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A Similar Anomaly Has Showed up in Three Physics Experiments Since 2009. What’s Happening?

A small but persistent discrepancy between the rates at which particles called B mesons decay into different pairs of leptons has physicists intrigued.

A typical LHCb event fully reconstructed during data taking on May 9, 2016. Caption and credit: CERN

The Large Hadron Collider accelerates billions of protons to very nearly the speed of light and then smashes them together head-on. These high-energy collisions happen at four specific points around the LHC’s ring – each straddled by one detector. One of these detectors is the LHCb. The ‘b’ in its name stands for ‘b-physics’, a branch of particle physics that studies larger particles that contain a smaller particle called the bottom quark.

In new results discussed at a seminar at CERN last week, physicists noted a discrepancy between the behaviour of one of the larger ‘b’ particles as observed by the LHCb and the theoretical prediction. They don’t exactly know why the difference exists but they have some ideas. The bigger question is if it could be a sign of the ‘new physics’ that physicists have been looking for.

They’ve also conceded that the results extracted from the LHCb data aren’t statistically significant enough. In other words, because the events being studied happen quite rarely, the data sample has to be big enough to comfortably eliminate the possibility of statistical fluctuations. This can be eliminated to a greater extent by collecting even more data. So why pay attention to it now?

There is a good reason: the discrepancy has been observed before in three different experiments across eight years.

“Although only mildly significant on their own, together the anomalies in the B-decays seem to be telling us that this is more than a statistical effect and that there is something in them that we do not understand,” Siim Tolk, a particle physicist at Cambridge University, UK, told The Wire.

The larger ‘b’ particle in question is a B meson. It contains one antimatter version of the bottom quark and one matter version of some other quark (there are five other types to pick from). When protons collide in the billions under the watchful gaze of the LHCb, the ‘up’ and ‘down’ quarks that they’re made of come loose and interact with other quarks produced in the collisions. In this chaotic splatter, a B meson is formed. And a millionth of a millionth of a second later, it has about a one-in-a-million chance of decaying into particles called kaons.

Physicists have their eye on this decay process. It is mediated by a phenomenon called flavour-changing neutral currents (FCNC).

The Standard Model, a clutch of theories and rules that physicists currently use to understand how particles behave, has room for FCNCs. While some kinds of FCNCs have been found, there are rarer others that continue to evade experimental verification. So efforts to make sense of them have often led scientists to create new theories and rules that haven’t been confirmed yet. This…

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