Neutralino-stop coannihilation in the CMSSM

Neutralino as the lightest supersymmetric particle (LSP) is a candidate for supersymmetric dark matter. It is known that coannihilation effects could be important in neutralino relic density calculation. Here we present some results on neutralino-stop coannihilation in the CMSSM. In this model, the lighter stop can be degenerate with the lightest neutralino chi when |A_0| is not equal to 0 and large, more specifically when the lighter stop mass is suppressed by large off-diagonal terms in the stop square mass matrix. In the region where the stop is slightly heavier than the neutralino, the coannihilation effect brings the relic density into the range favored by cosmology. While the neutralino-stop coannihilation channels do not increase the range of m_chi, they do extend the cosmologically preferred region to larger values of m_0.