Great excitement at CERN, after strange measurements that cannot be explained with the Standard Model. Is there a fifth force of nature?
The four (or three, or two) known forces of nature
There are currently four known forces of nature. The electromagnetic force, the strong nuclear force, the weak nuclear force and the gravity. But wait. According to general relativity, gravity is not a force, but a pseudo force. An effect caused by the curvature of spacetime. Time slows down and the volume shrinks around heavy objects. The electromagnetic force and the weak nuclear force also in fact form one (electroweak) force, Abdus Salam and Sheldon Glashow showed. At higher energies these two forces merge. So in fact there are only two known forces, and one pseudo force. These two forces are accurately described by the Standard Model, and gravity by the general theory of relativity.
Why do some think there is a fifth force of nature?
Until now, the Standard Model could accurately predict the outcomes of all experiments. Although quantum mechanics (and thus the Standard Model) cannot accurately predict the outcome of an individual collision, the statistical distribution of a large number of collisions can. If the measured distribution deviates strongly from the prediction, this is a sign that there is an unknown physical effect. For example, caused by a fifth force.
This is what has likely been observed in the breakdown of bottom quarks. The bottom quark and top quark are the heaviest (third generation) quarks. The Standard Model predicts that a fixed percentage of the bottom quarks will disintegrate into lepton pairs (electron pairs or muon pairs; tauon pairs are too heavy), and strange quarks. The actual distribution deviates from the prediction: more electron-positron pairs are formed (and fewer muon / antimuon pairs) than expected. There are theoretical models to explain this. These only require particles that do not exist or cannot exist in the Standard Model. In other words: particles that transmit an as yet unknown force. This has been determined to an accuracy of 3.1 sigma (with a probability of around 1/1000 that this is a coincidence).
One of the theories to explain these measurements is the existence of leptoquarks. These are particles that can react with both leptons and quarks and can convert leptons into quarks (and vice versa). This is not possible according to the Standard Model.
The fifth force of nature
The discovery of a fifth force of nature would be sensational news. The discovery of electromagnetism brought us the world of today, that of the strong nuclear force nuclear energy. A fifth force of nature means a new world view and new techniques. Possibly even a complete technical revolution, such as after the discovery of electricity. But 3.1 sigma is not impressively reliable. The odds are about 1: 1000 that this observation is coincidental, and because a huge number of measurements are analyzed, these kinds of coincidences are common. That's why physicists go for the five sigma here to make sure it's not a false alarm. That corresponds to a chance of less than 1 in a million. Unfortunately, the LHC is now being stepped up (early 2021), so that the experiments cannot start again for another year and the five sigma will be within reach. Until then, the physics world has curved toes.