@phdthesis{oai:tsukuba.repo.nii.ac.jp:00008608,
 author = {知久, 季倫 and Chiku, Suenori},
 month = {},
 note = {application/pdf, The naive perturbation theory is known to break down at high temperature (T). This is because higher order terms are enhanced by the powers of T and eventually exceed the lower order terms even if the expansion parameter in the perturbation is small. These large terms at high T are called the hard thermal loops (HTLs).
Therefore, we need to resum HTLs to obtain sensible results at high T. So far, several methods have been proposed to carry out this resummation. Asone of the promising candidates, self-consistent resummation method has been studied for a long time. However, it was found that the method has difficulties in the
renormalization at finite T and in the proof of the Nambu-Goldstone theorem at finite T. In this thesis, we develop an optimized perturbation theory (OPT) at finite temperature in the O(N) Φ4 theory, which can resum higher order terms at finite T without the problems mentioned above. It has the following features: 1. Hard thermal loops are correctly resummed at high T. 2. The renormalization of the ultra-violet divergences can be carried out systematically in any given order of OPT. 3. The Nambu-Goldstone theorem is fulfilled for arbitrary N and the any givenorder of OPT., 1999, Bibliography: p. 85-90},
 school = {筑波大学, University of Tsukuba},
 title = {有限温度における最適化された摂動論},
 year = {2000}
}