Physics 2, 54
A Viewpoint commentary discussing recent results from the Fermi Gamma-Ray Telescope on cosmic rays from dark matter may have dismissed an unusual spectral feature too soon; the authors respond.

We agree with Martin Israel that, to resolve the discrepancy, both experiments need to work to be sure that the systematic uncertainties are correct. Still, we conclude that the evidence for a prominent feature in the spectrum has been cast in serious doubt. And since our Viewpoint appeared, the HESS team has released a measurement of the $e+e-$ flux in the energy range 400 GeV – 5 TeV [1], overlapping with the Fermi result. HESS and Fermi agree (on the lack of a feature), each with very high statistical significance. It is possible that both HESS and Fermi have treated their systematics incorrectly, causing each to miss the feature observed by ATIC, but the evidence at this point is leaning in the direction of Fermi and now HESS: there is no prominent feature in the 400–800 GeV range. As Israel points out, the most crucial systematic uncertainties involve the behavior of the calorimeters, and those of Fermi and ATIC have relative strengths and weaknesses. Experiments with high statistics are generally better able to probe unforeseen systematic uncertainties than are those with low statistics. We, along with Israel, look forward to having these discrepancies resolved by the experiments in question.

## References

1. F. Aharonian et al., arXiv:0905.0105 (2009)

Bruce Winstein is the Samuel K. Allison Professor of Physics at The University of Chicago where he has been since 1972. For the bulk of his career he was a particle experimenter doing most of his work at Fermilab. While maintaining an interest in fundamental physics, he joined the field of observational cosmology about a decade ago. In 2001 he founded the NSF Physics Frontier Center for Cosmological Physics (now KICP). Currently he is the U.S. PI on the QUIET experiment measuring the polarization of the Cosmic Microwave Background Radiation. He was elected to the National Academy of Sciences in 1995, the American Academy of Arts and Sciences in 2007, and he shared the 2007 APS Panofsky Prize for his work studying CP violation in neutral kaons at Fermilab.

Kathryn Zurek is currently the David Schramm fellow in the theoretical astrophysics group at Fermi National Accelerator Laboratory. Previously, she focused on collider phenomenology as a postdoc at the University of Wisconsin Phenomenology Institute, having completed her Ph.D. in 2006 at the University of Washington in neutrino phenomenology. Her research interests center on the boundary of particle physics with cosmology and astrophysics, especially the hunt for dark matter.

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