Determination of the rigidity scale of the Alpha Magnetic Spectrometer

The Alpha Magnetic Spectrometer (AMS) experiment aboard the International Space Station has been collecting data since May 2011. The AMS silicon tracker accurately determines the trajectory of charged particles. Together with the magnet, it measures the rigidity (momentum per unit charge) of cosmic rays in the range from ∼0.5 GV to several TV. The precise knowledge of the rigidity scale is of critical importance to the physics goals of the experiment. The method to establish the tracker rigidity scale by using electron and positron events is presented. It allowed validation of the tracker alignment and estimation of the potential rigidity scale shifts. Using 5 years of AMS data, the tracker rigidity scale was measured with an accuracy of ±0.030(stat)±0.017(sys) TV−1, limited mostly by available positron statistics.