# [NCTS-NTU String Theory Seminars] 1. Soft photon theorem from a CFT Ward identity & 2. Holomorphic CFTs, topological modular forms, and Moonshine

• Event Date: 2022-05-13
• Speaker: Dr. Karan Fernandes (NTU) & Dr. Ying-Hsuan Lin (Harvard Univ.)  /  Host: Prof. Yu-tin Huang (NTU)
Place: RM815, Dept. of Physics, NTU (Hybrid)

Titles:
[NCTS-NTU String Theory Seminars]
1. Soft photon theorem from a CFT Ward identity
2. Holomorphic CFTs, topological modular forms, and Moonshine

Speakers:
1. Dr. Karan Fernandes (NTU)
2. Dr. Ying-Hsuan Lin (Harvard Univ.)

Time:
May 13, 2022 (Fri)
14:30-15:30 (Taiwan time)
16:00-17:00 (Taiwan time)

Venue: RM815, Dept. of Physics, NTU (Hybrid)
Classical soft theorems applied to a probe particle scattering on AdS$_4$ spacetimes reveal the existence of $\frac{1}{\omega^2 L^2}$ corrections to the leading soft photon factor on asymptotically flat spacetimes. In this talk, I will discuss how one can obtain this result from the Ward identity of a conformal field theory with global $U(1)$ symmetry. We follow the approach of Hijano and Neuenfeld wherein the Weinberg soft photon theorem was derived in the $L \to \infty$ limit of the Ward identity. By considering $1/L^2$ corrections, we find an expression of a corresponding soft photon creation operator in terms of the current on the boundary. The corrected soft photon theorem resulting from the Ward identity involves an additional angular integration over low frequency modes compared to the flat spacetime result. By approximating this integral over its dominant contribution, we recover the corrected soft theorem derived previously using classical soft theorems.