Fifth Force  

In General > s.a. Hyperphoton.
* Idea: In principle, any hypothetical force in addition to the four interactions of the standard model of particle physics; In practice, there are a few types of fifth force that have been considered more than others, such as a hypercharge-dependent one studied in the late 1980s in connection with a reanalysis of the Eötvös experiment by Fischbach et al, and various dilaton-field-dependent forces motivated by low-energy limits of Kaluza-Klein theories and string theory, or by scalar-tensor theories of gravity more generally; The latter need not violate the weak equivalence principle.
* Hypercharge-dependent: A type of force that could be coupled to a combination B cosθ + S sinθ of baryon number and strangeness (hypercharge seems antiquated) which, for bulk matter, gives rise to a dependence on the material, e.g., because near the center of the periodic table nuclear binding is tightest, and B/mass highest; If one interprets the fifth force as a component of gravity, then the fifth force violates the weak equivalence principle; The potential is parametrized by a Yukawa-type form given by, if Y1 and Y2 are the hypercharges of the two particles involved,

U(r) = f 2 Y1 Y 2 exp{–r/λ}/ r .

Phenomenology and Tests > s.a. Eötvös Experiment; tests of general relativity and of newtonian gravitation.
* History: Some indications of the possible existence of a fifth force had been coming from geophysical methods of testing newtonian gravitation, but the question received a lot of attention with the reanalysis of the Eötvös data by Fischbach et al (1986), following anomalous results in neutral K-meson experiments at Fermilab, which could be explained by a long-range force coupled to Y; It stimulated many new gravitational gradiometer, free fall, Cavendish or Eötvös-type experiments.
* Bounds: 2005, At the AU-scale, planet motion studies constrain the strength of this force to a 10–12–10–13 level; 2018, from the MICROSCOPE space mission.
@ Tests and bounds: Iorio PSS(07)gq/05 [AU-scale, from planet motion]; Farrar & Rosen PRL(07) + pw(07)may [galaxy cluster-scale, from Bullet Cluster]; Ferreira et al PRD(17)-a1612 [no fifth force in a scale-invariant universe]; news pw(18)apr [from the MICROSCOPE space mission].
> Related topics: see dark matter; Galileon Field; Unparticles.

@ Books: Franklin 93; Franklin & Fischbach 16.
@ General: Fischbach et al PRL(86), PRL(86), AP(88); De Rújula PLB(86); Eckhardt PRL(86); Nussinov PRL(86); Thieberger PRL(87); Nieto et al PRD(88); Burgess & Cloutier PRD(88) [from binary precession]; Fischbach & Talmadge Nat(92)mar; Romaides et al PRD(97) [tower experiment].
@ News: Stubbs Nat(89)mar; Waldrop Sci(89)nov; NS(90)jul21, p18.
@ From 5th dimension: Wesson et al PLB(99); Dahia et al MPLA(03).
@ Related topics: Krauss PT(08)oct [re spoof].
> Online resources: see Wikipedia page.

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