Brownian force noise from molecular collisions and the sensitivity of advanced gravitational wave observatories

TL;DR

This paper analyzes how residual gas damping causes Brownian force noise that limits the sensitivity of advanced gravitational wave detectors like LIGO, especially at small test mass distances, and discusses mitigation strategies.

Contribution

It provides a detailed numerical and analytical analysis of gas damping force noise in LIGO, supported by experimental data, highlighting its impact on detector sensitivity and potential mitigation.

Findings

Force noise increases significantly when test mass is close to surrounding structures.

Residual gas damping noise rivals quantum fluctuations in the 10-30 Hz range.

Mitigation strategies can reduce the impact of gas damping on sensitivity.

Abstract

We present an analysis of Brownian force noise from residual gas damping of reference test masses as a fundamental sensitivity limit in small force experiments. The resulting acceleration noise increases significantly when the distance of the test mass to the surrounding experimental apparatus is smaller than the dimension of the test mass itself. For the Advanced LIGO interferometric gravitational wave observatory, where the relevant test mass is a suspended 340 mm diameter cylindrical end mirror, the force noise power is increased by roughly a factor 40 by the presence of a similarly shaped reaction mass at a nominal separation of 5 mm. The force noise, of order 20 fN\rthz\ for $2 \times 10^{-6}$ Pa of residual H$_2$ gas, rivals quantum optical fluctuations as the dominant noise source between 10 and 30 Hz. We present here a numerical and analytical analysis for the gas damping force noise for Advanced LIGO, backed up by experimental evidence from several recent measurements. Finally, we discuss the impact of residual gas damping on the gravitational wave sensitivity and possible mitigation strategies.