1. Fermi LAT Limits on High-Energy Gamma Lines from WIMP Annihilation
Y. Edmonds KIPAC/SLAC/Stanford
on behalf of the Fermi-LAT Collaboration
Abstract
Annihilating or decaying dark matter particles can produce monochromatic gamma-rays, a distinctive signature for Dark Matter. We present limits on the flux from gamma-ray lines for 11 months of Fermi Large Area Telescope data. The data selection cuts are specific to the line search. Annihilation cross-section and decay lifetime limits for dark matter producing two gammas, and a gamma and Z are presented for two different regions of the Milky Way.
Introduction
WIMP annihilation directly into a photon and a another particle will produce a monochromatic gamma-ray line. This narrow line is a convincing signature of dark matter and is distinguishable from the spectra of astrophysical sources. For annihilation to gamma-Y the photon energy is given by
However, any coupling between dark matter and the photon must be very small and at least loop-suppressed.  This results in a branching ratio for annihilation or decay to photons of not more than 10-1 to 10-4 depending on the theory.  Limits on branching fractions can help constrain dark matter models.
Statistical Analysis and Results
The data is binned in sliding energy windows that reflect the instrument resolution. The likelihood for signal and background events is given by:
We use an unbinned maximum likelihood fit to determine line signal fraction f, and the nuisance parameter g. The background is represented by B(E,g), a powerlaw with index g. The signal pdf, S(E) is defined using fits to MC simulations of the LAT’s response to monochromatic photons generated uniformly over the LAT face. The figure below shows the normalized signal pdf for a number of energies.
The 95% CLUL for signal is calculated from the error on the signal fraction. MINUIT is used for the fitting and error calculation. The coverage and power are shown for a representative energy below.
This figure shows the 95% CLUL on the flux from lines for the two regions of interest.
Dark Matter Constraints
The 95% CLUL on flux from lines can be converted to upper limits on velocity-averaged cross-section <sv> for the gamma-gamma annihilation channel and lower limits on the lifetime t for a decaying WIMP. The flux produced by annihilating WIMPs is given by the integral:
Nγ gives the number of gamma-rays produced in the annihilation. <sv> is the cross-section for a particular annihilation channel. The integral is over the solid angle W for the region of interest and over the line of sight l. We use the NFW, Einasto and Isothermal density profiles for the Milky Way.
s is chosen to give 0.4GeVcm-3 at the sun. For a decaying dark matter particle:
The figures below show the 95%CLUL cross-section for the gamma-gamma and gamma-Z annihilation channels for our two regions of interest. While the all-sky minus GP (|b|>10°) region gives slightly lower flux limits, the all-sky minus GP + GC (|b|>10° | |l|<10°) gives lower cross-section limits. This is due to the cuspy nature of the NFW and Einasto density profiles. The Galactic Center is also the region where the profiles differ the most, causing a larger spread in cross-section values when the GC is included in the search region.
The figures below show the 95%CLLL lifetimes for decay into two gammas and gamma-Z for both regions. Since the line-of-sight integral is linear in dark matter density instead of quadratic the differences between density profiles are less significant than in the case for cross-sections. Fluctuations in the limits reflect statistical fluctuations in the data.
rs = 20 kpc
rs = 20 kpc, a =0.17
rs = 5 kpc
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Data Selection
The dataset uses 11 months of LAT data, from August 7, 2008 to July 21, Events used in this analysis pass diffuse class cuts plus additional cuts: a) a small average charge deposited in the tracker planes (veto against heavy ions); b) a transverse shower size in the calorimeter within a size range expected for electromagnetic showers (veto against hadronic showers and minimum ionizing particles).
In selecting a region for the line search we seek to maximize dark matter signal over background. Most dark matter density profiles show the greatest density towards the Galactic Center. However, gamma-rays sources along the Galactic Plane contribute to the background level. Therefore, we use a high latitude cut to remove the Galactic Plane. Our search regions are all-sky minus the Galactic Plane (|b|>10°) and all-sky minus the Galactic Plane plus the Galactic Center (|b|>10° | |l|<10°).
We remove point sources in the eleven month catalog with a 0.2° radius cut motivated by the LAT’s high energy PSF. In the region including the Galactic Center point sources are not removed.
The data used is for the energy range from 20 to 300 GeV. We use one of the LAT’s energy measurements that is well-suited for the line search. This method uses the longitudinal and transverse shower profile in reconstructing the photon energy.
Below are the counts maps for the all-sky - GP (|b|>10°) and all-sky – GP + GC (|b|>10° | |l|<10°) regions and photons between 20 and 300 GeV.
Conclusion
Cross section upper limits for WIMP annihilation into gamma-gamma are shown for 11 months of Fermi-LAT data in the Milky Way. The cross-sections are on the order of cm3s-1 and are of interest in constraining models with large annihilation cross-sections, such as the “Wino LSP”. For the case of a decaying WIMP we show lifetime limits which may constrain some gravitino models. Paper in preparation.
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