Theoretical Physics Seminar: Biswajit Sahoo

Status of Soft Theorem in D=4 (Its classical limit and understanding as Ward identity)

In recent years we explored the understanding of the soft factorization property of the S-matrix for a theory containing massless particles(photon/graviton) when the energy of external massless particles are small (soft particles). Though the leading factorization is discovered long ago (1965) by Weinberg, it's understanding in the subleading order (for all loop order in S-matrix) was not much explored prior to our (with Sen) work due to the infrared divergence of S-matrix in D=4. For loop corrected S-matrix, we found that the subleading soft factorization contains terms logarithmic in soft energy. The classical limit of this logarithmic terms in soft graviton theorem provides a new classical tail memory with the known permanent shift between the mirrors of gravitational wave detector. Currently, we are trying to understand whether this soft expansion can be understood as the Ward identity of any asymptotic symmetry.