Thermodynamic Properties of the Relativistic Composite String - Expository Remarks

The Casimir energy for the transverse oscillations of a piecewise uniform closed string is calculated. The great adaptability of this string model with respect to various regularization methods is pointed out. We survey several regularization methods: the cutoff method, the complex contour integration method, and the zeta-function method. The most powerful method in the present case is the contour integration method. The Casimir energy turns out to be negative, and more so the larger is the number of pieces in the string. The thermodynamic free energy and the critical Hagedorn temperature are calculated for a two-piece string. Mass and decay spectra are calculated for quantum massive excitations and the physical meaning of the critical temperatures characterizing the radiation in the decay of a massive microstate is discussed.