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# Polarization Vectors

In

*Probing the linear polarization of photons in ultraperipheral heavy ion collisions*and*Impact parameter dependence of the azimuthal asymmetry in lepton pair production in heavy ion collisions*it is proposed to study the collision between linearly polarized photons via the azimuthal angle between $q_\perp$ and $P_\perp$ where: $$ q_\perp = p_{1\perp} + p_{2\perp} = k_{1\perp} = k_{2\perp} $$ and $$ P_\perp = (p_{1\perp} - p_{2\perp})/2 $$ Experimentally this is accessible as: $$ \Delta\phi = \Delta\phi[ (p_{1}^+ + p_{2}^-), (p_{1}^+p_{2}^-) ] $$ This observable is a bit hard to visualize, so I put together the widget below. The red line represents the vector sum $(q_\perp)$ and the blue line is $P_\perp$
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