16.3 Luescher analysis

The phase shift can be computed from the formula

$$\cot{ \delta}=\frac{Z_{00}(1,q^2)}{\pi^{3/2}\gamma q}$$ where $\gamma=E/E_{CM}$, $q=kL/2\pi$ with $k$ the scattering momentum $$k^2=\frac{E_{CM}^2}{4}-m^2 = \frac{E^2-\vec{P}^2}{4}-m^2.$$ The Energy in the center of mass is related to the one in a generic frame with total momentum $\vec{P}$ via $$E_{CM}^2=E^{2}_{imp}-\vec{P}^2\,.$$ $E_{imp}$ is the energy measured in the lattice $$E=E^{measured}-E^{free-latt}+E^{free-cont}$$ $$E^{free-latt} = \cosh^{-1}{\left( \cosh(m) +\frac{1}{2}\left( \sum_{i=1}^{3}4 \sin\left(\frac{ p_{1i}}{2}\right)^2\right)\right)} \\ + \cosh^{-1}{\left( \cosh(m) +\frac{1}{2}\left( \sum_{i=1}^{3}4 \sin\left(\frac{ p_{2i}}{2}\right)^2\right)\right)} \,.$$

For the $Z$ function we use the rzeta package. The fit function for the phase shift is

$$\frac{k}{m} \cot{ \delta}=\frac{1}{a_0m}+\frac{r_0 m }{2}\frac{k^2}{m^2}$$

$P[0]=am$ , $P[1]=r_0m$

$$\begin{gather} \chi^2/d.o.f.=0.976672 \\ P[0]=-0.15212\pm (0.0014) \\ P[1]=-2.83566\pm (0.19) \\ \end{gather}$$ { $$\begin{gather} C=\begin{pmatrix} 1& 0.000941\\ 0.000941& 1\\ \end{pmatrix} \end{gather}$$ }

16.3.1 $M_L$ in zeta func and $M_\infty$ as normalization

$$\begin{gather} \chi^2/d.o.f.=0.975176 \\ P[0]=-0.152073\pm (0.0014) \\ P[1]=-2.83815\pm (0.19) \\ \end{gather}$$ { $$\begin{gather} C=\begin{pmatrix} 1& 0.000938\\ 0.000938& 1\\ \end{pmatrix} \end{gather}$$ }

16.3.2 Using $M_\infty$

$$\begin{gather} \chi^2/d.o.f.=2.31189 \\ P[0]=-0.15966\pm (0.0036) \\ P[1]=-3.15533\pm (0.24) \\ \end{gather}$$ { $$\begin{gather} C=\begin{pmatrix} 1& 0.00283\\ 0.00283& 1\\ \end{pmatrix} \end{gather}$$ }