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1 pb-1 June 29, 2006

Posted by gordonwatts in physics, science.

Physics, and Particle physics, in particular, is all about data. Well, the experiments are. Usually, the more the better.

Some disturbing rumors (pdf link) are floating around the LHC right now. Actually, Evans helps run the LHC, so this is more than rumors. But it isn’t official policy yet.

The current, official, plan is to do a short run sometime in 2007 at full energy (7 on 7 TeV) and collect a decent amount of data. 100 pb-1 are the numbers I’ve heard around. Well, this talk is a bit of a bummer in that respect: we are talking 450 GeV on 450 GeV. This is 0.9 TeV center of mass energy – exactly half what the Tevatron did in Run I – as opposed for the hoped for 14 TeV. Slide 37 in that talk delivers the harsh reality: First collisions in November 2007. There is no way they can get from first collisions to full energy in several short months. The new schedule claiming full energy collisions in early April I would consider suspect as well, given the complexity of the LHC.

The reasons for the delay are varied. As pointed out over on Cosmic Variance, the main problem is not enough of the magnets will be commissioned and certified to run at the currents required to bend a 7 TeV beam of protons. It would be foolish to push them further before you carefully test them. In a system this large and complex is pays in the long run to do it bit-by-bit no matter how frustrating.

Now, when I first heard this news I thought to myself “Ah, as expected.” I’ve been part of a very slow and difficult accelerator turn on: Tevatron at Run 2. I serious doubt that LHC will repeat that performance. On the other hand I do remember a fellow from the machine group, in direct response to a question from me, saying that it would be a matter of days between magnets installed individually tested to a full turn of the beam, and another week or so before collisions. The truth is somewhere in between. No biggie.

But then in an ATLAS meeting on Monday morning people were talking about accumulating data of 1 pb-1. To put this in perspective I went and found a paper from the first engineering run of the Tevatron. At the time DZERO wasn’t running or built – but CDF was up. They collected 25.3 nb-1 of data. One pb-1 of data has 1000 nb-1 of data. And they were able to observe the W. Indeed, they had a total of 400,000 events collected (not much given today’s Tevatron performance — we collect that in several hours). About 22 of them are W boson candidate events. Though the LHC detectors aren’t tuned to run at 1.8 TeV, with x 40 times more data we should be able to see something as well (and the detectors are better than CDF’s — as long as they are fully operational!!). But there won’t be any new physics out of the LHC. That will have to wait for the 14 TeV run. I’m betting close to the end of 2008 before that happens in earnest. But hey, we’ll see! Maybe they will be earlier. That would be fantastic!

BTW, that talk has some awesome pictures of the LHC accelerator under construction. If you are a fan of that sort of thing you should definitely check out the talk.



1. Alejandro Rivero - June 29, 2006

On the other hand I do remember a fellow from the machine group…

Yep, I had already heard of similar reports from the people at the machine, even if not directly. It seems they are in the optimistic side.

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