Present performance and future upgrades of the AURIGA capacitive readout

The resonant bar gravitational wave detector AURIGA is taking data for its second run at T = 4.5 K. The readout has been largely upgraded with respect to the previous run. In particular, we have realized a 3-mode detection scheme by tuning the high- Q resonance of the electrical readout to the mechanical modes of bar and transducer. Moreover, a low noise two-stage SQUID amplifier has been implemented. Thanks to these improvements, the bandwidth has been largely increased and the strain sensitivity is now better than 10 −20 Hz −1/2 over more than 100 Hz. In this paper, we describe a 3-mode model of the detector, which allows us to distinguish between the different noise contributions. Some possible upgrades are discussed.