arXiv subjects

Ll. Marti

Publications and source records attributed to Ll. Marti.

At least 19 recordsLinked to original sources

arXiv2022

Search for solar electron anti-neutrinos due to spin-flavor precession in the Sun with Super-Kamiokande-IV

Due to a very low production rate of electron anti-neutrinos ($\barν_e$) via nuclear fusion in the Sun, a flux of solar $\barν_e$ is unexpected. An appearance of $\barν_e$ in solar neutrino flux opens a new window for the new physics beyond the standard model. In particular, a spin-flavor precession process is expected to convert an electron neutrino into an electron anti-neutrino (${ν_e\to\barν_e}$) when neutrino has a finite magnetic moment. In this work, we have searched for solar $\barν_e$ in the Super-Kamiokande experiment, using neutron tagging to identify their inverse beta decay signature. We identified 78 $\barν_e$ candidates for neutrino energies of 9.3 to 17.3 MeV in 2970.1 live days with a fiducial volume of 22.5 kiloton water (183.0 kton$\cdot$year exposure). The energy spectrum has been consistent with background predictions and we thus derived a 90% confidence level upper limit of ${4.7\times10^{-4}}$ on the $ν_e\to\barν_e$ conversion probability in the Sun. We used this result to evaluate the sensitivity of future experiments, notably the Super-Kamiokande Gadolinium (SK-Gd) upgrade.

hep-ex↗

arXiv2021

First Gadolinium Loading to Super-Kamiokande

In order to improve Super-Kamiokande's neutron detection efficiency and to thereby increase its sensitivity to the diffuse supernova neutrino background flux, 13 tons of $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ (gadolinium sulfate octahydrate) was dissolved into the detector's otherwise ultrapure water from July 14 to August 17, 2020, marking the start of the SK-Gd phase of operations. During the loading, water was continuously recirculated at a rate of 60 m$^3$/h, extracting water from the top of the detector and mixing it with concentrated $\rm Gd_2(\rm SO_4)_3\cdot \rm 8H_2O$ solution to create a 0.02% solution of the Gd compound before injecting it into the bottom of the detector. A clear boundary between the Gd-loaded and pure water was maintained through the loading, enabling monitoring of the loading itself and the spatial uniformity of the Gd concentration over the 35 days it took to reach the top of the detector. During the subsequent commissioning the recirculation rate was increased to 120 m$^3$/h, resulting in a constant and uniform distribution of Gd throughout the detector and water transparency equivalent to that of previous pure-water operation periods. Using an Am-Be neutron calibration source the mean neutron capture time was measured to be $115\pm1$ $μ$s, which corresponds to a Gd concentration of $111\pm2$ ppm, as expected for this level of Gd loading. This paper describes changes made to the water circulation system for this detector upgrade, the Gd loading procedure, detector commissioning, and the first neutron calibration measurements in SK-Gd.

physics.ins-det↗

arXiv2021

New Methods and Simulations for Cosmogenic Induced Spallation Removal in Super-Kamiokande-IV

Radioactivity induced by cosmic muon spallation is a dominant source of backgrounds for $\mathcal{O}(10)~$MeV neutrino interactions in water Cherenkov detectors. In particular, it is crucial to reduce backgrounds to measure the solar neutrino spectrum and find neutrino interactions from distant supernovae. In this paper we introduce new techniques to locate muon-induced hadronic showers and efficiently reject spallation backgrounds. Applying these techniques to the solar neutrino analysis with an exposure of $2790\times22.5$~kton.day increases the signal efficiency by $12.6\%$, approximately corresponding to an additional year of detector running. Furthermore, we present the first spallation simulation at SK, where we model hadronic interactions using FLUKA. The agreement between the isotope yields and shower pattern in this simulation and in the data gives confidence in the accuracy of this simulation, and thus opens the door to use it to optimize muon spallation removal in new data with gadolinium-enhanced neutron capture detection.

hep-ex↗

arXiv2021

Diffuse Supernova Neutrino Background Search at Super-Kamiokande

A new search for the diffuse supernova neutrino background (DSNB) flux has been conducted at Super-Kamiokande (SK), with a $22.5\times2970$-kton$\cdot$day exposure from its fourth operational phase IV. The new analysis improves on the existing background reduction techniques and systematic uncertainties and takes advantage of an improved neutron tagging algorithm to lower the energy threshold compared to the previous phases of SK. This allows for setting the world's most stringent upper limit on the extraterrestrial $\barν_e$ flux, for neutrino energies below 31.3 MeV. The SK-IV results are combined with the ones from the first three phases of SK to perform a joint analysis using $22.5\times5823$ kton$\cdot$days of data. This analysis has the world's best sensitivity to the DSNB $\barν_e$ flux, comparable to the predictions from various models. For neutrino energies larger than 17.3 MeV, the new combined $90\%$ C.L. upper limits on the DSNB $\barν_e$ flux lie around $2.7$ cm$^{-2}$$\cdot$$\text{sec}^{-1}$, strongly disfavoring the most optimistic predictions. Finally, potentialities of the gadolinium phase of SK and the future Hyper-Kamiokande experiment are discussed.

astro-ph.HE↗

arXiv2021

Search for neutrinos in coincidence with gravitational wave events from the LIGO-Virgo O3a Observing Run with the Super-Kamiokande detector

The Super-Kamiokande detector can be used to search for neutrinos in time coincidence with gravitational waves detected by the LIGO-Virgo Collaboration (LVC). Both low-energy ($7-100$ MeV) and high-energy ($0.1-10^5$ GeV) samples were analyzed in order to cover a very wide neutrino spectrum. Follow-ups of 36 (out of 39) gravitational waves reported in the GWTC-2 catalog were examined; no significant excess above the background was observed, with 10 (24) observed neutrinos compared with 4.8 (25.0) expected events in the high-energy (low-energy) samples. A statistical approach was used to compute the significance of potential coincidences. For each observation, p-values were estimated using neutrino direction and LVC sky map ; the most significant event (GW190602_175927) is associated with a post-trial p-value of $7.8\%$ ($1.4σ$). Additionally, flux limits were computed independently for each sample and by combining the samples. The energy emitted as neutrinos by the identified gravitational wave sources was constrained, both for given flavors and for all-flavors assuming equipartition between the different flavors, independently for each trigger and by combining sources of the same nature.

astro-ph.HE↗

arXiv2021

Search for Tens of MeV Neutrinos associated with Gamma-Ray Bursts in Super-Kamiokande

A search for neutrinos produced in coincidence with Gamma-Ray Bursts(GRB) was conducted with the Super-Kamiokande (SK) detector. Between December 2008 and March 2017, the Gamma-ray Coordinates Network recorded 2208 GRBs that occurred during normal SK operation. Several time windows around each GRB were used to search for coincident neutrino events. No statistically significant signal in excess of the estimated backgrounds was detected. The $\barν_e$ fluence in the range from 8 MeV to 100 MeV in positron total energy for $\barν_e+p\rightarrow e^{+}+n$ was found to be less than $\rm 5.07\times10^5$ cm$^{-2}$ per GRB in 90\% C.L. Upper bounds on the fluence as a function of neutrino energy were also obtained.

astro-ph.HE↗

arXiv2020

Search for proton decay via $p\to e^+π^0$ and $p\to μ^+π^0$ with an enlarged fiducial volume in Super-Kamiokande I-IV

We have searched for proton decay via $p\to e^+π^0$ and $p\to μ^+π^0$ modes with the enlarged fiducial volume data of Super-Kamiokande from April 1996 to May 2018, which corresponds to 450 kton$\cdot$years exposure. We have accumulated about 25% more livetime and enlarged the fiducial volume of the Super-Kamiokande detector from 22.5 kton to 27.2 kton for this analysis, so that 144 kton$\cdot$years of data, including 78 kton$\cdot$years of additional fiducial volume data, has been newly analyzed. No candidates have been found for $p\to e^+π^0$ and one candidate remains for $p\to μ^+π^0$ in the conventional 22.5 kton fiducial volume and it is consistent with the atmospheric neutrino background prediction. We set lower limits on the partial lifetime for each of these modes: $τ/B(p\to e^+π^0) > 2.4 \times 10^{34}$ years and $τ/B(p\to μ^+π^0) > 1.6 \times 10^{34}$ years at 90% confidence level.

hep-ex↗

arXiv2020

Neutron-Antineutron Oscillation Search using a 0.37 Megaton$\cdot$Year Exposure of Super-Kamiokande

As a baryon number violating process with $ΔB=2$, neutron-antineutron oscillation ($n\to\bar n$) provides a unique test of baryon number conservation. We have performed a search for $n\to\bar n$ oscillation with bound neutrons in Super-Kamiokande, with the full data set from its first four run periods, representing an exposure of 0.37~Mton-years. The search used a multivariate analysis trained on simulated $n\to\bar n$ events and atmospheric neutrino backgrounds and resulted in 11 candidate events with an expected background of 9.3 events. In the absence of statistically significant excess, we derived a lower limit on $\bar n$ appearance lifetime in $^{16}$O nuclei of $3.6\times{10}^{32}$ years and on the neutron-antineutron oscillation time of $τ_{n\to\bar n} > 4.7\times10^{8}$~s at 90\% C.L..

hep-ex↗

arXiv2020

Indirect Search for Dark Matter from the Galactic Center and Halo with the Super-Kamiokande Detector

We present a search for an excess of neutrino interactions due to dark matter in the form of Weakly Interacting Massive Particles (WIMPs) annihilating in the galactic center or halo based on the data set of Super-Kamiokande-I, -II, -III and -IV taken from 1996 to 2016. We model the neutrino flux, energy, and flavor distributions assuming WIMP self-annihilation is dominant to $ν\overlineν$, $μ^+μ^-$, $b\overline{b}$, or $W^+W^-$. The excess is in comparison to atmospheric neutrino interactions which are modeled in detail and fit to data. Limits on the self-annihilation cross section $\langle σ_{A} V \rangle$ are derived for WIMP masses in the range 1 GeV to 10 TeV, reaching as low as $9.6 \times10^{-23}$ cm$^3$ s$^{-1}$ for 5 GeV WIMPs in $b\bar b$ mode and $1.2 \times10^{-24}$ cm$^3$ s$^{-1}$ for 1 GeV WIMPs in $ν\bar ν$ mode. The obtained sensitivity of the Super-Kamiokande detector to WIMP masses below several tens of GeV is the best among similar indirect searches to date.

hep-ex↗

arXiv2020

Evaluation of Gadolinium's Action on Water Cherenkov Detector Systems with EGADS

Used for both proton decay searches and neutrino physics, large water Cherenkov (WC) detectors have been very successful tools in particle physics. They are notable for their large masses and charged particle detection capabilities. While current WC detectors reconstruct charged particle tracks over a wide energy range, they cannot efficiently detect neutrons. Gadolinium (Gd) has the largest thermal neutron capture cross section of all stable nuclei and produces an 8 MeV gamma cascade that can be detected with high efficiency. Because of the many new physics opportunities that neutron tagging with a Gd salt dissolved in water would open up, a large-scale R&D program called EGADS was established to demonstrate this technique's feasibility. EGADS features all the components of a WC detector, chiefly a 200-ton stainless steel water tank furnished with 240 photo-detectors, DAQ, and a water system that removes all impurities in water while keeping Gd in solution. In this paper we discuss the milestones towards demonstrating the feasibility of this novel technique, and the features of EGADS in detail.

physics.ins-det↗

arXiv2020

Search for proton decay into three charged leptons in 0.37 megaton-years exposure of the Super-Kamiokande

A search for proton decay into three charged leptons has been performed by using 0.37$\,$Mton$\cdot$years of data collected in Super-Kamiokande. All possible combinations of electrons, muons and their anti-particles consistent with charge conservation were considered as decay modes. No significant excess of events has been found over the background, and lower limits on the proton lifetime divided by the branching ratio have been obtained. The limits range between $9.2\times10^{33}$ to $3.4\times10^{34}$ years at 90$\,$% confidence level, improving by more than an order of magnitude upon limits from previous experiments. A first limit has been set for the $p\rightarrowμ^-e^+e^+$ mode.

hep-ex↗

arXiv2019

Search for Astronomical Neutrinos from Blazar TXS0506+056 in Super-Kamiokande

We report a search for astronomical neutrinos in the energy region from several GeV to TeV in the direction of the blazar TXS0506+056 using the Super-Kamiokande detector following the detection of a 100 TeV neutrino from the same location by the IceCube collaboration. Using Super-Kamiokande neutrino data across several data samples observed from April 1996 to February 2018 we have searched for both a total excess above known backgrounds across the entire period as well as localized excesses on smaller time scales in that interval. No significant excess nor significant variation in the observed event rate are found in the blazar direction. Upper limits are placed on the electron and muon neutrino fluxes at 90\% confidence level as $6.03 \times 10^{-7}$ and $4.52 \times 10^{-7}$ to $9.26 \times 10^{-10}$ [${\rm erg}/{\rm cm}^2/{\rm s}$], respectively.

astro-ph.HE↗

arXiv2019

Sensitivity of Super-Kamiokande with Gadolinium to Low Energy Anti-neutrinos from Pre-supernova Emission

Supernova detection is a major objective of the Super-Kamiokande (SK) experiment. In the next stage of SK (SK-Gd), gadolinium (Gd) sulfate will be added to the detector, which will improve the ability of the detector to identify neutrons. A core-collapse supernova will be preceded by an increasing flux of neutrinos and anti-neutrinos, from thermal and weak nuclear processes in the star, over a timescale of hours; some of which may be detected at SK-Gd. This could provide an early warning of an imminent core-collapse supernova, hours earlier than the detection of the neutrinos from core collapse. Electron anti-neutrino detection will rely on inverse beta decay events below the usual analysis energy threshold of SK, so Gd loading is vital to reduce backgrounds while maximising detection efficiency. Assuming normal neutrino mass ordering, more than 200 events could be detected in the final 12 hours before core collapse for a 15-25 solar mass star at around 200 pc, which is representative of the nearest red supergiant to Earth, $\mathrmα$Ori (Betelgeuse). At a statistical false alarm rate of 1 per century, detection could be up to 10 hours before core collapse, and a pre-supernova star could be detected by SK-Gd up to 600 pc away. A pre-supernova alert could be provided to the astrophysics community following gadolinium loading.

astro-ph.HE↗

arXiv2019

Measurement of neutrino-oxygen neutral-current quasi-elastic cross section using atmospheric neutrinos at Super-Kamiokande

Neutral current (NC) interactions of atmospheric neutrinos on oxygen form one of the major backgrounds in the search for supernova relic neutrinos with water-based Cherenkov detectors. The NC channel is dominated by neutrino quasi-elastic (NCQE) scattering off nucleons inside $^{16}$O nuclei. In this paper we report the first measurement of NCQE cross section using atmospheric neutrinos at Super-Kamiokande (SK). The measurement used 2,778 live days of SK-IV data with a fiducial volume of 22.5 kiloton water. Within the visible energy window of 7.5-29.5 MeV, we observed $117$ events compared to the expected $71.9$ NCQE signal and $53.1$ background events. Weighted by the atmospheric neutrino spectrum from 160 MeV to 10 GeV, the flux averaged NCQE cross section is measured to be $(1.01\pm0.17(\text{stat.})^{+0.78}_{-0.30}(\text{sys.}))\times10^{-38}$ cm$^2$.

hep-ex↗

arXiv2019

Atmospheric Neutrino Oscillation Analysis With Improved Event Reconstruction in Super-Kamiokande IV

A new event reconstruction algorithm based on a maximum likelihood method has been developed for Super-Kamiokande. Its improved kinematic and particle identification capabilities enable the analysis of atmospheric neutrino data in a detector volume 32% larger than previous analyses and increases sensitivity to the neutrino mass hierarchy. Analysis of a 253.9 kton-year exposure of the Super-Kamiokande IV atmospheric neutrino data has yielded a weak preference for the normal hierarchy, disfavoring the inverted hierarchy at 74% assuming oscillations at the best fit of the analysis.

hep-ex↗

arXiv2018

Dinucleon and Nucleon Decay to Two-Body Final States with no Hadrons in Super-Kamiokande

Using 0.37 megaton$\cdot$years of exposure from the Super-Kamiokande detector, we search for 10 dinucleon and nucleon decay modes that have a two-body final state with no hadrons. These baryon and lepton number violating modes have the potential to probe theories of unification and baryogenesis. For five of these modes the searches are novel, and for the other five modes we improve the limits by more than one order of magnitude. No significant evidence for dinucleon or nucleon decay is observed and we set lower limits on the partial lifetime of oxygen nuclei and on the nucleon partial lifetime that are above $4\times 10^{33}$ years for oxygen via the dinucleon decay modes and up to about $4 \times 10^{34}$ years for nucleons via the single nucleon decay modes.

hep-ex↗

arXiv2018

Atmospheric neutrino oscillation analysis with external constraints in Super-Kamiokande I-IV

An analysis of atmospheric neutrino data from all four run periods of \superk optimized for sensitivity to the neutrino mass hierarchy is presented. Confidence intervals for $Δm^2_{32}$, $\sin^2 θ_{23}$, $\sin^2 θ_{13}$ and $δ_{CP}$ are presented for normal neutrino mass hierarchy and inverted neutrino mass hierarchy hypotheses based on atmospheric neutrino data alone. Additional constraints from reactor data on $θ_{13}$ and from published binned T2K data on muon neutrino disappearance and electron neutrino appearance are added to the atmospheric neutrino fit to give enhanced constraints on the above parameters. Over the range of parameters allowed at 90% confidence level, the normal mass hierarchy is favored by between 91.5% and 94.5% based on the combined result.

hep-ex↗

arXiv2018

Search for Boosted Dark Matter Interacting With Electrons in Super-Kamiokande

A search for boosted dark matter using 161.9 kiloton-years of Super-Kamiokande IV data is presented. We search for an excess of elastically scattered electrons above the atmospheric neutrino background, with a visible energy between 100 MeV and 1 TeV, pointing back to the Galactic Center or the Sun. No such excess is observed. Limits on boosted dark matter event rates in multiple angular cones around the Galactic Center and Sun are calculated. Limits are also calculated for a baseline model of boosted dark matter produced from cold dark matter annihilation or decay.

hep-ex↗