The LHC is about to start colliding protons at 7 TeV (3.5 TeV per beam) or with about three times more energy than has ever been achieved by man. This is really exciting stuff! We’ll have a big media day on Tuesday to make sure everyone has a front row seat to the event!
Last night I was asked an interesting question – interesting enough that I thought I’d share the answer.
Will we collide the beams at many energies, or only at 7 TeV?
The LHC is really a discovery machine. Imagine that you’re back in 1490, and we’ve built a new ship that is capable of going seven times further than any previous ship in history before it needs to land (to refresh its stores, etc). The first thing we want to do with this is take it as far as it will go to see what’s out there! It could be that we’ll find a whole mess of new particles – that would be wonderful! It could be that we find nothing at all. That really would be like Columbus sailing for the new world and coming to the edge of the earth! It seems impossible and would fly in the face of everything we know – for physicists, it would be almost as interesting as finding a load of new particles! And, of course, when you’re sailing that far, you might pass some interesting things along the way…
Once you’ve searched for new high energy physics, you might want to try collisions at a few different energies. That would be something like looking for islands in the Atlantic. They might not be as exciting as a new continent, but they’re worth searching for all the same. Technically, just like if we were to sail out, we have no choice but to pass all the energies in between. But we do so for only a moment, and don’t really pause there to do any significant search in the middle. Side note: the middle energies aren’t quite as exciting at the LHC as they would be at, say LEP, because protons are “composite”, rather than single objects (as one professor put it, it’s like colliding two garbage cans). You can get some sense of what’s going on at lower energies from the higher energy collisions.
So on Tuesday, you should expect to see the beams go up from 450 GeV each (when they go into the machine) to 3500 GeV each (when they are colliding) without stopping in the middle – unless they plan something I don’t know about, of course!
One fun (very) technical note. Computers have a clock that keeps them in time (your computer is probably 2 GHz, for example). The whole LHC acts like a giant set of computers, all of which are timed together. It’s as though the entire thing has a single heart beat, around 40 MHz. We actually keep the heartbeat going at just the right pace to always have collisions “on time.” But the protons are changing energy from when they are injected at 450 GeV to when they collide at 3.5 TeV!! That means the entire heartbeat of the machine speeds up just a little bit to keep up (because of relativity, it’s only a fraction of a percent, but it is noticeable!!). To make sure things are safe, ATLAS usually stops collecting data while the heartbeat is actually changing – it’s a delicate operation, and we don’t want to have to stop to fix something right before the data arrives! So we may or may not actually see any of the collisions at energies between 900 GeV and 7 TeV!