### Abstract

We calculate the ratio η / s, the shear viscosity (η) to entropy density (s), which characterizes how perfect a fluid is, in weakly coupled real scalar field theories with different types of phase transitions. The mean-field results of the η / s behaviors agree with the empirical observations in atomic and molecular systems such as H_{2}O, He, N, and all the matters with data available in the NIST database. These behaviors are expected to be the same in N component scalar theories with an O (N) symmetry. We speculate these η / s behaviors are general properties of fluid shared by QCD and cold atoms. Finally, we clarify some issues regarding counterexamples of the conjectured universal bound η / s ≥ 1 / 4 π found in Refs. [T.D. Cohen, Phys. Rev. Lett. 99 (2007) 021602, hep-th/0702136; A. Cherman, T.D. Cohen, P.M. Hohler, arXiv: 0708.4201 [hep-th]; A. Dobado, F.J. Llanes-Estrada, Eur. Phys. J. C 51 (2007) 913, hep-th/0703132].

Original language | English |
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Pages (from-to) | 18-21 |

Number of pages | 4 |

Journal | Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics |

Volume | 670 |

Issue number | 1 |

DOIs | |

Publication status | Published - 2008 Dec 4 |

Externally published | Yes |

### ASJC Scopus subject areas

- Nuclear and High Energy Physics

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## Cite this

*Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics*,

*670*(1), 18-21. https://doi.org/10.1016/j.physletb.2008.10.024