson events.” Perhaps completely new findings will emerge in the future that will provide new instructions for understanding the universe.

Matthias Schott has been Lichtenberg Professor of Experimental Particle Physics at Johannes Gutenberg University Mainz since 2013. The professorship is funded by the Volkswagen Foundation and focuses on questions relating to the origin of the mass of elementary particles, in particular the high-precision measurement of the mass of W bosons.