Handbook on string decay

We explain simple semi-classical rules to estimate the lifetime of any givenhighly-excited quantum state of the string spectrum in flat spacetime. Wediscuss both the decays by splitting into two massive states and by masslessemission. As an application, we study a solution describing a rotating andpulsating ellipse which becomes folded at an instant of time -- the ``squashingellipse''. This string interpolates between the folded string with maximumangular momentum and the pulsating circular string. We explicitly compute thequantum decay rate for the corresponding quantum state, and verify the basicrules that we propose. Finally, we give a more general (4-parameter) family ofclosed string solutions representing rotating and pulsating elliptical strings.Comment: 18 pages, 9 figures. Final version appeared in JHE