We examine the ability for the Large Area Telescope (LAT) to constrain Minimal Supersymmetric Standard Model (MSSM) dark matter through a combined analysis of Milky Way dwarf spheroidal galaxies. We examine the Lightest Supersymmetric Particles (LSPs) for a set of ~71k experimentally valid supersymmetric models derived from the phenomenological-MSSM (pMSSM). We find that none of these models can be excluded at 95% confidence by the current analysis; nevertheless, many lie within the predicted reach of future LAT analyses. With two years of data, we find that the LAT is currently most sensitive to light LSPs (m_LSP < 50 GeV) annihilating into tau-pairs and heavier LSPs annihilating into b-bbar. Additionally, we find that future LAT analyses will be able to probe some LSPs that form a sub-dominant component of dark matter. We directly compare the LAT results to direct detection experiments and show the complementarity of these search methods.
Astrophysical evidence suggesting that non-baryonic dark matter (DM) comprises nearly 25% of the energy density of the Universe is one of the most compelling arguments for particle physics beyond the Standard Model (SM) [1]. At present, experimental tests of this DM component are almost exclusively limited to gravitational interactions, and few constraints exist on the character of DM. Axions, dark photons, sterile neutrinos and even more exotic theoretical constructs are all plausible DM candidates [2], though models containing a new neutral and stable weakly-interacting massive particle (WIMP) of mass ∼ 100 GeV are by far the most studied. WIMPs are a favorable candidate because their mass and couplings to the SM can naturally give a cosmological relic density in agreement with the experimentally measured value [3]. Additionally, WIMPs point to new physics at the weak scale (∼ 100 GeV -1 TeV), a scale that has been the focus of much theoretical work to explain the stability of the Higgs potential and the origin of electroweak symmetry breaking.
In the past several decades, supersymmetry (SUSY) has been the most widelystudied, and arguably the best-motivated, theoretical framework for physics beyond the SM [4][5][6][7][8][9][10]. In the most attractive SUSY models, an extra matter parity (“Rparity”) symmetry is used to simultaneously explain the stability of the proton and of the Lightest Supersymmetric Particle (LSP). In viable SUSY models the LSP is often the lightest neutralino ( χ0 1 ), which is one of the most widely studied examples of WIMP DM. Generic predictions of SUSY are difficult to obtain, since the minimal consistent SUSY extension of the SM, the Minimal Supersymmetric Standard Model (MSSM) introduces more than 100 free parameters. A typical strategy for overcoming this difficulty is to highly constrain this set of parameters by employing aesthetic assumptions about the physical origin of SUSY at a very high energy (i.e., mSUGRA [11,12]). In contrast, here we study a broader and more comprehensive subset of the MSSM, the phenomenological-MSSM (pMSSM) [13]. The pMSSM is derived from the MSSM using experimental data to eliminate parameters that are free in principle, but highly constrained by observations (e.g., sources of flavor violation in the new physics flavor sector). Thus, the pMSSM provides a compromise between the need to remain flexible and somewhat agnostic in assumptions about yet-undiscovered physics and the need to categorize the range of predictions made by well-motivated models. The LSPs of the pMSSM are viable candidates to comprise some or all of DM, and they may be probed through a variety of experimental approaches.
The possibility of DM-SM interactions having weak-force strength allows an exciting opportunity to detect and characterize the nature of DM via a combination of experimental efforts. For example, weak-strength interactions might lend themselves to study at the LHC, where DM particles could be produced and studied indirectly through missing energy signatures. Additionally, the DM halo permeating our galaxy could be detected directly through scattering interactions between DM particles and nuclei in detectors on Earth. Finally, indirect detection of DM is possible through the astrophysical observation of anomalous energetic SM particles resulting from DM particle annihilation (or decay).
One of the most sensitive instruments for the indirect detection of DM is the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi ). Gamma rays from the final state of DM annihilation (or decay) would be produced preferentially in regions of high DM density and may be detectable by the LAT. Dwarf spheroidal satellite galaxies (dSphs) of the Milky Way are promising targets for the detection of such a signal. These dSphs are DM-dominated and lack active astrophysical production of γ-rays [14,15], a troublesome background in many other searches for DM annihilation. The LAT Collaboration recently presented results constraining the annihilation cross section for a small set of prototypical DM models from a joint likelihood analysis of 10 dwarf spheroidal galaxies [16]. In the present paper, we extend this analysis to an investigation of ∼ 71k pMSSM models previously discussed in the literature [13].
We begin by briefly discussing the techniques employed to generate ∼ 71k pMSSM models and the various constraints imposed in their selection. We next describe the combined likelihood procedure for setting upper limits on the annihilation cross section for each pMSSM DM model using LAT observations of ten Milky Way dSphs. We compare the LAT cross section limits to the actual cross section for each pMSSM model and study the SUSY model dependence of these results in detail. The main findings are: (i ) that the LAT is currently most sensitive to light LSPs (m χ0 1 < 50 GeV) annihilating primarily to τ -pairs, (ii ) that annihilations to τ -pairs are actually harder
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