(Proposal ID) S17A-056 (PI) Moriya, Takashi (Proposal Title) Spectroscopic confirmation of high-redshift superluminous supernovae (Abstract) We propose spectroscopic follow-up observations of high-redshift superluminous supernovae (SLSNe) at z~1-4 found by the HSC-SSP transient survey. The survey is planned from Nov 2016 to Apr 2017 at the COSMOS field, and we expect to detect about 20 SLSNe at z~ 1-4 during the survey. We will conduct spectroscopic follow-up observations of the detected SLSN candidates to confirm their redshifts and spectral types. By confirming them, we will be able to construct the first high-redshift SLSN sample with the secure spectroscopic confirmation. We are likely to break the highest redshift record of spectroscopically confirmed SNe. This first high-redshift SLSN sample will enable us to investigate the SLSN rate at high redshifts with which we can directly investigate the massive star formation up to z ~ 4. We can also estimate the high-redshift SLSN rates of the different SLSN subtypes which will be helpful to obtain a better understanding of their progenitors. The spectroscopically confirmed SLSNe may also be used as distance indicators to construct the Hubble diagram up to z ~ 4. The HSC-SSP SN survey in S17A is a very rare opportunity to investigate high-redshift SLSN properties that cannot be missed.