Thanks to more measurements, researchers from the LHCb experiment at CERN are becoming increasingly confident that they are seeing a real deviation from the Standard Model. “The hint is now a clue.”
Last week, researchers from LHCb discussed the decay of B0 mesons, particles consisting of a beauty quark and a down quark, at the CERN seminar in Geneva. B0 particles are created in collisions between protons in CERN’s LHC accelerator. Immediately afterwards, they can decay into a kaon, plus a muon and its antiparticle, among other things.
Among theorists, this decay is seen as an opportunity to accurately test predictions of the Standard Model. The angle at which the particles fly away is particularly important in this regard.
Shadow ahead
Interestingly, the process is thought to be sensitive to deviations from the Standard Model at collision energies beyond the range of the LHC accelerator. Such deviations can sometimes cast their shadow ahead at lower energies. This seems to be the case with B0 decay.
Nikhef is one of the partners in the LHCb collaboration, with important contributions to the designs and hardware, as well as the software and analysis of measurements.
LHCb was built specifically to track particles and their antiparticles. Previously, LHCb had already found tentative indications of differences from the predicted values that follow from the Standard Model of particle physics.
Statistically stronger
With new data from the period 2016-2018, these indications now appear to be statistically stronger. The previous result was about 3.3 standard deviations from the expected value. Physicists call this a hint, but want more certainty.
That certainty is now growing steadily. The new result is 4 standard deviations from the predicted value, something physicists call an “indication” of a real deviation. At five sigma or more, it is considered a discovery.
The LHCb collaboration is still extremely cautious about the result. The predictions are complex calculations, and a detail may have been missed. Deviations below 5 sigma can still be a random outlier on paper and can be averaged out again with more measurements.
Unknown particles
This happened a few years ago with another result from B0 decay measurements, which initially seemed to show a difference between electrons and muons, a heavier version of the electron. That result went away with more data coming in. This seemed to mark the end of the search at LHCb for deviations from the Standard Model via B0 mesons, partly because, at the same time, no further certainty was found in B0 muon decay despite more measurement data.
Now, however, that growing confidence is there. An important step in the right direction, says Nikhef-LHCb physicist Niels Tuning. “This is the strongest result to date for an indication of new physics beyond the Standard Model.”
What that new physics looks like is less clear. There are numerous theoretical variations on the Standard Model, which is known to be incomplete. New, as yet unknown particles are one option. Tuning: “Of course, we have to be cautious. But if this turns out to be a solid result further down the line, we can probably rule out a number of ideas again.”
