Life is getting very exciting again at CERN. In conversations about the future we are now using the words “days” and “weeks” instead of “months” and “years”. Now that beam is back in the LHC, we are eagerly awaiting the first collisions. The ATLAS detector has been operating 24 hours/7 days a week for a while now, and is ready. The next milestone will be proton beams going all the way around the LHC ring. Then the big milestone will be the first collisions of protons in the LHC.
While obviously we want collisions, the question is what can we do with the small amount of data we expect to collect this year? We will not make any big discoveries in 2009. We mainly want to establish that the detector is working well and to begin doing some basic checks. This starts with timing. All of the different parts of the detector (that work pretty much independently of one another) have to be synchronized with each other down to the nanosecond level, something that is very hard to do without an absolute reference. One really nice reference is a particle traveling through several different subdetectors.
The next thing we can do is find the first “jets” (sprays of particles formed from quarks and gluons), electrons, muons, and so on, the basic particles that our detector is built to detect. After that we can start to calibrate the detector. This is mainly done by comparing measurements of particles from one part of the detector to measurements of similar particles from other parts of the detector, and also by checking that we measure basic things like the masses of well-known particles to be the same as measured many times by previous experiments.
After being so close last fall, it has been a long wait to get back to this point. When the explosion happened last September, few people would have guessed it would take this long to get back to this point. But the excitement of last fall has returned. The line to get through the gates into CERN was a little longer this morning, parking has been much tougher to find the last couple of weeks, and (of course) the number of meetings is increasing. After all the years of preparation, we are finally at the start of a wonderful new era of discovery.