arXiv subjects

K. Ozaki

Publications and source records attributed to K. Ozaki.

15 recordsLinked to original sources

arXiv2023

Updated constraints on sterile neutrino mixing in the OPERA experiment using a new $ν_e$ identification method

This paper describes a new $ν_e$ identification method specifically designed to improve the low-energy ($< 30\,\mathrm{GeV}$) $ν_e$ identification efficiency attained by enlarging the emulsion film scanning volume with the next generation emulsion readout system. A relative increase of 25-70% in the $ν_e$ low-energy region is expected, leading to improvements in the OPERA sensitivity to neutrino oscillations in the framework of the 3 + 1 model. The method is applied to a subset of data where the detection efficiency increase is expected to be more relevant, and one additional $ν_e$ candidate is found. The analysis combined with the $ν_τ$ appearance results improves the upper limit on $\sin^2 2θ_{μe}$ to 0.016 at 90% C.L. in the MiniBooNE allowed region $Δm^2_{41} \sim 0.3\,\mathrm{eV}^2$.

hep-ex↗

arXiv2021

Characterization of New Silicon Photomultipliers with Low Dark Noise at Low Temperature

Silicon photomultipliers (SiPMs) have a low radioactivity, compact geometry, low operation voltage, and reasonable photo-detection efficiency for vacuum ultraviolet light (VUV). Therefore it has the potential to replace photomultiplier tubes (PMTs) for future dark matter experiments with liquid xenon (LXe). However, SiPMs have nearly two orders of magnitude higher dark count rate (DCR) compared to that of PMTs at the LXe temperature ($\sim$ 165 K). This type of high DCR mainly originates from the carriers that are generated by band-to-band tunneling effect. To suppress the tunneling effect, we have developed a new SiPM with lowered electric field strength in cooperation with Hamamatsu Photonics K. K. and characterized its performance in a temperature range of 153 K to 298 K. We demonstrated that the newly developed SiPMs had 6--54 times lower DCR at low temperatures compared to that of the conventional SiPMs.

physics.ins-det↗

arXiv2021

The future lifespan of Earth's oxygenated Atmosphere

Earth's modern atmosphere is highly oxygenated and is a remotely detectable signal of its surface biosphere. However, the lifespan of oxygen-based biosignatures in Earth's atmosphere remains uncertain, particularly for the distant future. Here we use a combined biogeochemistry and climate model to examine the likely timescale of oxygen-rich atmospheric conditions on Earth. Using a stochastic approach, we find that the mean future lifespan of Earth's atmosphere with oxygen levels more than 1% of the present atmospheric level is 1.08+-0.14 billion years. The model projects that a deoxygenation of the atmosphere, with atmospheric oxygen dropping sharply to levels reminiscnet of the Archaean Earth, will most probably be triggered before the inception of moist greenhouse conditions in Earth's climate system and before the extensive loss of surface water from the atmosphere. We find that future deoxygenation is an inevitable consequence of increasing solar fluxes, whereas its precise timing is modulated by the exchange flux of reducing power between the mantle and the ocean-atmosphere-crust system. Our results suggest that the planetary carbonate-silicate cycle will tend to lead to terminally CO2-limited biospheres and rapid atmospheric deoxygenation, emphasizing the need for robust atmospheric biosignatures applicable to weakly oxygenated and anoxic exoplanet atmospheres and highlighting the potential importance of atmospheric organic haze during the terminal stages of planetary habitability.

astro-ph.EP↗

arXiv2020

First observation of a tau neutrino charged current interaction with charm production in the Opera experiment

An event topology with two secondary vertices compatible with the decay of short-lived particles was found in the analysis of neutrino interactions in the Opera target. The observed topology is compatible with tau neutrino charged current (CC) interactions with charm production and neutrino neutral current (NC) interactions with $c\overline{c}$ pair production. However, other processes can mimic this topology. A dedicated analysis was implemented to identify the underlying process. A Monte Carlo simulation was developed and complementary procedures were introduced in the kinematic reconstruction. A multivariate analysis technique was used to achieve an optimal separation of signal from background. Most likely, this event is a $ν_τ$ CC interaction with charm production, the tau and charm particle decaying into 1 prong and 2 prongs, respectively. The significance of this observation is evaluated.

hep-ex↗

arXiv2019

Final results on neutrino oscillation parameters from the OPERA experiment in the CNGS beam

The OPERA experiment has conclusively observed the appearance of tau neutrinos in the muon neutrino CNGS beam. Exploiting the OPERA detector capabilities, it was possible to isolate high purity samples of $ν_{e}$, $ν_μ$ and $ν_τ$ charged current weak neutrino interactions, as well as neutral current weak interactions. In this Letter, the full dataset is used for the first time to test the three-flavor neutrino oscillation model and to derive constraints on the existence of a light sterile neutrino within the framework of the $3+1$ neutrino model. For the first time, tau and electron neutrino appearance channels are jointly used to test the sterile neutrino hypothesis. A significant fraction of the sterile neutrino parameter space allowed by LSND and MiniBooNE experiments is excluded at 90% C.L. In particular, the best-fit values obtained by MiniBooNE combining neutrino and antineutrino data are excluded at 3.3 $σ$ significance.

hep-ex↗

arXiv2018

Latest results of the OPERA experiment on nu-tau appearance in the CNGS neutrino beam

OPERA is a long-baseline experiment designed to search for $ν_μ\toν_τ$ oscillations in appearance mode. It was based at the INFN Gran Sasso laboratory (LNGS) and took data from 2008 to 2012 with the CNGS neutrino beam from CERN. After the discovery of $ν_τ$ appearance in 2015, with $5.1σ$ significance, the criteria to select $ν_τ$ candidates have been extended and a multivariate approach has been used for events identification. In this way the statistical uncertainty in the measurement of the oscillation parameters and of $ν_τ$ properties has been improved. Results are reported.

hep-ex↗

arXiv2018

Measurement of the cosmic ray muon flux seasonal variation with the OPERA detector

The OPERA experiment discovered muon neutrino into tau neutrino oscillations in appearance mode, detecting tau leptons by means of nuclear emulsion films. The apparatus was also endowed with electronic detectors with tracking capability, such as scintillator strips and resistive plate chambers. Because of its location, in the underground Gran Sasso laboratory, under 3800 m.w.e., the OPERA detector has also been used as an observatory for TeV muons produced by cosmic rays in the atmosphere. In this paper the measurement of the single muon flux modulation and of its correlation with the seasonal variation of the atmospheric temperature are reported.

hep-ex↗

arXiv2018

Final results of the search for $ν_μ \to ν_{e}$ oscillations with the OPERA detector in the CNGS beam

The OPERA experiment has discovered the tau neutrino appearance in the CNGS muon neutrino beam, in agreement with the 3 neutrino flavour oscillation hypothesis. The OPERA neutrino interaction target, made of Emulsion Cloud Chamber, was particularly efficient in the reconstruction of electromagnetic showers. Moreover, thanks to the very high granularity of the emulsion films, showers induced by electrons can be distinguished from those induced by $π^0$s, thus allowing the detection of charged current interactions of electron neutrinos. In this paper the results of the search for electron neutrino events using the full dataset are reported. An improved method for the electron neutrino energy estimation is exploited. Data are compatible with the 3 neutrino flavour mixing model expectations and are used to set limits on the oscillation parameters of the 3+1 neutrino mixing model, in which an additional mass eigenstate $m_{4}$ is introduced. At high $Δm^{2}_{41}$ $( \gtrsim 0.1~\textrm{eV}^{2})$, an upper limit on $\sin^2 2θ_{μe}$ is set to 0.021 at 90% C.L. and $Δm^2_{41} \gtrsim 4 \times 10^{-3}~\textrm{eV}^{2}$ is excluded for maximal mixing in appearance mode.

hep-ex↗

arXiv2018

Final results of the OPERA experiment on $ν_τ$ appearance in the CNGS beam

The OPERA experiment was designed to study $ν_μ\toν_τ$ oscillations in appearance mode in the CNGS neutrino beam. In this letter we report the final analysis of the full data sample collected between 2008 and 2012, corresponding to $17.97\cdot 10^{19}$ protons on target. Selection criteria looser than in previous analyses have produced ten $ν_τ$ candidate events, thus reducing the statistical uncertainty in the measurement of the oscillation parameters and of $ν_τ$ properties. A multivariate approach for event identification has been applied to the candidate events and the discovery of $ν_τ$ appearance is confirmed with an improved significance level of 6.1 $σ$. $Δm^2_{23}$ has been measured, in appearance mode, with an accuracy of 20%. The measurement of $ν_τ$ CC cross-section, for the first time with a negligible contamination from $\barν_τ$, and the first direct observation of the $ν_τ$ lepton number are also reported.

hep-ex↗

arXiv2016

Determination of the muon charge sign with the dipolar spectrometers of the OPERA experiment

The OPERA long-baseline neutrino-oscillation experiment has observed the direct appearance of $ν_τ$ in the CNGS $ν_μ$ beam. Two large muon magnetic spectrometers are used to identify muons produced in the $τ$ leptonic decay and in $ν_μ^{CC}$ interactions by measuring their charge and momentum. Besides the kinematic analysis of the $τ$ decays, background resulting from the decay of charmed particles produced in $ν_μ^{CC}$ interactions is reduced by efficiently identifying the muon track. A new method for the charge sign determination has been applied, via a weighted angular matching of the straight track-segments reconstructed in the different parts of the dipole magnets. Results obtained for Monte Carlo and real data are presented. Comparison with a method where no matching is used shows a significant reduction of up to 40\% of the fraction of wrongly determined charges.

physics.ins-det↗

arXiv2015

Discovery of tau neutrino appearance in the CNGS neutrino beam with the OPERA experiment

The OPERA experiment was designed to search for $ν_μ \rightarrow ν_τ$ oscillations in appearance mode, i.e. by detecting the $τ$-leptons produced in charged current $ν_τ$ interactions. The experiment took data from 2008 to 2012 in the CERN Neutrinos to Gran Sasso beam. The observation of $ν_μ \rightarrow ν_τ$ appearance, achieved with four candidate events in a sub-sample of the data, was previously reported. In this paper, a fifth $ν_τ$ candidate event, found in an enlarged data sample, is described. Together with a further reduction of the expected background, the candidate events detected so far allow assessing the discovery of $ν_μ\rightarrow ν_τ$ oscillations in appearance mode with a significance larger than 5 $σ$.

hep-ex↗

arXiv2015

Limits on muon-neutrino to tau-neutrino oscillations induced by a sterile neutrino state obtained by OPERA at the CNGS beam

The OPERA experiment, exposed to the CERN to Gran Sasso $ν_μ$ beam, collected data from 2008 to 2012. Four oscillated $ν_τ$ Charged Current interaction candidates have been detected in appearance mode, which are consistent with $ν_μ\to ν_τ$ oscillations at the atmospheric $Δm^2$ within the "standard" three-neutrino framework. In this paper, the OPERA $ν_τ$ appearance results are used to derive limits on the mixing parameters of a massive sterile neutrino.

hep-ex↗

arXiv2014

Observation of nu_tau appearance in the CNGS beam with the OPERA experiment

The OPERA experiment is searching for nu_mu -> nu_tau oscillations in appearance mode i.e. via the direct detection of tau leptons in nu_tau charged current interactions. The evidence of nu_mu -> nu_tau appearance has been previously reported with three nu_tau candidate events using a sub-sample of data from the 2008-2012 runs. We report here a fourth nu_tau candidate event, with the tau decaying into a hadron, found after adding the 2012 run events without any muon in the final state to the data sample. Given the number of analysed events and the low background, nu_mu -> nu_tau oscillations are established with a significance of 4.2sigma.

hep-ex↗

arXiv2014

Procedure for short-lived particle detection in the OPERA experiment and its application to charm decays

The OPERA experiment, designed to perform the first observation of $ν_μ\rightarrow ν_τ$ oscillations in appearance mode through the detection of the $τ$ leptons produced in $ν_τ$ charged current interactions, has collected data from 2008 to 2012. In the present paper, the procedure developed to detect $τ$ particle decays, occurring over distances of the order of 1 mm from the neutrino interaction point, is described in detail. The results of its application to the search for charmed hadrons are then presented as a validation of the methods for $ν_τ$ appearance detection.

hep-ex↗

arXiv2014

Measurement of TeV atmospheric muon charge ratio with the full OPERA data

The OPERA detector, designed to search for $ν_μ \to ν_τ$ oscillations in the CNGS beam, is located in the underground Gran Sasso laboratory, a privileged location to study TeV-scale cosmic rays. For the analysis here presented, the detector was used to measure the atmospheric muon charge ratio in the TeV region. OPERA collected charge-separated cosmic ray data between 2008 and 2012. More than 3 million atmospheric muon events were detected and reconstructed, among which about 110000 multiple muon bundles. The charge ratio $R_μ \equiv N_{μ^+}/N_{μ^-}$ was measured separately for single and for multiple muon events. The analysis exploited the inversion of the magnet polarity which was performed on purpose during the 2012 Run. The combination of the two data sets with opposite magnet polarities allowed minimizing systematic uncertainties and reaching an accurate determination of the muon charge ratio. Data were fitted to obtain relevant parameters on the composition of primary cosmic rays and the associated kaon production in the forward fragmentation region. In the surface energy range 1-20 TeV investigated by OPERA, $R_μ$ is well described by a parametric model including only pion and kaon contributions to the muon flux, showing no significant contribution of the prompt component. The energy independence supports the validity of Feynman scaling in the fragmentation region up to $200$ TeV/nucleon primary energy.

hep-ex↗