Proposal for Using DAΦNE as Pulse Stretcher for the Linac Positron Beam

The PADME experiment [1] aims at searching for the dark photon (A') in the e + e - → Aγ ′ process in a positron-on-target experiment, exploiting the positron beam of the DAΦNE Linac at the INFN Frascati National Laboratory (Italy). The Linac can provide a number of positrons in excess of 1010 in a 200 ns long pulse but, in order to keep the pile-up probability in the calorimeter at an acceptable level, the number of positrons for PADME has to be below 105/pulse, in particular by using the attenuating target in the BTF beam-line. The experiment is then essentially limited by the low duty factor (10-5 = 200 ns/20 ms). In this paper, an alternative proposal to use the DAΦNE positron ring as a Linac pulse stretcher, by injecting each pulse into the ring and extracting it by a slow resonant extraction using the m/3 resonance, is presented. This would allow for distributing the positrons of a single Linac pulse over a much longer time interval (0.2-0.5 ms), increasing the duty factor up to ∼1%. The required modifications of the DAΦNE positron transfer line and ring are described. A dedicated lattice for the positron ring has been designed and tracking of the positrons injected in the ring has been performed in order to optimize the extraction parameters and to give a preliminary estimate of the extracted beam characteristics.