When the satellite-borne instrument PAMELA observed an unexpectedly large flux of positrons in cosmic rays beyond the earth’s atmosphere, the results created a ripple in the particle physics and astrophysics communities [1]. Spurring the interest was the possibility that the extra positrons were formed in the annihilation of dark matter particles (although other, astrophysical explanations were also offered). A challenge to this interpretation came from a concurrent report by the PAMELA collaboration: the ratio of antiprotons to protons, which most dark matter theories would have predicted to also be in excess, did not exceed what was expected from secondary production (the interaction of cosmic-ray protons with hydrogen and helium nuclei in the interstellar medium) [2]. Thus a challenge was presented: to explain the positron excess without predicting too many antiprotons [3,4].
Now, in a new paper appearing in Physical Review Letters, the PAMELA Collaboration reports additional data on the antiproton abundance in cosmic rays. The data extend to higher and lower energies than the collaboration’s first report, and include data on the absolute size of the antiproton flux, not just the ratio of antiprotons to protons.
The results are still consistent with the view that all of the antiprotons come from secondary production. However, the secondary production calculations are sufficiently uncertain that theories of the positron excess that also imply an antiproton excess—including one involving dark matter [5] and one involving supernova remnants [6]—are also consistent with the data. Both more data and more precise theories of secondary production will be very welcome to help resolve this puzzle. – Stanley Brown
[1] O. Adriani et al. (PAMELA Collaboration), Nature 458, 607 (2009)
[2] O. Adriani et al. (PAMELA Collaboration), Phys. Rev. Lett. 102, 051101 (2009); see also Physics 2, 10 (2009)
[3] M. Cirelli, M. Kadastik, M. Raidal, and A. Strumia, Nucl. Phys. B 813, 1 (2009).
[4] F. Donato et al., Phys. Rev. Lett. 102, 071301 (2009); see also http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.102.071301
[5] G. Kane, R. Lu, and S. Watson, Phys. Lett. B 681, 151 (2009).
[6] P. Blasi, Phys. Rev. Lett. 103, 051104 (2009); P. Blasi and P. D. Serpico, Phys. Rev. Lett. 103, 081103 (2009).
Note added (15 September 2010): A new reference [3] has been included, which also discusses the implications of the antiproton data for models which seek to explain the positron data.
Note added (10 March 2011): Reference 1 has been corrected.