Q-Ball
Encyclopedia : Q : QB : QBA : Q-Ball
In theoretical physics, Q-ball refers to a type of non-topological soliton. A soliton is a localized field configuration that is stable -- it cannot spread out and dissipate. In the case of a non-topological soliton, the stability is guaranteed by a conserved charge: the soliton has lower energy per unit charge than any other configuration. (In physics, charge is often represented by the letter "Q", and the soliton is spherically symmetric, hence the name.)
Constructing a Q-ball
In its simplest form, a Q-ball is constructed in a field theory of a complex scalar field [\phi], in which the particle number is conserved. Such a theory is defined by an energy function of the fields, the Hamiltonian, which includes kinetic and gradient terms, and also a potential [V(\phi^*\phi)]. Expanding the potential in powers of [\phi], the lowest-order term is [m^2\phi^*\phi], which determines the mass [m] of the particle. This implies that each particle carries energy of at least [m].
The theory contains Q-ball solutions if there are any values of [\phi^*\phi] at which the potential is less than [m^2\phi^*\phi]. In this case, a volume of space with the field at that value can have an energy per unit charge that is less than [m], meaning that it cannot decay into a gas of individual particles. Such a region is a Q-ball. If it is large enough, its interior is uniform, and is called "Q-matter". For a review see (Lee et al 1992 T.D. Lee, Y. Pang, "Nontopological solitons", Phys. Rept. 221:251-350 (1992)).
History
Configurations of a charged scalar field that are classically stable (stable against small perturbations) were constructed by Rosen in 1968 G. Rosen, J. Math. Phys. 9:996 (1968). Stable configurations of multiple scalar fields were studied by Friedberg, Lee, and Sirlin in 1976 R. Friedberg, T.D. Lee, A. Sirlin, Phys. Rev. D13:2739 (1976). The name "Q-ball" and the proof of quantum-mechanical stability (stability against tunnelling to lower energy configurations) come from Sidney Coleman (Coleman 1986 S. Coleman, "Q-Balls", Nucl. Phys. B262:263 (1985); erratum: B269:744 (1986)).
Occurrence in Nature
It has been theorized that dark matter might consist of Q-balls (Frieman et al 1988 J. Frieman, G. Gelmini, M. Gleiser, E. Kolb "Solitogenesis: Primordial Origin Of Nontopological Solitons", [Phys. Rev. Lett. 60:2101 (1988)], Kusenko et al 1997 A. Kusenko, M. Shaposhnikov, "Supersymmetric Q balls as dark matter", [[http://www.arxiv.org/abs/hep-ph/9709492 Phys. Lett. B418:46-54 (1998)]]) and that Q-balls might play a role in baryogenesis, i.e. the origin of the matter that fills the universe (Dodelson et al 1990 S. Dodelson, L. Widrow, "Baryon Symmetric Baryogenesis", Phys. Rev. Lett. 64:340-343 (1990), Enqvist et al 1997 K. Enqvist, J. McDonald, "Q-Balls and Baryogenesis in the MSSM", [Phys.Lett. B425 309-321 (1998)]). Interest in Q-balls was stimulated by the suggestion that they arise generically in supersymmetric field theories (Kusenko 1997 A. Kusenko, "Solitons in the supersymmetric extensions of the Standard Model", [[http://www.arxiv.org/abs/hep-ph/9704273 Phys. Lett. B405:108 (1997)]]), so if nature really is fundamentally supersymmetric then Q-balls might have been created in the early universe, and still exist in the cosmos today.
External links
- [Cosmic anarchists], by Hazel Muir. A popular account of the proposal of Alexander Kusenko.
References
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