Explaining PAMELA and WMAP data through Coannihilations in Extended SUGRA with Collider Implications

The PAMELA positron excess is analyzed within the framework of nonuniversal SUGRA models with an extended $U(1)^n$ gauge symmetry in the hidden sector leading to neutralino dark matter with either a mixed Higgsino-wino LSP or an essentially pure wino dominated LSP. The Higgsino-wino LSP can produce the observed PAMELA positron excess and satisfy relic density constraints in the extended class of models due to a near degeneracy of the mass spectrum of the extended neutralino sector with the LSP mass. The simultaneous satisfaction of the WMAP relic density data and the PAMELA data is accomplished through a co-annihilation mechanism ($B_{\rm Co}-mechanism$), and leads to predictions of a neutralino and a chargino in the mass range (180-200) GeV as well as low lying sparticles accessible at colliders. We show that the models are consistent with the antiproton constraints from PAMELA as well as photon flux data from EGRET and FERMI-LAT. Predictions for the scalar neutralino proton cross section relevant for the direct detection of dark matter are also discussed and signatures at the LHC for these PAMELA inspired models are analyzed. It is shown that the mixed Higgsino-wino LSP model will be discoverable with as little as 1 fb$^{-1}$ of data and is thus a prime candidate for discovery in the low luminosity runs at the LHC.

💡 Research Summary

The paper addresses the long‑standing tension between the PAMELA positron excess and the relic‑density constraint from WMAP within a well‑motivated supersymmetric framework. The authors work in a non‑universal supergravity (SUGRA) setting and extend the hidden sector by a set of Abelian gauge groups $U(1)^n$. This extension generates an enlarged neutralino sector containing several hidden‑sector neutralinos that are nearly degenerate in mass with the lightest supersymmetric particle (LSP) of the visible sector. Two concrete realizations are studied: (i) a mixed Higgsino–wino LSP (referred to as the H‑W LSP) and (ii) an almost pure wino LSP (the W LSP). Both scenarios predict a light LSP in the 180–200 GeV range, but the mixed case is phenomenologically richer because the Higgsino component enhances the spin‑independent scattering cross‑section while the wino component drives the dominant annihilation channel into $W^+W^-$.

A central problem for any wino‑dominated model is that the thermal relic density is typically too low: the large $SU(2)L$ coupling makes the annihilation cross‑section $\langle\sigma v\rangle$ much larger than the value required to obtain $\Omega{\rm DM}h^2\simeq0.11$. The authors solve this by invoking a co‑annihilation mechanism they label the $B_{\rm Co}$‑mechanism. Because the hidden‑sector neutralinos and the lightest chargino $\chi^\pm$ are within a few percent of the LSP mass, they remain in thermal equilibrium with the LSP well after the usual freeze‑out temperature. Their presence effectively reduces the overall annihilation rate of the LSP ensemble, allowing a relic density consistent with WMAP. This “near‑degeneracy” is a natural outcome of the $U(1)^n$ extension: the hidden gaugino masses are tied to the visible‑sector gauginos through the SUGRA boundary conditions, and small kinetic mixing parameters keep the mass splittings tiny.

The same annihilation channel that rescues the relic density—$LSP,LSP\to W^+W^-$—also produces high‑energy positrons when the $W$ bosons decay. By solving the diffusion‑loss equation with standard galactic propagation parameters, the authors demonstrate that the predicted positron fraction matches the PAMELA data for boost factors of order unity to a few, far smaller than the $\mathcal{O}(10^2–10^3)$ boosts required in many alternative models. Importantly, the $W^+W^-$ final state yields a suppressed antiproton flux, keeping the model safely within the PAMELA antiproton limits. The associated gamma‑ray spectrum, dominated by final‑state radiation and inverse‑Compton scattering, is shown to be compatible with EGRET and Fermi‑LAT measurements.

Direct detection prospects are also discussed. The Higgsino admixture in the H‑W LSP raises the spin‑independent neutralino–proton cross‑section to the $10^{-9}$–$10^{-8}$ pb range, within reach of upcoming experiments such as LZ, XENONnT, and DARWIN. The pure wino case yields a smaller cross‑section, but still above the neutrino floor for the mass range considered.

Collider signatures at the LHC are a highlight of the work. Because the LSP and the lightest chargino are nearly degenerate, the traditional missing‑energy plus jets searches lose efficiency. Instead, the authors propose focusing on leptonic final states arising from $\chi^\pm\to W^\pm\chi^0$ decays, where the $W$ subsequently decays leptonically. The resulting signature consists of multiple isolated leptons (often two or three), modest missing transverse energy, and possibly soft jets from initial‑state radiation. Monte‑Carlo simulations indicate that with as little as $1,$fb$^{-1}$ of 13 TeV data, a $5\sigma$ excess can be achieved over the Standard Model background for the mixed Higgsino–wino scenario. The pure wino case requires a somewhat larger integrated luminosity but remains discoverable in the early LHC run.

Finally, the paper outlines a roadmap for future tests. The AMS‑02 and DAMPE experiments will refine the high‑energy positron spectrum, potentially discriminating between the mixed and pure wino cases through subtle spectral features. The Cherenkov Telescope Array (CTA) will improve constraints on the associated gamma‑ray flux, while next‑generation direct‑detection experiments will probe the predicted spin‑independent cross‑sections. At the LHC, higher luminosities (≥ 100 fb$^{-1}$) will allow mass reconstruction of the compressed electroweakinos, providing a direct measurement of the Higgsino–wino mixing angle.

In summary, the authors present a coherent and testable framework that simultaneously explains the PAMELA positron excess and the WMAP relic density by exploiting co‑annihilation in an extended SUGRA model with hidden $U(1)^n$ gauge symmetries. The model makes concrete predictions for indirect detection (positrons, antiprotons, gamma rays), direct detection (spin‑independent cross‑sections), and collider observables (compressed electroweakino spectra). Its compatibility with existing data and its clear discovery prospects in the low‑luminosity LHC runs make it a compelling candidate for new physics beyond the Standard Model.