Test Run Report of the P348 Collaboration

The P348 Collaboration preformed test measurements in the H4 SPS beam line during a period of 2 weeks in September-October. The measurements were designed with two main objectives. The first was to test the new detector components such as Micromegas and straw tube tracker, high-energy electron tagging system, ECAL, and HCAL. The second was to explore the feasibility of searching for invisible decays of dark photons in the sub- GeV mass range. Here we report various results of the test measurements. The main outcome of the run is that by using simple selection criteria, without good track definition, we reject background in the signal region down to the level of a few ×10^−8 per incident electron. As we do not see any other issues that represent fundamental problems for the experiment, our conclusion is that after small modifications the P348 detector is ready to start data taking in the year 2016. We recall that the A‘ with a mass in sub-GeV range is still one of the favorable explanation of the muon g-2 anomaly. This mass range is not yet probed by the direct experimental searches. Accumulation of about ≃ 5 × 10^9 e−s would allow either to observed A‘ or completely exclude this explanation for the discrepancy next year.