Journal
PHYSICAL REVIEW D
Volume 102, Issue 7, Pages -Publisher
AMER PHYSICAL SOC
DOI: 10.1103/PhysRevD.102.076011
Keywords
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Funding
- IISN-Belgium [4.4503.15]
- F. R. S.-FNRS under the Excellence of Science EOS be.h Project [30820817]
- Spanish Ministerio de Ciencia, Innovacion y Universidades [PGC2018-096894-B-100]
- Principado de Asturias [GRUPIN-IDI/2018/000174]
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We study a paradigmatic model in field theory where a global U(1) and scale symmetries are jointly and spontaneously broken. At zero density the model has a noncompact flat direction, which at finite density needs to be slightly lifted. The resulting low-energy spectrum is composed by a standard gapless U(1) Nambu-Goldstone mode and a light dilaton whose gap is determined by the chemical potential and corrected by the couplings. Even though U(1) and scale symmetries commute, there is a mixing between the U(1) Nambu-Goldstone and the dilaton that is crucial to recover the expected dynamics of a conformal fluid and leads to a phonon propagating at the speed of sound. The results rely solely on an accurate study of the Ward-Takahashi identities and are checked against standard fluctuation computations. We extend our results to a boosted superfluid, and comment the relevance of our findings to condensed matter applications.
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