The Sciences

Highest energy ever

Cosmic VarianceBy Daniel HolzMar 19, 2010 1:10 AM


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At this very moment the LHC is busy trying to set a new world record. The goal is to achieve beams circulating at 3.5 TeV, bringing collisions between protons to 3.5+3.5=7 TeV center-of-mass energy. This would be the highest particle energy ever accomplished by humans (nature somehow routinely manages to produce cosmic rays at energies 8 orders of magnitude higher!). This news is hot off the press: we had a talk today by Lyn Evans, and he gave us the latest update. He should know what's going on, since he's project leader of the LHC. Evans shared some entertaining anecdotes from the last few years of commissioning, including:

They use superfluid helium to cool the superconducting magnets. One of the many weird properties of this stuff is that it has zero viscosity. Which means that, if there's any sort of hairline fracture anywhere in the 27 kilometer long tunnel, the stuff comes spewing out, and very, very bad things happen. Every component, every joint, every one of the tens of thousands of tiny connections has to be perfect. It is this sort of failure which brought the machine to its knees shortly after commissioning, over a year ago. The magnets are kept very, very cold; the superfluid helium is at 1.9 Kelvin (-271 Celsius), or a couple of degrees above absolute zero. We're not talking a little vial in a laboratory being kept at this temperature. We're talking many thousands of tonnes of magnets, kept just above absolute zero (using 96 tonnes of liquid helium). As things cool down, they naturally contract. The decks on bridges do the same thing, hence those serrated grills at the ends of bridges to absorb the expansion and contraction due to weather (if you've ever motorcycled across a bridge, you know exactly what I'm talking about). There are equivalent serrated joints in the LHC beam pipe to ensure that it doesn't contract and rip open upon cooling (which, needless to say, would be bad). But upon reheating a section of the LHC, it turned out some of these devices left little fibers in the beam tube. Not good. How to find them, without ripping open the entire collider (costing millions of dollars and setting the project back precious months)? They ended up blowing a ping pong ball (with electronics embedded) down the tube, and tracking where it would get stuck. A simple, elegant, cheap solution to fix a multi-billion dollar enterprise. For a while during the construction they ended up with roughly a billion dollars worth of superconducting magnets being stored in a parking lot at CERN. For reference, this is comparable to the entire GDP of many small countries (Bhutan, Guyana, Burundi, etc.), sitting out in the rain and snow. Big science. Hopefully sometime in the next few days they'll be running at 3.5 TeV. Apparently it's been slow going because the system to prevent catastrophic quenching of the magnets (which is what "broke" the machine previously) is on a hair-trigger, setting off all sorts of false alarms (and when it goes off it quenches the magnets [in a controlled manner]). You can keep track of the progress on the LHC webpage (clicking on the image of the ring gives real-time data on the temperature of the magnets). Although this would be the highest energy ever achieved, it still doesn't significantly surpass the science reach of Fermilab's Tevatron, since the latter has run for many years (albeit at a lower energy of 1 TeV+1TeV). Both energy and (integrated) luminosity matter in this game, and the Tevatron has gotten more than 8 inverse fb (femtobarns; one of the best units in all of science [think "there's no way to miss it, it's as big as a barn"]). The LHC is shooting for 1 inverse fb. All being well, in a few months they'll bump the energy up to 5 Tev on 5 TeV. This should significantly open up the scientific discovery space, and could conceivably kick off the next revolution in particle physics. Exciting times!

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