arXiv subjects

The KamLAND Collaboration

Publications and source records attributed to The KamLAND Collaboration.

7 recordsLinked to original sources

arXiv2013

Reactor On-Off Antineutrino Measurement with KamLAND

The recent long-term shutdown of Japanese nuclear reactors has resulted in a significantly reduced reactor $\bar{nu}_{e}$ flux at KamLAND. This running condition provides a unique opportunity to confirm and constrain backgrounds for the reactor $\bar{nu}_{e}$ oscillation analysis. The data set also has improved sensitivity for other $\bar{nu}_{e}$ signals, in particular $\bar{nu}_{e}$'s produced in $β$-decays from $^{238}$U and $^{232}$Th within the Earth's interior, whose energy spectrum overlaps with that of reactor $\bar{nu}_{e}$'s. Including constraints on $θ_{13}$ from accelerator and short-baseline reactor neutrino experiments, a combined three-flavor analysis of solar and KamLAND data gives fit values for the oscillation parameters of $tan^{2} θ_{12} = 0.436^{+0.029}_{-0.025}$, $Δm^{2}_{21} = 7.53^{+0.18}_{-0.18} \times 10^{-5} {eV}^{2}$, and $sin^{2} θ_{13} = 0.023^{+0.002}_{-0.002}$. Assuming a chondritic Th/U mass ratio, we obtain $116^{+28}_{-27}$ $\bar{nu}_{e}$ events from $^{238}$U and $^{232}$Th, corresponding to a geo $\bar{nu}_{e}$ flux of $3.4^{+0.8}_{-0.8} \times 10^{6} {cm^{-2}s^{-1}}$ at the KamLAND location. We evaluate various bulk silicate Earth composition models using the observed geo $\bar{nu}_{e}$ rate.

hep-ex↗

arXiv2012

Search for extraterrestrial antineutrino sources with the KamLAND detector

We present the results of a search for extraterrestrial electron antineutrinos ($\barν_{e}$'s) in the energy range $8.3 MeV < E_{\barν_{e}} < 31.8 MeV$ using the KamLAND detector. In an exposure of 4.53 kton-year, we identify 25 candidate events. All of the candidate events can be attributed to background, most importantly neutral current atmospheric neutrino interactions, setting an upper limit on the probability of $^{8}$B solar $ν_{e}$'s converting into $\barν_{e}$'s at $5.3 \times 10^{-5}$ (90% C.L.), if we assume an undistorted $\barν_{e}$ shape. This limit corresponds to a solar $\barν_{e}$ flux of $93 cm^{-2} s^{-1}$ or an event rate of $1.6 events (kton-year)^{-1}$ above the energy threshold $(E_{\barν_{e}} > 8.3 MeV)$. The present data also allows us to set more stringent limits on the diffuse supernova neutrino flux and on the annihilation rates for light dark matter particles.

astro-ph.HE↗

arXiv2011

Constraints on $θ_{13}$ from A Three-Flavor Oscillation Analysis of Reactor Antineutrinos at KamLAND

We present new constraints on the neutrino oscillation parameters $/textyen Delta m^{2}_{21}$, $/textyen theta_{12}$, and $/textyen theta_{13}$ from a three-flavor analysis of solar and KamLAND data. The KamLAND data set includes data acquired following a radiopurity upgrade and amounts to a total exposure of $3.49 \textyen times 10^{32}$ target-proton-year. Under the assumption of {\textyen it CPT} invariance, a two-flavor analysis (/textyen mbox{$\textyen theta_{13} = 0$}) of the KamLAND and solar data yields the best-fit values $\textyen tan^{2} \textyen theta_{12} = 0.444^{+0.036}_{-0.030}$ and $\textyen Delta m^{2}_{21} = 7.50^{+0.19}_{-0.20} \textyen times 10^{-5} ~ {\textyen rm eV}^{2}$; a three-flavor analysis with $\textyen theta_{13}$ as a free parameter yields the best-fit values $\textyen tan^{2} \textyen theta_{12} = 0.452^{+0.035}_{-0.033}$, $\textyen Delta m^{2}_{21} = 7.50^{+0.19}_{-0.20} \textyen times 10^{-5} ~ {\textyen rm eV}^{2}$, and $\textyen sin^{2} \textyen theta_{13} = 0.020^{+0.016}_{-0.016}$. This $\textyen theta_{13}$ interval is consistent with other recent work combining the CHOOZ, atmospheric and long-baseline accelerator experiments. We also present a new global $\textyen theta_{13}$ analysis, incorporating the CHOOZ, atmospheric and accelerator data, which indicates $\textyen sin^{2} \textyen theta_{13} = 0.009^{+0.013}_{-0.007}$. A nonzero value is suggested, but only at the 79\textyen% C.L.

hep-ex↗

arXiv2010

Production of Radioactive Isotopes through Cosmic Muon Spallation in KamLAND

Radioactive isotopes produced through cosmic muon spallation are a background for rare-event detection in $ν$ detectors, double-$β$-decay experiments, and dark-matter searches. Understanding the nature of cosmogenic backgrounds is particularly important for future experiments aiming to determine the pep and CNO solar neutrino fluxes, for which the background is dominated by the spallation production of $^{11}$C. Data from the Kamioka liquid-scintillator antineutrino detector (KamLAND) provides valuable information for better understanding these backgrounds, especially in liquid scintillators, and for checking estimates from current simulations based upon MUSIC, FLUKA, and GEANT4. Using the time correlation between detected muons and neutron captures, the neutron production yield in the KamLAND liquid scintillator is measured to be $(2.8 \pm 0.3) \times 10^{-4} μ^{-1} g^{-1} cm^{2}$. For other isotopes, the production yield is determined from the observed time correlation related to known isotope lifetimes. We find some yields are inconsistent with extrapolations based on an accelerator muon beam experiment.

hep-ex↗

arXiv2009

The KamLAND Full-Volume Calibration System

We have successfully built and operated a source deployment system for the KamLAND detector. This system was used to position radioactive sources throughout the delicate 1-kton liquid scintillator volume, while meeting stringent material cleanliness, material compatibility, and safety requirements. The calibration data obtained with this device were used to fully characterize detector position and energy reconstruction biases. As a result, the uncertainty in the size of the detector fiducial volume was reduced by a factor of two. Prior to calibration with this system, the fiducial volume was the largest source of systematic uncertainty in measuring the number of anti-neutrinos detected by KamLAND. This paper describes the design, operation and performance of this unique calibration system.

physics.ins-det↗

arXiv2008

Precision Measurement of Neutrino Oscillation Parameters with KamLAND

The KamLAND experiment has determined a precise value for the neutrino oscillation parameter $Δm^{2}_{21}$ and stringent constraints on $θ_{12}$. The exposure to nuclear reactor anti-neutrinos is increased almost fourfold over previous results to 2.44$\times10^{32}$ proton-yr due to longer livetime and an enlarged fiducial volume. An undistorted reactor $\barν_{e}$ energy spectrum is now rejected at >5$σ$. Extending the analysis down to the inverse beta decay energy threshold, and incorporating geo-neutrinos, gives a best-fit at $Δm^{2}_{21}$= $7.58^{+0.14}_{-0.13}(stat)^{+0.15}_{-0.15}(syst)\times10^{-5}$ eV$^{2}$ and $\tan^2 θ_{12}$=$0.56^{+0.10}_{0.07}(stat)^{+0.10}_{-0.06}(syst)$. Local $Δχ^2$-minima at higher and lower $Δm^{2}_{21}$ are disfavored at >4$σ$. Combining with solar neutrino data, we obtain $Δm^{2}_{21}$= $7.59^{+0.21}_{-0.21}\times10^{-5}$ eV$^{2}$ and $\tan^2 θ_{12}$=$0.47^{+0.06}_{-0.05}$.

hep-ex↗

arXiv2004

Measurement of Neutrino Oscillation with KamLAND: Evidence of Spectral Distortion

We present results of a study of neutrino oscillation based on a 766 ton-year exposure of KamLAND to reactor anti-neutrinos. We observe 258 \nuebar\ candidate events with energies above 3.4 MeV compared to 365.2 events expected in the absence of neutrino oscillation. Accounting for 17.8 expected background events, the statistical significance for reactor \nuebar disappearance is 99.998%. The observed energy spectrum disagrees with the expected spectral shape in the absence of neutrino oscillation at 99.6% significance and prefers the distortion expected from \nuebar oscillation effects. A two-neutrino oscillation analysis of the KamLAND data gives \DeltaMSq = 7.9$^{+0.6}_{-0.5}\times10^{-5}$ eV$^2$. A global analysis of data from KamLAND and solar neutrino experiments yields \DeltaMSq = 7.9$^{+0.6}_{-0.5}\times10^{-5}$ eV$^2$ and \ThetaParam = 0.40$^{+0.10}_{-0.07}$, the most precise determination to date.

hep-ex↗