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Anomalies may be regarded with skepticism, but they often open the door for theorists to play. One of the most promising sandboxes for model builders has been anomalies in B physicsinteractions involving B mesons, which are particles composed of a bottom quark or antiquark plus another type of quark. A coterie of results from LHCb at CERN, Belle in Japan, and Babar in the US, point to potential problems with the standard model predictions for some rare B meson decays.

Alone, each notable B physics result is only a few-sigma discrepancy. But taken together, the aggregate of the results isdepending on whom you aska 5- to 7-sigma deviation from the standard model estimates. Ive worked in the field for a long time, says Isidori. Weve seen a lot of anomalies here and there popping up and going back, but this time I think its different . For the first time, its not just one thing that doesnt fit with the other, but its a coherent set of things.

If the anomalies are a hint of something real, the simplest explanation is a new particle called the Z, a partner to the Z boson that differs only slightly in its interactions with other particles (see Synopsis: Closing in on the Z' Boson). Isidori is not a big fan of the Z; he prefers a leptoquark. This hypothetical particle would form a bridge between leptons (electrons, muons, and taus) and quarks (see Viewpoint: A Challenge to Lepton Universality).

Many theorists attempt to link anomalies together in models. For example, a new anomaly from KOTO, an experiment at JPARC in Japan, measuring the lifetime of neutral kaons, has piqued theorists attention. Jia Liu, a theoretical physicist at the University of Chicago, wrote a paper that proposed a light, Higgs-like particle, or scalar boson, that would interact with muons and would explain both the KOTO anomaly and the muon anomaly. While theorists like finding one explanation for multiple anomalies, its often difficult to match all the data. Attempts to find a combined explanation for both the B physics and muon anomalies have mostly fallen flat. Two anomalies to deal with is my limit, because it is not easy, Liu says jokingly.

The best models, according to theorists, are those that fit the data naturally, without too much finagling. Neutrinos have been the focus of several recent anomalies, such as unexpected oscillations in the flavors of neutrinos observed by MiniBooNE at Fermilab in 2018 (see Viewpoint: The Plot Thickens for a Fourth Neutrino). To explain neutrino anomalies, the most straightforward thing to do is to introduce one new neutrino says Mona Dentler, a neutrino physicist at the University of Gttingen, Germany. The trouble is that this addition, called a sterile neutrino, is a possible dark matter candidate, which means it must agree with cosmological data. Constraints like this can require highly tailored solutions from theorists. You normally have to kind of stand on your head and add a bunch of different epicycles to somehow make the data fit your models, says Patrick Meade, a theorist at Stony Brook University, New York.

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The Era of Anomalies - Physics