Mainly axion cold dark matter in the minimal supergravity model

We examine the minimal supergravity (mSUGRA) model under the assumption that the strong CP problem is solved by the Peccei-Quinn mechanism. In this case, the relic dark matter (DM) abundance consists of three components: {\it i}). cold axions, {\it ii}). warm axinos from neutralino decay, and {\it iii}). cold or warm thermally produced axinos. To sustain a high enough re-heat temperature (T_R\agt 10^6 GeV) for many baryogenesis mechanisms to function, we find that the bulk of DM should consist of cold axions, while the admixture of cold and warm axinos should be rather slight, with a very light axino of mass \sim 100 keV. For mSUGRA with mainly axion cold DM (CDM), the most DM-preferred parameter space regions are precisely those which are least preferred in the case of neutralino DM. Thus, rather different SUSY signatures are expected at the LHC in the case of mSUGRA with mainly axion CDM, as compared to mSUGRA with neutralino CDM.

💡 Research Summary

The paper investigates the minimal supergravity (mSUGRA) framework under the assumption that the strong CP problem is solved by the Peccei‑Quinn (PQ) mechanism. In this setting the dark‑matter (DM) sector contains three components: (i) cold axions, (ii) warm axinos produced non‑thermally from neutralino decays, and (iii) axinos produced thermally in the early universe. The authors are motivated by the need for a relatively high reheating temperature (TR ≳ 10⁶ GeV) in order to accommodate popular baryogenesis mechanisms such as non‑thermal leptogenesis or Affleck‑Dine leptogenesis, while simultaneously avoiding the gravitino problem that plagues supersymmetric models with low‑scale supersymmetry breaking.

Axions are generated via the vacuum‑misalignment mechanism. For a PQ breaking scale fₐ/N in the range 10¹¹–10¹² GeV the resulting relic density matches the observed ΩCDM h²≈0.11, provided the axion mass lies in the conventional window 10⁻⁵–10⁻² eV. Axinos, the fermionic superpartner of the axion, can be the lightest supersymmetric particle (LSP). Their abundance receives contributions from (a) neutralino (χ̃₁⁰) decays χ̃₁⁰→ã γ (non‑thermal production, NTP) and (b) thermal scattering/decay processes (thermal production, TP). The NTP component is proportional to the neutralino relic density and the ratio m_ã/m_χ̃₁⁰; for a neutralino mass of order 100 GeV and a typical over‑abundant neutralino density (Ω_χ̃₁⁰ h²∼10) an axino mass below ∼1 GeV reduces the total DM density to the observed value. However, axinos with m_ã ≲ 1–10 GeV are warm or hot DM, which is disfavored by structure‑formation constraints. Thermal production yields an axino relic density Ω_TP h²≈5.5 gₛ⁶ ln(1.211/gₛ) (fₐ/N)⁻² (m_ã/0.1 GeV) (TR/10⁴ GeV). For m_ã > 0.1 MeV the TP axinos behave as cold DM.

To reconcile a high TR with the gravitino bound, the authors assume a heavy gravitino (m_{3/2} ≫ 5 TeV), which decays before big‑bang nucleosynthesis, allowing TR up to 10⁹ GeV. Nevertheless, successful thermal leptogenesis would require TR > 10⁹ GeV, in tension with the axion/axino sector. Consequently the paper focuses on non‑thermal leptogenesis (inflaton‑driven production of heavy right‑handed neutrinos) and Affleck‑Dine leptogenesis, both of which are viable for TR ≈ 10⁶–10⁸ GeV. These temperatures are also compatible with the production of axions and axinos as described above.

Scanning the mSUGRA parameter space (m₀, m_{1/2}, A₀, tan β, sign μ), the authors find that the regions which yield a predominantly axion CDM (with a small admixture of ∼100 keV axinos) are precisely those that are disfavored when neutralino CDM is assumed. In particular, large scalar masses (m₀ ≈ 2–5 TeV) and moderate to large gaugino masses (m_{1/2} ≈ 1–2 TeV) are required to achieve TR ≳ 10⁶ GeV while keeping the thermally produced axino density under control. In this regime the neutralino is the next‑to‑lightest supersymmetric particle (NLSP) and decays rapidly to the axino, so it does not contribute significantly to the relic density.

The altered spectrum has clear implications for LHC phenomenology. Heavy squarks and sleptons suppress direct scalar production, making gluino pair production the dominant supersymmetric process. Gluinos, being relatively lighter, decay via three‑body channels (e.g., g̃→qq̄ χ̃₁⁰→qq̄ ã γ) or through cascades involving off‑shell squarks, ultimately yielding final states with multiple jets, missing transverse energy carried by the invisible axino, and possibly soft photons from the χ̃₁⁰→ã γ transition. This signature pattern differs markedly from the classic neutralino‑LSP scenario, where leptonic cascades and clean dilepton edges are expected.

In summary, the paper demonstrates that a high reheating temperature compatible with viable baryogenesis can be achieved in mSUGRA if the dark sector is dominated by cold axions, with a sub‑dominant component of very light (∼100 keV) axinos. This requirement pushes the preferred supersymmetric parameter space into regions with heavy scalars and relatively light gluinos, leading to distinctive LHC signatures dominated by gluino production and atypical decay chains. The work highlights the importance of considering mixed axion/axino dark matter in supersymmetric model building and suggests new directions for experimental searches at the LHC.