Monday, September 14, 2015

Surprises from the LHC's "Beauty Factory"

In an attempt to unravel how matter and antimatter differ—and why we seem to have more of one than the other in our universe—scientists at the Large Hadron Collider have been studying the production and decay of particles called B mesons. Baryons, (from the greek barus, meaning heavy) such as protons and neutrons, each contain three valence quarks, but mesons (as in meso, or middle) are two-quark systems—one quark and one antiquark. They’re much less stable, contain equal amounts matter and antimatter, and tend to decay quickly into other particles, so they’re a promising tool for trying to ferret out the decay asymmetries that might have led to the state of the universe as we know it today. B mesons in particular are so-named because they contain a “bottom” antiquark, also known as a “beauty” antiquark, leading to the LHCb experiment’s name. However, as so often happens, the result the LHCb researchers found was not the one they were looking for.

When particles like B mesons decay, there's a whole host of potential products, depending on the particle’s energy and constituents. Oftentimes, these decay reactions produce leptons (from lept meaning thin or fine). Leptons, such as electrons and neutrinos, contain no quarks; as far as we understand it, they are elementary particles.

There are two types of charged lepton besides the electron, though: muons and taus. All three have the same charge, but muons are much heavier than electrons, and as a result they’re less stable. Taus are heavier still, with a correspondingly short lifetime. Regardless of this mass difference, the standard model of particle physics suggests that B mesons' decay should yield electrons, muons, and taus with equal frequency, a principle called "lepton universality".

The LHCb experiment works by creating huge numbers of B/anti-B pairs and observing the decay products of their constituents. If the results yielded more stable particles than antiparticles, even by a slight fraction, it would practically be the “holy grail” of modern particle physics; evidence of such a decay asymmetry could explain the matter/antimatter imbalance that remains one of the largest open questions in the standard model.

While no such tendency has yet turned up, researchers at LHCb noticed an unusual number of tau particles coming from their reactions. Rather than yielding equal numbers of electrons, muons, and taus, the high-energy proton collisions they conducted yielded 20-30% more taus than expected.

Similar studies conducted around the world—namely the “Babar” and “Belle” experiments—have come out with similar results, making this anomaly look less like a statistical fluke and more like a signpost pointing toward new physics. The combined data from these three experiments don’t yet qualify as a “discovery” by the physics community’s rigorous standards, but the chances are slim (less than 0.3%) that the result is due to random experimental variation.

So what could this mean? If it’s true that tau leptons are produced more readily than their lighter counterparts, it could tell us something about how the heavy leptons form, possibly indicating the existence of a new particle which mediates B mesons’ decay. It’ll be a while before there’s enough data to understand this result’s true significance, but as of right now it’s another exciting clue that our current picture of the universe isn’t yet complete.

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