The Sciences

The Protons Are Back in Town

Cosmic VarianceBy Sean CarrollApr 2, 2012 11:43 AM


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Zooming around the LHC, colliding at unprecedentedly high energies: 8 trillion electron volts total, in comparison with last year’s 7 TeV. The ultimate goal is to reach an amazing 14 TeV, although that won’t happen soon — the plan is to shut down for quite a while after the end of this year’s run, tighten the gaskets and so forth, and then resume the march to higher and higher energies.

This year’s run is all about luminosity, i.e. getting as many collisions in the can as they can. Last year they reached about 5 inverse femtobarns, while this year they’re shooting for 15 inverse femtobarns. Yes, those are the goofiest units in all of physics. Think of it this way: imagine the protons entering a detector are shooting at a tiny target with some fixed size, measured in units of area. Then we can measure the luminosity by counting the number of protons passing through that area in a fixed moment of time: i.e., the number of protons per square centimeter per second. That’s at any one moment; if we integrate up over the course of a year, the “per second” disappears and leaves us with the total number of protons that have passed through the target area, i.e. a certain number of protons per square centimeter. But that number would be enormously huge, so rather than using square centimeters, particle physicists like to use “barns,” defined as 10-24 cm2. (Broad side of a barn, get it?) But even measuring the luminosity in inverse barns would be really big, so they go for inverse femtobarns (1 fb = 10-39 cm2). Long story short: 10 inverse femtobarns is equivalent to 1040 protons passing through a 1 cm target area. (That’s much larger than the number of collisions — to get the number of collisions for any particular process, you need to multiply by the cross-section for that process, which is often quite tiny. That’s why particle physics is hard! Still, there will be a buttload of collisions.)

Anyway, I’m pretty sure the LHC is back to colliding protons after this year’s winter shutdown, and they’re smashing together at 8 TeV. But to be honest my only hard evidence is from Twitter, where the ATLAS collaboration has tweeted this image.

Meanwhile, results are still coming out from last year's run. Sadly, they're doing a great job at constraining possible new physics, but no convincing discoveries as yet. Here's a recent result from LHCb, the experiment that looks at decays of mesons containing b quarks. This plot is from David Straub, from a talk at Moriond, based on this paper.

Horizontal axis is the fraction of time (the branching ratio) bottom/strange mesons decay into two muons, while the vertical axis is the fraction of time bottom/down mesons do the same thing. These numbers have specific predictions within the good old Standard Model, but it's very easy for new physics such as supersymmetry to enhance the numbers quite a bit. LHCb has put an upper limit on both quantities, which rules out all the gray area of the plot, leaving only the colorful part at the bottom left. The colors correspond to possible predictions in different versions of supersymmetry. As you see, it would have been very easy to have detected a substantial deviation from the Standard Model by now, but no such luck. This doesn't mean some other version of supersymmetry isn't right, just that we'll have to try harder. No question that a proper update of our likelihood functions will have to decrease the chance that we expect fo find SUSY at the LHC compared to what we would have thought a few years ago, however. This is why the march to higher energies will be so important.

If you want to ask some detailed questions about the accelerator and the experiment, the CMS and ATLAS collaborations are having a Google+ hangout this Wednesday that you are welcome to join. It starts at 7 am Los Angeles time, so I'm unlikely to make it, but let us know if anyone here participates.

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