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Archive for July, 2008

One Reason I Love My Job

Thursday, July 31st, 2008

One of the things I like best about being a physicist is that, when I wander aimlessly through the halls, unaware of my surroundings, with disheveled hair, bare feet, and a look somewhere between intense concentration and lunacy on my face, this is considered entirely normal behavior.

Well, mostly normal, at least.

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Today marks the start of the 34th International Conference on High Energy Physics, the biggest particle-physics conference of the year. It is held in even-numbered years, and when the conference started, it was always held at the University of Rochester, and hence some people still refer to it as “the Rochester meeting.” But now it moves from country to country, and this year it is in the United States, jointly hosted by Princeton and Penn in Philadelphia. (I’d be there myself but for some other exciting events going on; see a future post on that.) It’s a big deal; people work hard to produce new science results specifically for this conference. Obviously, at this point the LHC experiments only have so much to say, as we don’t have any LHC data yet. There will be several talks about the status of the machine and detectors, plus presentations on the prospects for various measurements.

But even though a lot of people are focused on the jungle, meanwhile back in the states, there is a lot of physics going on! (See here for this week’s obscure reference.) In particular, the two big experiments at Fermilab’s Tevatron, CDF and D0, continue to put out new results every week. This week there are more results than usual; they were presented in a pair of seminars last Friday. Of particular note (so far) is D0’s observation of pairs of Z bosons. This process is not unexpected, but definitely rare; the fact that the Tevatron experiments can even observe this final state shows that CDF and D0 have enough data that they can think seriously about the possibility of excluding — or observing! — a standard-model Higgs boson.

Increasingly rare processes observed at the Tevatron

OK, so what about that Higgs boson? As of a few months ago, the two experiments together had accumulated enough data to be able to come oh-so-close to being able to exclude some range of Higgs boson masses. Since then, more data has been analyzed, and the experimenters have been working hard to improve their data-analysis techniques to be sensitive to processes with even smaller rates. It’s possible that the analyses to be shown at this conference will actually be able to exclude the Higgs at some masses. If that can be done, it will be the first new direct information about the mass of the Higgs since the end of CERN’s LEP program.

As of this writing, not all the numbers have been crunched yet and not all the results have been approved for release, so we don’t yet know what the answer is! But keep an eye out for news sometime next week, probably. Here is a plot that shows the Higgs results from earlier this year. A new plot will be shown at ICHEP. If the solid black line goes below 1 on the vertical axis, it will indicate that the data do not support the existence of a Higgs boson at the mass values on the horizontal axis. We await the news….

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Large Hadron Rapper

Wednesday, July 30th, 2008

The NY Times is now hip to the work of ATLAS’s own alpinekat (a writer/editor for the ATLAS e-News). She was also our editor on this blog for a while, so we can say we knew her before she got internet famous. Go Katie!

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Large Hadron Rap

Monday, July 28th, 2008

alpinekat has produced a really phat physics video about the LHC. Enjoy!

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How a Pixel Detector Works

Friday, July 25th, 2008

If you read about the LHC detectors a lot, then you’ve heard about pixel detectors from time to time. Both ATLAS and CMS have one. But what is a pixel detector? Ken explained briefly here—and mentioned, in the process, that when he was my age, their trackers only had 26,000 channels, dinosaurs roamed the earth, they had to walk ten miles to the lab in the snow, and it was uphill both ways—but, anyway, I thought it might be worth expanding on the explanation a bit.

A schematic of the CMS pixel detectorI’ve heard several times that a pixel detector is like a digital camera. Indeed, they both have pixels, but how far does the parallel really go? The biggest, most obvious difference is the shape, which follows from different purposes. Your camera is designed to collect all the light that reaches a particular point, namely the camera lens, and so there’s a flat rectangle of pixels at the spot where the light is focused. A pixel detector at a collider is designed to collect information on particles coming from a particular point, namely the place where the particles collide, so it’s built to surround that point as completely as possible. The schematic of the CMS detector in the upper right illustrates this point nicely; obviously, on a large scale, it looks nothing like the inside of your camera. (A pixel detector has more pixels too, but it’s about the same scale—ATLAS has about 80 million compared with 6 million in my digital camera.)

But the real question is how the individual pixels differ, and the answer turns out to be: not by as much as you might guess. I’m no expert on digital cameras, so I had to look it up, but it turns out that digital camera sensors are actually little arrays of silicon detectors. When a visible photon hits the layer of silicon that makes up a pixel, it knocks an electron out of its place in the silicon. The electron is pulled in one direction by an electric field, while the hole (the empty space where the electron should be) is pulled in the other. The charge thus collected by each pixel is proportional to the number of photons that hit it, and hence the intensity of the visible light; this charge is eventually read out by (for example) a charge-coupled device, and the picture can be assembled.

How a silicon detector worksThe basic idea of the silicon detectors we use in particle physics is the same, but we’re looking at fewer particles with much higher energy. Whereas a visible photon has an energy of a few electron volts (eV), the interesting particles passing through our silicon detectors at the LHC will have an energy somewhere from several hundred million eV to several hundred billion eV. Thus, as illustrated at left, when a particle passes through our silicon detector, it knocks loose a bunch (thousands or tens of thousands) of electron-hole pairs and doesn’t stop at all. That’s exactly what we want, actually; this type of detector is for telling us where a particle went, not for absorbing it. (This is called tracking, and it’s a topic for another day.)

That’s step one. In step two, the electron-hole pairs are pulled in opposite directions by an electric field, and pulled into “contacts.” (Actually, specially-doped regions of silicon, if you’re curious.) In step three, the charge built up on those contacts produces a current that flows into our electronics—another topic for another day.

So far I’ve discussed silicon detectors in general; I could just as well be talking about the “silicon strip” detectors that are also used in ATLAS and CMS, for example. The key feature of a pixel detector is that the individual contacts are two-dimensional; for every 0.05 by 0.4 millimeter pixel, there’s a separate circuit and separate electronics. This gives us a very precise measurement of where, exactly, the particle passed through the detector.

A little piece of pixel detector

Of course, those pixels aren’t so very small—I’m pretty sure they’re actually larger than the ones in your camera. But they have to read out much faster than your camera does, since the LHC produces collisions forty million times a second. They also have to withstand the intense radiation found right next to the collisions, which can damage the silicon structure, for years and still work. It’s challenges like this that make pixel detectors such a complex and expensive job, but they’re vitally important for our physics program—but that, yet again, is a topic for another day.

Update (November 25): “Another day” has arrived, or at least one of them: How Tracking Works

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pictures of muons

Friday, July 25th, 2008

Below are some nice pictures the ATLAS detector took of muons created in the atmosphere that passed through Meyrin, Switzerland on July 21, 2008 (or technically “Event Displays from M8 Week”). The lines are the paths the muons took through the ATLAS detector, according to the reconstruction software. You can see the detector elements that actually recorded something are lit up along those paths. The reconstruction software “connects the dots”.

In each image are several views from different perspectives, or projections. The top projection in each image is the view down the beam axis, sort of what will be the “proton’s eye view” during collisions. The one below that is the view from alongside the detector.

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Let’s party

Thursday, July 24th, 2008

We are all getting anxious for particle collisions (see here and here).
I know the time must be getting near since I have recently received a few emails about parties to celebrate the start-up. There will be one for ATLAS October 4, and one for the LHC the evening of October 21. That one follows the VVIP CERN event earlier in the day.

But we still don’t know exactly when we will have beam in the LHC. The last update I saw said that there won’t be circulating beam in the LHC before September. Collisions are expected to be 1-2 months behind that. The LHC will shut down for the winter in December. So it is getting tight, but even a few weeks of collision data this year will keep us busy for a while as we can use the data to calibrate the detector, and find and fix problems.

The next big clue as to when we will see protons will be when the experiments are given the official 4-week notice. Then we will know exactly when beam will come to ATLAS. That will also trigger ATLAS to start staffing the control room 24 hours a day (overnight shifts, here we come!). Right now we are only taking shifts during the day and evening, mainly on the weekends.

Speaking of shifts, here is a picture of the online shift booking system:

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Italian BBQ

Wednesday, July 23rd, 2008

Tile is rather notorious for its frequent parties. And every time we usually have some theme. Sometimes the Brazilians will cook, or the Georgians, or the Lebanese, or the Americans. But much to our shock and dismay, we realized that we have never had an Italian BBQ.

Now. The word BBQ in itself sort of implies a laissez-faire, relaxed, let’s-all-gather-around-the-grill kind of an attitude. But not for the Italians. They take food, in any form, very seriously.

The BBQ was Tuesday night. They started cooking on Saturday. And they have been cooking since.

And the Italians make up quite a large fraction of Tile. So basically, no work was getting done for Tile this week.

For example, here is a typical conversation with any Tile Italian this week.

Me: Do you have some time to meet in order to discuss the upgrades to the monitoring we need for beam?
Tile Italian: Yes… But I am extremely busy until Tuesday. Can we meet after Tuesday?
Me: Sure no problem. What is keeping you so occupied?
Tile Italian: It is the Tile Italian BBQ and the Bolognese sauce needs my attention!

But as can be imagined, the food was incredible. We even had our own personal pizza chef (who actually works on the high-level trigger but we made him an honorary Tile person for the night).

Tile Italian BBQ

And as can be imagined as well, the Italians cooked WAY too much food. So we are now having ‘Italian Lunch’ today. And tomorrow. And Friday…

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Cosmic Variance pointed me to it, and now I can’t stop using Wordle to analyze everything I am working on.  Here’t the current version of this page, and the message seems to be loud and clear:

“ATLAS beam good LHC now point really ready still work”

or maybe

“beam now ready”

Sounds good to me, but one thing still mystifies me: where are the other experiments?

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A month ago, I remember seeing a lot of red on the LHC cool down status, but now it is all BLEU. Almost all sectors except for 7-8 & 8-1 are now close to the 1.9 K target and the two sectors are around 20 K. Very impressive progress, almost no big hiccups. So the beam is coming…

The first-first-first beam in the LHC is anticipated in the first week of August in sector 2-3!!! But, hardware commissioning is not finished in this sector, so clock is ticking faster for some than others… I mean people working underneath. Quite ambitious program for two days and first timers, btw the beam gets injected at point 2 and stopped by collimators before point 3. Don’t expects collisions yet, lot more hardware commissioning still to go. More details here if you are interested.

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