Performance of the diamond active target prototype for the PADME experiment at the DAΦNE BTF

The PADME experiment at the DAΦNE Beam-Test Facility (BTF) is designed to search for the gauge boson of a new U(1) interaction in the process e+e−→γ+A′, using the intense positron beam hitting a light target. The A′, usually referred as a dark photon, is assumed to decay into invisible particles of a secluded sector and it can be observed by searching for an anomalous peak in the spectrum of the missing mass measured in events with a single photon in the final state. The measurement requires the determination of the 4-momentum of the recoil photon, performed by a homogeneous, highly segmented BGO crystals calorimeter. A significant improvement of the missing mass resolution is possible using an active target capable of determining the average position of the positron pulse with a resolution of less than 1 mm. This report presents the performance of a real size (2 × 2 cm2) PADME active target made of a thin (50 μm) diamond sensor, with graphitic strips produced via laser irradiation on both sides. The measurements are based on data collected in a beam test at the BTF in November 2015.