So what exactly is a sigma anyway?

Particle physics isn’t the most comprehensible thing at the best of times. So if you’re one of the large percentage of the population who don’t really know what a sigma is or why a boson is so important, you might feel slightly left out. Not to worry. That’s what the internet is for!

Obviously, a lot of people think Physics is Brilliant as you can see from this screengrab of Twitter trends from earlier today (posted by @astronomyblog). “Higgs” and “CERN” were actually global trends on the internet earlier, which makes me rather happy. Though not everyone is a particle physicist, or even a scientist for that matter. So for anyone who isn’t quite so savvy with the physics lingo, here’s a primer…

Sigma

Sigma (or σ) is physics talk for standard deviation, which has to do with statistics. Imagine you’re at a house party. There’s music playing and there’s a steady murmur of voices all around you, as people hold their conversation. That basic background noise level is 1σ. Most of the random noise falls within this level. Occasionally, something might happen which is louder than this – the guy announcing that the pizzas are cooked or the girl by the window laughing at something, for instance. Even including this, some 99% of all noise will fall within three standard deviations or 3σ.

Now imagine you’re trying to hold a conversation. You need to stand out from the background noise, so for anyone nearby who you’re taking to, you need to speak loud enough that you reach more than 1σ or they simply won’t hear you. The more you stand out against the background noise, the more likely you are to be heard. It’s that simple. Different scientists like different levels, depending on how much noise they have to deal with. At CERN they like 5σ to be certain of something, though I know astronomers who work with noisy high redshift data who are quite happy with 2σ.

Note that standing on a chair with a megaphone will probably get you at least 10σ at any house party.

Boson

A boson is a specific type of subatomic particle. There are two main classes of these – bosons and fermions – and they’re defined by a quantum mechanical property called spin. What spin is exactly is difficult to describe, but the easiest way to visualise it is use the old fashioned picture of protons, electrons, et al being like little tiny spheres which rotate like planets. Now, those spins only have certain values they can take – either whole numbers or half numbers. With our planet analogy, suppose those numbers give you the number of days that planet takes to rotate. Then we have planets with spins of 1 day, 2 days, 3 days, etc. These would be bosons. For bosons, 0 also counts as a whole number, so a planet taking 0 days to rotate would still be a boson. Any planets which have rotation rates of ½ a day, 1½ days, etc would be fermions.

Unfortunately, this is just a metaphor, and not a very good one either. We now know that subatomic particles don’t actually spin like this, so precisely what causes “spin” is still very much a moot point – and one for philosophers at that.

GeV

GeV means giga electron volts, and it’s used to refer to the mass of particles detected in colliders like the LHC. One giga electron volt is one billion electron volts, and confusingly, as the name might imply, electron volts are actually a unit of energy. Which relates directly to mass, as Einstein’s equation E = mc² tells us.

Specifically, an electron volt is the energy change of a single electron in a 1 volt circuit. Electrons are tiny, so 1 eV is a tiny amount of energy. So tiny that actually 125 GeV is still a tiny amount of energy by our standards. But for anything the size of an atom or smaller, that’s huge.

So there you have it. Hopefully, any non-specialists reading this will now have a better understanding of precisely what the news stories mean when they talk about a 4.9σ detection of a boson with a mass of 126 GeV!

About Invader Xan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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