A Calculation of the Sigma Meson Mass in Nuclear Matter
  We calculate the mass and width of the sigma meson in nuclear medium by considering that it couples to two virtual pions and to a pair of nucleon-antinucleon states and to particle-hole states. The mass is calculated by using the spectral function in the Walecka model, finding that it is about 380MeV. In addition, we obtain the value of 340MeV for the width of its spectral function. We find that there is a reduction in the mass value compared with that in vacuum. This result is consistent with those reported by other authors who have used different models predicting a decreasing of the mass as a function of the density. 
J. R. Morones-Ibarra and Ayax Santos-Guevara