Radiation pressure excitation and cooling of a cryogenic MEMS-cavity Mario Siciliani de Cumis, 1 Alessandro Farsi, 2 Francesco Marino, 3 Giuseppe D’Arrigo, 4 Francesco Marin, 3 Francesco Saverio Cataliotti, 5 and Emanuele Rimini 1, 4 1 Dipartimento di Fisica, Universit`a di Catania, Via S. Sofia 64, I-95100 Catania (CT), Italy 2 Dipartimento di Fisica, Universit`a degli studi di Milano Via Celoria 16, I-20100 Milano (MI), Italy 3 Dipartimento di Fisica, Universit`a di Firenze, INFN, Sezione di Firenze, and LENS, Via Sansone 1, I-50019 Sesto Fiorentino (FI), Italy 4 IMM-CNR, Stradale Primosole 50, I-95100 Catania (CT), Italy 5 Dipartimento di Energetica, Universit`a di Firenze, INFN, Sezione di Firenze, and LENS, Via Santa Marta 3, I-50139 Firenze, Italy (Dated: September 1, 2009) Abstract We describe an experiment achieving radiation pressure excitation and cooling of a mechanical mode in a cryogenic Fabry-Perot cavity with a micro-mechanical oscillator (MEMS) as end mirror. The response function to periodic modulations of the intracavity power provides an independent measurement of the effective modal mass allowing an accurate estimate of the mode temperature from the corresponding displacement noise spectrum. We also obtained optical cooling of the MEMS fundamental mode at 110 kHz from 11 K to 4.4 K, limited only by the optical Finesse and the mechanical quality of the system. These results represent a step towards the observation of quantum opto-mechanical effects and motivate further experiments with improved performances of the MEMS samples. PACS numbers: 42.50.Wk, 07.10.Cm, 42.50.-p 1