LHCb experiment hints at a small difference between two types of leptons
An intriguing deviation
Recent measurements at the LHCb experiment at CERN, Geneva suggest a small difference between two types of leptons (elementary particles). In the Standard Model, the three types of leptons, namely electrons, muons and taus, are all equal. This means that after correcting for their different masses, a muon is produced just as often as a tau in the decay of a heavy particle. The LHCb experiment studies the decay of heavy B mesons. These particles contain a heavy b quark. The b stands for 'beauty' or 'bottom', depending on who you talk to. These particles often decay into a lighter D meson (consisting of a charm quark), a charged lepton and a neutrino. The neutrino flies through the detector without being detected, but the D meson and lepton are registered in the sensors of the LHCb detector. The lepton is usually an electron or muon, as they are much lighter than their heavy brother, the tau. But once you correct for this mass effect, the three leptons are equal. At least, that is what the Standard Model predicts. If there are new forces or particles, they can change the relative fractions between the leptons. An example of such a hypothetical particle is a charged Higgs boson; the Higgs boson of the Standard Model (that has been observed by the LHC) is in fact neutral. The results were published today in the prestigious journal Physics Review Letters
With the data from Run 1, LHCb has very accurately determined the ratio between the number of B mesons decaying into a muon and the number of B mesons decaying into a tau. The outcome is that the decay to a tau occurs more frequently than expected. In itself, this measurement is not yet significant, but it is intriguing that two other experiments find the same anomaly. "The measured value agrees well with that of the BaBar experiment in the US and the Belle experiment in Japan,” says Greg Ciezarek, Nikhef researcher and co-author of the article. In figure 1, the results from these three experiments are shown, including the expected value of the Standard Model. If you combine these results, the probability that this is a statistical fluctuation is only 0.01%. That is a very small chance, but not small enough to throw away the Standard Model. Further research is needed to determine whether this anomaly is real or a statistical fluctuation. In the coming years, the new data from Run 2 of the LHC which has just begun, will tell.
About LHCb
Following a break for maintenance and improvements, the Large Hadron Collider (LHC) at CERN near Geneva was started up again last March for a second period of three years. The LHC is the largest and most powerful particle accelerator in the world and an enormous Big Science project. Nikhef contributes to three of the four large LHC experiments, including the detectors ALICE, ATLAS and LHCb. Many Nikhef researchers are involved in this. Run 2 of the LHC follows after a period of two years during which the machine was prepared for almost a doubling of the energy level achieved in Run 1. Furthermore, in the coming period the intensity of the collisions will increase considerably. After extensive tests and improvements, the restarting of the beams in the LHC and the first test collisions with a total energy of 13 TeV became a fact. This has ushered in episode two of LHC physics and has paved the way for new physics discoveries.
References
For further information about the LHCb experiment you can look at:
FOM Institute Nikhef
arXiv
Physics Review Letters
LHCb
More information
Science Communications Department, FOM Institute Nikhef, +31 20 592 50 75
Jeroen van Tilburg, researcher LHCb-Nikhef, +31 20 592 21 31
Marcel Merk, programme leader LHCb-Nikhef, +31 20 592 51 07