Last year, a new discovery in particle physics stunned scientists: a fundamental particle responsible for one of the four fundamental forces of the Universe was heavier than expected.
The discovery of a discrepancy between the theoretical and experimental masses of the W boson has promised new insights beyond the Standard Model, the theoretical model that describes the behavior of matter.
Now the scientists have run the same numbers again using an updated technique, this time finding that the particle’s mass matches the Standard Model’s predictions after all.
While this means we may not need a revolutionary overhaul of our current theory of particle physics, we can only be a little disappointed. The Standard Model of particle physics remains a hypothetical interpretation of the universe around us, but so far it has stood up well to the battery of tests we’ve managed to put it through. At the same time, we know that there are unexplained shortcomings: the Standard Model does not account for dark matter, for example, or even gravity.
Although the W boson cannot be measured directly, the mass and energy released during its decay can. Putting the pieces back together requires a thoughtful approach and a solid starting point for how the colliding particles hold together.
The latest research re-analyzed 2011 data from the ATLAS experiment at CERN’s Large Hadron Collider (LHC) in Switzerland, using a revised statistical approach based on a better understanding of the processes.
The researchers say their new reading is 16% more accurate than before and with a lower level of uncertainty, calling into question the 2022 results from the now-shutdown Tevatron collider in the US state of Illinois.
“Although the understanding of the detector as well as the effects of contributions from electroweak background processes and top quarks have not changed, significant progress has been made in the statistical framework on the extraction of W boson mass from data,” the researchers write. .
For this new research, the team focused on particle collision events where the W boson breaks up into lighter particles: electrons, muons and neutrinos. Additional data collected in 2017 validated the results.
The Tevatron measurement came out at 80.4335 gigaelectronvolts, a seemingly small but significant difference at the 80.357 gigaelectronvolts predicted by the standard model. The latest mass measurement of the W boson is 80.360 gigaelectronvolts, bringing it much closer to the theoretically predicted mass.
As a class of particles, gauge bosons like the W boson essentially facilitate interactions between other fundamental particles. Along with the Z boson, the W boson is crucial in processes such as radioactive decay and nuclear fusion.
“Due to an undetected neutrino in the decay of the particle, measuring the mass W is one of the most difficult precision measurements performed at hadron colliders,” explains particle physicist Andreas Hoecker from the ATLAS team at the CERN laboratory.
“This requires extremely precise calibration of measured particle energies and momenta, as well as careful assessment and excellent control of modeling uncertainties.”
It should be borne in mind that this is only a preliminary conclusion at this time. Other tests are currently in progress on more recent data. If the Standard Model turns out to be wrong about the mass of the W boson, that would hint at as yet unknown particles and forces at play. For now though, it seems that the reputation of this fundamental assumption is secure.
“This updated result from ATLAS provides a rigorous test and confirms the consistency of our theoretical understanding of electroweak interactions,” says Hoecker.
You can read an article detailing the new findings on the CERN website.
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