A year ago, particle physicists found evidence that called the so-called Standard Model into question. Now they have evaluated the data more closely – and are sobered.
With the LHCb experiment at Cern, physicists are investigating the decay of so-called B mesons.
“Have your Kleenex ready.” With these words, the particle physicist Martín González-Alonso announced on Tuesday in a tweet a lecture at Cern. And what was announced shortly afterwards at the Laboratory for Particle Physics in Geneva was actually a reason for tears for particle physicists. As an evaluation of new data shows, the earlier indications of the violation of a fundamental symmetry of nature cannot be substantiated. Once again, hopes of breaking new physical ground with the world’s largest particle accelerator have been dashed.
Particle decays show a list
The disappointment among particle physicists is also great because it looked very promising just over a year ago. At that time, the members of the LHCb working group had investigated decays of so-called B mesons. These particles do not occur in nature. However, they are created during particle collisions in the Large Hadron Collider (LHC). At that time, researchers were interested in decays that ultimately leave an electron and its antiparticle or a muon (a close relative of the electron) and its antiparticle.
According to the standard model of particle physics – this model describes the elementary particles known today and the forces between them – the two decays should be equally frequent. This is because electrons and muons are practically identical. Although they differ in their mass, they should interact in the same way with the other particles in the Standard Model.
Because electrons and muons both belong to the class of leptons, this “equality before the law” is also known as lepton universality. It is assumed that this is a fundamental symmetry of nature.
The LHCb experiment at CERN a year ago raised doubts about this.
More often an electron, less often a muon
how the evaluation of the data showed that in the decays of the B mesons, an electron and its antiparticle remained more often than a muon and its antiparticle. So lepton universality seemed to be violated.
The result did not yet have the statistical significance at which particle physicists speak of a discovery. Nevertheless, the result attracted a lot of attention. Quite a few researchers saw this as the strongest indication to date that the Standard Model does not comprehensively describe the world of elementary particles and does not need to be supplemented.
One possibility would be, for example, that in addition to the four known natural forces (gravity, the electromagnetic force, the strong and the weak nuclear force), there is a previously unknown fifth natural force. If this force acts differently on electrons than on muons, one could explain why the decays of the B mesons list.
Why physicists are dissatisfied with the Standard Model
Most particle physicists would welcome such a discovery. Because their relationship to the standard model is ambiguous. On the one hand, they are impressed that this model has been confirmed again and again for decades. On the other hand, they are dissatisfied because the model leaves many questions unanswered.
For example, the Standard Model does not provide a particle candidate for dark matter, a still unknown form of matter that holds galaxies like the Milky Way together through its gravitational pull. And the imbalance between matter and antimatter in the universe cannot be explained with the Standard Model either. That’s why particle physicists react euphorically every time cracks appear in the foundation of particle physics – no matter how small they are.
This time, however, it looks like the physicists were happy too soon. In recent months, the members of the LHCb working group have evaluated all the data they have collected since the Large Hadron Collider (LHC) went into operation in 2008. Nicola Serra from the University of Zurich, who is part of the LHCb working group, said he hoped that stronger evidence of the violation of lepton universality would now be found than a year ago. But the opposite is the case. The new results are in good agreement with the standard model.
The number of electrons was probably overestimated
The fact that the indications of new physics have evaporated is due to an improved evaluation of the data. Before one can compare the decays of the B mesons with one another, one has to estimate the frequency of so-called background events. This is what particle physicists call events that look like the signal they are looking for, but actually stem from other particle-physical processes.
A year ago, simulations were used to estimate the subsoil. Now the researchers relied on real data. This allowed the contributions to the background to be recorded more completely. As it turned out, the number of electrons had probably been overestimated in the earlier evaluation. This gave the impression that there were more decays that ended with an electron and its antiparticle.
However, Serra does not want to say goodbye to the search for a violation of lepton universality. Other decays of the B mesons would also have pointed in this direction in the past. None of these anomalies were statistically significant enough on their own. And yet a pattern has emerged that must now be further investigated.
Serra does not expect a definitive answer until after the third measurement period at CERN. This recently started and is expected to last until 2025 with some interruptions. However, Serra makes no secret of the fact that he is now significantly less optimistic. There is a significant chance that the standard model will once again be the happy winner.
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