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A 'New Physics' Postcard from the LHC's Earliest Days

Newly crunched results from 2011 and 2012 point to bizarre, un-Standard Model behavior among B mesons.

by Michael Byrne
Sep 5 2015, 1:00pm

Image: Guillaume Jeanneret/CERN

The latest source of intrigue from the Large Hadron Collider traces all the way back to 2011/2012 and the experiment's opening salvos of accelerated protons. According to results posted to the arXiv pre-print server and slated for publication this month in the Physical Review Letters, data collected at the collider reveal unusual distributions among the decay products of bizarre particles known as B Mesons. While the effect is small enough to be a statistical fluke, a hint of New Physics remains.

The observation rates a mere 2.1 sigma, while a full-on discovery rates a 5 sigma. That's pretty weak on its own, but the LHC observation is supported by similar finds elsewhere, including at the "BaBar" experiment at the SLAC National Accelerator Laboratory in California and the the "Belle" experiment at Japan's High Energy Accelerator Research Organization in Tsukuba.

"A 2-sigma difference in a single measurement isn't interesting by itself," Tara Shears, a particle physicist at the University of Liverpool and a member of the LHCb collaboration, tells Nature News. "But a series of 2-sigma differences, found in different types of decay and independently by different people in a different experiment, become very intriguing indeed."

Image: Nature

So, what are we even talking about? New physics?

B mesons are naturally short-lived particles of a sort usually only produced in cosmic ray collisions (and particle collision experiments). Mesons are composed of a quark and an antiquark, making them stunted relatives of more familiar three-quark particles like neutrons and protons. This class of particle is extremely unstable and, depending on a given meson's charge, it will decay into variously electrons, photons, and, in the case of B mesons, tau particles and muons, which are similar to electrons. Those last two should be produced in equal numbers, per the Standard Model of Physics, the helpful but incomplete map of fundamental particles and the relationships between them.

It would seem that they're produced with a particular bias, however, which is not predicted by the Standard Model. And this is where we start thinking about New Physics. "Both biases could potentially be explained, for example, by positing another kind of Higgs boson, which possesses charge and interacts differently with the various particles involved in the decays," Nature offers.

We'll have to wait a while for any sort of confirmation, however. These results are from the earliest days of the LHC and results from its current run, which began this past June, are likely more than a year away.

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