Theoretical achievements and questions in the quenching problem*

A systematic reduction of spectroscopic factors was obtained by nucleon removal reaction analysis as in Ref.[1,2]. Triggered by those results, the quenching problem has been discussed over almost a decade. Recently, new results with different types of reaction have been reported and less systematics was found.
In this talk I will overview recent achievements and remaining questions to resolve this problem, from knockout reaction theory point of view in particular. For extracting spectroscopic factors from knockout experiment, quantitative accuracy of the reaction theory adopted to reaction analysis is, of course, essential.
I will introduce our recent study on validity of three reaction theories for knockout analysis, i.e., Distorted Wave Impulse Approximation (DWIA), Transfer-to-the-Continuum model (TC), and Faddeev/Alt-Grassberger-Sandhas method (FAGS). In this study it has been shown that DWIA, which is the model most commonly adopted to knockout reaction analyses, gives consistent knockout cross section with those of TC and FAGS, once the same ingredients are given to the reaction calculation.
I also introduce a possible reason for the systematic quenching found in nucleon removal reaction analysis. The key will be the adiabatic approximation.

[1] A. Gade et al., Phys. Rev. C 90, 057602 (2014).
[2] J. A. Tostevin and A. Gade, Phys. Rev. C 77, 044306 (2008).