Sunday, July 20, 2008

Argonne Scientist to Become ATLAS Physics Coordinator for CERN - Part 1

ATLAS is one of the detector already ready to go at the LHC. Tom LeCompte will be starting as deputy physics coordinator for ATLAS before moving up to the coordinator position the following year.

The ATLAS Collaboration Board recently approved the appointment of LeCompte as deputy physics coordinator of the ATLAS experiment. He will be deputy starting Oct. 1 for one year and then become physics coordinator on Oct 1, 2009.

I was privileged to be able to ask Tom a few questions not only about his new job and move to CERN, but also on the physics issue. So here is Part 1 of my Q&A with him.

1. What will be your primary function as the science coordinator of ATLAS?

Tom: I start as Deputy Physics Coordinator, under the Physics Coordinator who is presently Professor Dave Charlton of the University of Birmingham. So for a year, it's easy - I do whatever he tells me to do. The following year, I become the Physics Coordinator. Ultimately, we're responsible for the physics results in the experiment - they obviously have to be correct, but they also have to be timely.

There's a certain amount of coordination out there. Suppose you have one group that is measuring events of a particular type - call it A. They pick a reasonable way to select their events. Now suppose there is another group measuring B, and they pick a different reasonable way. Now C comes along, and this group realizes that A and B are backgrounds to the events they are interested in. But they can't simply combine them, because they were selected differently. So they have to repeat the work of at least one of the two groups. Commonality across different analyses can save time, and reduces the chance of making a mistake, so we want to encourage this. On the other hand, we don't want to squelch individual creativity, so there's a balance we have to strike.

There are also cases where we have to prioritize. After we will have run for a while and gained some experience, our reconstruction algorithms will naturally improve. We will want to rerun them on the data we have already taken, a process that takes a couple of months. The experiment will have to decide which events to process first (not everybody can be first) and physics coordination will have to forge this consensus.

As you might imagine, getting 2000 people to work together on a scientific experiment without accidentally getting in each other's way is not something that happens automatically.

2. Besides the search for the Higgs (or many Higgs) boson, what other physics can be studied with ATLAS?

Tom: I don't really like calling it "the Higgs search", because it presupposes the answer. The problem is understanding what we call "electroweak symmetry breaking", which is a fancy name for "why are the W and Z bosons so heavy when the photon is so light", or alternatively, "why is the weak force so weak and so short range, while the electromagnetic force is long range and powerful." One possible answer is that there are one or more Higgs bosons. Another is that the W's and Z's interact very strongly at short distances and form resonances and these resonances break the symmetry. A third possibility is that the top quark, much heavier than the other five quarks, places a special role in this. There are many variations on these basic ideas, and as experimenters our job will be to gather the evidence that lets us distinguish between the various possibilities.

So, here's an incomplete list of some of the things we will have to do - besides figuring out what's up with electroweak symmetry breaking: We will need to understand the structure of the proton, because we collide beams of protons and we need to understand our initial state. We will need to make precision measurements of quantities well predicted by today's theories, to look for inconsistencies: these inconsistencies will point us in the direction of the new physics. We will need to search for new particles and new phenomena, and because the LHC has more energy than any previous accelerator, it's a likely place to look. We will need to better study the properties of the top quark, because we will have thousands of times more of them than the existing experiments. Finally, for about one month out of the year, instead of beams of protons we will collide beams of atomic nuclei, and that will let us study the properties of hot, dense nuclear matter, which behaves very differently than ordinary nuclei.

You can see that 2000 people sounds like a lot, but there are many things we will have to measure - by the time you are done dividing things up, it's not as many as you would think.

Stay tune for Part 2.


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