Physics

The W boson threat to the Standard Model

A new experimental result has shaken the world of particle physics. The W boson, it turns out, is a lot heavier than we thought. This poses a challenge to our most successful and tested theory about the fabric of the universe so far: the Standard Model. And while this one experimental result might not be enough on its own to overthrow the theory, it already points in the direction of a theory that could, writes Martin Bauer.

Recently the mass of the W boson has been measured by the Collider Detector at Fermilab with unprecedented precision and a surprising result. The recent result disagrees wildly with all previous measurements of the W boson’s mass, but this result is no fluke. To give you an idea of ​​the precision of this latest measurement and how unlikley it is that this result is a mistake, consider this: if you weigh yourself multiple times with different scales you would expect to see some discrepancy. But an equivalent discrapancy like the one between the most recent measurement and the previous measurements of the W boson’s mass, would statistically occur only after you’ve weighed yourself 1 billion times.


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Measuring the properties of the W boson precisely is very important. From stellar fusion to carbon dating, the mass of the W boson affects many calculations that underly our understanding of the Universe. For instance, its mass is linked to the lifetimes of other particles, which in turn are important for understanding how the Universe developed after the Big Bang. But perhaps the most important consequence of this latest measurement of the W boson’s mass is that it puts it in tension with our most successful theory about particle physics: the Standard Model.

At this point you are probably wondering: What is a W boson, what is it good for and what are the actual implications of this surprising result?

The W boson is a fundamental particle mediating the weak force, a force that we never experience directly because it only acts on subatomic distances. All other known fundamental forces give rise to bound systems: eg solar systems bound together by gravity, atoms bound together by the electromagnetic force and atomic nuclei bound by the strong force. The weak force does not give rise to such bound systems, but is crucial for many natural phenomena that affect our everyday life. The fusion process in the sun is initiated by hydrogen transforming into heavy hydrogen via the weak force. Without the W boson the sun would be very dim. All atoms we know are made from protons, neutrons and electrons yet there are many different particles around. Why do they never make atoms or more complicated structures? Because they decay rapidly due to the weak force.

What makes the weak force so unique is the fact that the W boson can change the charges of other particles it interacts with. It can turn an electron (charge -1) into a neutrino (charge 0), or a neutron (charge 0) into a proton (charge +1). Methods like carbon dating directly rely on this property. The slow decay of neutrons into protons in carbon isotopes lets us date archeological artefacts and would be impossible without the W boson.

Measuring the mass of the W boson directly is extremely challenging.

So much so that my experimental colleagues dubbed it ‘the hardest measurement in high-energy physics’. There are a number of reasons that make it so difficult:

First, W bosons are very heavy for elementary particles. A single W boson weighs 80 times as much as a proton. Accelerators are the only place in the world where the enormous energy can be leveraged to produce them. But once they are produced, W bosons decay immediately and determining their mass requires measurements of the decay products and reconstructing it from these measurements. To make things worse, the colliders that produce W bosons inevitably also produce hundreds and hundreds of other particles. Figuring out whether a W boson has been produced and making a measurement of its mass is rather like finding connecting puzzle pieces in a huge box full of them and then putting them together correctly – and some are missing and there is more than one way they could fit.

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In contrast to many anomalies that have challenged the status of the Standard Model before, it is statistically almost impossible for the CDF result to be a fluke.

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The Collider Detector at Fermilab (CDF) Collaboration has done this exercise with a dataset of four million W bosons collected from 2002 to 2011 at the Tevatron collider at Fermilab, about an hour outside of Chicago. Weighing the W boson took the collaboration over a decade to complete. And the results are spectacular. Not only did CDF deliver the most precise measurement of the W mass every achieved, their result disagrees with all previous measurements, putting it in direct tension with countless experiments that have confirmed the most successful theory high-energy physicists has ever developed: the Standard Model of Particle Physics. It cannot be overstated how remarkable this is: The CDF measurement is off by less than one in a thousand, but the precision achieved is one in ten thousand!

This enormous precision is the reason why this measurement caught the attention of physicists worldwide. In contrast to many anomalies that have challenged the status of the Standard Model before, it is statistically almost impossible for the CDF result to be a fluke. But if it is not a fluke, what is it then? Did the CDF experimentalists discover a first sign of new physics in their data?

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While a single measurement rarely establishes a new theory of physics, it can identify structures that could replace the Standard Model prediction.

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In the wake of the CDF announcement a twofold challenge emerges: A comprehensive analysis of all the tools and techniques applied to extract the W mass from the mountain of data is underway. Experimentalists will test again and again whether there is any unaccounted uncertainty, any explanation in the measurement process for this almost impossible discrepancy.

In the meantime, theoretical physicists are asking the question about a different, more speculative origin. The structure of boson masses is a prediction of the Standard Model. This structure is very rigid. It is not possible to simply change the mass of the W boson without changes in other observables such as decays of heavy quarks and leptons, the mass measurement of the Z boson, and various other measurements that have confirmed the Standard Model structure over and over again . This is unless there is something else that changes this structure, something that has been missed by other experiments so far, something new and unexpected, manifesting itself in the collisions of the Tevatron collider.

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A number of initial studies show that the presence of another Higgs boson, or novel heavy particles interacting with the W boson, as predicted in some theories of Dark Matter, could both be responsible for the discrepancy.

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Theorists will carefully scrutinize the Standard Model calculations.It is clear that if the CDF measurement stands, the Standard Model alone will not be enough to explain all of our experimental data anymore. And while a single measurement rarely establishes a new theory of physics, it can identify structures that could replace the Standard Model prediction.

A number of initial studies show that the presence of another Higgs boson or novel heavy particles interacting with the W boson, as they are predicted in some theories of Dark Matter, could both be responsible for the discrepancy. If these new particles really shift the W mass, they can also be produced elsewhere. Guided by theoretical studies, data from the Large Hadron Collider (LHC) at Cern will be combed for signs of these new particles and the results will narrow down possible explanations or produce further hints about the nature of the underlying physics. The LHC collides protons with a record 7 times the energy of the Tevatron with which CDF took their data. It has just started its third run and is projected to collect more data than the first two runs combined.

Even if these searches come up empty-handed, eventually LHC data will allow for an equally or even more precise measurement of the W mass and confirm or contradict the CDF measurement.

The complexity of the ‘hardest measurement in high-energy physics’ makes this a very hard puzzle to solve. It will be with us for a while. But we physicists have long given up the belief that we can always predict where new physics will be found. Too often have we been blindsided by unexpected discoveries.

If we can say anything with certainty right now, it is that continued funding for fundamental research is indispensable. It took ten years to take the data and another ten years for the analysis and if we had cut funding at any point the ‘hardest measurement in high-energy physics’ would never have been completed and what we will learn from the impressive measurement by CDF would have remained buried in the data.

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