Validity of Goldstone Theorem at Two Loops in Noncommutative U(N) Linear   Sigma Model
  The scalar theory is ultraviolet (UV) quadratically divergent on ordinary spacetime. On noncommutative (NC) spacetime, this divergence will generally induce pole-like infrared (IR) singularities in external momenta through the UV/IR mixing. In spontaneous symmetry breaking theory this would invalidate the Goldstone theorem which is the basis for mass generation when symmetry is gauged. We examine this issue at two loop level in the U(N) linear sigma model which is known to be free of such IR singularities in the Goldstone self-energies at one loop. We analyze the structures in the NC parameter (\theta_{\mu\nu}) dependence in two loop integrands of Goldstone self-energies. We find that their coefficients are effectively once subtracted at the external momentum p=0 due to symmetry relations between 1PI and tadpole contributions, leaving a final result proportional to a quadratic form in p. We then compute the leading IR terms induced by NC to be of order p^2\ln(\theta_{\mu\nu})^2 and p^2\ln\tilde{p}^2 (\tilde{p}_{\mu}=\theta_{\mu\nu}p^{\nu}) which are much milder than naively expected without considering the above cancellation. The Goldstone bosons thus keep massless and the theorem holds true at this level. However, the limit of \theta\to 0 cannot be smooth any longer as it is in the one loop Goldstone self-energies, and this nonsmooth behaviour is not necessarily associated with the IR limit of the external momentum as we see in the term of p^2\ln(\theta_{\mu\nu})^2. 
Yi Liao (ITP, Leipzig)