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Scientific Method / Science & Exploration

New Large Hadron Collider data may thin out theories in particle physics

Exceedingly rare decay of B mesons shows up largely as expected.

Although the Large Hadron Collider is often viewed as a Higgs discovery machine—a task for which it turned out to be admirably suited—the collider isn't a one-trick pony. Its general purpose detectors, ATLAS and CMS, should be able to spot any other unusual particles out there, while the ALICE detector is specialized for heavy ion collisions. But this week, attention fell on LHCb, the Large Hadron Collider beauty experiment.

Beauty is the alternate name for the bottom quark, which was discovered back in the 1980s and is the second heaviest of the quarks. Bottom quarks are often found in particles, called B mesons, in which they're paired with another quark (or an antimatter equivalent). LHCb is designed specifically to track how these B mesons decay, since their pattern of decays provides a sensitive test of the Standard Model of particle physics. Now, the LHCb team has announced they've spotted a number of rare decays—not one-in-a-billion, but close—and they've found that the rate at which decays occur agrees remarkably well with that predicted by the Standard Model. This in turn puts some limits on alternative theories.

With bottom quarks being 30 years old, you might think there would be little left to learn from them. But the fact is that they were heavy enough that they weren't produced in vast numbers by earlier particle colliders. That means that very rare events involving a bottom quark either weren't detected at all or were detected in such small numbers that it was impossible to say anything about these events with any statistical certainty.

The LHC had the potential to change that. For starters, it operates at the highest energy yet, meaning that bottom quarks will be produced in large numbers. But it also has the highest luminosity (particle collisions per time) of anything we've yet built. In this year alone, the LHC has generated as much data as the Tevatron did in its entire history—twice. In comparison to what's come before, the LHC is a bottom quark factory. And since quarks rarely exist without partners, many of these will immediately pair up to form B mesons.

The LHCb detector is designed to make sure this factory is productive. It only samples collisions where B mesons are produced (if you look at the LHC status page while it's in operation, you'll see these represent only a fraction of the collisions tracked by CMS and ATLAS). Once spotted, the detector watches how they decay, tracing the particles they produce and the energies involved.

The newly described data comes from tracking cases where a specific type of B meson (the Bs) decays into two muons, or negatively charged particles that are a bit like heavier versions of the electron. This decay pathway is so rare that the Standard Model predicts that it should occur only 3.5 times for every billion B meson decays. The rate measured at LHCb comes out to be 3.23 times per every billion. Within experimental errors, the two values match. That means that, at least as far as we can measure, the Standard Model nails this prediction.

That's not a bad thing to know, in that further support of an existing model is a good thing. But it's also very useful because lots of models of physics beyond the Standard Model predict subtle differences that will cause deviations from this rate. That means that we can use these results to eliminate some of these models from consideration. This includes a number of forms of supersymmetry, one of the more popular alternatives to the Standard Model. But it can also put limits on less popular ones as well.

That doesn't mean supersymmetry is ready for the trash heap. But these results, combined with the inability of the other detectors to find new particles beyond the Higgs, do seem to take some of the simpler (and therefore more appealing) versions of supersymmetry off the table. If evidence later turns up for the more complicated versions, then it just means that nature sees no reason to respect the taste of physicists. (For much more on this topic, physicist Matt Strassler responded to some problematic press coverage of the results.)

In further good news for the Standard Model, the presentations from other detector teams at the meeting are indicating that the Higgs looks like the Standard Model predicts it should. Again, the more information we have saying the Standard Model is right, the less space there is for alternative theories to maneuver.

It appears that most of the new analyses were done with data gathered before a cutoff in the summer. The LHC has been running continuously since, and has more than doubled the amount of data already, so there will be a lot more data for people to sift through during the machine's long shutdown for upgrades (scheduled to start before the winter is over).

Meanwhile, this general approach—looking at large populations of particles we already know about for hints of new physics—is likely to become ever more common as it gets harder and harder to build a machine that can reach new energies. In fact, with the shutdown of the Tevatron, Fermilab's entire plan for its future depends on it.

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