Almost ten years ago, the European Laboratory for Particle Physics (CERN) made headlines with the discovery of the Higgs particle. These days, the LHCb experiment at CERN has published results that also have the makings of a real scientific sensation: studies of the so-called beauty quark suggest that there could be a fifth, super-weak force in addition to the four known forces of nature (electromagnetism, gravitation, strong and weak interaction). However, CERN researchers still have a lot of work ahead of them to confirm this spectacular hypothesis. At the forefront: particle physicists from the University of Zurich and EPF Lausanne
Researchers are working on four major experiments at the Large Hadron Collider (LHC), the CERN particle accelerator. When the Higgs particle was discovered in 2012, the ATLAS and CMS experiments were at the forefront. These days, the LHCb experiment is making headlines with remarkable results. The scientists of this experiment are investigating the nature of beauty quarks, or b-quarks for short. b-quarks are elementary components of matter with a mass about 1000 times heavier than the mass of the up and down quarks that make up the protons and neutrons. b-quarks are created in proton-proton collisions at the LHC and then decay almost immediately into lighter particles.
According to the known laws of particle physics, the so-called 'Standard Model', b-quark decays involving electrons should occur with the same frequency as those involving muons. But this does not seem to be the case, as the latest results from the LHCb experiment suggest. The analysis of the data collected since 2011 shows that decays with electrons are slightly more frequent than those with muons. Prof. Nicola Serra, a particle physicist at the University of Zurich who has been working on the LHCb experiment for years, sees this as fantastic news: "When the Higgs particle was discovered almost ten years ago, it was ultimately the confirmation of a particle whose existence had been predicted long before and with which the Standard Model was once again confirmed. Our measurements at the LHCb experiment, instead, point to something completely new that goes beyond the Standard Model."
Anomaly in the decay of b quarks
According to current knowledge, there are three families (also known as "generations" or "flavours") of elementary particles. The first family makes up the known matter: it includes the up and the down quarks, which form the nucleus of all atoms and, together with the electrons, make up the majority of the physical world we know. In addition, there is a second family of particles that corresponds to the strange quark, the charm quark and the muon, these particles are heavier than the members of the first family. Even heavier are the particles of the third family with the top quark, the beauty quark and the τ-lepton. Each of the three families contains in addition a corresponding neutrino.
The LHCb experiment at CERN can detect beauty quarks decaying into the lighter strange quarks and two oppositely charged leptons - either an electron-antielectron pair or a muon-antimuon pair. However, these decays are extremely rare. The LHCb experiment has analysed about a trillion events in the last seven years; only a few thousand involved one of these decays. Their analysis now shows that the decay with a muon-antimuon pair is about 15% rarer than that with an electron-antielectron pair. This result touches the very foundations of particle physics. Until now, the Standard Model has assumed that electrons and muons differ in their mass (the muon is about 200 times heavier), but otherwise behave in the same way. This equality - physicists call it 'lepton universality' – seems to be violated according to the new results. A central assumption of today's physics would be at stake.
Reference to a hitherto unknown force
We know from our school lessons on probability theory: A coin falls equally often on 'heads' and on 'tails'. But what happens when such a matter of course is suddenly called into question? This is precisely the problem physicists faced in 2015, when first measurement from the LHCb experiment provided evidence of an unequal decay rate of b-quarks for the first time. How could this inequality be understood? How would it change our understanding of matter? Experimental physicists had raised questions that theoretical physicists now sought to answer. One who took up this task very early on was Prof. Gino Isidori, a particle physicist at the University of Zurich like Nicola Serra. Isidori soon had an idea what could explain the different rate of the two decays: "The experimental data suggested that muons and electrons are subject to different forces. Apparently, a new kind of force is at play here, which we call the superweak force", Isidori outlines his thoughts.
The statement by the Zurich particle physicist is explosive, because up to now physics has known exactly four fundamental forces, namely electromagnetism, gravitation, strong and weak interaction. Should a fifth force actually exist in nature, it would need an exchange particle to mediate the force, as is the case with the four known forces. Gino Isidori: "We call this still hypothetical particle that mediates the superweak force the leptoquark. This new particle should be heavier than all other particles seen so far, but still we should be able to detect it experimentally. This opens up an intriguing prospect: indeed, our calculations suggest that the leptoquark, if it exists, has a good chance to be discovered at the LHC by the CMS and ATLAS experiments."
Certainty in three to five years
The theoretical explanation of a fifth force has the potential to revolutionise our understanding of the material world. However, it still stands on a rather weak foundation. There is still the possibility that the differences in the frequency of b-quark decays are merely a whim of statistics. According to the latest measurements from the LHCb experiment, the probability of a statistical fluctuation is only 0.1 %. But in particle physics, the gold standard for an observation taking into account all known uncertainties, is a probability of less than 0,00003%, corresponding to a significance of 5 σ."To achieve this level of reliability, we expect to need another three to five years," says Nicola Serra. "In this process, we will not only collect more data for the measurement described above, but we will also make novel measurements, using different decay channels, at the LHCb experiment and at other experiments that should give us the final certainty."
So there is still a lot to do for particle physicists. It is quite possible, however, that in a few years the scientific sensation will succeed. There is light at the horizon that the limitations of the Standard Model of particle physics could be overcome and fundamentally new insights be gained - as experts have long hoped for. This hope is not only nourished by the latest results from CERN. Several other results from the LHCb and at other experiments, hint to deviations of the measurements from the predictions following a similar pattern. They all indicate that particle physics could open a new chapter in the next few years.
Author: Benedikt Vogel