Direct measurement of cosmic-ray electrons into the TeV region with CALET on the ISS

The Calorimetric Electron Telescope (CALET) is an astroparticle physics observatory deployed on the Japanese Experiment Module Exposed Facility on the International Space Station since August 2015. After on-orbit checkout, it began taking scientific data in October 2015 and has maintained stable operations with 85% live time fraction since. The main instrument on CALET is the electromagnetic calorimeter (CAL), comprising a charge detector (CHD), an imaging calorimeter (IMC), and a total absorption calorimeter (TASC). The CHD, two crossed layers of plastic scintillating paddles, gives a coarse position and measurement of the incident particle charge. The IMC, eight pairs of plastic scintillating fibers with interspersed tungsten sheets, captures the incident particle track and early shower development with fine spatial resolution. The TASC, made of PbWO scintillator logs, effectively contains electromagnetic showers. At normal incidence, the CAL is 30 radiation lengths (but only 1.3 proton interaction lengths) deep, with energy deposit resolution on the order of 3% and effective hadron background rejection. The primary science goal for CALET is measurement of the cosmic-ray electron+positron spectrum from GeV energies to 10 TeV and beyond. As electrons lose energy quickly in propagation through the galaxy via bremsstrahlung and inverse Compton scattering, the mean lifetime and diffusion length (both decreasing with energy) for electrons at 1 TeV are on the order of 100 kyr and 1 kpc, respectively. The flux from the sum of all distant galactic sources is expected to fall quickly above a few TeV as the mean diffusion length becomes shorter than the distance to continuum sources. Features above these energies can then be modeled as contributions from nearby sources such as Vela, Monogem, and the Cygnus Loop. Some models of WIMP dark matter also could produce signatures in this energy range, depending on the particle mass. We report here on the CALET measurement of the electron spectrum using 7.5 years of data. Systematic errors on the measurement are tabulated and reported. The spectrum is consistent below 1 TeV with the results from AMS-02, although there is an unexplained tension between these results and those from DAMPE and Fermi-LAT. Above 1 TeV, suppression of the flux is observed as expected, but the spectral shape appears to flatten above a few TeV. We report on the consistency of this measurement with a modeled contribution including nearby sources and ongoing efforts to scrutinize the highest energy candidates in an event-by-event analysis to maintain low contamination from cosmic-ray protons.