Thermal noise from icy mirrors in gravitational wave detectors

The detection of gravitational waves has established a new and very exciting field of astronomy in the past few years. To increase the number of detections and allow observation of a wider range of sources, several future gravitational wave detectors will operate at cryogenic temperatures. Recent investigations of a mirror in one of the cryostats of the Japanese KAGRA detector showed a decrease in reflectivity due to ice growth, induced by residual water molecules moving from the warm to the cold sections of the detector''s vacuum system. Based on the optical measurements made in KAGRA, in this paper we calculate the implications of an ice layer on coating thermal noise for the planned European Einstein Telescope. We find coating thermal noise to oscillate, due to periodic reflectivity changes as the ice layer grows. The average coating thermal noise increases significantly over a time of one year with a larger increase at higher temperatures.