Status of the J-PARC E16 experiment for the measurement of mass spectrum of vector mesons at finite density

The J-PARC E16 experiment aims at measuring the spectral change of vector mesons in nuclei to study the finite density effect on the hadron mass. We will obtain mass spectra via di-electron decays of vector mesons produced in 30 GeV proton-nucleus reactions. Electrons are clean probes that are free from final state interaction, whereas the branching ratio of dilepton decays of vector mesons is small at the order of 10^-4. To accumulate high statistics, we have developed a spectrometer with large acceptance and a high-rate capability to utilize a high intensity proton beam of 10¹⁰ proton/spill (2-sec duration) at J-PARC. Enormous backgrounds of pions are eliminated by two-stage electron identification detectors, a hadron blind detector (HBD) together with a lead-glass calorimeter (LG). The invariant mass is reconstructed with a resolution of 6 MeV/c², realized with the tracking devices of a GEM tracker (GTR) and a silicon strip detector (SSD).

Since 2020, we have continued the construction and commissioning runs of the spectrometer and beamline. In June 2023, the construction of the spectrometer for the first physics data acquisition was completed. In this presentation, we will show the staging strategy and expected physics results, and report the results of the commissioning runs including the observed beam microstructure.