A theory-defying anomaly has persisted in the latest results from a Large Hadron Collider experiment, according to new results.
The world’s largest particle accelerator, the Large Hadron Collider in Geneva, Switzerland, contains a host of experiments that seek to answer the unanswered questions about the nature of the universe. Mostly, these experiments have ruled out theories describing various exotic particles to explain dark matter. But one of the experiments, called LHCb, has discovered a small deviation between what they’ve measured and what’s predicted by the core theory of particle physics, called the Standard Model. After three years of data analysis, the discrepancy remains—a potential sign of new physics.
Particle accelerators hunt for new particles essentially by using the E=mc2 equation (which essentially that says energy and mass are equivalent): They accelerate particles to nearly the speed of light and smash them together inside of detectors, where the released energy turns into particles not often seen on Earth. This is how physicists discovered the Higgs boson, for example. But as this direct production method fails to yield new particles, other experiments are looking for hints of new physics indirectly—such as by observing how particles decay into other particles.
Among the most intensely studied decays is the rare B0→ K*0µ+µ− decay, or, put simply, a B meson decaying to a kaon and two muons. For a little background: Atoms are made from electrons, protons, and neutrons; protons and neutrons are made from quarks. There are six kinds of quarks (each of which has an antiparticle, which is basically the same particle with the opposite charge). The six quarks are called up, down, strange, charm, top, and bottom. The B0 particle contains a down quark and anti-bottom quark. After the LHC creates these B0 particles, they decay. Physicists are most interested in the rare event when it decays to a K*0, consisting of a down quark and anti-strange quark (which further decays), plus two muons (muons are like a heavier cousin of the electron).
What’s so exciting about the decay? In some aspects of the decay, what physicists actually measure differs slightly from their expectations (one of which we wrote about here). These differences haven’t passed the so-called five-sigma test yet; the physics community has agreed upon a five standard deviation difference between experiment and theory as denoting a true discovery. Basically, think of each of the billions of collisions per second that happen in the LHC as its own experiment. Some of those collisions will produce B0 particles, and some of those B0 particles will decay in the specific way that physicists want to study. Physicists need to run the experiment many, many times to build up enough statistics to tell whether what they observe agrees with theory or disagrees with it.
This week, LHCb physicists announced that one such discrepancy between theory and experiment has persisted with more data. It doesn’t move us closer to the announcement discovery, because it doesn’t raise the statistical significance or move us closer to five standard deviations. But at least it provides a consistency check, given that more data has made other LHC discrepancies disappear.
For this analysis, the tension focuses on the combination of angles that the particles travel after the B0 decays. While LHCb physicist Patrick Koppenburg gets into the specifics here, basically, these angles represent where the resulting particles go during the decay. Physicists can use these angles to calculate asymmetries, such as between the two muons moving forward and backward. The muon asymmetry mostly agrees with the Standard Model, but for one asymmetry calculated based on a combination of the remaining angles in the decay system, the Standard Model predicts a value different from what the experimentalists have measured.
As for what could cause the discrepancy, that’s still unclear. Perhaps unknown particles are the culprit. But physicists haven’t ruled out more mundane explanations, like interactions between quarks that might be exhibiting their own effect.
Still, these kinds of tensions can be the makings of exciting new physics stories. Koppenburg told Gizmodo that they’re looking to incorporate data taken in 2017 and 2018 into the analysis. “The more data there are, the more care is needed,” he said.