add function for computing GL FSE correction factors

R/gl_fse_corr.R

@@ -0,0 +1,18 @@
+#' apply parameter-free Gasser-Leutwyler FSE corrections
+#' @details Apply FSE correction of a meson mass and decay constant
+#' based on eqs. 8 and 9 of Colangeo, Duerr, Haefeli [Nucl.Phys.B 721 (2005)]
+#' going back to Gasser and Leutwyler. Note that we employ the normalisation
+#' in which f_pi = 130.4.
+#' @param xi Numeric vector, the ratio \eqn{M_{ps}^2 / (4 \pi f_\pi)^2}. 
+#' @param lambda Numeric, the dimensionless product \eqn{M_{ps} \cdot L}. 
+#' @return Data frame with two columns containing the correction factors \code{K_mps}
+#' and \code{K_fps} for which we have:
+#' \deqn{ M_{ps}(L) = M_{ps}(\infty) \cdot K_{M_{ps} + \mathcal{O}(M_{ps} \xi^2) }
+#' \deqn{ f_{ps}(L) = f_{ps}(\infty) \cdot K_{f_{ps} + \mathcal{O}(f_{ps} \xi^2) }
+#' @export
+gl_fse_corr <- function(xi, lambda){
+  stopifnot( length(xi) == length(lambda) )
+  g1_tilde <- g1(lambda)
+  return(data.frame(K_Mps = (1+0.5*xi*g1_tilde), K_fps = (1-2*xi*g1_tilde)))
+}
+