Performance of SK-Gd's upgraded real-time supernova monitoring system

Among multi-messenger observations of the next galactic core-collapse supernova, Super-Kamiokande (SK) plays a critical role in detecting the emitted supernova neutrinos, determining the direction to the supernova (SN), and notifying the astronomical community of these observations in advance of the optical signal. On 2022, SK has increased the gadolinium dissolved in its water target (SK-Gd) and has achieved a Gd concentration of 0.033%, resulting in enhanced neutron detection capability, which in turn enables more accurate determination of the supernova direction. Accordingly, SK-Gd's real-time supernova monitoring system (Abe te al. 2016b) has been upgraded. SK_SN Notice, a warning system that works together with this monitoring system, was released on December 13, 2021, and is available through GCN Notices (Barthelmy et al. 2000). When the monitoring system detects an SN-like burst of events, SK_SN Notice will automatically distribute an alarm with the reconstructed direction to the supernova candidate within a few minutes. In this paper, we present a systematic study of SK-Gd's response to a simulated galactic SN. Assuming a supernova situated at 10 kpc, neutrino fluxes from six supernova models are used to characterize SK-Gd's pointing accuracy using the same tools as the online monitoring system. The pointing accuracy is found to vary from 3-7∘ depending on the models. However, if the supernova is closer than 10 kpc, SK_SN Notice can issue an alarm with three-degree accuracy, which will benefit follow-up observations by optical telescopes with large fields of view

Measurements of the charge ratio and polarization of cosmic-ray muons with the Super-Kamiokande detector

We present the results of the charge ratio (R) and polarization (Pμ0) measurements using the decay electron events collected from 2008 September to 2022 June by the Super-Kamiokande detector. Because of its underground location and long operation, we performed high precision measurements by accumulating cosmic-ray muons. We measured the muon charge ratio to be R=1.32±0.02 (stat.+syst.) at EμcosθZenith=0.7+0.3−0.2 TeV, where Eμ is the muon energy and θZenith is the zenith angle of incoming cosmic-ray muons. This result is consistent with the Honda flux model while this suggests a tension with the πK model of 1.9σ. We also measured the muon polarization at the production location to be Pμ0=0.52±0.02 (stat.+syst.) at the muon momentum of 0.9+0.6−0.1 TeV/c at the surface of the mountain; this also suggests a tension with the Honda flux model of 1.5σ. This is the most precise measurement ever to experimentally determine the cosmic-ray muon polarization near 1 TeV/c. These measurement results are useful to improve the atmospheric neutrino simulations

First gadolinium loading to Super-Kamiokande

International audienceIn 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 Gd2(SO4)3⋅8H2O (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 m3/h, extracting water from the top of the detector and mixing it with concentrated Gd2(SO4)3⋅8H2O 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 m3/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±1 μs, which corresponds to a Gd concentration of 111±2 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

Searching for Supernova Bursts in Super-Kamiokande IV

International audienceSuper-Kamiokande has been searching for neutrino bursts characteristic of core-collapse supernovae continuously, in real time, since the start of operations in 1996. The present work focuses on detecting more distant supernovae whose event rate may be too small to trigger in real time, but may be identified using an offline approach. The analysis of data collected from 2008 to 2018 found no evidence of distant supernovae bursts. This establishes an upper limit of 0.29 yr$^{−1}$ on the rate of core-collapse supernovae out to 100 kpc at 90% C.L. For supernovae that fail to explode and collapse directly to black holes the limit reaches to 300 kpc

Observation of B meson decays to b(1)pi and b(1)K

We present the results of searches for decays of B mesons to final states with a b(1) meson and a charged pion or kaon. The data, collected with the BABAR detector at the Stanford Linear Accelerator Center, represent 382x10(6) B(B)over bar pairs produced in e(+)e(-) annihilation. The results for the branching fractions are, in units of 10(-6), B(B+ -> b(1)(0)pi(+)) = 6.7 +/- 1.7 +/- 1.0, B(B+ -> b(1)(0)K(+)) = 9.1 +/- 1.7 +/- 1.0, B(B-0 -> b(1)(-/+)pi +/- ) = 10.9 +/- 1.2 +/- 0.9, and B(B-0 -> b(1)(-)K(+)) = 7.4 +/- 1.0 +/- 1.0, with the assumption that B(b(1) -> omega pi) = 1. We also measure charge and flavor asymmetries A(ch)(B+ -> b(1)(0)pi(+)) = 0.05 +/- 0.16 +/- 0.02, A(ch)(B+ -> b(1)(0)K(+)) = -0.46 +/- 0.20 +/- 0.02, A(ch)(B-0 -> b(1)(-/+)pi +/- ) = -0.05 +/- 0.10 +/- 0.02, C(B-0 -> b(1)(-/+)pi +/- ) = -0.22 +/- 0.23 +/- 0.05, Delta C(B-0 -> b(1)(-/+)pi +/- ) = -1.04 +/- 0.23 +/- 0.08, and A(ch)(B-0 -> b(1)(-)K(+)) = -0.07 +/- 0.12 +/- 0.02. The first error quoted is statistical, and the second systematic

Evidence for the rare decay B+→Ds+π0

We have searched for the rare decay B+ -> D-s(+) pi(0). The analysis is based on a sample of 232 X 10(6) Y(4S) -> B (B) over bar decays collected with the BABAR detector at the SLAC PEP-II e(+)e(-) storage ring. We find 19.6 signal events, corresponding to a significance of 4.7 sigma. The extracted signal yield including statistical and systematic uncertainties is 20.1(-6.0-1.5)(+6.8+0.4) and we measure B(B+ -> D-s(+) pi(0)) = (1.5(-0.4)(+0.5) +/- 0.1 +/- 0.2) X x 10(-5), where the first uncertainty is statistical, the second is systematic, and the last is due to the uncertainty on the D-s(+) decay and its daughter decay branching fractions

Measurements of CP-violating asymmetries in B0→a1±(1260)π∓ Decays

We present measurements of CP-violating asymmetries in the decay B-0 -> a(1)(+/-)(1260)pi(-/+) with a(1)(+/-)(1260)->pi(-/+)pi(+/-)pi(+/-). The data sample corresponds to 384x10(6) B(b) over bar pairs collected with the BABAR detector at the PEP-II asymmetric B factory at SLAC. We measure the CP-violating asymmetry A(CP)(a1 pi)=-0.07 +/- 0.07 +/- 0.02, the mixing-induced CP violation parameter S-a1 pi=0.37 +/- 0.21 +/- 0.07, the direct CP violation parameter C-a1 pi=-0.10 +/- 0.15 +/- 0.09, and the parameters Delta C-a1 pi=0.26 +/- 0.15 +/- 0.07 and Delta S-a1 pi=-0.14 +/- 0.21 +/- 0.06. From these measured quantities we determine the angle alpha(eff)=78.6 degrees +/- 7.3 degrees

Branching fraction and CP-violation charge asymmetry measurements for B-meson decays to ηK±, ηπ±, η′K, η′π±, ωK, and ωπ±

We present measurements of the branching fractions for B-0-meson decays to eta'K-0 and omega K-0, and of the branching fractions and CP-violation charge asymmetries for B+-meson decays to eta pi(+), eta K+, eta'pi(+), eta'K+, omega pi(+), and omega K+. The data, collected with the BABAR detector at the Stanford Linear Accelerator Center, represent 383x10(6) B (B) over bar pairs produced in e(+)e(-) annihilation. The measurements agree with previous results; we find no evidence for direct CP violation

Improved limits on the lepton-flavor violating decays tau(-) -> l(-)l(+)l(-)

A search for the neutrinoless, lepton-flavor violating decay of the tau lepton into three charged leptons has been performed using 376 fb(-1) of data collected at an e(+)e(-) center-of-mass energy around 10.58 GeV with the BABAR detector at the SLAC PEP-II storage rings. In all six decay modes considered, the numbers of events found in data are compatible with the background expectations. Upper limits on the branching fractions are set in the range (4-8)x10(-8) at 90% confidence level

Production and decay of Ωc0

We present an analysis of inclusive Omega(0)(c) baryon production and decays in 230.5 fb(-1) of data recorded with the BABAR detector. Omega(0)(c) baryons are reconstructed in four final states (Omega(-)pi(+), Omega(-)pi(+)pi(0), Omega(-)pi(+)pi(+)pi(-), Xi K--(-)pi(+)pi(+)) and the corresponding ratios of branching fractions are measured. We also measure the momentum spectrum in the e(+)e(-) center-of-mass frame. From the spectrum, we observe Omega(0)(c) production from B decays and in c (c) over bar events, and extract the two rates of production