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Particle physics

Startling deviations

München, 06/08/2017

The Standard Model of particle physics has passed many stringent tests. Nevertheless, all is not well – and a new and striking discrepancy with the model has been discovered. Its confirmation would imply the existence of new physics.

Belle detector at the High Energy Accelerator Research Organisation (KEK). Image: KEK, Japan

The Standard Model of particle physics provides a highly precise description of all the properties of elementary particles that have so far been observed and quantified by experiment. The history of the theory goes back to the early 1960s, and in 2012 the last of its predicted components – the famous Higgs boson – was finally detected.

Notwithstanding its many successes, there are several phenomena for which the Standard Model provides no satisfactory explanations. One of them is the riddle posed by the dominance of matter over antimatter in the Universe: Why did the matter and antimatter created by the Big Bang not annihilate each other entirely? Instead, a small fraction of the matter produced in the Big Bang survived to provide the building blocks of the visible Universe. This conundrum and other enigmas continue to motivate new hypotheses and new theories. One of these is the idea that the Standard Model only accounts for part of the whole picture, and must be embedded in a more comprehensive theory – one based on the concept of supersymmetry, for instance.

In the quest for hints that might point toward the viability of such an all-embracing theory, experimental physicists are systematically probing the limits of the established model. One of its basic assumptions relates to the interactions of a class of elementary particles known as charged leptons, which comprises the electron, the muon and the tau particle, which differ from each other only in their masses.

Particles called B mesons offer an especially attractive way to test the validity of this assumption. B mesons are heavy quark-antiquark pairs (quarks are the building blocks of protons and neutrons), which can only be created in large-scale particle accelerators. They have a mass equivalent to that of five-and-a-half protons and are extremely unstable, decaying into lighter particles within a billionth of a second. Moreover, there are literally hundreds of alternative ways in which this decay can occur. The Standard Model, however, provides extremely precise predictions of the relative probability of any given decay pathway.

An international research consortium, which includes LMU’s Professor Thomas Kuhr, has now demonstrated that the tau particle decays via a specific pathway significantly more often than is predicted by the Standard Model. The effect has been seen in the datasets obtained by the BELLE collaboration at the KEKB accelerator in Tsukuba in Japan and in the BABAR experiment at the Stanford Linear Accelerator(SLAC) in California – both of which have now concluded –as well as in the ongoing LHCb experiment at CERN’s Large Hadron Collider am CERN in Geneva. The observations are reported in the latest issue of the journal Nature.

“The magnitude of the increment is absolutely astounding, in particular when one considers that it has been observed in three completely independent experiments,” says Thomas Kuhr, who is also a member of the Universe Excellence Cluster in Munich. One of the simplest explanations for the finding postulates the existence of a previously unknown particle –similar to the W- boson that carries the so-called weak nuclear force, but with a larger mass. Another possible solution assumes that there is an electrically charged version of the Higgs particle. “But before we can postulate the existence of new interactions or particles, we need to improve the level of statistical significance of the effect,” Kuhr cautions.

Further data from the LHCb experiment, together with results expected from the upcoming BELLE II experiment at the Super KEKB Accelerator in Japan, which will come online in 2018, should provide the necessary clarity in this respect. Naturally, the scientists involved very much hope that the new datasets will confirm the preliminary result, and provide further clues that point to new physics beyond the purview of the Standard Model.
Nature 2017