A tantalising discrepancy from the standard model of particle physics hasn鈥檛 persisted in new results from CERN鈥檚 Large Hadron Collider (LHC). While the previous result created a frenzy of hypotheses about new particles and adjustments to the standard model that could account for the discrepancy, there is growing evidence that it was not quite correct.
In 2022, researchers analysing archival data from the now-shuttered Tevatron collider in Illinois found that a fundamental particle called the W boson seemed to be ever-so-slightly more massive than predicted by the standard model. They found that it had a mass of 80.4335 gigaelectronvolts, while the widely accepted mass for the W boson was 80.379 gigaelectronvolts.
Members of the ATLAS collaboration at the CERN particle physics laboratory in Switzerland were in the process of reanalysing their own data, which had always agreed with the standard model in the past, when the Tevatron results came out. The CERN researchers used a more accurate method to reanalyse their results, improved by a stronger understanding of both the particles involved and the data itself. It involved smashing two beams of protons together and analysing the trajectories of the various particles produced in the collision, and it required careful analysis over the course of years to come to a result.
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鈥淭he precision that you need to have is 0.01 per cent. That鈥檚 the difficult part, to get it so precise,鈥� says , a member of the ATLAS team at CERN. 鈥淲e measure this with big detectors, which are several metres long, and we have to understand the particle track on the micrometre level.鈥�
The CERN researchers found a W boson mass of 80.360 gigaelectronvolts, in line with the predicted mass from the standard model and with all of the previous measurements, but in conflict with the result from the Tevatron collider. 鈥淚t鈥檚 hard to say what鈥檚 going on with the Tevatron result, but what you see is that every one of the other measurements sort of lines up in one column and they鈥檙e an outlier,鈥� says , another member of the ATLAS team. 鈥淚t could be a systematic error, it could be the data. We just don鈥檛 know.鈥�
at Duke University in North Carolina, who led the Tevatron research, says that this work does not make him doubt his team鈥檚 previous result. 鈥淲hile I look forward to detailed discussions of the ATLAS methodology, I stand by the [Tevatron] paper,鈥� he says. 鈥淲e need new analyses and a fresh look at new data.鈥� Because the ATLAS work is a reanalysis of old data, it isn鈥檛 surprising that it returned a similar result, he says.
But given that several other teams have performed experiments that agree with the ATLAS measurement, ATLAS researchers are confident in their result as well. 鈥淲hat鈥檚 nice about these precision measurements is that the more precise they become, the less wiggle room you have in other areas,鈥� says Dunford. 鈥淲e know the standard model is not complete, because there are lots of things that it can鈥檛 explain, like dark matter, but this means that any new physics theory that we come up with in order to explain those things has to also adhere to this new precise measurement of the W mass.鈥�
The researchers are now analysing fresh data taken in 2018, and other collaborations at CERN are also working on their own measurements. For now, though, the excitement from the Tevatron measurement is damped and it seems that the standard model continues to be the best description of particle physics as we know it. 鈥淯nfortunately, so far at the LHC it鈥檚 the same story 鈥� the standard model works far too well,鈥� says Schott.
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