New Physics search with Experiment E36 at J-PARC

We are potentially standing at the precipice in the quest for discovery of New Physics (NP) beyond the SM by performing a precision test of lepton universality. Experiment E36 conducted at J-PARC in Japan tests lepton universality in the $R_{K} = \Gamma(K_{e2})/\Gamma(K_{\mu2})$ ratio. In the SM, the ratio of leptonic $K^{+}$ decays is highly precise with an uncertainty of $\Delta R_{K} /R_{K} = 4\cdot10^{-4}$. Any observed deviation from the SM prediction would break the universality of the lepton couplings and provide a clear indication of NP beyond the SM. The E36 detector apparatus allows sensitivity to search for sterile neutrinos below 300 MeV/$c^2$ and light $U(1)$ gauge bosons, which could be associated with dark matter or explain established muon-related anomalies such as the muon g-2 value, and perhaps the proton radius puzzle. E36 data taking was completed in 2015. A scintillating fiber target was used to stop a beam of up to 1.2 Million $K^+$ per spill. $K^+$ decay products were detected with a large-acceptance toroidal spectrometer capable of tracking charged particles with high resolution, combined with a CsI(Tl) photon calorimeter with large solid angle covering $75\%$ of $4\pi$ and particle identification systems. The status of the data analysis will be presented.