Dark Matter and Pulsar Origins of the Rising Cosmic Ray Positron
  Fraction in Light of New Data From AMS
  The rise of the cosmic ray positron fraction with energy, as first observed
with high confidence by PAMELA, implies that a large flux of high energy
positrons has been recently (or is being currently) injected into the local
volume of the Milky Way. With the new and much more precise measurement of the
positron fraction recently provided by AMS, we revisit the question of the
origin of these high energy positrons. We find that while some dark matter
models (annihilating directly to electrons or muons) no longer appear to be
capable of accommodating these data, other models in which ~1-3 TeV dark matter
particles annihilate to unstable intermediate states could still be responsible
for the observed signal. Nearby pulsars also remain capable of explaining the
observed positron fraction. Future measurements of the positron fraction by AMS
(using a larger data set), combined with their anticipated measurements of
various cosmic ray secondary-to-primary ratios, may enable us to further
discriminate between these remaining scenarios.
