In Wino Veritas? Indirect Searches Shed Light on Neutralino Dark Matter

Abstract: Indirect detection constraints on gamma rays (both continuum and lines) have set strong constraints on wino dark matter. By combining results from Fermi-LAT and HESS, we show that: light nonthermal wino dark matter is strongly excluded; thermal wino dark matter is allowed only if the Milky Way dark matter distribution has a significant (>~0.4 kpc) core; and for plausible NFW and Einasto distributions the entire range of wino masses from 100 GeV up to 3 TeV can be excluded. The case of light, nonthermal wino dark matter is particularly interesting in scenarios with decaying moduli that reheat the universe to a low temperature. Typically such models have been discussed for low reheating temperatures, not far above the BBN bound of a few MeV. We show that constraints on the allowed wino relic density push such models to higher reheating temperatures and hence heavier moduli. Even for a flattened halo model consisting of an NFW profile with constant-density core inside 1 kpc and a density near the sun of 0.3 GeV/cm^3, for 150 GeV winos current data constrains the reheat temperature to be above 1.4 GeV. As a result, for models in which the wino mass is a loop factor below the gravitino mass, the data favor moduli that are more than an order of magnitude heavier than the gravitino. We discuss some of the sobering implications of this result for the status of supersymmetry. We also comment on other neutralino dark matter scenarios, in particular the case of mixed bino/higgsino dark matter. We show that in this case, direct and indirect searches are complementary to each other and could potentially cover most of the parameter space.

• The paper demonstrates that indirect gamma-ray observations robustly exclude significant parameter space for both thermal and non-thermal wino dark matter.
• It combines Fermi-LAT and HESS data over 100 GeV to 3 TeV to derive strict constraints on the Milky Way’s dark matter halo profiles.
• The findings challenge supersymmetric models by imposing requirements for dark matter cores and heavier moduli, affecting reheating temperatures.

Analyzing Indirect Detection Constraints on Wino Dark Matter

The paper "In Wino Veritas? Indirect Searches Shed Light on Neutralino Dark Matter" by JiJi Fan and Matthew Reece examines the indirect detection constraints on wino dark matter, primarily using gamma-ray emissions as a probe. This analysis leverages data from Fermi-LAT and HESS to place stringent bounds on the viability of wino dark matter across a wide mass spectrum, from 100 GeV to 3 TeV. The study adeptly combines various observational datasets, leading to significant exclusions of both thermal and non-thermal wino dark matter under canonical halo profile assumptions.

Key Findings and Numerical Constraints

First and foremost, the study highlights that for non-thermal light wino dark matter, current observational data leads to a strong exclusion. In more detail, thermal wino dark matter can only be accommodated if the dark matter distribution within the Milky Way possesses a significant core, specifically around 0.4 kpc. Notably, for plausible Navarro-Frenk-White (NFW) and Einasto profiles, covering the entire mass range of winos from 100 GeV to 3 TeV, the data trends towards exclusion.

The research also investigates the implications of such profiles for non-thermal cosmological histories. Within scenarios featuring decaying moduli—scalar fields from string theories which decay and supposedly fill the universe with winos—the required reheat temperatures are effectively constrained. Given an NFW profile with a constant-density core, for instance, a range of wino masses leads to reheat temperatures bounded above 1.4 GeV. This has the practical implication of pushing moduli masses to be more than an order of magnitude heavier than the gravitino mass ($m_{3/2}$) in models where wino masses are loop-factors below $m_{3/2}$.

Implications for Supersymmetric Models

The understanding garnered from this paper plays a pivotal role in commenting on the status of supersymmetry (SUSY). Specifically, the constraints imply substantial pressure on certain supersymmetric models, particularly those that favored winos as dark matter candidates due to their efficient self-annihilation and minimal interactions with nucleons, leading them to be less detectable in direct detection experiments.

The emphasis on the indirect detection through gamma rays underscores complementary probing possibilities alongside direct detection methods. This approach could potentially envelope a significant portion of the parameter space in mixed dark matter scenarios, like mixed bino/higgsino compositions.

Future Directions and Potential Challenges

One of the sobering takeaways from this work is the significant tension these constraints place on models of mini-split SUSY, specifically regarding the allowable mass hierarchy between scalars, gauginos, and gravitinos. The bounded necessity for higher reheating temperatures suggests heavier intermediary fields than those allowed by such models, thus sating some of the theoretical appeal of these scenarios.

Moving forward, the tension between the data's implications and theoretical models calls for refined observational strategies and more precise determinations of halo profiles. The resolution of whether the Milky Way's halo contains a cored density profile versus the canonical cuspy profiles has profound importance, not just for constraining dark matter properties but also for supporting particular supersymmetric frameworks.

The paper sets a rigorous benchmark for theoretical and observational advancements. It posits the determination of dark matter's nature as a confluence of high-energy particle physics and astrophysical observations, where future surveys and theoretical insights must seek harmony. This pursuit will hone our understanding of weakly interacting massive particles (WIMPs) and refine our cosmological history narratives, specifically concerning the intersection of dark matter production mechanisms and the universe's thermal history.