## Basic Physics in ATLAS July 17, 2008

Posted by gordonwatts in ATLAS, physics, Uncategorized.

There are times when I worry that things I have taught in introductory physics – like electricity and magnetism – aren’t really used in particle physics (At UW these are called Physics 121, 122, and 123).

The biggest example is momentum conservation. We use this all the time. In fact, one of the primary ways we will discover a new beyond-the-standard-model particle is via momentum conservation. A common line of reasoning is that we’ve not been able to detect this particle up to now because it doesn’t interact with our matter and our detectors as we expect it to. This is where basic physics comes to the rescue. We know the initial momentum of the collision in our detector. If this new particle were to fly off into the distance and not interact with our detector, then when we summed up the momentum of all of the outgoing particles… well, there would be some missing momentum! Score! Of course, it isn’t quite that simple, things like neutrinos will mimic exactly that signal, but there are ways around it.

The second place basic physics often comes into play is in detector construction and operation. For example, ATLAS has two large and very powerful magnetic fields. The first is the inner tracking field, and the second is the outer toroid field. Magnetic fields interact – think of bringing together two North pole magnets. So these two fields were carefully designed not to interact.

Except, one has to pump current through the outer toroid field to the inner solenoid magnet. As anyone who has taken a basic E&M course will tell you, a current generates a magnetic field. This means the cables that carry the current have to be able to withstand the force of the magnetic field interaction! At these field strengths and the 1000’s of amps of current flowing – that is a lot.

Of course, the engineers knew about this, and designed the cable housing to withstand this. Trickier than it sounds since all of this is superconducting. Still, it was nice to hear the reported successful test of this.

In the picture the 8 large tubes that surround the ATLAS detector generate the toroid field – they are 8 really giant superconducting magnets.

1. phalid - July 18, 2008

“There are times when I worry that things I have taught in introductory physics – like electricity and magnetism – aren’t really used in particle physics”

You have to build a solid foundation before you can begin the process of making them physicists. I began a self study plan which started with basic Newtonian physics. Slowly but surely I’ve been able to work my up to more abstract concepts and topics and the knowledge I learned from my earlier reading did indeed help out.

2. gordonwatts - July 18, 2008

Oh, I agree. Each step, when learning, is connected to the next. But when you are 100 steps down sometimes you don’t need that first step to make it. Sort of like the way a lot of chemistry is done – very rarely do chemists rely directly on E&M or Newton’s laws…

3. D - July 23, 2008

Now would that these engineers could design power supplies that could withstand B fields!

4. gordonwatts - July 23, 2008

True — but considering how much power these supplies are converting in such a small space, I guess we are pushing the edge of tech there too.