US Participation in the Construction of the T2K SuperFGD Detector as Part of the T2K ND280 Upgrade

There have been astonishing advances in neutrino physics  during the last twodecades. Before 1998, there was no neutrino oscillation, meaning no neutrino mass and mixing.Today, neutrino oscillation is firmly established.  Oscillations connect the three neutrino generations with three mixing angles. There are three neutrino masses, but oscillation measurements can only determine the differences between the squares of the masses so there are only two unique mass parameters measurable.  We have in fact  measured all three mixing and two mass parameters. Furthermore, these findings pave the way toward determine themass ordering/hierarchy and to explore Charge-Parity (CP) violation in the lepton sector, whichmay hold a critical key to our understanding of the matter-antimatter asymmetry in the universe,one of the most profound mysteries in science. In fact, T2K, a long baseline neutrino oscillation experiment based in Japan, has recently released results thatshow an initial hint of CP violation (closed 3sranges ofdCPfor both Normal and Inverted massordering centered around -90o) making the prospects of DUNE, a next generation long baseline neutrino oscillation experiment based in U.S., for discoveringCP violation in neutrinos very bright. Neutrino oscillation which requires non-zero neutrinomass is considered the only phenomenon beyond the Standard Model observed in laboratoryvenue today. The T2K experiment uses a 30 GeV proton beam accelerated by the J-PARC accelerator facility.The facility comprises (1) the neutrino beamline, (2) the Near Detector (ND) complex, which islocated 280 m downstream of the neutrino production target and (3) the far detector, Super-Kamiokande, which detects neutrinos at a baseline distance of 295 km from the target.T2K data taking started in January 2010 and stable beam operation at 480 kW beam powerhas been achieved. In all, the total integrated data collected for physics analysis so far is 1.51x1021POT (Protons-On-Target) in neutrino-mode and 1.65x1021POT in antineutrino-mode giving a POT total of 3.16x1021which is 41% of the total approved POT for T2K (7.8x1021). The most recent results from theT2K oscillation analysis are consistent with ?23at maximal mixing,dCP= -p/2 and normal massordering. The CP conservingdCP= 0 and  are now disfavored at 95% C.L.In order to increase the sensitivity for CPV, T2K proposes an extended run ("T2K-II"). Theplan calls for accelerator and beamline upgrades for 1.3 MW beam power and accumulation of datacorresponding to >10x1021POT. The aim of T2K-II is to achieve  3 sensitivity for CPV at thecurrently favored oscillation parameters. In addition, T2K proposes to upgrade the ND280 detectorin order to reduce systematic uncertainty forelectron neutrinoappearance to the 4% level, matching the needs ofthe T2K physics for the extended run period. This can be obtained with the addition of a novelhighly granular scintillator detector, SuperFGD, which is composed of fully active small scintillatorcubes, with a  1 cm side, each read out with three WLS fibers. This detector is sandwichedbetween two High-Angle TPC, read out by resistive Micromegas detectors, with a compact andlight field cage. These detectors are surrounded by six large TOF planes to determine the trackdirection and improve the PID. We request DOE support for US participation in the construction of the SuperFGD detectoras part of the T2K ND280 upgrade for the period April 1, 2019 - March 31, 2022 With this proposal, however, we are looking beyond T2K, namely, DUNE. In 2017-2018, recognizing a need for an advanced fully active scintillator tracker in the DUNE ND, we pushed forwarda plan to synthesize the T2K SuperFGD effort and DUNE ND design effort by proposing a "3D-projection Scintillator Tracker (3DST)" that has essentially the same detector characteristics asSuperFGD. In this scheme, the T2K SuperFGD becomes a fully working prototype of the DUNE3DST allowing the US groups involved in both projects to utilize the overall resources efficientlyand also transfer the expertise and knowledge gained from SuerFGD to 3DST most effectively. Thiswill also allow the groups to attract and train students and postdocs for DUNE, while continuingintellectual leadership roles within T2K via effective contributions to the upgrade. Recently, a"hybrid" detector with a Liquid Argon TPC + High Pressure gas TPC + 3DST with KLOE magnet has been adoptedas the baseline design for DUNE ND.