# Polarization profile

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# Measuring Beam Polarization Profile with p-Carbon Polarimeters   # Two-dimensional case

Let's assume the polarization and intensity profiles have a gaussian shape:

{\begin{aligned}I_{B}(x,y)&=I_{0,B}\exp \left\{-{\frac {x^{2}}{\sigma _{x,I}^{2}}}-{\frac {y^{2}}{\sigma _{y,I}^{2}}}\right\}\\I_{Y}(x,y)&=I_{0,Y}\exp \left\{-{\frac {x^{2}}{\sigma _{x,I}^{2}}}-{\frac {y^{2}}{\sigma _{y,I}^{2}}}\right\}\\P_{B}(x,y)&=P_{0,B}\exp \left\{-{\frac {x^{2}}{\sigma _{x,P}^{2}}}-{\frac {y^{2}}{\sigma _{y,P}^{2}}}\right\}\\P_{Y}(x,y)&=P_{0,Y}\exp \left\{-{\frac {x^{2}}{\sigma _{x,P}^{2}}}-{\frac {y^{2}}{\sigma _{y,P}^{2}}}\right\}\end{aligned}} Since we are interested only in the width of the polarization profile with respect to the intensity one we can use the following relations:

{\begin{aligned}\sigma _{x,I}^{2}&\equiv \sigma _{y,I}^{2}\equiv 1\\\sigma _{x,P}^{2}&\equiv 1/R_{x}\\\sigma _{y,P}^{2}&\equiv 1/R_{y}\end{aligned}} Integrating from $-\infty$ to $\displaystyle \infty$ over both dimensions we get for the polarization weighted with intensity of either one or both beams:

\displaystyle \begin{align} \frac{\iint P(x,y) I(x,y) dx dy}{\iint I(x,y) dx dy} &= \frac{P_{0}}{\sqrt{1 + R_x} \sqrt{1 + R_y}} \\ \frac{\iint P(x,y) I_B(x,y) I_Y(x,y) dx dy}{\iint I_B(x,y) I_Y(x,y) dx dy} &= \frac{P_{0}}{ \sqrt{1 + \frac{R_x}{2}} \sqrt{1 + \frac{R_y}{2}} }\\ \frac{\iint P_B(x,y) P_Y(x,y) I_B(x,y) I_Y(x,y) dx dy}{\iint I_B(x,y) I_Y(x,y) dx dy} &= \frac{P_{0,B} P_{0,Y}}{\sqrt{1 + \frac{R_{x,B}}{2} + \frac{R_{x,Y}}{2} } \sqrt{1 + \frac{R_{y,B}}{2} + \frac{R_{y,Y}}{2} }}\end{align}

As we normaly measure the average polarization $\langle P\rangle$ given by it is trivial to get the equations for re-weighting factors $k_{SSA}$ and $k_{DSA}$ :

{\begin{aligned}\langle P\rangle _{SSA}&=k_{SSA}\times \langle P\rangle \\\langle P_{B}\cdot P_{Y}\rangle _{DSA}&=k_{DSA}\times \langle P_{B}\rangle \cdot \langle P_{Y}\rangle \end{aligned}} where

{\begin{aligned}k_{SSA}&={\frac {{\sqrt {1+R_{x}}}{\sqrt {1+R_{y}}}}{{\sqrt {1+{\frac {R_{x}}{2}}}}{\sqrt {1+{\frac {R_{y}}{2}}}}}}\\k_{DSA}&={\frac {{\sqrt {1+R_{x,B}}}{\sqrt {1+R_{y,B}}}{\sqrt {1+R_{x,Y}}}{\sqrt {1+R_{y,Y}}}}{{\sqrt {1+{\frac {R_{x,B}}{2}}+{\frac {R_{x,Y}}{2}}}}{\sqrt {1+{\frac {R_{y,B}}{2}}+{\frac {R_{y,Y}}{2}}}}}}\end{aligned}} It is interesting to study the difference between the scale factors $k_{SSA}$ and $\displaystyle k_{DSA}$ . To make things easier we assume the same value for all $\displaystyle R$ 's which is $\sim 0.2$ .

\displaystyle \begin{align} \frac{k_{DSA}}{k_{SSA,B} k_{SSA,Y}} = 1 + \frac{R^2}{4(1+R)}\end{align}

where the last term gives a correction on the order of $\lesssim 1\%$ . Therefore, with good precision we have

{\begin{aligned}\langle P_{B}\cdot P_{Y}\rangle _{DSA}&\approx \langle P\rangle _{SSA,B}\langle P\rangle _{SSA,Y}\end{aligned}} # Time dependent P_SSA

$\displaystyle P_{SSA} = \left(1 + \frac12 R(t)\right) P(t) = (1 + \frac12 R_0 + \frac12 R' t)(P_0 + P't) \approx P_0 + P't + \frac12 R_0 P_0 + \frac12 (R_0 P' + R' P_0) t =P_0 (1 + \frac12 R_0) + (P' + \frac12 (R_0 P' + R' P_0) ) t$