# sigma v for Majorana annihilation

Hi,

Using CalcHEP, I've computed cross-sections (sigma and sigma*v with v->0) for fermion DM annihilating to b quarks through scalar, vector, and axial vector couplings. I've also done these by hand. The CalcHEP numerical results agree with the by-hand results for all cases /except sigma*v with v->0/ for fermion DM annihilating through a vector with a purely axial coupling. Interestingly, sigma and sigma*v with v->0 for fermion DM annihilating through a spin-1 particle with vector couplings or with both vector and axial couplings agrees with the by-hand result, as does the result for sigma (not sigma*v) for the DM annihilating with only axial couplings.

Furthermore, if I extract the symbolic results (e.g. from symb1.m) and plug in the four-vectors that are required for the zero-velocity limit, then I /do/ agree with the by-hand calculation of sigma*v with v->0. So, seemingly, there is something that goes wrong when CalcHEP evaluates that symbolic result.

Any ideas what might be going wrong?

Thanks,

Sam

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