Stop co-annihilation in the minimal supersymmetric standard model revisited

We reexamine the stop co-annihilation scenario of the minimal supersymmetric standard model, wherein a binolike lightest supersymmetric particle has a thermal relic density set by co-annihilations with a scalar partner of the top quark in the early universe. We concentrate on the case where only the top partner sector is relevant for the cosmology, and other particles are heavy. We discuss the cosmology with focus on low energy parameters and an emphasis on the implications of the measured Higgs boson mass and its properties. We find that the irreducible direct detection signal correlated with this cosmology is generically well below projected experimental sensitivity, and in most cases lies below the neutrino background. A larger, detectable, direct detection rate is possible, but is unrelated to the co-annihilation cosmology. LHC searches for compressed spectra are crucial for probing this scenario.

Figure 1

Impact of $\delta \rho$ (excluded red region), vacuum stability (excluded purple region) and Higgs phenomenology (excluded orange region) on the $\tilde{t}$ sector. Also shown in green is the region that yields a observationally consistent Higgs boson mass $122<m_h<128\mathrm{GeV}$; there are two overlapping regions corresponding to different sign choices for the gluino mass. (a) $m_{{\tilde{t}}_1}=600\mathrm{GeV}$, (b) $m_{{\tilde{t}}_1}=1.5\mathrm{TeV}$. Also shown are contours where the sbottom is degenerate with the stop (solid) or within $\pm 5\mathrm{GeV}$ [dashed $(-)$/dotted $(+)$].

Figure 2

The mass difference $\mathrm{\Delta }m$ required to obtain the correct relic density as a function of the LSP mass $m_{\chi }$. (a) All allowed masses, only constrained by the stability of the vacuum. (b) Requiring a Higgs mass between 122 and 128 GeV. The blue (green) band indicates the range of mass splittings for a ${\tilde{t}}_1$ (${\tilde{b}}_L$) NLSP due to the different values of the stop mixing angle. The solid black lines indicate the naive expectation if only QCD processes contribute to the relic density calculation. The dot-dashed black line marks the mass splitting consistent with only a right-handed light stop contributing to co-annihilations.

Figure 3

Triangle $\chi \chi h$ [(a) and (b)] and box $gg\chi \chi$ (c) diagrams contributing to the dark matter nucleon coupling.

Figure 4

The spin-independent direct detection cross section ${\sigma }_{SI}$ as a function of $m_{\chi }$. In red are all points in our scan, which include points that violate the vacuum stability constraint on $X_t$ by up to 50%. Blue points satisfy indirect constraints ($\delta \rho$, Higgs signal strength, and vacuum stability), see Sec. 2a, as well as direct searches at the LHC, see Sec. 2c. Green points also realize a Higgs mass in the window $122\mathrm{GeV}<m_h<128\mathrm{GeV}$, see Sec. 2b. The expected sensitivity of the future LZ experiment and the ultimate sensitivity achievable in light of the irreducible neutrino background are indicated by a solid and dashed line respectively, taken from Ref. [80].