[1][2]: 57 The reanalysis was sparked by theoretical work in 1971 by Fujii [3][4]: 3 proposing a model that changes distance dependence with a Yukawa potential-like term: The parameter
[5]: 26 The effect of this potential can be described equivalently as exchange of vector and/or scalar bosons, that is a predicting as yet undetected new particles.
Another kind of fifth force, which arises in Kaluza–Klein theory, where the universe has extra dimensions, or in supergravity or string theory is the Yukawa force, which is transmitted by a light scalar field (i.e. a scalar field with a long Compton wavelength, which determines the range).
This requires extremely sensitive experiments which search for a deviation from the inverse-square law of gravity over a range of distances.
A further experiment measuring the gravitational constant in a deep borehole in the Greenland ice sheet found discrepancies of a few percent, but it was not possible to eliminate a geological source for the observed signal.
[14] Jain et al. (2012)[15] examined existing data on the rate of pulsation of over a thousand cepheid variable stars in 25 galaxies.
[16] A comprehensive review by Ephraim Fischbach and Carrick Talmadge suggested there is no compelling evidence for the fifth force,[17] though scientists still search for it.
Forces that depend on the composition of an object can be very sensitively tested by torsion balance experiments of a type invented by Loránd Eötvös.
In 2015, Attila Krasznahorkay at ATOMKI, the Hungarian Academy of Sciences's Institute for Nuclear Research in Debrecen, Hungary, and his colleagues posited the existence of a new, light boson only 34 times heavier than the electron (17 MeV).
[20] In an effort to find a dark photon, the Hungarian team fired protons at thin targets of lithium-7, which created unstable beryllium-8 nuclei that then decayed and ejected pairs of electrons and positrons.
[21] Feng et al. (2016)[22] proposed that a protophobic (i.e. "proton-ignoring") X-boson with a mass of 16.7 MeV with suppressed couplings to protons relative to neutrons and electrons and femtometer range could explain the data.