Search for the Decay K_L → π^0\nu\bar{\nu} at the J-PARC KOTO Experiment (Springer Theses)

This book reports on a new result from the __K~L~__→__π__^0^__ν____ν__ search at the J-PARC KOTO experiment, which sets an upper limit of 3×10^-9^ for the branching fraction of the decay at the 90% confidence level, improving the previous best limit by an order of magnitude. To explain the matter–antimatter asymmetry in the universe, still unknown new physics beyond the standard model (SM) that breaks CP symmetry is necessary. The rare decay of a long-lived neutral K meson, __K~L~__→__π__^0^__ν____ν__, is a CP-violating decay. It is an excellent probe to search for new physics because new physics can contribute to the decay and change its branching fraction, while the SM is as small as 3×10^-11^. However, it is extremely difficult to search for because all of the decay products are neutral and two neutrinos are undetectable. The __K~L~__→__π__^0^__ν____ν__ signal is identified by measuring two photons from a __π__^0^ with a calorimeter and confirming the absence of any other detectable particles with hermetic veto counters. The book contributes to the analysis of neutron-induced backgrounds which were the dominant background sources in the search. For the background caused by two consecutive hadronic showers in the calorimeter due to a neutron, the author evaluated the background yield using a data-driven approach. For another background caused by an __η__ meson production—__η__ decays two photons—by a neutron that hits a veto counter near the calorimeter, the author developed an original analysis technique to reduce it. The book also contributes to the analysis of the normalization modes (__K~L~__→3__π__^0^, __K~L~__→2__π__^0^, __K~L~__→2__γ__) to measure __K~L~__ yield, the estimation of the signal acceptance based on a simulation, and the evaluation of the trigger efficiency. As a result, significant improvements in the measurement were achieved, and this is an important step in the continuing higher sensitivity search, which can reach new physics with the energy scales up to __O__(100-1000 TeV). This book reports on a new result from the K L → π 0 ν ν search at the J-PARC KOTO experiment, which sets an upper limit of 3×10 -9 for the branching fraction of the decay at the 90% confidence level, improving the previous best limit by an order of magnitude. To explain the matter–antimatter asymmetry in the universe, still unknown new physics beyond the standard model (SM) that breaks CP symmetry is necessary. The rare decay of a long-lived neutral K meson, K L → π 0 ν ν , is a CP-violating decay. It is an excellent probe to search for new physics because new physics can contribute to the decay and change its branching fraction, while the SM is as small as 3×10 -11 . However, it is extremely difficult to search for because all of the decay products are neutral and two neutrinos are undetectable. The K L → π 0 ν ν signal is identified by measuring two photons from a π 0 with a calorimeter and confirming the absence of any other detectable particles with hermetic veto counters. The book contributes to the analysis of neutron-induced backgrounds which were the dominant background sources in the search. For the background caused by two consecutive hadronic showers in the calorimeter due to a neutron, the author evaluated the background yield using a data-driven approach. For another background caused by an η meson production— η decays two photons—by a neutron that hits a veto counter near the calorimeter, the author developed an original analysis technique to reduce it. The book also contributes to the analysis of the normalization modes ( K L →3 π 0 , K L →2 π 0 , K L →2 γ ) to measure K L yield, the estimation of the signal acceptance based on a simulation, and the evaluation of the trigger efficiency. As a result, significant improvements in the measurement were achieved, and this is an important step in the continuing higher sensitivity search, which can reach new physics with the energy scales up to O (100-1000 TeV). Front Matter ....Pages i-xvi Introduction (Kota Nakagiri)....Pages 1-13 The KOTO Experiment (Kota Nakagiri)....Pages 15-40 Data Taking (Kota Nakagiri)....Pages 41-47 Event Reconstruction (Kota Nakagiri)....Pages 49-64 Monte Carlo Simulation (Kota Nakagiri)....Pages 65-71 Analysis of the \(K_L \!\rightarrow \! \pi ^0 \nu \overline{\nu }\) and \(K_L \!\rightarrow \! \pi ^0 X^0\) Searches (Kota Nakagiri)....Pages 73-135 Discussion (Kota Nakagiri)....Pages 137-144 Conclusion (Kota Nakagiri)....Pages 145-146 Back Matter ....Pages 147-151 This book reports on a new result from the KL 0 search at the J-PARC KOTO experiment, which sets an upper limit of 3x10-9 for the branching fraction of the decay at the 90% confidence level, improving the previous best limit by an order of magnitude.