Fermilab Says Particle Is Heavy Enough to Break the Standard Model

Physicists have found a small discrepancy in the weight of the W boson, an elementary particle.

The measurement comes from a particle collider that smashed its final protons a decade ago. The roughly 400 members of the CDF collaboration have continued to analyze W bosons produced by the collider, chasing down numerous sources of error to reach an unparalleled level of precision.

If the excess heft can be independently confirmed, it would imply the existence of undiscovered particles or forces, which would bring about the first major rewrite of the laws of quantum physics in half a century.

This would be a complete change in how we see the world and it could even be as important as the 2012 discovery of the Higgs boson. This would be a completely new area.

The Standard Model of particle physics, the long-reigning set of equations capturing all known particles and forces, is in dire need of repair. The nature of dark matter is one of the grand mysteries left unanswered by the Standard Model. A credible threat to the Standard Model is made by the CDF collaboration's strong track record.

Aida El-Khadra is a theoretical physicist at the University of Illinois.

There is no champagne yet. The new W mass measurement departs from the Standard Model, but other experiments weighing the W have produced less dramatic results. The W particle was found to be only a hair heavier than what the Standard Model says, thanks to the ATLAS experiment. One or both groups overlooked some subtle quirk of their experiments according to the clash between CDF and ATLAS.

Guillaume Unal is a physicist at the Large Hadron collider and a member of the ATLAS experiment.

It's a monumental piece of work, but it's very hard, said Frank Wilczek, a physicist who won a Nobel Prize.

Weak Bosons

The weak force is one of the four fundamental forces. The weak force does not push or pull so much as it transforms heavier particles into lighter ones. A muon spontaneously decays into a W boson and a neutrino, for instance, and the W becomes an electron and another neutrino. The sun shines because of the subatomic shape-changing that causes radioactivity.

Over the last 40 years, various experiments have measured the W and Z bosons. The mass of the W boson has proved to be an attractive target. The W mass can be predicted by combining several measurable quantum properties in the Standard Model equations.

The web of connections surrounding the W boson has been exploited by experimentalists for decades. Researchers could start to sense the smaller effects of the W particle's mass once they had accurate measurements of the terms that most heavily influence it.

Physicists can predict the mass of a particle called the top quark just ahead of the discovery of the top quark. In the 2000s, they did it again to anticipate the mass of the boson before it was found.

The theory has no missing pieces, despite the fact that theorists expected the top quark and the Higgs to exist and to be connected to the W boson through the equations of the Standard Model. The mass of the W boson would point toward the unknown.

Catching W’s

The new mass measurement is based on an analysis of 4 million W bosons produced at the Tevatron between 2002 and 2011. When the Tevatron crashed protons into antiprotons, a W boson popped up. The W could decay into a muon or an electron, both easy to detect. The muon or electron is heavier when it is faster.

Ashutosh Kotwal is a physicist at Duke University and the driving force behind the CDF collaboration. The CDF researchers can see the path and speed of a muon or electron through a chamber filled with 30,000 high-voltage wires. Getting an accurate trajectory depends on knowing the exact position of each wire. The new analysis took advantage of the muons that rained down from the sky. The researchers can see any wires that are not straight by using the bulletlike particles to rip through the detector.

They spent a lot of time between data releases doing cross-checks to make sure they understood what they were reading. All the time, W boson measurements increased in speed. CDF's last analysis covered data from the first five years. The data doubled over the next four years.

It came at us like a fire hose, faster than you could drink from.

The collaboration has been going on for nearly a decade. In a November 2020 meeting over the internet, the team's result was decrypted with the press of a button.

The physicists absorbed the answer. They found that the W boson weighs 80,433 million electron volts. The discrepancy is seven times larger than the margin of error of the measurement or prediction.

Physicists normally must clear a seven-sigma discrepancy to claim a definitive discovery. In this case, they are given pause by the lower measurement from the ATLAS experiments.

Chris Quigg, a theoretical physicist at Fermilab who was not involved in the research, said that this is not a discovery, but a provocation.

Clash of Experiments

The CDF measurement will fall to the Large Hadron Collider if the onus of confirmation or disproving is not taken into account. The higher collision rate complicates the analysis of the mass of the W. The tension can be resolved by collecting additional data at lower beam intensities.

Figuring out what an oversize W boson might mean is something theorists can't help but ponder.

When a muon emits a W boson as it decays into an electron, it can interact with other particles. This fraternization with the unknown could skew the mass.

There is a chance that a heavy W boson is due to a second Higgs boson. Or it could be due to a new massive boson, a new force to bind them together and a variant of the weak force.

Some theorists think particles are predicted by a theory called supersymmetry. This framework links matter particles and force-carrying particles, and creates an undiscovered partner of the opposite type for each of the known particles. Physicists still believe that Supersymmetry is true despite the fact that it failed to come to fruition at the LHC.

The muon g-2 anomaly is a discrepancy between the Standard Model and certain supersymmetric particles. The particles would help us with g-2 by nudging the W boson's mass up a bit.

The work of experimentalists in honing their precision measurements makes researchers more optimistic that a long-awaited breakthrough is coming.

We are getting close to the point where something is going to break.

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