TeV gamma-rays and the largest masses and annihilation cross sections of neutralino dark matter

TL;DR

The paper tackles the question of how massive neutralino dark matter can be within MSSM frameworks given TeV gamma-ray observations and the WMAP relic density. Using a model independent gamma-ray flux analysis and exploring minimal SUSY breaking schemes (mSUGRA, mAMSB) and their non universal Higgs mass extensions (NUHM), it derives upper bounds on the annihilation cross section and identifies the most favorable regions in parameter space, including resonant and coannihilation scenarios. Nonperturbative electroweak and QCD effects are shown to significantly modify the viable mass ranges, allowing neutralinos from a few hundred GeV up to over 100 TeV under special conditions, while maintaining consistency with the relic density and (in some cases) with direct detection prospects. The work provides practical benchmarks for indirect detection rates, clarifies the interpretation of gamma-ray data from the Galactic center, and highlights the complementary reach of gamma-ray observations and direct detection experiments for probing heavy neutralinos. $\langle\sigma v\rangle$, $m_χ$, NUHM, gluino coannihilation, Higgs resonance, Sommerfeld enhancements, and J-factor analyses are central to the study and its implications for future searches.

Abstract

Motivated by the interpretation of the recent results on the TeV gamma radiation from the Galactic center, including the new 2004 HESS data, as a by-product of dark matter particles annihilations, we address the question of the largest possible neutralino masses and pair annihilation cross sections in supersymmetric models. Extending the parameter space of minimal models, such as the mSUGRA and the mAMSB scenarios, to general soft SUSY breaking Higgs masses gives access to the largest possible pair annihilation rates, corresponding to resonantly annihilating neutralinos with maximal gaugino-higgsino mixing. Adopting a model-independent approach, we provide analytical and numerical upper limits for the neutralino pair annihilation cross section. A possible loophole is given by the occurrence of non-perturbative electro-weak resonances, a case we also consider here. We then show that a thorough inclusion of QCD effects in gluino (co-)annihilations can, in extreme scenarios, make neutralinos with masses in the hundreds of TeV range, well beyond the s-wave unitarity bound, viable dark matter candidates. Finally, we outline the ranges of neutralino masses and cross sections for models thermally producing a WMAP relic abundance, thus providing reference values for ``best-case'' indirect SUSY dark matter detection rates.

Figures (13)

• Figure 1: The minimal $\chi^2$ resulting from fits to the 2003 HESS data (left panel), and to the full 2003+2004 HESS data set (right panel) on the gamma-rays flux from the Galactic Center, as a function of the annihilating Dark Matter particle mass, for different final state channels (see the text for details), including, as a thick black line, the " best spectral function" case. The two horizontal lines in each panel indicate the 90% and 95% C.L. exclusion limits corresponding to the set of data under consideration.
• Figure 2: Iso-confidence-level contours of " best spectral functions" fits to the Cangaroo-II and to the 2003 and 2004 HESS data, in the plane defined by the annihilating particle mass and by the quantity $(\sigma J)$, defined in Eq. (\ref{['eq:sigmaj']}).
• Figure 3: Confidence level contours in the $(\langle\sigma v\rangle,J)$ plane, for the full HESS data set. The upper $x$-axis indicates the "boost factor", with respect to the NFW halo profile, i.e.$J/J_{NFW}$. The red arrows indicate the values of $J$ for a sample of viable Milky Ways halo profiles. In the upper right panel we show the confidence level regions on the $(m_{\chi},(\sigma J))$ plane for the 2003+2004 HESS data, in the " best spectral functions" approach.
• Figure 4: (Left panel): Points, in the mSUGRA parameter space at $A_0=0$ and at $m_{\chi}=m_{\widetilde{\tau}_1}$, giving $\Omega_\chi h^2=0.13$, i.e. the largest possible neutralino masses in the stau coannihilation region, for positive (solid blue line) and negative (dashed red line) $\mu$. (Right panel): Points, in the mSUGRA parameter space at $A_0=0$ and at $m_{\chi}=m_{A}/2$, giving $\Omega_\chi h^2=0.13$, i.e. the largest possible neutralino masses in the funnel region, for positive (solid blue line) and negative (dashed red line) $\mu$.
• Figure 5: The pair annihilation cross sections of the largest mass neutralinos in the mSUGRA and mAMSB models giving a WMAP neutralino relic density. The shaded areas correspond to the 90% C.L. contours for the Cangaroo-II data (left, gauge bosons final state case) and for the HESS data (right, best spectral function case, the gauge boson final state being ruled out, see Fig. \ref{['fig:chi2']}, right), for a NFW profile (upper shaded areas) and for the adiabatically contracted N03 profile (lower shaded areas). The upper (lower) dotted lines correspond to extrapolations of the cross section of mAMSB (HB/FP region of mSUGRA) neutralinos giving a relic abundance outside the 2-$\sigma$ WMAP range.