Fermilab finds discrepancy with the mass of the W boson

Sometimes the headlines in the consumer press about physics can be misleading. They're often sexed up a bit to grab attention. However, the latest reports about an experiment in particle physics causing a 'physics revolution' might not be far from the truth.

It is seemingly no big deal because what it has discovered is that the weight of a particular particle is 0.1% heavier than theory predicts. That doesn't seem like something to make much fuss about. If I was only 0.1% heavier than I was ten years ago I'd be mighty pleased.

Just a quick refresher before we get into this. The subatomic world is sometimes described as particles and sometimes described as waves. Things at that level can be both of these things at the same time, and whether you detect a particle or a wave depends on how you look at it. Ultimately, they're more generally described as fields these days, and you could see a particle as an excitation of those fields. None of that really nails it, though. Don't get too hung up on it, just think of it as a way to look at things — sometimes a particle model provides useful information and sometimes a wave model does.

The current 'standard model' of particle physics describes four fundamental forces. Three of those — electromagnetism, gravity and the strong nuclear force — push or pull things, broadly speaking. The fourth force — the weak nuclear force — doesn't really push or pull anything. Instead it makes one type of particle transform into another type of particle. In doing so we get the force responsible for radiation, and it's the force that drives nuclear fusion in the sun. So it's quite important.

Forces, like the bits of matter they act on, can be described as particles too. The weak nuclear force has three such particles, unimaginatively called W+, W- and Z particles. I always feel sorry for the weak nuclear force. The particles associated with the other forces get names — gluon, photon, graviton — but the weak nuclear force only gets a few letters.

The weight of a lot of particles just is, by which I mean there's no way to calculate what it should be. We just weigh it and then plug that measured weight into all the equations. It's different for the W particles. The standard model of quantum physics predicts what the weight of the particles should be. If the theory is correct, we would find that when we weigh those particles they match what the theory predicts.

This is where the discrepancy of 0.1% comes in. There are lots of things to rule out before we get too excited about this, particularly that experimental error might be responsible for the discrepancy. The result comes from the Fermilab's particle accelerator in the US, and they have a good record of diligence in these matters, which means the result looks promising. It still needs further confirmation, though, and hopefully that will come (or not) from the particle accelerator at CERN.

Small deviations, such as this 0.1%, can lead to a big problem with the theory. Bear in mind that the margins of error in particle physics are often described many digits after the decimal point. 0.1% is a big deal. When you unravel a theory to account for that deviation there are opportunities for new things to emerge, and that's what gets physicists excited.

The thing to understand about physics, though, is that it's a process of refining things. Einstein's theory of gravitation supplanted Newton's, but Newton's theory is still used all the time, and that's because the fine differences predicted by Einstein are irrelevant to most Earth-bound physics. Newton may not have completely grasped the fundamentals of what gravity is, but his maths was good enough to put a man on the moon. Likewise, the standard model is phenomenally accurate, and even a complete conceptual overhaul will not stop the maths of the standard model being effective for the things it already works well for.

It's not as if physicists didn't suspect the standard model had problems anyway. It has so far failed to describe dark matter, and there's six times as much dark matter as there is ordinary matter, which makes it fairly important. Sometimes scientists get clues to outstanding issues like this when they have to overhaul a theory.

I emphasise, though, that despite Fermilab's excellent reputation, this still needs confirmation.

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