LHCb experiment: once more room for doubt about Standard Model of Particle Physics

19 October 2021

Once more, new results from the LHCb experiment at CERN leave room for doubts about the Standard Model of particle physics. It is possible that the electron and its 200 times heavier version, the muon, differ more than thought after all.

On Tuesday, the LHCb collaboration made public measurements of a specific decay of produced b-mesons in proton collisions at the LHC accelerator at CERN. These particles can decay into so-called kaons and, in theory, equally often into an electron or a muon.

Nikhef is a participant in LHCb. Nikhef’s Dutch physicist Niels Tuning is currently physics coordinator of the experiment.

In principle, decay can take place to four types of kaons. LHCb looks for any differences in each type of decay between the route to an electron and to a muon.

On Tuesday, LHCb announced measurements of two decay routes (called KOS and K*+) from the period 2011-2018. The results are expressed as a ratio between observations with muons and with electrons, the quantity R_k, which in theory should be 1.

The Standard Model of particle physics knows no difference except for the mass of the two leptons. The LHCb experiment is seen as an important test of this so-called lepton universality.

The new measurements are at 0.66 and 0.70. That’s lower than 1 but the uncertainty in the results is too great for dramatic conclusions. “Formally, we say it agrees with the standard model,” says Tuning. At the same time, it does fit the pattern of a lower R_k value than the theory predicts.

Back in March, LHCb published results from the main kaon pathway that could indicate different behavior of electrons and muons. The earlier measurement came to R_k 0.85 and had a significance of 3.1 sigma. For physicists, that is a signal to be taken seriously, but not yet hard evidence of an anomaly.

The new results do not even reach 1.5 sigma and could still easily be a random anomaly, rather than an indication that the theory is wrong.

Any difference between electrons and muons does not fit the standard model of particle physics, and could be an indication of an unknown fifth force in nature. According to some theorists, such a thing could point to an unknown new particle, the leptoquark.

Programme leader Marcel Merk of the LHCb group at Nikhef: “It remains a bit of a cliff hanger which way this is going to go. For some it is still not hard enough, for others it confirms the possibility of a real anomaly.”