Hans Heinrich Euler (b. 6 October 1909 in Merano, d. 1941) was a German physicist. He received his PhD in 1935 at the University of Leipzig under Werner Heisenberg with a thesis Über die Streuung von Licht an Licht nach der Diracschen Theorie (On the scattering of light by light based on Dirac's theory). Euler was the first physicist who was able to show that Paul Dirac's introduction of the positron opens the possibility that photons in electron-positron pair production scatter with each other and calculated the cross section for this process in his PhD thesis.