Commissioning of the recoil detector for the day-one experiment at HESR with pp elastic scattering at COSY

The conceptual design of the luminosity monitor for the PANDA experiment is based on measuring the differential elastic antiproton-proton scattering rate by 4 planes of HV-MAPS tracking detector. The absolute precision is limited by the lack of existing data in the relevant momentum region. Therefore, the so-called Day-One experiment at HESR will measure antiproton-proton elastic scattering. The goal of this experiment is to measure a wide range of 4-momentum transfer t (0.0008-0.1 GeV²) so that the contribution of the physical differential distributions to the absolute luminosity uncertainty is less than 1%. The polar angle of scattered antiprotons and the energy of recoil protons will be measured at forward angles by tracking detectors and by thick energy detectors near 90 , respectively. In order to test the method proposed for the Day-One experiment one of the recoil arms has been designed and built. The commissioning of the recoil arm by measuring proton-proton elastic scattering has been performed at COSY in July and September of 2013. The preliminary results of the commissioning are presented.