First spin correlation measurement for a pair of relativistic massive fermions

Top view showing the left Mott polarimeter. The polarised electron beam enters from the right and scatters on the Be target in the Moller chamber (the quasi-triangular piece). Møller scattering occurs for less than 10-5 beam electrons. Most of the beam electrons continue down the beam pipe towards the beam dump where they are absorbed. The left Møller electrons of interest move in the direction towards the left lower corner of the photo. Their spin polarization is analysed through scattering off an Au target in the Mott polarimeter. The irregular hexagonal chamber of this polarimeter is seen just after the short connecting pipe. Most of the Moller electrons continue down the thick pipe and are absorbed in the Møller dump. The Au target is viewed by two (here: scintillation) counters based on photomultipliers) which detect the Moller electrons scattered to the left or to the right w.r.t. the incoming direction, at 120 degrees (i.e. backwards). Scattering of interest happens for a fraction again less than 10-5 of Møller electrons and occurs in a plane defined by theses two counters and the spin projection on the direction perpendicular to this plane can be measured from asymmetry of counts in these two counters. The two blocks with 4 screws sticking out are made of Pb and make the shielding of the scintillation counters. The tower perpendicular to the Mott block is the Au target moving device to enable intertwining target – no target measurements. Such a procedure is necessary to enable extracting a rather small effect over a significant background.

In the Feb. 2020 data taking the photomultipilers were replaced by photodiodes, arranged in a 4×4 matrix, in optical contact with the scintillator, acting as a silicon photomultiplier for each of the 4 counters in the final configuration.