[Paper Review] Search for relic neutralinos with Milagro

Lazar Fleysher|arXiv (Cornell University)|May 5, 2003

Astrophysics and Cosmic Phenomena15 references4 citations

TL;DR

This paper presents a search for relic neutralinos—candidate dark matter particles—via their annihilation into gamma rays using the Milagro air shower detector. Despite no significant signal, the study sets a stringent upper limit on the product of neutralino-proton cross-section, local dark matter density, and photon yield, constraining supersymmetric dark matter models for neutralino masses up to 1 TeV.

ABSTRACT

The neutralino, the lightest stable supersymmetric particle, is a strong theoretical candidate for the missing astronomical "dark matter". Depending on their annihilation cross section, relic neutralinos from early formation of the Universe trapped in orbits around massive objects may currently be annihilating at measurable rates. The Minimal Supersymmetric extension of the Standard Model predicts that the gamma rays emerging from one of the annihilation modes will give a distinct monochromatic signal with energy between 100GeV and 10TeV, depending on the neutralino mass. An additional "continuum" spectrum signal of photons will be produced by the decay of secondaries produced in the non-photonic annihilation modes. Milagro is an air shower array which uses the water Cherenkov technique and is capable of detecting TeV gamma rays from the direction of the Sun with an angular resolution of less than a degree. It is the first instrument capable of establishing a limit on the gamma-ray flux from neutralino annihilations near the Sun. In this report results of a search for neutralino to photon annihilation with the Milagro gamma-ray observatory are presented.

Motivation & Objective

• To test the hypothesis that relic neutralinos, predicted by supersymmetry, could produce detectable gamma-ray signals via annihilation.
• To establish the first experimental limit on gamma-ray flux from neutralino annihilations near the Sun using a ground-based TeV gamma-ray observatory.
• To constrain the product of the neutralino-proton scattering cross-section, local dark matter density, and photon yield per annihilation event.
• To evaluate the detectability of monochromatic and continuum gamma-ray signals from neutralino annihilations in the solar system.

Proposed method

• Utilized the Milagro water Cherenkov detector to observe extensive air showers initiated by high-energy gamma rays from the Sun.
• Applied event reconstruction techniques including angular resolution, energy estimation, and particle type identification to distinguish gamma rays from cosmic-ray backgrounds.
• Performed Monte Carlo simulations of neutralino annihilation density in the Solar System, showing 40–50% of annihilations occur outside the Sun.
• Used statistical hypothesis testing with Neyman-Pearson criteria to set upper limits on the gamma-ray flux, assuming no prior knowledge of background intensity.
• Constructed a test based on the likelihood ratio of signal versus background, independent of unknown background levels, using a conservative critical region.
• Translated the observed flux upper limit into a constraint on the product of σpχ, ρ₀, and bγ for different neutralino masses.

Experimental results

Research questions

• RQ1 Can relic neutralinos trapped in the Solar System produce a detectable monochromatic gamma-ray signal via annihilation?
• RQ2 What is the expected contribution of neutralino annihilations occurring outside the Sun to the total gamma-ray flux in the solar region?
• RQ3 What upper limit can be placed on the gamma-ray flux from neutralino annihilation near the Sun using Milagro data?
• RQ4 How does the absence of a signal constrain the product of the neutralino-proton cross-section, local dark matter density, and photon yield?

Key findings

• No significant gamma-ray signal was observed from the direction of the Sun, leading to a stringent upper limit on the flux of annihilating neutralinos.
• For a 1 TeV neutralino, the upper limit on the product of the neutralino-proton cross-section, local dark matter density, and photon yield is less than 2.3, assuming no continuum background.
• Monte Carlo simulations indicate that 40–50% of neutralino annihilations occur outside the Sun, potentially enhancing detectability of a diffuse signal.
• The statistical method successfully set an upper limit without prior knowledge of the background level, using a likelihood ratio test invariant to unknown background intensity.
• The study provides the first experimental limit on gamma-ray flux from neutralino annihilations near the Sun using a ground-based TeV detector.