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The rise of the cosmic ray positron fraction with energy, as first observedwith high confidence by PAMELA, implies that a large flux of high energypositrons has been recently (or is being currently) injected into the localvolume of the Milky Way. With the new and much more precise measurement of thepositron fraction recently provided by AMS, we revisit the question of theorigin of these high energy positrons. We find that while some dark mattermodels (annihilating directly to electrons or muons) no longer appear to becapable of accommodating these data, other models in which ~1-3 TeV dark matterparticles annihilate to unstable intermediate states could still be responsiblefor the observed signal. Nearby pulsars also remain capable of explaining theobserved positron fraction. Future measurements of the positron fraction by AMS(using a larger data set), combined with their anticipated measurements ofvarious cosmic ray secondary-to-primary ratios, may enable us to furtherdiscriminate between these remaining scenarios.

Dark Matter and Pulsar Origins of the Rising Cosmic Ray Positron Fraction in Light of New Data From AMS