Is there a fifth force in Nature?

The standard model of particle physics recognizes the existence of four fundamental forces (their correct name is interactions):

• Gravitation: Newton called thus the force of attraction between any two masses at a distance. For Einstein, according to General Relativity, gravitation is the curvature of space as a consequence of the presence of a mass, which affects the movement of nearby masses. This force, which is always attractive, has an infinite range, although its effect decreases in inverse ratio of the square of the distance, and is the weakest of the four, but its effect is dominant at cosmic and planetary distances, as well as on the Earth's surface.
• Electromagnetic interaction: It affects bodies and particles with an electric charge, such as protons and electrons, but not particles without charge, such as neutrons and neutrinos. This force, which can be attractive (between particles and bodies with charges of different signs) or repulsive (between bodies with charges of the same sign), has an infinite range, although its effect decreases in inverse ratio of the square of the distance, and is 7×10³⁵ times more intense than gravity, but its effect is negligible at cosmic and planetary distances, as well as on the surface of the Earth, where almost all bodies lack electric charge.
• Weak interaction: Also called weak nuclear force, it affects all particles called leptons (such as electrons and neutrinos) and hadrons (such as protons, neutrons, and mesons). Gauge bosons, which serve as intermediaries in fundamental interactions, are not affected. The weak interaction is 10²⁵ times more intense than gravity, but its range is very small, less than an attometer (10^-18 meters), so it only affects elementary particles, and cannot be detected in macroscopic bodies.
• Strong interaction: Also called strong nuclear interaction, it affects hadrons, but not leptons or gauge bosons. It is the most intense of all (hence its name): 10³⁸ times stronger than gravity. Its range is also very small (a femtometer, 10^-15 meters), although it is 1000 times larger than that of the weak interaction. This force is responsible for the stability of atomic nuclei, as it allows protons to coexist within the nucleus. Having the same charge, they should repel each other if they were subject only to the electromagnetic interaction, but the strong force, much more intense at that distance, is attractive.

CERN LHC tunnel

The possible existence of a fifth fundamental force, which would be outside the standard model of particle physics and would open the way to its extension, is a fairly old idea. First there was talk about a repulsive gravitation. In a 1987 Science News article, mention was made of the discovery, one year earlier, of some gravity anomalies in mines and deep wells in Australia and Michigan, which led some physicists to propose the existence of a repulsive force that would act at distances between 10 and 1000 meters and would be 50 or 100 times less intense than normal gravity. The same year came the proposal of a sixth fundamental force, this time attractive, which would increase gravity. Some physicists were happy, because some proposals of Grand Unified Theories that were being made during the 80s predicted the existence of two more forces, one attractive and the other repulsive. However, new interpretations of the experiments that gave anomalous results led to the conclusion that these results could be explained without resorting to new forces, by the simple presence of rocks and other geological formations in the places where the anomalies had been discovered. Two years later, in 1990, the evidence provided by new experiments caused the idea of ​​the existence of new fundamental forces to be abandoned for the time being.

The fifth force reappeared in the year 2000 as a result of experiments carried out with particle accelerators that suggested the existence of a new gauge boson, which was given the provisional name Z', because it would be related to the Z boson of the weak interaction. It was originally thought that this supposed discovery could give some confirmation to string theory. After a year, in 2001, another experiment with neutrinos also gave an anomalous result that was attributed to the possible existence of the elusive fifth force. In neither of these two cases the results of the experiments were confirmed, and the question never reappeared.

Until 2015, when Attila Krasznahorkay and his team from the Hungarian Nuclear Research Institute bombarded lithium with protons and discovered an abnormality in the particle paths generated by the decay of the resulting beryllium atoms. They proposed that the abnormalities could be due to the existence of an unknown particle. They later found a similar anomaly in the decay of helium. A team at the University of California proposes that these anomalies could be due to a new gauge boson, which would serve as intermediary for an unknown fifth fundamental force. So far, all this has served to propose new experiments to be carried out. We’ll see what comes out of them.

The same post in Spanish
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Manuel Alfonseca