Abstract The SNO $$+$$ + collaboration reports its first spectral analysis of long-baseline reactor antineutrino oscillation using 114 tonne-years of data. Fitting the neutrino oscillation probability to the observed energy spectrum yields constraints on the neutrino mass-squared difference $$\Delta m^2_{21}$$ Δ m 21 2 . In the ranges allowed by previous measurements, the best-fit $$\Delta m^2_{21}$$ Δ m 21 2 is ( $$8.85^{+1.10}_{-1.33}$$ 8 . 85 - 1.33 + 1.10 ) $$\times $$ × $$10^{-5}$$ 10 - 5 $$\hbox {eV}^2$$ eV 2 . This measurement is continuing in the next phases of SNO+ and is expected to surpass the present global precision on $$\Delta m^2_{21}$$ Δ m 21 2 with about three years of data.

The half-life of ^39Ar is measured using the DEAP-3600 detector located 2 km underground at SNOLAB. Between 2016 and 2020, DEAP-3600 used a target mass of (3269 ± 24) kg of liquid argon distilled from the atmosphere in a direct-detection dark matter search. Such an argon mass also enables direct measurements of argon isotope properties. The decay of ^39Ar in DEAP-3600 is the dominant source of triggers by two orders of magnitude, ensuring high statistics and making DEAP-3600 well-suited for measuring this isotope's half-life. Use of the pulse-shape discrimination technique in DEAP-3600 allows powerful discrimination between nuclear recoils and electron recoils, resulting in the selection of a clean sample of ^39Ar decays. Observing over a period of 3.4 years, the ^39Ar half-life is measured to be (302 ± 8_ stat± 6_ sys) years. This new direct measurement suggests that the half-life of ^39Ar is significantly longer than the accepted value, with potential implications for measurements using this isotope's half-life as input.

Abstract The knowledge of scintillation quenching of $$\alpha $$ α -particles plays a paramount role in understanding $$\alpha $$ α -induced backgrounds and improving the sensitivity of liquid argon-based direct detection of dark matter experiments. We performed a relative measurement of scintillation quenching in the MeV energy region using radioactive isotopes ( $$^{222}$$ 222 Rn, $$^{218}$$ 218 Po and $$^{214}$$ 214 Po isotopes) present in trace amounts in the DEAP-3600 detector and quantified the uncertainty of extrapolating the quenching factor to the low-energy region.

The SNO+ detector operated initially as a water Cherenkov detector. The implementation of a sealed cover gas system midway through water data taking resulted in a significant reduction in the activity of Rn-222 daughters in the detector and allowed the lowest background to the solar electron scattering signal above 5 MeV achieved to date. This paper reports an updated SNO+ water phase B-8 solar neutrino analysis with a total livetime of 282.4 days and an analysis threshold of 3.5 MeV. The B-8 solar neutrino flux is found to be (2.32(-0.17)(+0.18)(stat)(-0.05)(+0.07) (syst)) x 10(6) cm(-2) s(-1) assuming no neutrino (scillations, or (5.36(-0.39)(+0.41) (stat)(+0.17)(-0.16) (syst) x 10(6) cm(-2) s(-1) assuming standard neutrino (scillation parameters, in good agreement with both previous measurements and standard solar model calculations. The electron recoil spectrum is presented above 3.5 MeV.

The direction of individual $^8$B solar neutrinos has been reconstructed using the SNO+ liquid scintillator detector. Prompt, directional Cherenkov light was separated from the slower, isotropic scintillation light using time information, and a maximum likelihood method was used to reconstruct the direction of individual scattered electrons. A clear directional signal was observed, correlated with the solar angle. The observation was aided by a period of low primary fluor concentration that resulted in a slower scintillator decay time. This is the first time that event-by-event direction reconstruction in high light-yield liquid scintillator has been demonstrated in a large-scale detector.

The SNO+ Collaboration reports the first evidence of reactor antineutrinos in a Cherenkov detector. The nearest nuclear reactors are located 240 km away in Ontario, Canada. This analysis uses events with energies lower than in any previous analysis with a large water Cherenkov detector. Two analytical methods are used to distinguish reactor antineutrinos from background events in 190 days of data and yield consistent evidence for antineutrinos with a combined significance of 3.5σ.

The specific activity of the β decay of ^39 Ar in atmospheric argon is measured using the DEAP-3600 detector. DEAP-3600, located 2 km underground at SNOLAB, uses a total of (3269 ± 24) kg of liquid argon distilled from the atmosphere to search for dark matter. This detector is well-suited to measure the decay of ^39 Ar owing to its very low background levels. This is achieved in two ways: it uses low background construction materials; and it uses pulse-shape discrimination to differentiate between nuclear recoils and electron recoils. With 167 live-days of data, the measured specific activity at the time of atmospheric extraction is (0.964 ± 0.001 _stat ± 0.024 _sys ) Bq/kg _atmAr , which is consistent with results from other experiments. A cross-check analysis using different event selection criteria and a different statistical method confirms the result.

Received 24 December 2021DOI:https://doi.org/10.1103/PhysRevD.105.029901© 2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCosmologyDark matterParticle astrophysicsParticle dark matterPhysical SystemsGalactic halosWeakly interacting massive particlesTechniquesCosmic ray & astroparticle detectorsDark matter detectorsParticles & FieldsGravitation, Cosmology & Astrophysics

This paper reports results from a search for single and multi-nucleon disappearance from the $^{16}$O nucleus in water within the \snoplus{} detector using all of the available data. These so-called"invisible"decays do not directly deposit energy within the detector but are instead detected through their subsequent nuclear de-excitation and gamma-ray emission. New limits are given for the partial lifetimes: $\tau(n\rightarrow inv)>9.0\times10^{29}$ years, $\tau(p\rightarrow inv)>9.6\times10^{29}$ years, $\tau(nn\rightarrow inv)>1.5\times10^{28}$ years, $\tau(np\rightarrow inv)>6.0\times10^{28}$ years, and $\tau(pp\rightarrow inv)>1.1\times10^{29}$ years at 90\% Bayesian credibility level (with a prior uniform in rate). All but the ($nn\rightarrow inv$) results improve on existing limits by a factor of about 3.

Dark matter with Planck-scale mass (≃10^{19}  GeV/c^{2}) arises in well-motivated theories and could be produced by several cosmological mechanisms. A search for multiscatter signals from supermassive dark matter was performed with a blind analysis of data collected over a 813 d live time with DEAP-3600, a 3.3 t single-phase liquid argon-based detector at SNOLAB. No candidate signals were observed, leading to the first direct detection constraints on Planck-scale mass dark matter. Leading limits constrain dark matter masses between 8.3×10^{6} and 1.2×10^{19}  GeV/c^{2}, and ^{40}Ar-scattering cross sections between 1.0×10^{-23} and 2.4×10^{-18}  cm^{2}. These results are interpreted as constraints on composite dark matter models with two different nucleon-to-nuclear cross section scalings.

The DEAP-3600 detector searches for the scintillation signal from dark matter particles scattering on a 3.3 tonne liquid argon target. The largest background comes from 39 Ar beta decays and is suppressed using pulse-shape discrimination (PSD). We use two types of PSD estimator: the prompt-fraction, which considers the fraction of the scintillation signal in a narrow and a wide time window around the event peak, and the log-likelihood-ratio, which compares the observed photon arrival times to a signal and a background model. We furthermore use two algorithms to determine the number of photons detected at a given time: (1) simply dividing the charge of each PMT pulse by the mean single-photoelectron charge, and (2) a likelihood analysis that considers the probability to detect a certain number of photons at a given time, based on a model for the scintillation pulse shape and for afterpulsing in the light detectors. The prompt-fraction performs approximately as well as the log-likelihood-ratio PSD algorithm if the photon detection times are not biased by detector effects. We explain this result using a model for the information carried by scintillation photons as a function of the time when they are detected.

SNO+ is a large-scale liquid scintillator experiment with the primary goal of searching for neutrinoless double beta decay, and is located approximately 2 km underground in SNOLAB, Sudbury, Canada. The detector acquired data for two years as a pure water Cherenkov detector, starting in May 2017. During this period, the optical properties of the detector were measured in situ using a deployed light diffusing sphere, with the goal of improving the detector model and the energy response systematic uncertainties. The measured parameters included the water attenuation coefficients, effective attenuation coefficients for the acrylic vessel, and the angular response of the photomultiplier tubes and their surrounding light concentrators, all across different wavelengths. The calibrated detector model was validated using a deployed tagged gamma source, which showed a 0.6% variation in energy scale across the primary target volume.

Theγ-ray pulsar PSR B1706 −44 and the adjacent supernova remnant (SNR) candidate G 343. 1−2. were observed by H.E.S.S. during a dedicated observation campaign in 2007. As a result of this observatio n campaign, a new source of very-high-energy (VHE; E > 100 GeV)γ-ray emission, HESS J1708 −443, was detected with a statistical significance of 7 σ, although no significant point-like emission was detected a t the position of the energetic pulsar itself. In this paper, the morphologic al and spectral analyses of the newly-discovered TeV source are presented. The centroid of HESS J1708 −443 is considerably o ffset from the pulsar and located near the apparent center of th e SNR, atαJ2000 = 17h08m11s ± 17s and δJ2000 = −44◦20′ ± 4′. The source is found to be significantly more extended than th e H.E.S.S. point spread function ( ∼0.1◦), with an intrinsic Gaussian width of 0.29 ◦ ± 0.04◦. Its integral flux between 1 and 10 TeV is ∼ 3.8× 10−12 ph cm−2 s−1, equivalent to 17% of the Crab Nebula flux in the same energy range. The measured energy spectrum is wel l-fit by a power law with a relatively hard photon index Γ = 2.0± 0.1stat± 0.2sys. Additional multi-wavelength data, including 330 MHz VLA ob servations, were used to investigate the VHE γ-ray source’s possible associations with the pulsar wind nebula of PSR B1706 −44 and/or with the complex radio structure of the partial shell-typ e SNR G 343.1 −2.3.

The SNO+ experiment is located 2 km underground at SNOLAB in Sudbury, Canada. A low background search for neutrinoless double beta (0νββ) decay will be conducted using 780 tonnes of liquid scintillator loaded with 3.9 tonnes of natural tellurium, corresponding to 1.3 tonnes of ^130Te. This paper provides a general overview of the SNO+ experiment, including detector design, construction of process plants, commissioning efforts, electronics upgrades, data acquisition systems, and calibration techniques. The SNO+ collaboration is reusing the acrylic vessel, PMT array, and electronics of the SNO detector, having made a number of experimental upgrades and essential adaptations for use with the liquid scintillator. With low backgrounds and a low energy threshold, the SNO+ collaboration will also pursue a rich physics program beyond the search for 0νββ decay, including studies of geo- and reactor antineutrinos, supernova and solar neutrinos, and exotic physics such as the search for invisible nucleon decay. The SNO+ approach to the search for 0νββ decay is scalable: a future phase with high ^130Te-loading is envisioned to probe an effective Majorana mass in the inverted mass ordering region.

A liquid scintillator consisting of linear alkylbenzene as the solvent and 2,5-diphenyloxazole as the fluor was developed for the SNO+ experiment. This mixture was chosen as it is compatible with acrylic and has a competitive light yield to pre-existing liquid scintillators while conferring other advantages including longer attenuation lengths, superior safety characteristics, chemical simplicity, ease of handling, and logistical availability. Its properties have been extensively characterized and are presented here. This liquid scintillator is now used in several neutrino physics experiments in addition to SNO+.

Measurements of electron drift properties in liquid and gaseous xenon are reported. The electrons are generated by the photoelectric effect in a semi-transparent gold photocathode driven in transmission mode with a pulsed ultraviolet laser. The charges drift and diffuse in a small chamber at various electric fields and a fixed drift distance of 2.0 cm. At an electric field of 0.5 kV/cm, the measured drift velocities and corresponding temperature coefficients respectively are 1.97 +/- 0.04 mm/mu s and (-0.69 +/- 0.05)%/K for liquid xenon, and 1.42 +/- 0.03 mm/mu s and (+0.11 +/- 0.01)%/K for gaseous xenon at 1.5 bar. In addition, we measure longitudinal diffusion coefficients of 25.7 +/- 4.6 cm(2)/s and 149 +/- 23 cm(2)/s, for liquid and gas, respectively. The quantum efficiency of the gold photocathode is studied at the photon energy of 4.73 eV in liquid and gaseous xenon, and vacuum. These charge transport properties and the behavior of photocathodes in a xenon environment are important in designing and calibrating future large scale noble liquid detectors.

The SNO+ experiment collected data as a low-threshold water Cherenkov detector from September 2017 to July 2019. Measurements of the 2.2-MeV gamma's produced by neutron capture on hydrogen were made using an Am-Be calibration source, for which a large fraction of emitted neutrons are produced simultaneously with a 4.4-MeV gamma. Analysis of the delayed coincidence between the 4.4-MeV gamma and the 2.2-MeV capture gamma revealed a neutron detection efficiency that is centered around 50% and varies at the level of 1% across the inner region of the detector, which to our knowledge is the highest efficiency achieved among pure water Cherenkov detectors. In addition, the neutron capture time constant was measured and converted to a thermal neutron-proton capture cross section of 336.3(-1.5)(+1.2) mb.

We present studies of proton fluxes in the T10 beamline at CERN. A prototype high pressure gas time projection chamber (TPC) was exposed to the beam of protons and other particles, using the 0.8 GeV/c momentum setting in T10, in order to make cross section measurements of low energy protons in argon. To explore the energy region comparable to hadrons produced by GeV-scale neutrino interactions at oscillation experiments, i.e., near 0.1 GeV of kinetic energy, methods of moderating the T10 beam were employed: the dual technique of moderating the beam with acrylic blocks and measuring scattered protons off the beam axis was used to decrease the kinetic energy of incident protons, as well as change the proton/minimum ionising particle (MIP) composition of the incident flux. Measurements of the beam properties were made using time of flight systems upstream and downstream of the TPC. The kinetic energy of protons reaching the TPC was successfully changed from ∼0.3 GeV without moderator blocks to less than 0.1 GeV with four moderator blocks (40 cm path length). The flux of both protons and MIPs off the beam axis was increased. The ratio of protons to MIPs vary as a function of the off-axis angle allowing for possible optimisation of the detector to select the type of required particles. Simulation informed by the time of flight measurements show that with four moderator blocks placed in the beamline, (5.6 ± 0.1) protons with energies below 0.1 GeV per spill traversed the active TPC region. Measurements of the beam composition and energy are presented.