KOTO Experiment Research Articles

Suppression of neutron background using deep neural network and Fourier frequency analysis at the KOTO experiment

We present two analysis techniques for distinguishing background events induced by neutrons from photon signal events in the search for the rare KL0→π0νν̄ decay at the J-PARC KOTO experiment. These techniques employed a deep convolutional neural network and Fourier frequency analysis to discriminate neutrons from photons, based on their variations in cluster shape and pulse shape, in the electromagnetic calorimeter made of undoped CsI. The results effectively suppressed the neutron background by a factor of 5.6×105, while maintaining the efficiency of KL0→π0νν̄ at 70%.

Probing long-lived axions at the KOTO experiment

Probing long-lived axions at the KOTO experiment

The flavor of QCD axion dark matter

We argue that demanding a consistent cosmological history, including the absence of domain walls and strongly interacting relics at the Peccei-Quinn scale, singles out two concrete realizations of hadronic QCD axions as viable dark matter models. These realizations generally feature flavor-violating axion couplings to Standard Model quarks that are unsuppressed at low energies. As a consequence, experiments looking for flavor-violating hadronic processes involving the axion can be sensitive probes of QCD axion dark matter models. In particular, we show that the NA62 and KOTO experiments could detect the K → π + a decay for axions consistent with the observed dark matter abundance via the post-inflationary misalignment mechanism.

Search for the Pair Production of Dark Particles X with K_{L}^{0}→XX, X→γγ.

We present the first search for the pair production of dark particles X via K_{L}^{0}→XX with X decaying into two photons using the data collected by the KOTO experiment. No signal was observed in the mass range of 40-110 MeV/c^{2} and 210-240 MeV/c^{2}. This sets upper limits on the branching fractions as B(K_{L}^{0}→XX)<(1-4)×10^{-7} and B(K_{L}^{0}→XX)<(1-2)×10^{-6} at the 90%confidence level for the two mass regions, respectively.

Reduction of charged kaon background in the KOTO experiment

The KOTO experiment at J-PARC searches for the rare kaon decay . In the analysis of the data collected from 2016 to 2018, the largest background was caused by decays of K ±. To detect K ± in the beam line, we developed an in-beam charged particle detector in 2020. After installing it in the KOTO detector system, we collected physics data in 2021. Using a control sample of K ± events, we estimated the inefficiency to be , which reduces the K ± background by a factor of 13. The number of K ± background events was suppressed to be at the single event sensitivity of 7.9 × 10−10.

Effect of low-energy neutrons on accidental counting rate in the KOTO experiment

For further sensitivity improvement in the KOTO experiment, we studied the origin and mechanism of accidental hits of the VETO counters. A simulation study shows that a half of the counts of Front-Barrel (FB) VETO counter is stemmed from “delayed photons”, which is produced by the (n,γ) reaction due to the thermal neutrons moderated in the KOTO detector. A boron-carbide absorber is a possible candidate to suppress the delayed photons.

Search for the decay at the J-PARC KOTO experiment

The KOTO experiment at the J-PARC 30GeV Main Ring aims to study the rare decay . This decay directly breaks the CP symmetry and is highly suppressed in the Standard Model (SM). In addition, the theoretical uncertainties on the decay is only a few percent. Those features make this decay one of the best probes to search for new physics beyond the SM.We had found new background sources from the analysis of the 2016-2018 data set. Thus, we took measures against the new background events.In this paper, we briefly introduce the results of the 2016-2018 data analysis. We thus report the new detectors and analysis tools that we introduced to reduce the new background events and also the other types of background events. Finally, we report the status of the 2021 data analysis.

Development of a PMT base used for the new charged veto detector in the J-PARC KOTO experiment

The KOTO experiment searches for the rare decay at J-PARC. The largest background in the data collected in 2016-2018 was caused by decays of K ±. To detect K ±, we are developing an in-beam charged veto detector to be installed upstream of the KOTO detector. It uses a 0.2mm-thick thin plastic scintillator. The escaped light is reflected and collected by a 12-μm-thick aluminized mylar, and read out by an array of PMTs placed outside the beam. We newly developed PMT bases to be used for this detector. Based on an electron beam test, we expect the K ± background to be reduced by more than a factor 100. We plan to install this detector in the KOTO beam line, in December 2022.

Thin scintillation counter with a new readout method for the KOTO experiment

The KOTO experiment at J-PARC is dedicated to the search for the rare decay . While analyzing the data taken in 2018-2018, we found that some background events were caused by K ± decays in the beam. To reduce this background, we are developing a new charged particle detector. It uses a 0.2-mm-thick plastic scintillator. The scintillation light escaping from the surface of the scintillator without total reflection is collected by 12-μm-thick aluminized mylar and is read out by multiple PMTs on the side. Based on the performance evaluated with an electron beam, we expect to have 1/100 K ± background reduction.

Online Event Selection and GPU-based Waveform Compression at the High Level Trigger of the KOTO Experiment

The Data Acquisition (DAQ) system of the KOTO experiment is currently undergoing a major upgrade, following an also ongoing accelerator’s beam intensity upgrade. The throughput of the current DAQ is limited by the hardware, and different prescalings have to be applied to reduce the data rate to a manageable level. The DAQ upgrade aims to remove the hardware bottleneck, increasing the amount of data that reaches the high level trigger system. The new high level trigger will make use of parallelization over CPU and GPU threads to perform event reconstruction, selection, and compression, to eventually allow for more variate physics searches in KOTO.

Hadron shower background in KOTO 2019 - 2021 data

One of the keys for the search of KOTO experiment is to suppress the backgrounds caused by hadron showers in CsI calorimeter. We developed three methods to discriminate the hadron showers from gamma showers. The methods use 1) pulse shape of signals, 2) shower shape on the calorimeter, and 3) depth of the showers in the calorimeter. We report on the discrimination power of these methods. The performance of the third method remained the same even if the dark current of silicon photomultipliers increased by a factor of 85 due to irradiation.

Search at the J-PARC KOTO Experiment

We present the study of massless dark photons in the decay of in the J-PARC KOTO experiment. The massless dark photon is different from the massive dark photon because it does not mix directly with ordinary photons, but it could interact with SM particles through direct coupling with quarks. In some theoretical predictions, the can be as large as \U0001d4aa(10-3), which is well within the sensitivity of KOTO. Because of the lack of kinematic constraints, searching for could be challenging, but the hermetic veto system of KOTO provides a unique opportunity to probe for this decay. We used three techniques to suppress the neutron background, based on machine learning, Fourier analysis, and both-end readout. We will present the background suppression result based on data collected in 2020.

Analysis Techniques for Neutron Background Suppression at KOTO

We present two analysis techniques based on novel machine learning and classic Fourier frequency analysis that suppress the neutron background in the search at the J-PARC KOTO experiment by \U0001d4aa(10−6).

First-order electroweak phase transition from weakly coupled sub-GeV physics and possible connection to fermion flavor

We propose that the dynamics of a scalar $\phi$ of mass $O(10)$ MeV that is weakly coupled to the Higgs can lead to a first order electroweak phase transition, fulfilling a key requirement for baryogenesis. Stability of the model near the weak scale requires a suppressed - possibly vanishing - top Yukawa coupling to the Higgs before the transition which rises to the Standard Model value afterwards. This can be accomplished through the dynamics of $\phi$ via a dimension-5 operator. We conjecture that the entire Standard Model flavor structure could turn on, mutatis mutandis, after the electroweak phase transition, via dimension-5 interactions of $\phi$ suppressed by scales ranging from $O(10^3)$ TeV to near Planck mass. Due to its suppressed couplings, $\phi$ is long-lived and can lead to missing energy signals in rare kaon decays, which can be probed by the KOTO experiment.

Study of the K_{L}→π^{0}νν[over ¯] Decay at the J-PARC KOTO Experiment.

The rare decay K_{L}→π^{0}νν[over ¯] was studied with the dataset taken at the J-PARC KOTO experiment in 2016, 2017, and 2018. With a single event sensitivity of (7.20±0.05_{stat}±0.66_{syst})×10^{-10}, three candidate events were observed in the signal region. After unveiling them, contaminations from K^{±} and scattered K_{L} decays were studied, and the total number of background events was estimated to be 1.22±0.26. We conclude that the number of observed events is statistically consistent with the background expectation. For this dataset, we set an upper limit of 4.9×10^{-9} on the branching fraction of K_{L}→π^{0}νν[over ¯] at the 90%confidence level.

Pionium as a source of false events in the K→πνν¯ decays

We suggest that the decay modes of kaons with a pion and a pionium ($\pi^+\pi^-$ atom) in the final state can constitute a not yet considered background to the very rare decay $K\to\pi\nu\bar\nu$. In fact, a part of pioniums may escape the decay region before decaying into two $\pi^0$s (or to $\pi^0\pi^0\gamma$ in the case of excited pionium). To illustrate the importance of this background, we show that it may even explain, under some assumptions, the unexpected $K_L$ decay events that appeared in the KOTO experiment.

Pulse shape discrimination of photons and neutrons in the energy range of 0.1 – 2 GeV with the KOTO un-doped CsI calorimeter

A pulse shape difference between photons and neutrons was observed in the output signals of scintillation light from the un-doped CsI calorimeter of the KOTO experiment for the KL→π0νν̄ decay. We developed a discrimination method to reject neutrons and to accept photons in the deposited energy range from 0.1 to 2 GeV. The method rejects 67% of neutron-induced single hadronic clusters and 86% of neutron-induced two-cluster events while keeping more than 90% of photon-induced single electromagnetic clusters and two-photon events.

Cosmological implications of the KOTO excess

The KOTO experiment has reported an excess of $K_L\to\pi^0\nu\bar\nu$ events above the Standard Model prediction, in tension with the Grossman-Nir (GN) bound. The GN bound heavily constrains new physics interpretations of an excess in this channel, but another possibility is that the observed events originate from a different process entirely: a decay of the form $K_L\to\pi^0X$, where $X$ denotes one or more new invisible species. We introduce a class of models to study this scenario with two light scalars playing the role of $X$, and we examine the possibility that the lighter of the two new states may also account for cosmological dark matter (DM). We show that this species can be produced thermally in the presence of additional interactions apart from those needed to account for the KOTO excess. Conversely, in the minimal version of the model, DM must be produced nonthermally. In this case, avoiding overproduction imposes constraints on the structure of the low-energy theory. Moreover, this requirement carries significant implications for the scale of reheating in the early Universe, generically preferring a low but observationally permitted reheating temperature of O(10 MeV). We discuss astrophysical and terrestrial signatures that will allow further tests of this paradigm in the coming years.

The light sgoldstino phenomenology: explanations for the muon (g \u2212 2) deviation and KOTO anomaly

In this work, we study the long-standing experimental anomaly in muon (g −2) and also recent anomalous excess in {K}_Lto {pi}^0+voverline{v} at the J-PARC KOTO experiment with sgoldstino. After supersymmetry breaking, the interactions between quarks and sgoldstino (s) make the decays K → π + s sizable through loop diagrams, which affects the measurements of decays K → π + invisible. Furthermore, the couplings between photons and sgoldstino contribute to ∆aμ as well as the bino-slepton contribution. With satisfying all known experimental constraints such as from NA62, E949, E137, Orsay, KTEV and CHARM experiments, these two anomalies can be explained simultaneously. The mass of CP-even sgoldstino is close to the neutral pion mass which does not violate the Grossman-Nir bound. The parameter space can be further tested in future NA62, DUNE experiments, as well as experiments in the LHC.

First search for KL→π0γ

We report the first search for the $K_L \to \pi^0 \gamma$ decay, which is forbidden by Lorentz invariance, using the data from 2016 to 2018 at the J-PARC KOTO experiment. With a single event sensitivity of $(7.1\pm 0.3_{\rm stat.} \pm 1.6_{\rm syst.})\times 10^{-8}$, no candidate event was observed in the signal region. The upper limit on the branching fraction was set to be $1.7\times 10^{-7}$ at the 90\% confidence level.