Form 8

Research Result Report
ICRR Inter-University Research Program 2022
Research Subject:
Data Taking, Calibrations, Measurements & Analysis with Super-Kamiokande I-VII
Principal Investigator:
L. Labarga (University Autonoma Madrid)

Participating Researchers:
Nataly Ospina (University Autonoma Madrid)
Bryan Zaldivar (also IFIC, Valencia, Spain)

Summary of Research Result

The items in the title are fundamental for the collaborative work to get out the most of
the Super-Kamiokande experiment, SuperK-Gd and the preparation for the next
generation Hyper-Kamiokande. The period spanned by JPF2022 is characterized by:- It starts with a ~0.02% Gd2(SO4)3 concentration (nominal is 0.2%) dissolved in thewater since ~22 months of running with no major change being carried out in SK’s
water system.
- A second Gd2(SO4)3 loading was carried out in June 2022 (T1.5); it was 26 tons
approximately. Since then, SK is running with a ~0.06% Gd2(SO4)3 concentration.
Calibrations and related

We have evaluated with autoXenon and Nickel data the evolution with time of SuperKamiokande detector’s performance, mainly from the point of view of lighttransmission and detection. Main conclusions and results are:- Already since the end of 2020, the SK water system is keeping the Top-BottomAsymmetry (TBA) in light transmission at 2%, within ± 1% at any time (includingGd loading). During stable running (i.e., no in-purpose extra action to the water
system for any reason), it keeps TBA almost constant (± 0.2%)- The changes with time of TBA is compared with different relevant variables: ID
water temperatures, transparency, changes in the Dark Noise; Only clear
correlation with ID temperature is observed.
- The variation with time of the gains of the PMTs, grouped by age (so called SKIIand SKIII) and by their position in the detector (at top, bottom, or barrel), havebeen estimated from the corresponding evolution of the 1 photo-electron peakmeasured with Nickel data. Since the start of SK-V, their effect on relevant
variables is already significant (f.i. it reduces the TB-Asymmetry measured with

Form 8

autoXenon by ~2%). In addition, gain corrected Ni and gain corrected auto-Xenon
TBAs match better than uncorrected ones, and also gain corrected autoXenon TBA
matches now reasonably well the autoXenon TBA using only pre-calibrated PMTs
SuperK-Gd:

The Gd2(SO4)3 concentration after the second load in June corresponds to a ~70%
neutron tagging efficiency. The 26 tons of salt dissolved were produced in batches of
1 ton. Of course, all batches were carefully screened before deciding to use them.
- An intensive campaign of measurement and cross checks of samples from all ~26
T1.5 Gd production lots has been performed at three Laboratories: Boulby,
Canfranc and Kamioka. In the Canfranc Underground Laboratory, we havescreened with high purity Germanium Detectors 12 T1.5 samples.
- No Radioactive Isotope other than small amounts of 176Lu were seen (no harm forphysics from this isotope is known).
- The SK detector with ~0.06% Gd2(SO4)3 is performing well, no deterioration has
been observed after the dissolving of the Gd. The signals from the neutron captureby the Gd are very well identified in all the analysis strategies.
- Among the several publications by the Collaboration or by some of its members,
I would like to highlight “Development of ultra-pure gadolinium sulfate for theSuper-Kamiokande gadolinium project”; K. Hosokawa, et al. PTEP 2023 (2023)
no.1, 013H01.
Physics Analyses

We are getting more and more involved in the main physics goal of SuperK-Gd: thesearch for the Diffuse Supernova Neutrino Background (DSNB).
- The first paper on DSNB search with Gd neutron tagging has been submitted (May9th) to Astrophysical Journal Letters with title “Search for astrophysical electron
antineutrinos in Super-Kamiokande with 0.01wt% gadolinium loaded water.”
We are continuing the R&D towards reliable quantitative estimates of the uncertainties
in the selection or identification procedures based on Neural Network or, in general,
AI algorithms. I.e. to obtain uncertainties estimates that have a mathematically correct
statistical meaning. These, and other works by our group, are intended to significantly
improve the precision of the systematic error estimates of our measurements,
particularly in the “extreme” cases (from the point of view of number of variables
involved) of neutron tagging and global neutrino oscillation analysis.
No. A005