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Unexpected particle mass may break the Standard Model

The Standard Model, the grand explanation that physicists use to describe how the universe works, may have been broken by some new experimental data. At the US government’s Fermi National Accelerator Lab (CDF), particle collisions over the past ten years have been examined by a team, and the mass of the W boson measured four million times over. But according to their research, published in Science, the fundamental particle doesn’t weigh what it should – if these results are confirmed, we might have to rewrite our understanding of physics.

What is the W boson? It’s an electrically-charged fundamental particle that governs what is called the weak force, one of the four fundamental forces of nature (as well as the strong force, electromagnetism, and gravity). Bosons are force-carrying particles that transfer discrete amounts of energy between particles of matter – as examples, the electromagnetic force is carried by bosons known as photons, while the Higgs boson transfers the force that endows particles with mass. They generally exist for milliseconds – as Dave Toback, particle physicist at Texas A&M University and CDF spokesman, notes, “they are constantly popping in and out of existence in the quantum froth of the universe”.

The findings carry a confidence level of 7 sigma, suggesting a 1-in-390 billion chance that this is a statistical fluke

Scientists have calculated that the W boson is about 80 times heavier than a proton, and the precise weight of the particle is one element of the Standard Model, the theory that explains how atoms are put together. But this experiment found that the difference between the theoretical prediction and the experimental value is 0.09%, significantly larger than the result’s error margins (of around 0.01%), and a huge difference at the subatomic level. Giorgio Chiarelli, part of the Fermi team, notes that the discrepancy is “not that much, but it’s enough”. Indeed, the findings carry a confidence level of 7 sigma, suggesting a 1-in-390 billion chance that this is a statistical fluke – it’s worth taking seriously.

What could this result mean? It’s possible that it’s simply an error of measurement – rerunning the experiment and altering the set-up could find that the W boson mass is closer to theoretical predictions. Matthias Schott, a physicist at the Johannes Gutenberg University of Mainz, Germany, said: “I would be cautious to interpret the significant difference to the Standard Model as a sign of new physics.” He noted that generating a W boson mass measurement from experimental data is famously a complex process, and suggested it would be useful to work out why the value differs from other recent results.

The finding may indicate the presence of an undiscovered fifth fundamental force, or suggest that the properties of the Higgs boson are different from those currently theorised

But what if the results are accurate? It would mean looking at the Standard Model yet again, and asking how it can account for more unexpected information. Although these rules have proven generally quite accurate, the model still has problems – the largest of which is its inability to account for dark matter, which may make up 95% of the universe. Dr Mitesh Patel of Imperial College, who works at the Large Hadron Collider, suggests this result could herald the biggest shift in our understanding of the universe since Einstein’s theories of relativity: “The hope is that these cracks will turn into chasms and eventually we will see some spectacular signature that not only confirms that the Standard Model has broken down as a description of nature, but also gives us a new direction to help us understand what we are seeing and what the new physics theory looks like.”

Exactly what those shifts may be, however, is open to a lot of speculation. The finding may indicate the presence of an undiscovered fifth fundamental force, or suggest that the properties of the Higgs boson are different from those currently theorised (for example, if it’s a composite particle, or if multiple versions exist). It may also lend credence to models of supersymmetry, which predict the existence of heavier partners for each Standard-Model particle.

At the moment, it’s much too early to say if this result is meaningful – the scientists involved are calling for further research to confirm or dismiss the findings. But if it turns out that the W boson is in fact heavier than expected, it’ll offer further insight into how physicists may need to revise the Standard Model. As Toback notes: “It’s now up to the theoretical physics community and other experiments to follow up on this and shed light on this mystery. If the difference between the experimental and expected value is due to some kind of new particle or subatomic interaction, which is one of the possibilities, there’s a good chance it’s something that could be discovered in future experiments.” And from that minor finding, we may rethink how we look at the entire universe.

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