Super-Kamiokande Detector Research Articles

Commissioning of the New Electronics and Online System for the Super-Kamiokande Experiment

The Super-Kamiokande detector is a ring imaging Cherenkov detector for neutrino physics and proton-decay search and consists of 50 000 tons of pure water equipped with about 13 000 photo-multipliers. The old front-end electronics and online system running for more than one decade were all upgraded in September, 2008 and the data acquisition was started successfully. The new front-end electronics is based on a charge to time converter and a multi-hit Time to Digital converter. TCP/IP based readout channel is implemented to handle large amounts of data. In the new data acquisition scheme, the hardware event-trigger for the data reduction is replaced by processing all the hits in the online farm, so that we are able to lower the threshold of the detection energy for solar neutrino and analyze consecutive events whose time interval is too long to detect in the previous system. To make the new online system to be capable of processing larger dataflow of up to 470 MB/s, we utilize Gigabit and 10-Gigabit Ethernet technologies and distribute the load over Linux PCs to handle a large amount of data. In this paper, we will describe the design and performance of the new system in the commissioning.

Development and production of the ionization chamber for the T2K muon monitor

We report on the development of an ionization chamber for the muon monitor in the T2K (Tokai-to-Kamioka) long baseline neutrino oscillation experiment.In the T2K experiment, we generate a muon neutrino beam using the 50-GeV proton synchrotron in J-PARC and detect it in the Super-Kamiokande detector, which is 295km away from J-PARC. We aim the neutrino beam off-axis by 2.5° from Super-Kamiokande in order to maximize the sensitivity to the neutrino oscillation measurement. Since the beam direction is required to be aimed within 1mrad, we need to monitor it with a precision better than 1mrad.The muon monitor is the only instrument that can monitor the neutrino beam direction in real time by measuring a profile of muons which are generated along with neutrinos. It is required to measure the intense muon beam precisely and stably in a high radiation area. Therefore, we use two independent systems for the muon monitor: an array of ionization chambers and another array of solid-state detectors. We developed the ionization chamber and performed beam tests with an electron beam at the Laboratory of Particle Beam Science in Kyoto University. We also had a long-term test in the NuMI beamline at Fermilab. Then we finalized the design of the ionization chamber and built it. We expect the ionization chambers as the muon monitor to monitor the beam direction with a precision of 0.2mrad.

Time projection chambers for the T2K experiment

The T2K experiment will study neutrino oscillation properties by directing a high intensity neutrino beam produced at J-PARC in Tokai, Japan, towards the large super-Kamiokande detector located 295km away, in Kamioka, Japan. The experiment includes a sophisticated near detector complex, 280m downstream of the neutrino production target. A key element of the near detectors is the ND280 tracker consisting of two active scintillator-bar target systems surrounded by three large time projection chambers (TPCs) for charged particle tracking. The tracker will provide high statistics samples for studying charged current neutrino interaction rates and kinematics prior to oscillation, so as to better understand backgrounds in the far detector. The tracker is surrounded by the UA1/Nomad dipole magnet and the TPCs will measure the charge, momentum, particle type (through ionization energy loss in the gas) of particles passing through them. Novel features of these TPCs include the rectangular box design, constructed from composite panels, the use of bulk micromegas detectors for gas amplification, electronics readout based on a new ASIC, and a photoelectron calibration system. This paper describes the design and construction of the TPCs, the readout electronics, the gas handling system and shows the performance of a completed TPC as deduced from measurements with particle beams, cosmic rays, and the calibration system.

Investigating light neutralinos at neutrino telescopes

We present a detailed analysis of the neutrino-induced muon signals coming from neutralino pair annihilations inside the Sun and the Earth with particular emphasis for light neutralinos. The theoretical model considered is an effective minimal supersymmetric extension of the standard model without gaugino-mass unification, which allows neutralinos of light masses (below 50 GeV). The muon events are divided into through-going and stopping muons, using the geometry of the Super-Kamiokande detector. In the evaluation of the signals, we take into account the relevant hadronic and astrophysics uncertainties and include neutrino oscillation and propagation properties in a consistent way. We derive the ranges of neutralino masses which could be explored at neutrino telescopes with a low muon-energy threshold (around 1 GeV) depending on the category of events and on the values of the various astrophysics and particle-physics parameters. A final analysis is focused on the upward muon fluxes which could be generated by those neutralino configurations which are able to explain the annual modulation data of the DAMA/LIBRA experiment. We show how combining these data with measurements at neutrino telescopes could help in pinning down the features of the dark matter particle and in restraining the ranges of the many quantities (of astrophysics and particle-physics origins) which enter in the evaluations and still suffer from large uncertainties.

High-speed charge-to-time converter ASIC for the Super-Kamiokande detector

A new application-specific integrated circuit (ASIC), the high-speed charge-to-time converter (QTC) IWATSU CLC101, provides three channels, each consisting of preamplifier, discriminator, low-pass filter, and charge integration circuitry, optimized for the waveform of a photomultiplier tube (PMT). This ASIC detects PMT signals using individual built-in discriminators and drives output timing signals whose width represents the integrated charge of the PMT signal. Combined with external input circuits composed of passive elements, the QTC provides full analog signal processing for the detector's PMTs, ready for further processing by time-to-digital converters (TDCs). High-rate ( > 1 MHz ) signal processing is achieved by short-charge-conversion-time and baseline-restoration circuits. Wide-range charge measurements are enabled by offering three gain ranges while maintaining a short cycle time. QTC chip test results show good analog performance, with efficient detection for a single photoelectron signal, four orders of magnitude dynamic range ( 0.3 mV ∼ 3 V ; 0.2 ∼ 2500 pC ), 1% charge linearity, 0.2 pC charge resolution, and 0.1 ns timing resolution. Test results on ambient temperature dependence, channel isolation, and rate dependence also meet specifications.

Kinematic reconstruction of atmospheric neutrino events in a large water Cherenkov detector with proton identification

We report the development of a proton identification method for the Super-Kamiokande (SK) detector. This new tool is applied to the search for events with a single proton track, a high purity neutral current sample of interest for sterile neutrino searches. After selection using a neural network, we observe 38 events in the combined SK-I and SK-II data corresponding to 22 85.1 days of exposure, with an estimated signal-to-background ratio of 1.6 to 1. Proton identification was also applied to a direct search for charged-current quasielastic (CCQE) events, obtaining a high precision sample of fully kinematically reconstructed atmospheric neutrinos, which has not been previously reported in water Cherenkov detectors. The CCQE fraction of this sample is 55%, and its neutrino (as opposed to antineutrino) fraction is 91.7{+-}3%. We selected 78{mu}-like and 47 e-like events in the SK-I and SK-II data set. With this data, a clear zenith angle distortion of the neutrino direction itself is reported in a sub-GeV sample of {mu} neutrinos where the lepton angular correlation to the incoming neutrino is weak. Our fit to {nu}{sub {mu}}{yields}{nu}{sub {tau}} oscillations using the neutrino (L/E) distribution of the CCQE sample alone yields a wide acceptance region compatible with our previousmore » results and excludes the no-oscillation hypothesis at 3-sigma.« less

Time Projection Chambers for the T2K near detector

T2K is a long baseline neutrino experiment aiming primarily at the measurement of the mixing angle θ13 through the observation of vμ ⟩ ve oscillations between the JPARC accelerator complex and the Super Kamiokande detector. A near detector, called ND280 and located 280 m from the neutrino production target, will be used to measure the neutrino energy spectrum, flavour content and interaction rates of the un-oscillated beam and hence to predict the neutrino interactions at Super Kamiokande. ND280 is a magnetized off-axis tracking detector that includes as key detector elements three large Time Projection Chambers (TPCs) that will play a crucial role in the measurement of the momentum, charge and identity of charged particles. The T2K TPCs are now being constructed and will be commissioned at JPARC at the end of 2009. These detectors use gaseous micro-pattern devices which are based on the novel bulk-MicroMegas technology. Design and construction aspects of the T2K TPCs are presented together with the expected detector performances. Recent tests of bulk-MicroMegas prototypes are discussed as well as the performance obtained with the TPC front-end electronics based on the new AFTER ASIC.

Search for indirect signal of WIMPs in super-kamiokande

We present the result of indirect search for Weakly Interacting Massive Particles(WIMPs). We used Super-Kamiokande detector for the search using neutrino-induced upward going muons. This search targets to detect the neutrino signals from WIMPs annihilation in the sun and the earth. This time, data set from SKI to SKIII(2828.3 days) are used for analysis, and looked for excess of neutrino signal from the sun as compared with expected atmospheric neutrino background.From the excess search, no significant excess was observed. Then, we calculate the flux limit of upward going muon in various angular cones around the Sun. We know the relation between WIMP mass and the cone half angle from the Sun. We calculated window angle which contain 90% of signal from WIMP annihilation in the Sun for several WIMP neutralino mass using this relation. And we can get 90% upper poissonian limit using all categories of upward going muon event for each WIMP mass with considering detection efficiency.In this analysis, improve the previous limit by ∼1.4 times. It's reasonable considering the increasing of statistics. We are almost approaching the expected flux region which is calculated from DARKSUSY simulation at all WIMP mass range.

Status of LCGT and CLIO

The Japanese gravitational wave proposal, LCGT (Large-scale Cryogenic Gravitational wave Telescope), is to construct a 3km underground interferometer with cryogenic mirrors in the Kamioka Mine. CLIO (Cryogenic Laser Interferometer Observatory) is a 100m-baseline underground cryogenic interferometer built at the same site to serve as a bridge connecting CLIK (7m-baseline prototype cryogenic interferometer) and the planed LCGT. The site of CLIO is near the Super-Kamiokande neutrino detector. The tunnel for CLIO was dug in 2002, and a strain meter for geophysics was installed in the same tunnel in 2003. CLIO was installed in 2005 and the operation started in 2006.

Comprehensive analysis of neutrino events in the Super-Kamiokande type detector from the view point of the Computer Numerical Experiment

Comparisons are made on the direction of the incident neutrino, zenith angle distribution of neutrino events produced both inside and outside the detector between the Super-Kamiokande Experiment and our Computer Numerical Experiment.

On search for eV hidden-sector photons in Super-Kamiokande and CAST experiments

If light hidden sector photons (γ′s) exist, they could be produced through kinetic mixing with solar photons in the eV energy range. We propose to search for this hypothetical γ′-flux with the Super-Kamiokande and/or upgraded CAST detectors. The proposed experiments are sensitive to the γ–γ′ mixing strength as small as 10−5≳χ≳10−9 for the γ′ mass region 10−4≲mγ′≲10−1 eV and, in the case of non-observation, would improve limits recently obtained from photon regeneration laser experiments for this mass region.

Search for matter-dependent atmospheric neutrino oscillations in Super-Kamiokande

We consider {nu}{sub {mu}}{yields}{nu}{sub {tau}} oscillations in the context of the mass varying neutrino (MaVaN) model, where the neutrino mass can vary depending on the electron density along the flight path of the neutrino. Our analysis assumes a mechanism with dependence only upon the electron density, hence ordinary matter density, of the medium through which the neutrino travels. Fully-contained, partially-contained and upward-going muon atmospheric neutrino data from the Super-Kamiokande detector, taken from the entire SK-I period of 1489 live days, are compared to MaVaN model predictions. We find that, for the case of 2-flavor oscillations, and for the specific models tested, oscillation independent of electron density is favored over density dependence. Assuming maximal mixing, the best-fit case and the density-independent case do not differ significantly.

Study of TeV neutrinos with upward showering muons in Super-Kamiokande

A subset of neutrino-induced upward through-going muons in the Super-Kamiokande detector consists of high-energy muons which lose energy through radiative processes such as bremsstrahlung, e + e - pair production and photonuclear interactions. These “upward showering muons” comprise an event sample whose mean parent neutrino energy is approximately 1 TeV. We show that the zenith angle distribution of upward showering muons is consistent with negligible distortion due to neutrino oscillations, as expected of such a high-energy neutrino sample. We present astronomical searches using these high-energy events, such as those from WIMP annihilations in the Sun, Earth and Galactic Center, some suspected point sources, as well as searches for diffuse flux from the interstellar medium.

Search for Supernova Neutrino Bursts at Super‐Kamiokande

The result of a search for neutrino bursts from supernova explosions using the Super-Kamiokande detector is reported. Super-Kamiokande is sensitive to core-collapse supernova explosions via observation of their neutrino emissions. The expected number of events comprising such a burst is ~10^4 and the average energy of the neutrinos is in few tens of MeV range in the case of a core-collapse supernova explosion at the typical distance in our galaxy (10 kiloparsecs); this large signal means that the detection efficiency anywhere within our galaxy and well past the Magellanic Clouds is 100%. We examined a data set which was taken from May, 1996 to July, 2001 and from December, 2002 to October, 2005 corresponding to 2589.2 live days. However, there is no evidence of such a supernova explosion during the data-taking period. The 90% C.L. upper limit on the rate of core-collapse supernova explosions out to distances of 100 kiloparsecs is found to be 0.32 SN/year.

GADZOOKS! The Future of Super–Kamiokande?

An inexpensive, effective technique is being developed to allow tagging of antineutrinos in water Cherenkov [WC] detectors via the addition to water of a solute with a large neutron cross section and energetic gamma daughters. Gadolinium is an excellent candidate since in recent years it has become very inexpensive, now less than $8 per kilogram in the form of commercially-available gadolinium trichloride. This non-toxic, non-reactive substance is highly soluble in water. Neutron capture on gadolinium yields an 8.0 MeV gamma cascade easily seen in detectors like Super–Kamiokande [Super–K, SK]. The uses of GdCl3 as a possible upgrade for the Super–Kamiokande detector — with a view toward improving its performance as an antineutrino detector for supernova neutrinos and reactor neutrinos — are discussed, as are the ongoing R&D efforts which aim to make this dream a reality within the next two years.

Large-aperture hybrid photo-detector

We have developed the first complete large-aperture (13-inch diameter) hybrid photo-detector (HPD). The withstanding voltage problem has been overcome and we were able to attain an HPD operating voltage of +20 kV. Adoption of our newly developed backside illumination avalanche diode (AD) was also critical in successfully countering the additional problem of an increase in AD leakage after the activation process. We observed single photon signal timing jitter of under 450 ps in FWHM, electron transit time of ∼12 ns, and clear pulse height separation up to several photoelectron peaks, all greatly superior to the performance of any conventional large-aperture photomultiplier tubes (PMTs). In addition, our HPD has a much simpler structure than conventional large-aperture PMTs, which simplifies mass production and lowers manufacturing cost. We believe that these attributes position our HPD as the most suitable photo-detector for the next generation mega-ton class water-Cherenkov detector, which is expected to be more than 20× larger than the Super-Kamiokande (SK) detector.

Observation of a TeV Cosmic Ray Induced Muon Flux Anisotropy Underground

We report the observation of an anisotropy in the muon flux induced by cosmic rays. The muons were observed about 3,000 meters water equivalent underground with the Super-Kamiokande detector resulting in an muon energy threshold of about 1 TeV. We also present an update on the oscillation of neutrinos produced from cosmic ray interaction in the atmosphere using the full Super-Kamiokande data.

Search for Diffuse Astrophysical Neutrino Flux Using Ultra–High‐Energy Upward‐going Muons in Super‐Kamiokande I

Many astrophysical models predict a diffuse flux of high-energy neutrinos from active galactic nuclei and other extra-galactic sources. At muon energies above 1 TeV, the upward-going muon flux induced by neutrinos from active galactic nuclei is expected to exceed the flux due to atmospheric neutrinos. We have performed a search for this astrophysical neutrino flux by looking for upward-going muons in the highest energy data sample from the Super-Kamiokande detector using 1679.6 live days of data. We found one extremely high energy upward-going muon event, compared with an expected atmospheric neutrino background of 0.46 plus or minus 0.23 events. Using this result, we set an upper limit on the diffuse flux of upward-going muons due to neutrinos from astrophysical sources in the muon energy range 3.16-100 TeV.

Precision measurement of solar neutrino oscillation parameters by a long-baseline reactor neutrino experiment in Europe

Abstract We consider the determination of the solar neutrino oscillation parameters Δ m 21 2 and θ 12 by studying oscillations of reactor anti-neutrinos emitted by nuclear power plants (located mainly in France) with a detector installed in the Frejus underground laboratory. The performances of a water Cerenkov detector of 147 kt fiducial mass doped with 0.1% of gadolinium (MEMPHYS-Gd) and of a 50 kt scale liquid scintillator detector (LENA) are compared. In both cases 3σ uncertainties below 3% on Δ m 21 2 and of about 20% on sin 2 θ 12 can be obtained after one year of data taking. The gadolinium doped Super-Kamiokande detector (SK-Gd) in Japan can reach a similar precision if the SK/MEMPHYS fiducial mass ratio of 1 to 7 is compensated by a longer SK-Gd data taking time. Several years of reactor neutrino data collected by MEMPHYS-Gd or LENA would allow a determination of Δ m 21 2 and sin 2 θ 12 with uncertainties of approximately 1% and 10% at 3σ, respectively. These accuracies are comparable to those that can be reached in the measurement of the atmospheric neutrino oscillation parameters Δ m 31 2 and sin 2 θ 23 in long-baseline superbeam experiments.

Science in culture: A modern megalith

Mariko Mori's glass sculpture responds to the death of stars. High-tech artist Mariko Mori is exhibiting Tom Na H-iu at the 2006 Singapore Biennale, a glass monolith taking the cosmic pulse. Or at least, driven by cosmic radiation. Its internal light source is controlled by the Super-Kamiokande neutrino detector, so the neutrinos governing its light emission come from a dying star, an echo of the spiritual or astronomical thinking that may have driven Neolithic humans to erect their stone monoliths.