Add comment on the code and use approximation

Core/include/Acts/Seeding/SeedFinder.ipp

@@ -369,10 +369,13 @@ SeedFinder<external_spacepoint_t, grid_t, platform_t>::getCompatibleDoublets(
       // We check the interaction point by evaluating the minimal distance
       // between the origin and the straight line connecting the two points in
       // the doublets. Using a geometric similarity, the Im is given by
-      // yNewFrame * rM / deltaR <= m_config.impactMax We use squared quantities
-      // to avoid square roots
-      if (yNewFrame * yNewFrame * rM * rM <=
-          m_config.impactMax * m_config.impactMax * deltaR2) {
+      // yNewFrame * rM / deltaR <= m_config.impactMax
+      // However, we make here an approximation of the impact parameter
+      // which is valid under the assumption xNewFrame / yNewFrame is small
+      // The correct computation would be:
+      // yNewFrame * yNewFrame * rM * rM <= m_config.impactMax *
+      // m_config.impactMax * deltaR2
+      if (std::abs(rM * yNewFrame) <= impactMax * xNewFrame) {
         // check if duplet cotTheta is within the region of interest
         // cotTheta is defined as (deltaZ / deltaR) but instead we multiply
         // cotThetaMax by deltaR to avoid division