Wednesday, December 3, 2008


Q: I get what accelerator physics is, but how can we really use it in real life? And why do you need to switch over from the Tevatron to the LHC? How are they different?

Hello Dr. Syphers,
My name is Shannon...
I was a little confused about the LHC accelerator. I know what it is, but how does it work? Also, how is this different from the Tevatron?

Why Accelerators, and how do they work?

Hi all,

This is in response to several questions, like the ones above, which are fairly similar to each other, namely ...

"How can we use accelerator physics in real life? Why do we need to switch over to the LHC from the Tevatron? How does the LHC work? How is it different from the Tevatron??"

The accelerators I work on use electric fields to accelerate charged particles and give them more and more kinetic energy. You may have learned (or will learn) that a particle can gain energy by doing work on it; work is basically "force times distance"; and the force here is the force due to an electric field. So, by subjecting particles like electrons or protons (charged!) to electric fields we can give them kinetic energy and they speed up.

I should point out, however, that eventually they get closer and closer to the speed of light, which is a "limit" that they cannot cross, in accordance with Einstein's theory of special relativity. But, they can (and do) continue to gain energy. We see and use Einstein's theory in our work every day. In fact, these accelerators wouldn't work at all if we didn't know about relativity.

Most particle accelerators were developed to study elementary particles like electrons, protons, ions, etc. The first ones were built in the 1920's and 1930's. But there have been many "spin-offs" of these devices. For instance, the older-style television sets (before "flat screen" TV's) use electron beams in them. They are actually particle accelerators! You might have one in your home today. In this case, the electrons are subjected to electric fields that produce total voltages of 10,000 volts or so. We say, then, that an electron in this scenario would gain a total kinetic energy of 10,000 electron volts (10 keV). This is just shorthand that we use in the accelerator business, because we tend to deal with elementary particles like electrons and protons, etc. In terms of Joules of energy, 1 eV = 1.6 x 10^(-19) Joule.

Other spin-offs of accelerator physics have been in the field of medicine, where x-ray machines (electron accelerators), MRI machines, PET scans, etc. use technologies developed for particle accelerators. There is even proton and neutron cancer therapy treatments that use particles from accelerators. Accelerators are also used in industry for welding, chemical analysis, and many other uses. But what has driven all of this has been the quest to examine nature's smallest particles and most fundamental forces.

The Tevatron is the highest energy accelerator in the world today. It accelerates protons through a total of 1 Trillion volts (10^(12) volts). Thus, the protons each have an energy of 1 TeV (which is how the Tevatron got its name). The LHC will make protons with energies of 7 TeV. Both of these accelerators are used, or will be used, to collide particles going in opposite directions at these high energies. Particles in nature have not had these kinds of energies since just after the Big Bang, so we are reproducing conditions from way back then. The purpose of both of these accelerators is to learn how the universe is put together by creating and studying particles that existed in great numbers long ago.

There isn't a very large fundamental difference between the LHC and the Tevatron. The LHC is larger, has stronger magnets and will give particles 7 times more energy than the Tevatron does. This just allows us to create more particles with more energy and study smaller and smaller things, hopefully gaining further insights into how the universe works. In each case, particles pass through electric fields, giving them energy (and momentum). Then, they are directed around in a circle using electromagnets so that they can pass through the electric fields again and gain MORE energy. The required strength of the electromagnets depends upon the momentum of the particles; as the particles gain momentum the magnets have to be turned on stronger and stronger. So, since we can only build magnets "so strong," then the circles get bigger and bigger for higher particle energies. The Tevatron is 4 miles in circumference. The LHC is 17 miles around!

I've left out a lot of details here, but these blogs can get rather long...
I'm sure you have more questions, so have at it!


-Mike Syphers

Hi this is Will
I was just wondering, when you collide the particles do you actually see anything or because its so fast you only see what happens with the ultra high speed cameras? And what do they look like, explosions or like fireworks or what? Thanks again for doing this program.

Hi Will,

That's a great question. First of all, what does it mean to "see" something? I mean, really physically. When you "see" something, physically what happens is that photons enter your eye through the iris (the detector's limiting aperture), get focused by the eye's lens, and interact with molecules in your retina that create electrical signals which are transmitted to your brain. Based upon which portions of the retina are activated, and with what "intensity," the brain interprets what it detects to decide what it was you just "saw". Might you agree with all that?

So, the way we "see" things in our experiment is to allow the particles to collide, which creates new particles moving in lots of directions. These new particles interact with different parts of our detectors, which generate electrical signals that are monitored by computers (the "brains" of the experiment). The computer signals are stored and reconstructed later. These detectors have magnetic fields built in so that we can monitor how the charged particles move around -- thus, we can determine their charge (pos or neg) and most of the time their momentum as well. We have blocks of metal that can absorb particles, too. When these blocks heat up, we can determine what energy the particles had. We put all of this type of information from a single collision together, and allow the computer to reconstruct what happened. (Of course, the computer only does what a scientist tells it to do, so it's actually the scientists who program the computers that diagnose what happened.)

At the bottome of this response is an image of what the computer might reconstruct from a collision. The lines and curves emanating from the center are "tracks" reconstructed by the computer program to show where particles went. The colored bars along the circumference of the program indicate the amount of energy that the particles had.

As you can see, they do indeed look a little bit like "fireworks." Cool? or not?

No comments: