**Nuclear matter** is an idealized system of interacting nucleons (protons and neutrons) that exists in several phases that as yet are not fully established.^{[2]} It is *not* matter in a nucleus, but a hypothetical substance consisting of a huge number of protons and neutrons interacting by only nuclear forces and *no* Coulomb forces.^{[3]}^{[4]} Volume and the number of particles are infinite, but the ratio is finite.^{[5]} Infinite volume implies no surface effects and translational invariance (only differences in position matter, not absolute positions).

A common idealization is *symmetric nuclear matter*, which consists of equal numbers of protons and neutrons, with no electrons.

When nuclear matter is compressed to sufficiently high density, it is expected, on the basis of the asymptotic freedom of Quantum chromodynamics, that it will become quark matter, which is a degenerate Fermi gas of quarks.^{[6]}

Some authors use "nuclear matter" in a broader sense, and refer to the model described above as "infinite nuclear matter",^{[1]} and consider it as a "toy model", a testing ground for analytical techniques.^{[8]} However, the composition of a neutron star, which requires more than neutrons and protons, is not necessarily locally charge neutral, and does not exhibit translation invariance, often is differently referred to, for example, as *neutron star matter* or *stellar matter* and is considered distinct from nuclear matter.^{[9]}^{[10]} In a neutron star, pressure rises from zero (at the surface) to an unknown large value in the center.

Methods capable of treating finite regions have been applied to stars and to atomic nuclei.^{[11]}^{[12]} One such model for finite nuclei is the liquid drop model, which includes surface effects and Coulomb interactions.

## See also

- QCD vacuum
- Quark–gluon plasma
- Degenerate matter
- Neutron-degenerate matter
- Strange matter
- Nuclear structure
- Neutronium
- Nuclear physics
- Nuclear spectroscopy

## References

- ^
^{a}^{b}Phillip John Siemens; Aksel S. Jensen (1994).*Elements Of Nuclei: Many-body Physics With The Strong Interaction*. Westview Press. ISBN 0-201-62731-0. **^**Dominique Durand; Eric Suraud; Bernard Tamain (2001).*Nuclear dynamics in the nucleonic regime*. CRC Press. p. 4. ISBN 0-7503-0537-1.**^**Richard D. Mattuck (1992).*A Guide to Feynman Diagrams in the Many-body Problem*(Reprint of 1974 McGraw-Hill second ed.). Courier Dover Publications. ISBN 0-486-67047-3.**^**John Dirk Walecka (2004).*Theoretical nuclear and subnuclear physics*(2 ed.). World Scientific. p. 18. ISBN 981-238-898-2.**^**Helmut Hofmann (2008).*The Physics of Warm Nuclei: With Analogies to Mesoscopic Systems*. Oxford University Press. p. 36. ISBN 0-19-850401-2.**^**Stefan B Rüster (2007). "Phase diagram of neutral quark matter at moderate densities". In Armen Sedrakian; John Walter Clark; Mark Gower Alford (eds.).*Pairing in fermionic systems*. World Scientific. ISBN 981-256-907-3.**^**Paweł Haensel; A Y Potekhin; D G Yakovlev (2007).*Neutron Stars*. Springer. ISBN 0-387-33543-9.**^**Herbert Müther (1999). "Dirac-Brueckner approach for finite nuclei". In Marcello Baldo (ed.).*Nuclear methods and the nuclear equation of state*. World Scientific. p. 170. ISBN 981-02-2165-7.**^**Francesca Gulminelli (2007). "Nuclear matter versus stellar matter". In A. A. Raduta; V. Baran; A. C. Gheorghe; et al. (eds.).*Collective Motion and Phase Transitions in Nuclear Systems*. World Scientific. ISBN 981-270-083-8.**^**Norman K. Glendenning (2000).*Compact stars*(2 ed.). Springer. p. 242. ISBN 0-387-98977-3.**^**F. Hofmann; C. M. Keil; H. Lenske (2001). "Density dependent hadron field theory for asymmetric nuclear matter and exotic nuclei".*Phys. Rev. C*.**64**(3). arXiv:nucl-th/0007050. Bibcode:2001PhRvC..64c4314H. doi:10.1103/PhysRevC.64.034314.**^**A. Rabhi; C. Providencia; J. Da Providencia (2008). "Stellar matter with a strong magnetic field within density-dependent relativistic models".*J Phys G*.**35**(12): 125201. arXiv:0810.3390. Bibcode:2008JPhG...35l5201R. doi:10.1088/0954-3899/35/12/125201.