[Paper Review] Implications of the Fermi-LAT Pass 8 Galactic Center Excess on Supersymmetric Dark Matter

A. Achterberg, Melissa van Beekveld+4 more|arXiv (Cornell University)|Sep 29, 2017

Dark Matter and Cosmic Phenomena4 citations

TL;DR

This paper re-evaluates the compatibility of the Fermi-LAT Pass 8 Galactic Center Excess (GCE) with supersymmetric dark matter, focusing on neutralino annihilation into W-bosons or top quarks. It finds that neutralino masses between 85–220 GeV can explain the GCE while remaining consistent with direct detection, LHC, and dwarf spheroidal galaxy constraints, with some models exhibiting low electroweak fine-tuning.

ABSTRACT

The Fermi Collaboration has recently updated their analysis of gamma rays from the center of the Galaxy. They reconfirm the presence of an unexplained emission feature which is most prominent in the region of $1-10$ GeV, known as the Galactic Center GeV excess (GCE). Although the GCE is now firmly detected, an interpretation of this emission as a signal of self-annihilating dark matter (DM) particles is not unambiguously possible due to systematic effects in the gamma-ray modeling estimated in the Galactic Plane. In this paper we build a covariance matrix, collecting different systematic uncertainties investigated in the Fermi Collaboration's paper that affect the GCE spectrum. We show that models where part of the GCE is due to annihilating DM can still be consistent with the new data. We also re-evaluate the parameter space regions of the minimal supersymmetric Standard Model (MSSM) that can contribute dominantly to the GCE via neutralino DM annihilation. All recent constraints from DM direct detection experiments such as PICO, LUX, PandaX and Xenon1T, limits on the annihilation cross section from dwarf spheroidal galaxies and Large Hadron Collider limits are considered in this analysis. Due to a slight shift in the energy spectrum of the GC excess with respect to the previous Fermi analysis, and the recent limits from direct detection experiments, we find a slightly shifted parameter region of the MSSM compared to our previous analysis that is consistent with the GCE. Neutralinos with a mass between $85-220$ GeV can describe the excess via annihilation into a pair of $W$-bosons or top quarks. Remarkably, there are low fine-tuning models among the regions that we have found. The complete set of solutions will be probed by upcoming direct detection experiments and with dedicated searches in the upcoming data of the Large Hadron Collider.

Motivation & Objective

• To assess whether the Fermi-LAT Pass 8 Galactic Center Excess (GCE) can be explained by self-annihilating supersymmetric dark matter particles.
• To update the parameter space of the minimal supersymmetric Standard Model (MSSM) consistent with the GCE using new Pass 8 data and recent experimental constraints.
• To evaluate the viability of neutralino annihilation into W-bosons or top quarks as the origin of the GCE, considering systematic uncertainties in the gamma-ray spectrum.
• To determine whether the viable MSSM models exhibit low electroweak fine-tuning, indicating naturalness.
• To identify regions of the MSSM parameter space that will be probed by upcoming direct detection experiments and LHC searches.

Proposed method

• A covariance matrix is constructed to incorporate systematic uncertainties in the GCE spectrum from the Fermi-LAT Pass 8 analysis, particularly affecting spectral shape and morphology.
• The analysis assumes that part of the GCE flux (1–10 GeV) originates from neutralino dark matter annihilation, with a free power law fitted to the high-energy tail (>10 GeV).
• The phenomenological MSSM (pMSSM) parameter space is scanned, including constraints from direct detection experiments (PICO, LUX, PandaX, Xenon1T), LHC limits, and dwarf spheroidal galaxy annihilation constraints.
• The electroweak fine-tuning is calculated using the method from Refs. [41, 92, 93], focusing on the higgsino component of the lightest neutralino as a key driver of tuning.
• Solutions are classified into two main regions: W⁺W⁻ annihilation (80–120 GeV) and t̄t annihilation (175–220 GeV), with distinct stop and Higgs boson mass dependencies.
• The compatibility of the models with LHC searches for compressed stops, heavy Higgs bosons, and chargino-neutralino cascades is evaluated.

Experimental results

Research questions

• RQ1 Can the Fermi-LAT Pass 8 Galactic Center Excess be explained by neutralino dark matter annihilation in the MSSM, given updated data and constraints?
• RQ2 What is the range of neutralino masses and annihilation channels that remain viable for explaining the GCE while satisfying direct detection and LHC limits?
• RQ3 Are there MSSM models that explain the GCE and simultaneously exhibit low electroweak fine-tuning, indicating naturalness?
• RQ4 How do the new Pass 8 data and systematic uncertainties affect the previously identified parameter regions for the GCE?
• RQ5 Which upcoming experiments—direct detection or LHC—will be able to probe the viable MSSM regions identified in this study?

Key findings

• Neutralinos with masses between 85–220 GeV can explain the GCE via annihilation into W⁺W⁻ or t̄t final states, consistent with Pass 8 data and systematic uncertainties.
• The W⁺W⁻ annihilation channel is viable for neutralinos of 80–120 GeV, with a dominant bino component and a small wino or higgsino admixture, and exhibits low electroweak fine-tuning (<20).
• The t̄t annihilation channel is viable for neutralinos of 175–220 GeV, with two sub-regions: one with compressed stop-neutralino spectra (stop mass near neutralino mass) and another with enhanced higgsino components and heavy Higgs bosons (500–1000 GeV).
• Models in the t̄t region with high higgsino components show low fine-tuning, while those with small higgsino components and light stops exhibit higher tuning.
• The complete W⁺W⁻ region and the higgsino-enhanced t̄t region will be probed by upcoming direct detection experiments, while the LHC can test the stop and heavy Higgs sectors.
• The analysis confirms that the GCE remains consistent with a dark matter interpretation, particularly in natural MSSM models, despite recent constraints from direct detection and LHC searches.