This isn't exactly my area of expertise, but I've gotten requests by both email and twitter to try to explain yesterday's news about the Higgs' boson.
- What is this Higgs' boson thing?
- How did they find it?
- What does the five sigma stuff mean?
- Why do they talk about it as a "Higgs'-like particle"?
So, first things first. What is a Higgs' boson?
When things in the universe interact, they usually don't actually interacts by touching each other directly. They interact through forces and fields. What that means is a bit tricky. I can define it mathematically, but it won't do a bit of good for intuition. But the basic idea is that space itself has some properties. A point in space, even when it's completely empty, it has some properties.
Outside of empty space, we have particles of various types. Those particles interact with each other, and with space itself. Those interactions are what end up producing the universe we see and live in.
Fields are, essentially, a property of space. A field is, at its simplest, a kind of property of space that is defined at every point in space.
When particles interact with fields, they can end up exchanging energy. They do that through a particular kind of particle, called an exchange particle. For example, think about an electromagnetic field. An electron orbits an atomic nucleus, due to forces created by the electromagnetic fields of the electrons and protons. When an electron moves to a lower-energy orbital, it produces a photon; when it absorbs a photon, it can jump to a higher orbital. The photon is the exchange particle for the electromagnetic field. Exchange particles are instances of a kind of particle called a boson.
So.. one of the really big mysteries of physics is: why do some particles have mass, and other particles don't? That is, some particles, like protons, have masses. Others, like photons, don't. Why is that?
It's quite a big mystery. Based on our best model - called the standard model - we can predict all of the basic kinds of particles, and what their masses should be. But we didn't have a clue about why there's mass at all!
So, following the usual pattern in particle physics, we predict that there's a field. Particles moving through that field, if they interact with the field, experience a sort of drag. That drag is mass. So - just like particles like neutrinos aren't affected by electromagnetic fields, some particles like photons won't have mass because they don't interact with the field that produces mass. We call that field the Higgs' field.
(The previous paragraph formerly contained an error. The higgs field produces mass, not gravity. Just a stupid typo; my fingers got ahead of my brain.)
So physicists proposed the existence of the Higgs' field. But how could they test it?
It's a field. Fields have exchange particles. What would the exchange particles of the Higgs' field be? Exchange particles are bosons, so this one is, naturally, called a Higgs' boson. So if the Higgs' field exists, then it will have an exchange particle. If the standard model of physics is right, then we can use it to predict the mass that that boson must have.
So - if we can find a particle whose mass matches what we predict, and it has the correct properties for a mass-field exchange particle, then we can infer that the Higgs' field is real, and is the cause of mass.
How did they find the Higgs' boson?
We have a pretty good idea of what the mass of the Higgs' boson must be. We can describe that mass in terms of a quantity of energy. (See the infamous Einstein equation!) If we can take particles that we can easily see and manipulate, and we can accelerate them up to super-super high speed, and collide them together. If the energy of a collision matches the mass of a particle, it can create that kind of particle. So we slam together, say, two protons at high enough energy, we'll get a Higgs' boson.
But things are never quite that easy. There are a bunch of problems. First, the kind of collision that can produce a Higgs' doesn't always produce one. It can produce a variety of results, depending on the specifics of the collision as well as purely random factors. Second, it produces a lot more than just a Higgs'. We're talking about an extremely complex, extremely high energy collision, with a ton of complex results. And third, the Higgs' boson isn't particularly stable. It doesn't really like to exist. So like many unstable things in particle physics, it decays, producing other particles. And many of those particles are themselves unstable, and decay into other particles. What we can observe is the last products of the collision, several steps back from the Higgs'. But we know what kind of things the Higgs' can decay into, and what they can decay into, etc.
So, we slam these things together a couple of thousand, or a couple of million times. And we look at the results. We look at all of the results of all of the collisions. And we specifically look for a bump: if there's really a specific collision energy level at which Higgs' bosons are produced, then we'll see a bump in the number of Higgs' decay products that are produced by collisions at that energy. And what the announcement yesterday showed is that that's exactly what they saw: a bump in the observations inside the expected range of values of the mass of a Higgs' boson.
What does five sigmas mean?
Whenever we're making observations of a complex phenomenon, there are all sorts of things that can confound our observations. There are measurement errors, calculation errors, random noise, among many other things. So we can't just look at one, or two, or ten data points. We need to look at a lot of data. And when you've got a lot of data, there's always a chance that you'll see what appears to be a pattern in the data, which is really just the product of random noise
For example, there are some people who've won the lottery multiple times. That seems crazy - it's so unlikely to win once! To win multiple times seems crazy. But probabilistically, if you keep observing lotteries, you'll find repeat winners. Or you'll find apparent patterns in the winning numbers, even though they're being drawn randomly.
We don't want to be fooled by statistics. So we create standards. We can compute how unlikely a given pattern would be, if it were occuring do to pure randomness. We can't even absolutely rule out randomness, but for any degree of certainty, we can determine just how unlikely a given observation is to be due to randomness.
We describe that in terms of standard deviations. An observation of a phenomenon has a roughly 68% chance of being measured within one standard deviation (one sigma) of the actual value, or a roughly 32% chance of being observed outside of one sigma. At two sigmas, there's only a roughly 5% chance of being outside. At three sigmas out, you're down to a roughly 0.3% chance of randomly observing an event outside. The odds continue that way.
So, the Higgs' hunters computed probabilities of observing the data that they found if they assumed that there was no Higgs'. The amount of data that they found exceeded 5 sigmas away from what you would expect by random chance if there was no Higgs'. That translates as damned unlikely. The ultimate choice of 5 sigmas is arbitrary, but it's accepted as a threshold for certainty in particle physics. At five sigmas, we realistically rule out random chance.
Why do they keep saying Higgs'-like particle?
Remember up above, I said: "So - if we can find a particle whose mass matches what we predict, and it has the correct properties for a mass-field exchange particle, then we can infer that the Higgs' field is real, and is the cause of mass"? There are two thing we need to show to conclude that we've found the mediator of the Higgs' field. There needs to be a particle with the right mass, and it needs to have the properties of a mass-mediator. What we've got right now is an observation that yes, there is a particle at the mass we'd expect for a Higgs'. But we don't have observations yet of any properties of the particle other than its mass. Assuming the standard model is right, the odds of finding another particle with that mass is damned unlikely, but the standard model could be wrong. It's not likely at this point, but people like to be careful. So at this point, to be precise, we've observed a Higgs'-like particle - a particle that according to all of the observations we've made so far appears to be a Higgs'; but until we observe some properties other than mass, we can't be absolutely certain that it's a Higgs'.