The Standard Model of particle physics has been developed over several decades to describe the properties and interactions of elementary particles. The model has been extended and modified with new information, but time and again, experiments have bolstered physicists’ confidence in it.
And yet, scientists know that the model is incomplete. It cannot predict the masses of certain particles, nor can it explain what most of the universe is made of. To discover what lies beyond the Standard Model, scientists are searching for its flaws—untenable assumptions and phenomena that it does not predict. A growing set of results from the study of bottom quarks may offer physicists a welcome chance to do just that.
“The Standard Model is very rigid,” says Marco Nardecchia, a theorist from Italy, “so the best way to break it is by precisely testing its predictions.”
The Standard Model makes many detailed predictions about how particles should interact or decay. Some subatomic processes are so complicated that even theorists aren’t quite sure exactly how they are supposed to work. For one: quarks—the constituents that make up elementary particles—should interact in the same way with the electron as with its heavier cousins, the muon or tau lepton.
There are six types of quarks. The lightest and most common are the up and down quarks, which together make up protons and neutrons. Particles carrying a bottom quark—which is much heavier—are short-lived. In its decay, the bottom quark transitions into a lighter quark, preferentially a charm quark and rarely an up quark, forming another known particle.
The remaining energy is carried by a charged lepton: an electron, a muon or a tau, each accompanied by its associated neutrino. According to the Standard Model, the rates of producing electrons, muons and taus differ only due to the very different masses of these three charged leptons. (The tau mass, for example, exceeds the electron mass by a factor of about 3500.)
“These predictions are straightforward and precise,” says Vera Lüth, a scientist on the BaBar experiment, “which is why we decided to pursue these measurements in the first place.”
Scientists working on three different experiments are testing these predictions by examining specific decays of particles that carry a bottom quark.
The first hint of an unexpected tau enhancement appeared in 2012 at the BaBar experiment at SLAC National Accelerator Laboratory, which studied close to 500 million events produced in electron-positron collisions, and reconstructed less than 2000 decays involving taus. In 2015, the Belle experiment in Japan reported a similar enhancement in the tau rate in data collected from electron-positron collisions at the same energy.
“A friend working on another experiment was sure that we had done something wrong,” Lüth says. “Then they observed the same effect.”
In 2015, scientists working on the LHCb experiment operating at CERN saw signs of the same phenomenon in very large samples of proton-proton collisions at much higher energy and collision rates.
“All these results point in the same direction,” says Hassan Jawahery, a professor at the University of Maryland working on LHCb. “That’s what puzzles everyone.”
On their own, these individual results have a significance below the level that would raise an eyebrow. But together, they are “intriguing,” according to Tom Browder, the spokesperson of the Belle experiment and its successor, Belle II. “We are pretty sure that something new is out there. Proving even a tiny deviation from the Standard Model could lead to a revolution in our field.”
The results accumulated so far have already inspired theorists to speculate about what kind of new physics processes might cause these enhancements.
Some theories suggest that perhaps there is a yet undiscovered charged Higgs boson that favors the heavy tau over the much lighter muon and electron. Other models predict the existence of at least one new particle outside the Standard Model. “We may need something which interacts with quarks and leptons simultaneously,” Nardecchia says.
Scientists won’t know what’s happening without further study, and gathering enough data to allow more detailed and precise studies will be a crucial step toward finding out.
Scientists at the LHCb experiment are only at the beginning of this study. They plan to analyze about four times as many events in the next few years. They hope to complete new and updated measurements by this summer. The LHC accelerator complex program foresees major upgrades that will enlarge the experiments’ datasets over the next decade. In parallel, Belle II is scheduled to start collecting data in 2019 and is expected to record enough to shed light on this query in a few years.
Physicists around the globe are eagerly waiting to compare notes.