Tuesday, March 30, 2010

LHC Produces a Dozen Mosquitoes

There is a lot of hoopla in the press today announcing that the LHC (large hadron collider) has broken all records for an atom-smasher at 7 TeV. There's good reason for that. Apart from a genuine energy threshold having been crossed by a particle accelerator, the LHC is also a year behind schedule, and since it cost the EU $10 billion equivalent, CERN is under a lot of pressure to demonstrate all kinds of pay-off, which inevitably leads to a stream of PR releases. What's amusing about this PR release is that most people don't even know what an electron-volt is, let alone what a hadron is (or as one unfortunate colleague of mine unwittingly entitled his slide: "Large Hardon Collisions"). So, as exhorted by atomic physicist Max Born's granddaughter: let's get physical.

Technically, one electron-volt (eV) is the energy gained by accelerating a single electron between the terminals of a 1 volt battery, assuming it doesn't collide with any air molecules along the way. Put differently, it takes about 10 eV to pull an electron off a hydrogen atom. Closer to what the LHC does, the CRT in a typical analog TV accelerates its beam of electrons to about 30,000 eV (KeV) using a high-tension transformer—the thing responsible for the hissing you hear when you turn the set on. By comparison, the LHC is capable of producing about 1 billion times that energy. Sounds impressive, but what does it correspond to on a more human scale.

For the bigger LHC experiments, there are typically two beams of protons (members of the Hadron family of nuclear particles) circulating in opposing directions to within 8 nines the speed of light in two separate ring-shaped tubes that are each about 20 miles long. At various locations around Geneva, they are brought together to produce the new physics (hopefully). Each beam comprises 2800 separate clouds of protons that are collimated like a "billiard cue" made out of 100 trillion protons. Each billiard cue of protons is about 10 cm long with a cross-sectional area of 4 or 5 protons. It's this exquisite squeezing of the beams that is really impressive. Each beam (i.e., sequence of billiard cues) is accelerated to 7 trillion eV prior to any collisions—the 7 TeV picked up by the press. Center of mass energy, where two opposing billiard cues collide, is twice that (14 TeV).

The total beam energy is on the order of 500 MegaJoules (MJ). 1 MJ is equivalent to the energy contained in a compact automobile traveling at 100 mph. So, 500 MJ would be something like an AmTrak train moving at 100 mph. As the beam energies are ramped up, that's certainly what the Swiss power grid around Geneva sees! On the other hand, 7 TeV per proton only constitutes about 1 microJoule (μJ), which has been likened to the energy of a dozen airborne mosquitoes.