A startling discovery threatens to upend the standard model of particle physics
After a decade of analyses, a collaboration of physicists has made the most accurate measurement of the mass of a key particle – and it could unravel physics as we know it. The new measurement differs dramatically from predictions based on the Standard Model, hinting at new physics.
Since it was developed in the 1970s, the Standard Model of particle physics has been extremely successful in explaining particle interactions and most fundamental forces. It doesn’t cover everything – major missing pieces include dark matter and even gravity – but what it covers, it covers very well, consistently resisting experiments that test its predictions.
But now, a well-studied particle may threaten to poke a hole in this neat model. The mass of particles can be calculated through their relationships to other particles in the Standard Model, and this predicted mass can then be compared to actual measurements made in particle colliders, to test the internal consistency of the Standard Model. This process has now led to a major divergence, thanks to an unassuming particle called the W boson.
W bosons are elementary particles carrying the weak force, mediators of nuclear processes like those at work in the Sun. According to the standard model, their mass is related to the masses of the Higgs boson and a subatomic particle called a top quark. In a new study, nearly 400 scientists from the Collider Detector at Fermilab (CDF) collaboration spent a decade examining 4.2 million candidate W bosons collected from 26 years of data at the Tevatron collider. From this treasure, the team was able to calculate the mass of the W boson to within 0.01%, making it twice as accurate as the previous best measurement.
According to their calculations, the W boson has a mass of 80,433.5 mega-electronvolts (MeV), with an uncertainty of just 9.4 MeV on either side. This is within the range of some previous measurements, but well outside that predicted by the Standard Model, which puts it at 80,357 MeV, plus or minus 6 MeV. This means that the new value is shifted by seven standard deviations.
Further cementing the anomaly, the mass of the W boson was also recently measured using data from the Large Hadron Collider, in a paper published in January. This team arrived at a value of 80,354 MeV (+/- 32 MeV), which is comfortably close to that given by the standard model.
So what’s going on? Some physicists not involved in the study are more comfortable with the Standard Model, which is understandable.
“All these measurements claim to measure the same quantity,” said experimental physicist Martin Grünewald, in a report by Science. “Someone must be, I won’t say wrong, but may have made a mistake or pushed the error assessment too aggressively.”
But the scientists in the new CDF analysis say the procedures they used to reach their figure have been properly scrutinized for many years. In fact, the final measured value was hidden from the analyzers until these quality checks were completed.
“The number of improvements and additional checks made to our result is huge,” said Ashutosh Kotwal, lead author of the CDF analysis. “We took into account our better understanding of our particle detector as well as advances in theoretical and experimental understanding of the interactions of the W boson with other particles. When we finally unveiled the result, we found that it differed from the Standard Model prediction.
If the new figure is verified, it could hint at unknown particles or new physics beyond the Standard Model, which interfere with expected interactions. After all, we already know this framework is incomplete, and further investigation might help unravel the mystery.
“It is now up to the theoretical physics community and other experiments to follow up on this and shed light on this mystery,” said CDF co-spokesman David Toback. “If the difference between the experimental value and the expected value is due to some sort of new particle or subatomic interaction, which is one of the possibilities, chances are it’s something that can be discovered. in future experiments.
The research was published in the journal Science.
Sources: Fermilab, UK Research and Innovation