Think Positive

Over the past few years, the astronomers monitoring the data from the PAMELA satellite mission have reported a surprising phenomenon: They identified a strong stream of positrons not far from Earth. Positrons are “anti-electrons” – particles that are identical to electrons in all ways save their charge, which is positive as compared to the electrons’ negative one. Where do these positrons come from? How are they created?

 

The existence of the positrons was not a surprise: The mission, which explores cosmic rays, was looking for positrons. Astrophysicists believe that the source of these positrons is cosmic ray collisions that occur in the interstellar gas. Such collisions create short-lived particles that, as they perish, produce various other particles, including positrons. The surprise was in their number, which was much higher than models had predicted. So the question was: Where do the excess positrons come from?

A number of theories have arisen; many of them are based on “dark matter” – the invisible stuff that, if it exists, would explain several anomalies in the astronomical observations of gravity.

Prof. Eli Waxman, and former research students Kfir Blum and Boaz Katz, wondered if dark matter – which has not been proven to exist – is needed to explain the extra positrons; and they wanted to understand how these positrons advance through the galaxy, which could yield clues as to their source. Though no existing model can explain how the positrons move through galactic space, Waxman and his group found a way to calculate the upper limit on the number of positrons that can be created and dispersed in the galaxy. Their model did not rely on the existence of any type of dark matter.

Recent data from another satellite mission – AMS – showed that the number of positrons at high energies comes close to the limit proposed by the Weizmann team, but does not exceed it. Analysis suggests that cosmic ray collisions in the interstellar gas can explain the existence of the positron stream, without involving dark matter. Armed with this knowledge, AMS is now gathering information on the emission of cosmic radiation out of the galaxy into intergalactic space.

 

Prof. Eli Waxman's research is supported by the Nella and Leon Benoziyo Center for Astrophysics, which he heads; and the Friends of the Weizmann Institute in memory of Richard Kronstein. Prof. Waxman is the incumbent of the Max Planck Professorial Chair of Quantum Physics.