Research topic: Theory | 93
Contact:
Mathias Garny, [email protected]
References:
[1] http://fermi.gsfc.nasa.gov
[2] T. Bringmann, X. Huang, A. Ibarra, S. Vogl, C. Weniger, JCAP 1207 054 (2012) [3] C. Weniger, JCAP 1208 007 (2012)
[4] W. Buchmuller, M. Garny, JCAP 1208 035 (2012) [5] M. Su, D. Finkbeiner, arXiv:1206.1616
Over the past years, the Large Area Telescope (LAT) on board the Fermi satellite [1] has observed the gamma-ray sky in the energy range from 20 MeV to 300 GeV with unprecedented accuracy. Recently, an analysis based on optimized search regions around the galactic centre using 43 months of data has reported tentative evidence for a gamma-ray feature around 130 GeV [2,3]. This observation has triggered an active debate, and is being scrutinized by the Fermi collaboration.
It is intriguing to compare this observation with features expected for supersymmetric dark matter. In particular, important constraints arise from annihilation or decay into weak gauge bosons or Higgs bosons as well as fermions. Typically, these channels dominate over the production of monochromatic photons, since dark matter is electrically neutral and annihilation into photons arises only at the loop level. For example, for neutralino dark matter, the branching ratio into monochromatic photons is at the permille level, while it can rise to up to 3% for decaying gravitino dark matter. The subsequent decay and fragmentation of e.g. W bosons leads to a broad spectrum of gamma rays in the range 10–100 GeV, limiting the allowed amount of continuum emission from dark matter. We find that, to explain the Fermi excess, a branching ratio into monochromatic photons of more than half a percent is necessary [4]. This excludes Higgsino- and Wino-like neutralino dark matter as an explanation. Similar constraints arise from the flux of antiprotons produced by the W bosons.
In view of the limited statistics, it is premature to draw a firm conclusion about the spatial distribution of the excess. Nevertheless, it has been found to be compatible with the radial dependence expected for conventional Navarro–Frenk– White or Einasto profiles in the case of annihilating dark matter [5]. In contrast, for decaying dark matter, these profiles would lead to a much broader emission and a significant contribution from the galactic halo. The absence of a feature around 130 GeV away from the galactic centre therefore disfavours explanations based on decaying dark matter. In the near future, the H.E.S.S. II Cherenkov telescope will test the apparent excess in the Fermi data, and offer the possibility to search for gamma-ray lines at TeV energies. The planned Cherenkov Telescope Array (CTA) and the satellite mission Gamma-400 also have an excellent potential to search for gamma-ray lines. Whether the Fermi excess is real or not, it has exemplified how much we can learn about dark matter by searching for features in the gamma spectrum.
Figure 2
Spatial profile around the galactic centre [5] and expectations for annihilating or decaying dark matter
94 | Research topic: Theory
Quantum gauge theories are incredibly successful in describing nature down to the very smallest scales that we can reach in high-energy collisions. But even after many decades of experience with gauge theories, the scientific community struggles to obtain accurate numerical predictions. Precision computations using conventional approaches need massive computer algebra, in which thousands of terms must be evaluated, and the answers they provide can fill dozens of pages – if they can be written out on paper at all. Yet, closer studies of the expressions obtained often reveal an astonishing simplicity. In a few cases, modern mathematical methods have been employed to reduce many pages of formulas to just a few lines. Since we usually expect that calculations and results possess similar complexity, scientists from all over the world are now searching for radically different and more economic ways to perform gauge theory computations.
In the last decade, developments in string theory have clarified powerful new hidden symmetries of gauge theory, which have allowed us to solve a few problems that seemed hopelessly complicated at the end of the last century. The relation between string theory and gauge theory is based on the so-called gauge-string correspondence of Juan
Maldacena. String theory is a deformation of gravity. Hence, in certain regimes, it can be studied with the geometric techniques of Einstein’s theory of general relativity. If string theory is to teach us about the scattering of particles, what geometric problem should we look at? The answer is quite beautiful – scattering becomes a minimal-area problem in the geometric regime of string theory.
Minimal-area problems are very common in everyday life. Imagine a wire that has been bent into some circular shape and dipped into a soapy solution. When we hold this wire parallel to the ground, a soap bubble will hang down from it, pulled by the gravitational attraction of the earth. The area of the bubble depends on the shape and size of the circular wire in a complicated but calculable manner. String theory has developed a very similar picture for scattering amplitudes. Consider for example a scattering process involving N gluons with four-momenta . In string theory, we are now instructed to build a wire in four-dimensional space by placing the momenta one after another such that the tail of
Figure 1
Gluon momenta are placed such that the tail of starts at the head of . The resulting figure forms a closed wire with piecewise linear sections.