Particle physics could be rewritten after the shock W-boson results from Fermilab

Tevatron particle accelerator in Fermilab, Illinois, pictured 1992

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A new measurement of a fundamental particle called the W boson seems to defy the standard model of particle physics, our current understanding of how the basic building blocks of the universe interact. The result, which was a decade on the way, will be scrutinized, but if it is true, it could lead to completely new theories about physics.

“It would be the biggest discovery since, well, since the start of the standard model 60 years ago,” said Martijn Mulders at CERN’s particle physics laboratory near Geneva, Switzerland, who has written a comment on the journal’s results. Science.

The standard model describes three distinct forces: electromagnetism, the strong force and the weak force. Particles called bosons serve as mediators for these forces between particles of matter. The weak force, which is responsible for radioactive decay, uses the W boson as one of its messengers.

The W boson is so central to the standard model that physicists have tried to measure its mass with increasing precision since it was first observed in 1983. These measurements have all largely agreed with each other, an obvious confirmation of the validity of the standard model.

But we know that the standard model is wrong. It has no explanation for gravity, dark matter and the absence of antimatter in our universe, so physicists are constantly on the lookout for deviant measurements that could lead to new theories.

Now Ashutosh Kotwal at Duke University in North Carolina and his colleagues have announced a new measurement of the W boson’s mass using data from the collider Tevatron in Illinois, which puts it at 80.4335 gigaelectron volts.

The generally accepted mass for the W boson is 80,379 gigaelectronvolts, and although the deviation may seem small, the new value is the most accurate to date, which is equivalent to measuring your body weight to within 10 grams.

More importantly, its difference from the generally accepted value of the W boson mass has a statistical significance of about 5 sigma, which corresponds to a probability of about 1 in 3.5 million that a data pattern like this would appear as a statistical coincidence.

Physicists normally use 5 sigma as the significance level to count something as a “discovery”, but the difference between the new mass measurement and that predicted by the standard model is even higher, at 7 sigma. This equates to about 1 in 780 billion probability of seeing a result like this by chance.

Kotwal and his team are aware that they are making an extraordinary statement that could overthrow the physics as we know it, but he says they have done every test they can to confirm it. A small amount of systematic uncertainty – mainly potential errors in the experimental design – remains, but now it is time for others to weigh in the results, he says. “We think the answer holds up to our own scrutiny,” he says.

Measurement of W boson mass

The team measured the mass of the boson by crushing beams of protons and antiprotons together and analyzing the particles produced in the collision. The analysis was so complex that the result took more than a decade to produce, after the Tevatron was shut down in 2011, but its potential implications are enormous.

“If the W boson mass deviates so much from the expectations of the standard model, and if we all understand [systematic] uncertainties, then it’s great, says Ulrik Egede at Monash University in Australia.

“If” is the important point for many physicists who, although excited about the result, are wary of its deviation from previous measurements. “We must first understand the discrepancy between [this result] and all other experiments before we think of explanations from physics beyond the standard model “, says Matthias Schott at CERN, who worked on a previous W-boson measurement with data from the ATLAS experiment collected at the Large Hadron Collider (LHC) until its shutdown during 2018.

Finding out the source of the deviation is not an easy task. W bosons decompose rapidly to other particles, either an electron and an electron neutrino, or a heavier muon and myon neutrino. Neutrinos are difficult to detect, so Kotwal and his team had to draw conclusions about where they were from large amounts of data. “[W boson masses] are considered to be some of the most experimentally difficult measurements to make, says Egede.

The 2018 ATLAS measurement for the W boson mass is the latest to date, but it may not help solve the mystery either. ATLAS used two beams of protons, rather than another of antiprotons, which made the results more difficult to compare, says Kotwal.

If physicists can not find a problem with the work of Kotwal and his team, then the next step will be to produce another measurement, which may come from three experiments at the LHC. “It’s the only collider with high enough energy to create W bosons,” said Harry Cliff of the University of Cambridge. The LHC is preparing for a new run this year after being offline since 2018, but Mulders says that data collected for the CMS experiment during the previous run can provide a new W-boson measurement for next year.

If the result is confirmed, it may be consistent with other unexplained anomalies such as those from the Muon g-2 experiment and abnormalities picked up at the LHC related to subatomic particles called bottom quarks, which may require new physics theories to explain. Although there are no clear challengers for such a theory at present, Kotwal says that certain variants of supersymmetry, which require a completely new set of particles, can accommodate the higher W boson mass.

Although the result takes 10 years to produce, Kotwal says this is just the beginning to understand its significance when physicists around the world get hold of data. “Science will be examined and we will continue to think about it,” he said.

Journal reference: ScienceDOI: 10.1126 / science.abk1781

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