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K. Mizukoshi

Publications and source records attributed to K. Mizukoshi.

18 recordsLinked to original sources
arXiv2026

Search for Diffuse Supernova Neutrino Background in the Full KamLAND Dataset with Neural-Network-Based Event Classification

We report a search for the diffuse supernova neutrino background (DSNB) with the KamLAND detector, targeting electron antineutrinos via inverse beta decay in the neutrino energy range of 8.3 to 30.8 MeV. Using liquid-scintillator exposures of 9.02 kton-year for 8.3 to 9.3 MeV and 9.42 kton-year for 9.3 to 30.8 MeV, we observe seven candidate events after applying a new deep-neural-network-based event classification technique. This result is consistent with the background-only expectation of 16.2 plus or minus 9.4 events, including systematic uncertainties associated with the neural-network selection. A spectral analysis of the energy and radial distributions finds no significant excess attributable to the DSNB. We therefore set 90 percent confidence-level upper limits on the DSNB flux of 38 to 43 per square centimeter per second, depending on the assumed DSNB model. We also derive model-independent 90 percent confidence-level upper limits on the electron-antineutrino flux, obtaining some of the most stringent constraints below 13.3 MeV. Beyond the DSNB search itself, this work demonstrates neural-network-based event classification as a promising approach for suppressing neutron-associated backgrounds in liquid-scintillator neutrino detectors.

astro-ph.HE↗
arXiv2023

Low-energy Calibration of XENON1T with an Internal $^{37}$Ar Source

A low-energy electronic recoil calibration of XENON1T, a dual-phase xenon time projection chamber, with an internal $^{37}$Ar source was performed. This calibration source features a 35-day half-life and provides two mono-energetic lines at 2.82 keV and 0.27 keV. The photon yield and electron yield at 2.82 keV are measured to be (32.3$\pm$0.3) photons/keV and (40.6$\pm$0.5) electrons/keV, respectively, in agreement with other measurements and with NEST predictions. The electron yield at 0.27 keV is also measured and it is (68.0$^{+6.3}_{-3.7}$) electrons/keV. The $^{37}$Ar calibration confirms that the detector is well-understood in the energy region close to the detection threshold, with the 2.82 keV line reconstructed at (2.83$\pm$0.02) keV, which further validates the model used to interpret the low-energy electronic recoil excess previously reported by XENON1T. The ability to efficiently remove argon with cryogenic distillation after the calibration proves that $^{37}$Ar can be considered as a regular calibration source for multi-tonne xenon detectors.

physics.ins-det↗
arXiv2023

Material radiopurity control in the XENONnT experiment

The selection of low-radioactive construction materials is of the utmost importance for rare-event searches and thus critical to the XENONnT experiment. Results of an extensive radioassay program are reported, in which material samples have been screened with gamma-ray spectroscopy, mass spectrometry, and $^{222}$Rn emanation measurements. Furthermore, the cleanliness procedures applied to remove or mitigate surface contamination of detector materials are described. Screening results, used as inputs for a XENONnT Monte Carlo simulation, predict a reduction of materials background ($\sim$17%) with respect to its predecessor XENON1T. Through radon emanation measurements, the expected $^{222}$Rn activity concentration in XENONnT is determined to be 4.2$\,(^{+0.5}_{-0.7})\,μ$Bq/kg, a factor three lower with respect to XENON1T. This radon concentration will be further suppressed by means of the novel radon distillation system.

physics.ins-det↗
arXiv2022

Search for New Physics in Electronic Recoil Data from XENONnT

We report on a blinded analysis of low-energy electronic-recoil data from the first science run of the XENONnT dark matter experiment. Novel subsystems and the increased 5.9 tonne liquid xenon target reduced the background in the (1, 30) keV search region to $(15.8 \pm 1.3)$ events/(tonne$\times$year$\times$keV), the lowest ever achieved in a dark matter detector and $\sim$5 times lower than in XENON1T. With an exposure of 1.16 tonne-years, we observe no excess above background and set stringent new limits on solar axions, an enhanced neutrino magnetic moment, and bosonic dark matter.

hep-ex↗
arXiv2022

Comparative pulse shape discrimination study for Ca(Br, I)$_2$ scintillators using machine learning and conventional methods

In particle physics experiments, pulse shape discrimination (PSD) is a powerful tool for eliminating the major background from signals. However, the analysis methods have been a bottleneck to improving PSD performance. In this study, two machine learning methods -- multilayer perceptron and convolutional neural network -- were applied to PSD, and their PSD performance was compared with that of conventional analysis methods. Three calcium-based halide scintillators were grown using the vertical Bridgman--Stockbarger method and used for the evaluation of PSD. Compared with conventional analysis methods, the machine learning methods achieved better PSD performance for all the scintillators. For scintillators with low light output, the machine learning methods were more effective for PSD accuracy than the conventional methods in the low-energy region.

physics.ins-det↗
arXiv2022

Effective field theory and inelastic dark matter results from XENON1T

In this work, we expand on the XENON1T nuclear recoil searches to study the individual signals of dark matter interactions from operators up to dimension-eight in a Chiral Effective Field Theory (ChEFT) and a model of inelastic dark matter (iDM). We analyze data from two science runs of the XENON1T detector totaling 1\,tonne$\times$year exposure. For these analyses, we extended the region of interest from [4.9, 40.9]$\,$keV$_{\text{NR}}$ to [4.9, 54.4]$\,$keV$_{\text{NR}}$ to enhance our sensitivity for signals that peak at nonzero energies. We show that the data is consistent with the background-only hypothesis, with a small background over-fluctuation observed peaking between 20 and 50$\,$keV$_{\text{NR}}$, resulting in a maximum local discovery significance of 1.7\,$\sigma$ for the Vector$\otimes$Vector$_{\text{strange}}$ ($VV_s$) ChEFT channel for a dark matter particle of 70$\,$GeV/c$^2$, and $1.8\,\sigma$ for an iDM particle of 50$\,$GeV/c$^2$ with a mass splitting of 100$\,$keV/c$^2$. For each model, we report 90\,\% confidence level (CL) upper limits. We also report upper limits on three benchmark models of dark matter interaction using ChEFT where we investigate the effect of isospin-breaking interactions. We observe rate-driven cancellations in regions of the isospin-breaking couplings, leading to up to 6 orders of magnitude weaker upper limits with respect to the isospin-conserving case.

hep-ex↗
arXiv2022

An approximate likelihood for nuclear recoil searches with XENON1T data

The XENON collaboration has published stringent limits on specific dark matter -nucleon recoil spectra from dark matter recoiling on the liquid xenon detector target. In this paper, we present an approximate likelihood for the XENON1T 1 tonne-year nuclear recoil search applicable to any nuclear recoil spectrum. Alongside this paper, we publish data and code to compute upper limits using the method we present. The approximate likelihood is constructed in bins of reconstructed energy, profiled along the signal expectation in each bin. This approach can be used to compute an approximate likelihood and therefore most statistical results for any nuclear recoil spectrum. Computing approximate results with this method is approximately three orders of magnitude faster than the likelihood used in the original publications of XENON1T, where limits were set for specific families of recoil spectra. Using this same method, we include toy Monte Carlo simulation-derived binwise likelihoods for the upcoming XENONnT experiment that can similarly be used to assess the sensitivity to arbitrary nuclear recoil signatures in its eventual 20 tonne-year exposure.

hep-ex↗
arXiv2022

Double-Weak Decays of $^{124}$Xe and $^{136}$Xe in the XENON1T and XENONnT Experiments

We present results on the search for double-electron capture ($2ν\text{ECEC}$) of $^{124}$Xe and neutrinoless double-$β$ decay ($0νββ$) of $^{136}$Xe in XENON1T. We consider captures from the K- up to the N-shell in the $2ν\text{ECEC}$ signal model and measure a total half-life of $T_{1/2}^{2ν\text{ECEC}}=(1.1\pm0.2_\text{stat}\pm0.1_\text{sys})\times 10^{22}\;\text{yr}$ with a $0.87\;\text{kg}\times\text{yr}$ isotope exposure. The statistical significance of the signal is $7.0\,σ$. We use XENON1T data with $36.16\;\text{kg}\times\text{yr}$ of $^{136}$Xe exposure to search for $0νββ$. We find no evidence of a signal and set a lower limit on the half-life of $T_{1/2}^{0νββ} > 1.2 \times 10^{24}\;\text{yr}\; \text{at}\; 90\,\%\;\text{CL}$. This is the best result from a dark matter detector without an enriched target to date. We also report projections on the sensitivity of XENONnT to $0νββ$. Assuming a $275\;\text{kg}\times\text{yr}$ $^{136}$Xe exposure, the expected sensitivity is $T_{1/2}^{0νββ} > 2.1 \times 10^{25}\;\text{yr}\; \text{at}\; 90\,\%\;\text{CL}$, corresponding to an effective Majorana mass range of $\langle m_{ββ} \rangle < (0.19 - 0.59)\;\text{eV/c}^2$.

hep-ex↗
arXiv2022

Application and modeling of an online distillation method to reduce krypton and argon in XENON1T

A novel online distillation technique was developed for the XENON1T dark matter experiment to reduce intrinsic background components more volatile than xenon, such as krypton or argon, while the detector was operating. The method is based on a continuous purification of the gaseous volume of the detector system using the XENON1T cryogenic distillation column. A krypton-in-xenon concentration of $(360 \pm 60)$ ppq was achieved. It is the lowest concentration measured in the fiducial volume of an operating dark matter detector to date. A model was developed and fit to the data to describe the krypton evolution in the liquid and gas volumes of the detector system for several operation modes over the time span of 550 days, including the commissioning and science runs of XENON1T. The online distillation was also successfully applied to remove Ar-37 after its injection for a low energy calibration in XENON1T. This makes the usage of Ar-37 as a regular calibration source possible in the future. The online distillation can be applied to next-generation experiments to remove krypton prior to, or during, any science run. The model developed here allows further optimization of the distillation strategy for future large scale detectors.

physics.ins-det↗
arXiv2021

Emission of Single and Few Electrons in XENON1T and Limits on Light Dark Matter

Delayed single- and few-electron emissions plague dual-phase time projection chambers, limiting their potential to search for light-mass dark matter. This paper examines the origins of these events in the XENON1T experiment. Characterization of the intensity of delayed electron backgrounds shows that the resulting emissions are correlated, in time and position, with high-energy events and can effectively be vetoed. In this work we extend previous S2-only analyses down to a single electron. From this analysis, after removing the correlated backgrounds, we observe rates < 30 events/(electron*kg*day) in the region of interest spanning 1 to 5 electrons. We derive 90% confidence upper limits for dark matter-electron scattering, first direct limits on the electric dipole, magnetic dipole, and anapole interactions, and bosonic dark matter models, where we exclude new parameter space for dark photons and solar dark photons.

hep-ex↗
arXiv2021

Low background measurement in CANDLES-III for studying the neutrino-less double beta decay of $^{48}$Ca

We developed a CANDLES-III system to study the neutrino-less double beta (0$νββ$) decay of $^{48}$Ca. The proposed system employs 96 CaF$_{2}$ scintillation crystals (305 kg) with natural Ca ($^{\rm nat.}$Ca) isotope which corresponds 350\,g of $^{48}$Ca. External backgrounds were rejected using a 4$π$ active shield of a liquid scintillator surrounding the CaF$_2$ crystals. The internal backgrounds caused by the radioactive impurities within the CaF$_2$ crystals can be reduced effectively through analysis of the signal pulse shape. We analyzed the data obtained in the Kamioka underground for a live-time of 130.4\,days to evaluate the feasibility of the low background measurement with the CANDLES-III detector. Using Monte Carlo simulations, we estimated the background rate from the radioactive impurities in the CaF$_{2}$ crystals and the rate of high energy $γ$-rays caused by the (n, $γ$) reactions induced by environmental neutrons. The expected background rate was in a good agreement with the measured rate, i.e., approximately 10$^{-3}$ events/keV/yr/(kg of $^{\rm nat.}$Ca), in the 0$νββ$ window. In conclusion, the background candidates were estimated properly by comparing the measured energy spectrum with the background simulations. With this measurement method, we performed the first search for 0$νββ$ decay in a low background condition using a detector with a Ca isotope, in which the Ca present was not enriched, in a scale of hundreds of kg. The $^{48}$Ca isotope has a high potential for use in 0$νββ$ decay search, and is expected to be useful for the development of a next-generation detector for highly sensitive measurements.

hep-ex↗
arXiv2021

Search for Coherent Elastic Scattering of Solar $^8$B Neutrinos in the XENON1T Dark Matter Experiment

We report on a search for nuclear recoil signals from solar $^8$B neutrinos elastically scattering off xenon nuclei in XENON1T data, lowering the energy threshold from 2.6 keV to 1.6 keV. We develop a variety of novel techniques to limit the resulting increase in backgrounds near the threshold. No significant $^8$B neutrino-like excess is found in an exposure of 0.6 t $\times$ y. For the first time, we use the non-detection of solar neutrinos to constrain the light yield from 1-2 keV nuclear recoils in liquid xenon, as well as non-standard neutrino-quark interactions. Finally, we improve upon world-leading constraints on dark matter-nucleus interactions for dark matter masses between 3 GeV/c$^2$ and 11 GeV/c$^2$ by as much as an order of magnitude.

hep-ex↗
arXiv2021

Search for inelastic scattering of WIMP dark matter in XENON1T

We report the results of a search for the inelastic scattering of weakly interacting massive particles (WIMPs) in the XENON1T dark matter experiment. Scattering off $^{129}$Xe is the most sensitive probe of inelastic WIMP interactions, with a signature of a 39.6 keV de-excitation photon detected simultaneously with the nuclear recoil. Using an exposure of 0.89 tonne-years, we find no evidence of inelastic WIMP scattering with a significance of more than 2$σ$. A profile-likelihood ratio analysis is used to set upper limits on the cross-section of WIMP-nucleus interactions. We exclude new parameter space for WIMPs heavier than 100 GeV/c${}^2$, with the strongest upper limit of $3.3 \times 10^{-39}$ cm${}^2$ for 130 GeV/c${}^2$ WIMPs at 90\% confidence level.

hep-ex↗
arXiv2020

Projected WIMP Sensitivity of the XENONnT Dark Matter Experiment

XENONnT is a dark matter direct detection experiment, utilizing 5.9 t of instrumented liquid xenon, located at the INFN Laboratori Nazionali del Gran Sasso. In this work, we predict the experimental background and project the sensitivity of XENONnT to the detection of weakly interacting massive particles (WIMPs). The expected average differential background rate in the energy region of interest, corresponding to (1, 13) keV and (4, 50) keV for electronic and nuclear recoils, amounts to $12.3 \pm 0.6$ (keV t y)$^{-1}$ and $(2.2\pm 0.5)\times 10^{-3}$ (keV t y)$^{-1}$, respectively, in a 4 t fiducial mass. We compute unified confidence intervals using the profile construction method, in order to ensure proper coverage. With the exposure goal of 20 t$\,$y, the expected sensitivity to spin-independent WIMP-nucleon interactions reaches a cross-section of $1.4\times10^{-48}$ cm$^2$ for a 50 GeV/c$^2$ mass WIMP at 90% confidence level, more than one order of magnitude beyond the current best limit, set by XENON1T. In addition, we show that for a 50 GeV/c$^2$ WIMP with cross-sections above $2.6\times10^{-48}$ cm$^2$ ($5.0\times10^{-48}$ cm$^2$) the median XENONnT discovery significance exceeds 3$σ$ (5$σ$). The expected sensitivity to the spin-dependent WIMP coupling to neutrons (protons) reaches $2.2\times10^{-43}$ cm$^2$ ($6.0\times10^{-42}$ cm$^2$).

physics.ins-det↗
arXiv2020

Excess Electronic Recoil Events in XENON1T

We report results from searches for new physics with low-energy electronic recoil data recorded with the XENON1T detector. With an exposure of 0.65 t-y and an unprecedentedly low background rate of $76\pm2$ events/(t y keV) between 1 and 30 keV, the data enables sensitive searches for solar axions, an enhanced neutrino magnetic moment, and bosonic dark matter. An excess over known backgrounds is observed at low energies and most prominent between 2 and 3 keV. The solar axion model has a 3.4$σ$ significance, and a 3D 90% confidence surface is reported for axion couplings to electrons, photons, and nucleons. This surface is inscribed in the cuboid defined by $g_{ae}<3.8 \times 10^{-12}$, $g_{ae}g_{an}^{eff}<4.8\times 10^{-18}$, and $g_{ae}g_{aγ}<7.7\times10^{-22} GeV^{-1}$, and excludes either $g_{ae}=0$ or $g_{ae}g_{aγ}=g_{ae}g_{an}^{eff}=0$. The neutrino magnetic moment signal is similarly favored over background at 3.2$σ$ and a confidence interval of $μ_ν \in (1.4,2.9)\times10^{-11}μ_B$ (90% C.L.) is reported. Both results are in strong tension with stellar constraints. The excess can also be explained by $β$ decays of tritium at 3.2$σ$ with a trace amount that can neither be confirmed nor excluded with current knowledge of its production and reduction mechanisms. The significances of the solar axion and neutrino magnetic moment hypotheses are reduced to 2.0$σ$ and 0.9$σ$, respectively, if an unconstrained tritium component is included in the fitting. With respect to bosonic dark matter, the excess favors a monoenergetic peak at ($2.3\pm0.2$) keV (68% C.L.) with a 3.0$σ$ global (4.0$σ$ local) significance. We also consider the possibility that $^{37}$Ar may be present in the detector and yield a 2.82 keV peak. Contrary to tritium, the $^{37}$Ar concentration can be tightly constrained and is found to be negligible.

hep-ex↗
arXiv2020

The energy calibration system for CANDLES using (n, γ) reaction

CAlcium fluoride for the study of Neutrinos and Dark matters by Low-energy Spectrometer (CANDLES) searches for neutrino-less double beta decay of $^{48}$Ca using a CaF$_2$ scintillator array. A high Q-value of $^{48}$Ca at 4,272 keV enabled us to achieve very low background condition, however, at the same it causes difficulties in calibrating the detector's Q-value region because of the absence of a standard high-energy $γ$-ray source. Therefore, we have developed a novel calibration system based on $γ$-ray emission by neutron capture on $^{28}$Si, $^{56}$Fe and $^{58}$Ni nuclei. In the paper, we report the development of the new calibration system as well as the results of energy calibration in CANDLES up to 9 MeV.

physics.ins-det↗
arXiv1997

The Associated Production of Weak Bosons and Jets by Multiple Parton Interactions

The sources of $W + n$-jet events in hadron collisions are higher-order QCD processes, but also multiple-parton interactions. A subprocess producing a $W + k$-jet final state, followed by one producing $l$ jets in the same nucleon-nucleon interaction, will result in a $W + n$-jet event if $k + l = n$. In the simplest case a $W + 2$-jet event can be produced by a quark-antiquark annihilation into $W$ and a 2-jet event occurring in the same proton-antiproton interaction. We compute that this happens at the 10% level of the higher-order QCD processes for the type of cuts made by the Tevatron experiments. For jet $p_T$ values of order $5{\sim}10$ GeV, multiple-parton interactions dominate higher-order QCD-processes. The emergence of this new source of $W + n$-jet events towards lower $p_T $ simulates the running of $α_s$; it is imperative to remove these processes from the event sample in order to extract information on the strong coupling constant. Also, BFKL studies of low-$p_T$ jet cross sections are held hostage to a detailed understanding of the multiple-parton interactions. We perform the calculations required to achieve these goals. A detailed experimental analysis of the data may, for the first time, determine the effective areas occupied by quarks and gluons in the nucleon. These are not necessarily identical. We also compute the multiple-parton contribution to $Z + n$-jet events.

hep-ph↗