Soft Collinear Effective Theory for Heavy WIMP Annihilation

In a large class of models for Weakly Interacting Massive Particles (WIMPs), the WIMP mass M lies far above the weak scale m_W.  This work identifies universal Sudakov-type logarithms ~ alpha log^2 (2 M/m_W) that spoil the naive convergence of perturbation theory for annihilation processes.  An effective field theory (EFT) framework is presented, allowing the systematic resummation of these  logarithms.  Another impact of the large separation of scales is that a long-distance wavefunction distortion from electroweak boson exchange leads to observable modifications of the cross section.  Careful accounting of momentum regions in the EFT allows the rigorous disentanglement of this so-called Sommerfeld enhancement from the short-distance hard annihilation process.   The WIMP is described as a heavy-particle field, while the electroweak gauge bosons are treated as soft and collinear fields. Hard matching  coefficients are computed at renormalization scale mu ~ 2 M, then evolved down to mu ~ m_W, where  electroweak symmetry breaking is incorporated and the matching onto the relevant quantum mechanical Hamiltonian is performed. The example of an SU(2)_W triplet scalar dark matter candidate annihilating to line photons is used for concreteness, allowing the numerical exploration of the impact of next-to-leading order corrections and log resummation.  For M ~ 3 TeV, the resummed Sommerfeld enhanced cross section is reduced by a factor of ~ 3 with respect to the tree-level fixed order result.