"We propose to realize an experiment sensitive to a large fraction of the parameter space for short distance neutrino flavor oscillations into sterile components.The experiment aims at the clear and unambiguous discovery, or at the definitive disproof, of the so called neutrino anomalies, a set of circumstantial evidences of electron neutrino disappearance at short distance from the source observed by several experiments. The interpretation of the anomalies as oscillations into sterile neutrino components is also supported by cosmological data, which consistently indicate that the total number of neutrinos might be larger than three.If successful, we will demonstrate the existence of sterile neutrinos, opening a brand new era in fundamental particle physics and in cosmology. A solid signal would mean the discovery of the first particles beyond the Standard Electroweak Model and would have profound implications in our understanding of the Universe.In case of a negative result, we would close a long standing debate about the reality of the neutrino anomalies, probe the existence of new physics in low energy neutrino interactions, provide a measurement or a limit of neutrino magnetic moment, and give Borexino a superb energy calibration, very beneficial for high-precision solar neutrino measurements.The experiment will be done by placing a well designed artificial neutrino (or antineutrino) source close or inside the Borexino solar neutrino detector at the Laboratori Nazionali del Gran Sasso. The superb Borexino sensitivity, its large size, and its very low radioactive background will be the key elements of the experiment.The expected sensitivity, calculated with high precision Monte Carlo simulations which implements the deep knowledge of the detector response developed by the collaboration and by the P.I. in particular, is sufficient to guarantee either a clear discovery or the complete exclusion of sterile neutrinos as an explanation of the neutrino anomalies."