NA64 Search for Dark Photons

The NA64 experiment, known as P348 before official approval, was proposed to the CERN SPSC on January 2014 with main goal the search for the following decay modes A′ → invisible A′ → e−e+ (1) In March 2016 P348 was granted approval by the CERN Research Board and received the title NA64. Since having been promoted, the experiment has been conducting the searches the searches for the processes mentioned above at the CERN SPS. NA64 is a fixed target experiment which utilizes the active beam dump. The detection of rare processes is based on the missing energy techniques. Such techniques are used for particles whose interaction with the detector is very small, so that they escape the detection and carry away some energy. A significant missing energy in the experiment means that such particles are produced. The method of the search for the A′ → invisible decay is as follows. If the A′ exists it could be produced via the kinetic mixing with bremsstrahlung photons in the reaction of high-energy electrons absorbed in an active beam dump (target) followed by the prompt A′ → invisible decay into Dark Matter particles in a hermetic detector: e−Z → e−ZA′ ; A′ → χχ¯ The experiment employed the optimized 100 GeV electron beam from the H4 beam line at the North Area (NA) of the CERN SPS. The H4 provided an essentially pure e− beam for fixed-target experiments. The beam was designed to transport the electrons with the maximal intensity up to ≃ 107 per SPS spill of 4.8 s in the momentum range between 50 and 150 GeV/c that could be produced by the primary proton beam of 400 GeV/c with the intensity up to a few 1012 protons on a beryllium target.The main contribution to the the e− yield from the target was the production of π0 followed by a process π0 → γγ → e−e+. The short-lived π0 decays inside the target, and the electrons were produced through the conversion of the decay photons in a separate converter. Protons and charged secondaries that did not interact in the convertor are separated from the neutrals be deflecting them in a magnetic field to a thick absorber. The electrons produced in the convertor are transported to the NA64 detector inside an evacuated beam-line tuned to an adjustable beam momentum. The set of specialised detectors is summarised below and a detailed schematic is included in Fig.1 1. Electromagnetic calorimeters ECAL 2. Hadronic calorimeters HCAL 3. Beam counters and Hodoscopes 4. Veto counter 5. Synchrotron radiation counter 6. Decay volume 7. Micromegas tracker MM 8. Straw tube chambers