Measurement of the CR e+/e- ratio with ground-based instruments Pierre Colin Max-Planck-Institut für Physik CR Moon shadow MPP retreat - 21 January 2014
Cosmic ray electrons Observation: Above the atmosphere: on-board Balloons or Satellites Using the atmosphere (air shower): Cherenkov Telescopes (discrimination diffuse gamma / electron is very difficult) ~1% of the Cosmic Rays Origin: Same as CR hadrons (acceleration in SNR shock) Secondary from CR-ISM int. (P+P π± µ± e± ) Pulsars Dark Matter annihilation/decay? Short live time at high E: Sources < 1kpc for E>100GeV
Unexpected positron excess Origin of the new component? Exotic DM scenario Expectation from CR physics Pulsar scenario Experimental techniques: - PAMELA & AMS: Strong magnet in space - Fermi: Earth magnetic field
How to probe e + /e - at E >500 GeV? Flux decrease dramatically in E -α withα= 3 or 4 - Need large effective area (x100-1000 for a decade in E) Separation electron - positron - Need a larger magnetic field (AMS = 0.15 tesla) - or a larger tracker Possible solutions: - A super AMS (size x10 100 tons satellite!) - Ground-based instrument using a natural spectrometer provided by the geomagnetic field
Earth-Moon spectrometer Earth Magnetosphere Cosmic rays Moon West-ward positive part. Moon 1 TeV Proton shadow East-ward negative part. Deviation 1.5 x (1TeV/E) x Z The Moon shadow position depends on: CR charge CR kinetic energy
The Moon shadow effect on cosmic rays CR Moon shadow seen by Tibet-AS P/P ratio upper limits from moon shadow asymmetry Detected by Ground-based EAS array experiments (ARGO-YBJ, Tibet-ASγ, MILAGRO, etc.) - Used for angular resolution determination. - Est/West Asymmetry of the shadow probes anti-p/p ratio.
The electron Moon shadow? Not possible with current EAS arrays: Not good enough e + /proton discrimination and energy resolution Future projects: HAWC: 95% hadron rejection at ~1TeV (energy resolution: ~100%) LHAASO: 4 ponds of water Cherenkov + ground scintillators, air fluorescence, underground muon detectors, etc HAWC in construction in Mexico (altitude 4100 m) LHAASO project in Tibet (4300 m) 100 m 1.2 km Planed for 2020 Other solutions: Imaging Atmospheric Cherenkov Telescopes! MAGIC hadron rejection at 1 TeV: 99% (energy resolution: ~15%)
Moon shadow observation with MAGIC e + shadow e - shadow γ-ray shadow MAGIC photodetectors: low gain PMT (6 dynodes) UV and blue sensitive no filter
Limit due to Moon light Hardware limit (reduced HV) (standard HV) Energy range of the Moon shadow in the MAGIC FoV Observation Observation Current observation with MAGIC: Target energy range: 300-700 GeV Distance to Moon: 3.5-5 Possible only with Moon phase < 50% (limited observation window)
MAGIC Performance Maximum observation time per year: ~20 h / shadow Expected detection time with MAGIC: e - shadow: ~40h (~2 years) e + shadow: ~600h ( = impossible) MAGIC can detect the e - shadow but cannot measure the e + /e - ratio One need to improve the sensitivity by ~4 to detect the e + shadow (CTA could do it if well design for Moon light observation)
Cosmic electron with CTA (e - + e + ) spectrum from ~50 GeV to ~20 TeV Energy resolution at 1TeV: ~10% Possible confusion with diffuse Gamma rays Moon shadow observations??? All electron spectrum Model from Phys. Rev. D 82, 092004 (2010) CTA : 10% hadron bgd (non-official)
Dedicated Moon shadow IACT Camera designed for bright moon light Fast sampling (as MAGIC) Small pixels Robust photo detectors (SiPM) UV filter FACT air shower image taken pointing full Moon Need large telescope (20m class) Low enough threshold with strong Moon (increase obs. time) High Altitude (~4000 m) Less Mie scattering of Moon light More UV-peaked Cherenkov light Larger source- image distance Site near the equator Stronger horizontal magnetic field Moon at higher elevation (low Zd)
Conclusion The excess of cosmic positrons is an unexpected experimental fact which impacts the fundamental astroparticle physics: CR origin and propagation in galaxy New astrophysical source of CR Possible Dark Matter signature Space-based instruments reached their limit Ground-based instruments can use the natural earth-moon spectrometer to probe e + /e - ratio. EAS arrays do not have good e + /Proton separation. IACT are strongly affected by the Moon light. A dedicated IACT array with SiPM/UV-filter camera could be a cheep solution.