A team of scientists working with the Large Hadron Collider particle-smasher near Geneva may have found signs of an as yet unexplored realm of physics, one that all particle physicists are looking for. In an experiment whose results are to be published in the journal Physical Review Letters in September, the scientists have described a process whose outcomes show a slight but important deviation from values predicted by the extant paradigm describing the behaviour of fundamental particles, the Standard Model.
While the signals are not strong enough for the scientists to be able to claim their evidence, it’s the third time they’ve been spotted. The paper notes that the previous two observations came at the Belle and BaBar experiments (first in 2007 and 2008, resp.), which use different kinds of particle collisions. So, observing the same kind of signal in three different experiments is enough to warrant attention, even if they’re not robust enough to have the physicists celebrating right away.
The process concerns the decay of a fundamental particle called the B meson into lighter particles, including two leptons – the tau and the muon. Leptons are fundamental particles whose most famous kind is the electron. However, unlike the electron, the tau and the muon are much less stable and decay in less than a second into smaller packets of energy. The decay of B mesons produces taus and muons equally (after adjusting for their different masses). The Standard Model calls this lepton universality, a rule that all leptons must be treated equally.
But in data from particles’ collisions filtered out of one of the detectors – LHCb – scientists may have found that slightly more taus than muons were being produced. The rate at which one particle decays into another reflects the strength of a force shared between the two particles, so an unexpected deviation in decay rates hints at previously unknown forces at play. And these forces could be part of a larger system of particle interactions that could help scientists answer currently unanswerable questions in physics.
Many of them have to do with the Standard Model, which has accounted for many features of fundamental particles but cannot explain certain things, like what dark matter is. In fact, the last major bit of the Model was discovered in 2012: the Higgs boson, but the Model hasn’t been able to explain why it was much lighter than it should’ve been. But with its discovery and the subsequent upgrades the LHC went through, scientists are hoping signs of ‘new physics’, like the claimed violation of lepton universality, are found more often. It’s not out of the question either: the latest finding was gleaned from data gathered in the first run of the LHC in 2011, while the second run of particle-smashing that began in May 2015 is happening at almost twice the energy.
The pre-print paper uploaded on arXiv on June 29 says the significance of the LHCb detection was 2.1 sigma, while 3 sigma marks the threshold to claim evidence (and 5 sigma to claim a discovery). However, the combined significance of the results from BaBar, Belle and LHCb is 3.9 sigma. But either way, scientists will keep an eye out to find more instances of B mesons decaying unequally into taus and muons. Because if the unequal decay doesn’t happen in the same conditions again, the experimenters could be alerted not to a previously undetected force but a previously undetected glitch.