 Threebody force

A threebody force is a force that does not exist in a system of two objects but appears in a threebody system. In general, if the behaviour of a system of more than two objects cannot be described by the twobody interactions between all possible pairs, as a first approximation, the deviation is mainly due to a threebody force.
The fundamental strong interaction does exhibit such behaviour, the most important example being the stability experimentally observed for the helium3 isotope, which can be described as a 3body quantum cluster entity of two protons and one neutron [PNP] in stable superposition. Direct evidence of a 3body force in helium3 is known: [1]. The existence of stable [PNP] cluster calls into question models of the atomic nucleus that restrict nucleon interactions within shells to 2body phenomenon. The threenucleoninteraction is fundamentally possible because gluons, the mediators of the strong interaction, can couple to themselves. In particle physics, the interactions between the three quarks that compose hadrons can be described in a diquark model which might be equivalent to the hypothesis of a threebody force. There is growing evidence in the field of nuclear physics that threebody forces exist among the nucleons inside atomic nuclei for many different isotopes (threenucleon force).
See also
 Faddeev equation
 Fewbody systems
 Nbody problem
References
 B.A. Loiseau and Y. Nogami, "ThreeNucleon Force", Nucl. Phys. B2, 470 (1967).
 H. Witala, W. Glöckle, D. Hüber, J. Golak, and H. Kamada, "Cross Section Minima in Elastic Nd Scattering: Possible Evidence for ThreeNucleon Force Effects", Phys. Rev. Lett. 81, 1183 (1998).
 E. Epelbaum, A. Nogga, W. Glöckle, H. Kamada, UlfG. Meissner, and H. Witala, "Threenucleon forces from chiral effective field theory", Phys. Rev. C 66, 064001 (2002).
 P. Mermod et al., "Search for threebody force effects in neutrondeuteron scattering at 95 MeV", Phys. Lett. B 597, 243 (2004).
Categories: Force
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