Friday, September 28, 2007

Working on the Compact Muon Solenoid

It sounds like the end of the six weeks is coming up, so I thought this week I’d cut you guys a break and share with you some of what it’s like to work at CERN as a grad student. In a way, all of these physics puzzles are a little misleading, because they really have nothing to do with what professional physicists actually “do”. At the edges of particle physics things are fairly ragged, and nobody’s making up nifty little problems because nobody knows quite how things work yet. Instead, a typical day for me will consist of coding at a computer, or working with hardware and circuits, or occasionally sitting through seven hours of meetings. (As I write this, I’m in a meeting about something called “level 1 trigger efficiencies”, which is exactly as exciting as it sounds.)

To take a small step back, I work at a detector called the Compact Muon Solenoid. If you remember my earlier post, the Large Hadron Collider is responsible for accelerating protons up to almost the speed of light and then colliding them together, and at the actual collision points we stick detectors to see what comes out. At startup, there will be two detectors: CMS, and at almost the opposite end of the ring, ATLAS, which earns my award for worst acronym ever (“A Toroidal LHC ApparaTus”). Anyway, these two detectors are both designed to discover new physics, and there’s a (usually) friendly rivalry between the two.

Okay, so CMS is a detector, but maybe you’d like to know what it actually looks like. Here’s a picture of (part of) the thing. (I can't show you the whole thing because it's not fully constructed yet.)

When it's done, it'll all be one giant machine, and yet it’s packed unbelievably densely with sensitive electronics. You can’t really get a great sense of scale from the picture, but this thing is 15 meters tall. That number rolled off me the first time I heard it, and maybe it isn’t impressing you much either, but when you’re standing in front of a piece of equipment that’s almost six stories high it hits you that the pictures don’t entirely do it justice.

The whole thing weighs 12,500 metric tons, which is a lot even for the volume. The reason it’s so dense is a component called the electromagnetic calorimeter, which is made up of 80,000 crystals of something called lead tungstate. Believe it or not, this stuff is 98% lead, an opaque metal, and yet it’s completely transparent. Seriously. Here’s a picture:

Yup. If you pick one up, it feels like lead, and if you bang two together they make a metallic ringing (and people will get mad at you, because they're very expensive). In total, they weigh about as much as 24 adult African elephants, but they’re supported by carbon fiber structures about 0.4 millimeters thick.

Everything about CMS is epic, but in my opinion nothing is more impressive than the rate of data flow: at the speed the protons are going, they’ll circle the LHC 40 million times per second. Each time they pass there will be about 25 collisions, so we’re looking at about a billion “events” per second. The amount of data stored for each event is about 1 MB, so the data is pouring in at a rate of a million GB per second. (I used GB in case any of you have a 100 GB hardrive on your computers; this thing could fill up ten thousand of those every second.) In fact, most of this stuff is useless and gets weeded out immediately, but about 500 Gbits/s is actually transferred through the “event builder”. Just to put this in perspective, this is equal to the total amount of data exchanged by the entire world’s telecom networks. I'm counting data from every phone conversation, every file download, every email and every internet video viewed on the entire planet.

CMS is cool, but I don’t really see it very often. For one thing, it’s 100 meters underground, and I don’t like working where there are no windows. For another, it’s in a town called Cessy (in France), and I typically work in my office in Meyrin, Switzerland. In fact, if it’s a nice day out, I’ll sometimes take my laptop to an outdoor table near the cafeteria because they’ve got good coffee.

One of the things I love about CERN is how easy it is to run into famous or important people. This week is CMS week, which mostly means lots of meetings, but it also means people from all over the world fly in to Geneva. Just sitting in the CERN cafeteria, you can see Nobel laureates, highly cited researchers, and world experts on pretty much anything that has to do with physics or hardware or computing. Yesterday a bunch of grad students and postdocs got together for a game of ultimate frisbee and the Deputy Physics Director for CMS skipped out of a meeting to play with us. He was much better than us, too, in spite of being forty years older.

I haven’t really talked much about what exactly I do, in part because I didn’t want this blog entry to be too long, but if enough people are interested I could talk a little about my research. Some of it’s a bit technical and wouldn’t be interesting to you guys, but I have to deliver a “preliminary defense” in mid-December, and I’d guess that kind of thing would resonate with anyone who has to write a senior thesis at the end of the year. Anyway, if you’re interested let me know.


ChrisLaz said...

The CERN cafeteria has good coffee??!! Man you've forgotten how real coffee tastes...

marc2718 said...

It turns out the secret is getting the coffee from the machine closest to the magazine shop. Different beans. Of course, for the best coffee, you just stay out at 904 and ask Dick to brew you a cup. :)

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