There is a lot of chatter about the Higgs out there at the moment, but unfortunately I think that a lot of it illustrated the poor understanding of statistics by journalists (and even scientists). p-values make me weep.
But I thought I would talk about something else, the the discovery of bottomium at the LHC. Funnily enough, it's not actually a new particle, and that's something I thought I would try and explain.
Let's start with a picture - what's this?
One of the other things you remember from high school science is that each of these orbitals has a different energy level (some have the same, but let's not worry about this at the moment), and that when atoms absorb energy, the it changes energy levels, and this energy can lost through the emission of photons. For lithium, the energy levels look like this
Now, it might seem we are going a little off topic, but let's keep going. What about the nucleus of an atom? We know that the nucleus is made of protons and neutrons, and that protons carry positive charge, while neutrons are, well, neutral. This leads to a bit of a conundrum, as that positively charged protons will repel each other - how does the nucleus hold itself together?
This is where the strong force comes in. It provides an attraction which overcome the repulsion of the electromagnetic force between the protons. But the nucleus, like the atom, is governed by the rules of quantum mechanics, and so has quantized energy levels. So we get a picture like this
Now, one thing that some people don't know is that if we have a Nickel atom in its ground state, and one in a higher energy level, the one at the higher energy level is more massive than the one in its ground state. This, of course, makes sense. We know from special relativity that mass and energy are interchangeable, and so the excited nucleus has more energy and so more mass.
OK - finally, let's get back to the announcement of bottomium. Bottomium is a meson, meaning that it is make of a quark and an anti-quark, held together by the strong force. Looks something like this -
quarkonium state, made of a bottom quark and an anti-bottom quark.
So, did the LHC discover bottomium in the previous week? Well, no. The ground-state bottomium particle was the ηb was discovered by the BaBar experiment in 2008. So what is the bally-hoo coming from the LHC.
Well, mesons (and they close companions, the baryons) obey the rules of quantum mechanics, and like atoms and nuclei, they have quantized states. They can be in their ground state, but if you give them energy, they can move into a higher energetic state. Just like nuclei, the more energy means more mass, and so the new particle, the χb(3P) is just the same old bottom-anti-bottom, but just in a higher energy state (in fact, the 3P gives it away, as those chemically minded would remember the P from SPDF). So, it's not really a new particle in the sense that it is made of something new, but is the same old particle but now with added energy. I guess that would not have been such an amazing press release :)
One last thing. As we've noted above, nuclei and mesons (and baryons) have more mass when they absorb energy and move to higher level. But the same must be true of atoms, and an excited hydrogen atom has more mass than an atom in its ground state. But the amount of energy held in an excited electron system is quite small compared to the overall atomic mass. Cool eh!
Note added in proof: Due to a convoluted history, the first excited bottomium state was discovered in 1977, with the ground state discovered in 2008. Here's the energy level diagram from the 2008 article above.