Solar neutrino measurements using the full data period of Super-Kamiokande-IV

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Solar neutrino measurements using the full data period of Super-Kamiokande-IV

K. Abe et al. (Super-Kamiokande Collaboration)

Phys. Rev. D 109, 092001 – Published 3 May 2024

Abstract

An analysis of solar neutrino data from the fourth phase of Super-Kamiokande (SK-IV) from October 2008 to May 2018 is performed and the results are presented. The observation time of the dataset of SK-IV corresponds to 2970 days and the total live time for all four phases is 5805 days. For more precise solar neutrino measurements, several improvements are applied in this analysis: lowering the data acquisition threshold in May 2015, further reduction of the spallation background using neutron clustering events, precise energy reconstruction considering the time variation of the PMT gain. The observed number of solar neutrino events in 3.49–19.49 MeV electron kinetic energy region during SK-IV is ${65,443}_{- 388}^{+ 390} (stat .) \pm 925 (syst .)$ events. Corresponding ${}^{8}B$ solar neutrino flux is $(2.314 \pm 0.014 (stat .) \pm 0.040 (syst .)) \times 10^{6} {cm}^{- 2} s^{- 1}$ , assuming a pure electron-neutrino flavor component without neutrino oscillations. The flux combined with all SK phases up to SK-IV is $(2.336 \pm 0.011 (stat .) \pm 0.043 (syst .)) \times 10^{6} {cm}^{- 2} s^{- 1}$ . Based on the neutrino oscillation analysis from all solar experiments, including the SK 5805 days dataset, the best-fit neutrino oscillation parameters are $\sin^{2} θ_{12, solar} = 0.306 \pm 0.013$ and $Δ m_{21, solar}^{2} = (6.1 0_{- 0.81}^{+ 0.95}) \times 10^{- 5} {eV}^{2}$ , with a deviation of about $1.5 σ$ from the $Δ m_{21}^{2}$ parameter obtained by KamLAND. The best-fit neutrino oscillation parameters obtained from all solar experiments and KamLAND are $\sin^{2} θ_{12, global} = 0.307 \pm 0.012$ and $Δ m_{21, global}^{2} = (7.5 0_{- 0.18}^{+ 0.19}) \times 10^{- 5} {eV}^{2}$ .

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Received 20 December 2023
Accepted 20 February 2024

DOI:https://doi.org/10.1103/PhysRevD.109.092001

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Neutrino oscillations Solar neutrinos

Neutrinos

Particles & FieldsGravitation, Cosmology & Astrophysics

Authors & Affiliations

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Article Text

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Issue

Vol. 109, Iss. 9 — 1 May 2024

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Images

Figure 1
The time variation of water transparency measured by decay electrons. This parameter is described as $L$ in Appendix pp1.
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Figure 2
Top: the time variation of the top-bottom asymmetry (TBA) throughout SK-IV full period. The red dots (blue upward-pointing triangle) show the TBA value measured by the auto-Xenon calibration system (Ni-Cf calibration source) [28]. The black vertical line shows when the temperature of the supply water was changed, as described in the main text. Bottom: the difference between two TBA measurements, which is normally within $\pm 0.5 %$ during the SK-IV dataset.
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Figure 3
The time variation of the relative PMT gain. Each panel shows PMTs from different production years.
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Figure 4
The time variation of the average dark rate of PMTs in each run. Each panel shows PMTs from different production years.
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Figure 5
A comparison of the shape of the 1 photoelectron (p.e.). the distribution used in the detector simulation. The original PMT gain (black solid) is shifted by $+ 5 %$ (in red dashed), $+ 10 %$ (in green dotted), $+ 15 %$ (in blue dotted-dashed). The vertical line shows the threshold set at 0.25 p.e.
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Figure 6
The probability density function of the timing residual ( $t_{i} - t_{tof} - t_{0} = Δ t$ ) for the single photoelectron signal. The second and the third peaks around 40 ns and 110 ns are caused by the PMT’s after pulses.
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Figure 7
The vertex resolution as a function of the true electron kinetic energy. The events whose reconstructed vertex is in the standard fiducial volume (more than 200 cm from the ID wall) are sampled.
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Figure 8
The PMT acceptance as a function of the incident angle of the photon. The dashed-black line shows the acceptance in the case of the PMT itself while the solid-red line shows that of the PMTs equipped with the acrylic cover and FRP case used after SK-II.
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Figure 9
The distribution of the opening angle between the direction of the generated electron and the vector from the reconstructed vertex to each hit-PMT’s position. The black solid, red dashed, green dotted, and blue dash-dotted lines show the distribution of 3.49 MeV, 6.49 MeV, 9.49 MeV, and 14.49 MeV in electron kinetic energy, respectively.
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Figure 10
The average directional resolution as a function of the true electron kinetic energy, for standard fiducial events. The typical angular resolutions are 35.5°, 27.4°, 23.0°, and 18.5° at 3.49 MeV, 6.49 MeV, 9.49 MeV, and 14.49 MeV of electron kinetic energy, respectively.
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Figure 11
Relative size of reconstructed $N_{eff}$ for simulated electrons with $10 MeV / c$ momentum generated uniformly in the entire ID volume. The top (bottom) panel shows results with the analysis method used for the previous (this) analysis. The vertical and horizontal axis represents reconstructed $Z$ and $R^{2} = X^{2} + Y^{2}$ positions, respectively.
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Figure 12
The difference of the $N_{eff}$ between the calibration data and the MC simulation. The markers show the position of the calibration. The red (dashed) horizontal lines show $\pm 0.5 %$ .
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Figure 13
The difference of the $N_{eff}$ between the calibration data and the MC simulation after taking the weighted average. The marker shows the position of the calibration. The blue (dashed) horizontal lines show $\pm 0.5 %$ .
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Figure 14
Top: the energy distributions of the monoenergy electrons, reconstructed in the standard fiducial volume. Bottom: the energy resolution [%] as a function of the electron’s true total energy.
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Figure 15
Typical energy distributions of the LINAC calibrations taken in 2016 at the position of $(X, Y, Z) = (- 8.13, - 0.707, - 0.06) m$ . The injected electron energies were measured by the Ge detector.
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Figure 16
The systematic uncertainties of the energy resolution are estimated by the LINAC calibration. The light-blue dashed line shows the fitting function for the combined values.
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Figure 17
The typical opening angle distributions of the LINAC calibration data and the MC simulation in SK-IV. The black solid (gray dashed) line shows the angular resolution of data (MC), which contains 68.3% of events. The data samples are the same as Fig. 15.
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Figure 18
The systematic uncertainties of the angular resolution estimated by the LINAC calibration. The definitions of the markers are the same as Fig. 16.
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Figure 19
The typical distribution of the DT calibration at position $(X, Y, Z) = (+ 0.353, - 0.707, 0.0) m$ taken on December 2016. The top panel shows the reconstructed kinetic energy of electrons, the middle panel shows the azimuthal angle dependence of the effective hits, and the bottom panel shows the zenith angle dependence of the effective hits. The effective hit is obtained by the peak position from the fitting with a Gaussian function.
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Figure 20
The time variation of the effective hit ( $N_{eff}$ ) of the DT calibration data and MC simulation (top) together with their difference (bottom). The vertical dashed line shows the period of the convection study. The two horizontal red-dotted lines show $\pm 0.5 %$ from the offset (green dot-dashed line).
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Figure 21
The position dependence of the energy scale estimated from the DT calibration in SK-IV. The top (bottom) panel shows the $Z$ -position ( $R^{2}$ -position) dependence. The two horizontal red-dashed lines show $\pm 0.4 %$ from the offset (green dotted line). There are 35 calibration positions, i.e. seven calibration holes for the $X − Y$ plane, and five positions in height. For the $X − Y$ plane, $(X, Y) = (+ 0.353, - 0.707)$ , $(- 3.889, - 0.707)$ , $(- 8.131, - 0.707)$ , $(+ 10.958, - 0.707)$ , $(+ 0.353, \pm 1201.9)$ , and $(- 12.370, - 0.707) m$ . For the height, $Z = 0$ , $\pm 6$ , and $\pm 12 m$ .
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Figure 22
The directional dependence of the difference of the effective hit between the calibration data and the MC simulation. The value is obtained by considering the volume weight of the calibration position. The two horizontal red-dashed lines show $\pm 0.1 %$ from the offset (green dotted line).
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Figure 23
The time variation of the trigger efficiencies estimated by the MC simulation of ${}^{8}B$ solar neutrinos. The trigger efficiency is highly correlated with the gain of PMTs (Fig. 3) and water transparency (Fig. 1). After changing the threshold of data acquisition (vertical light-blue dashed line), the efficiency of 3.49–3.99 MeV is improved significantly.
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Figure 24
Comparison of the log-likelihood for the signal and background of the non-neutron data period, without the WIT system. The spallation signal is shown by the red solid line, accidentals by the blue dotted line, and the tuned cut value is shown in the vertical green dashed line. The multiple spallation cut is already applied, so only 82% of remaining spallation needs to be removed to achieve 90% overall spallation removal effectiveness.
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Figure 25
Difference in solar neutrino signal events between 5.99–19.49 MeV using the updated spallation cuts compared to the previous method. The red solid (blue dashed) line is the difference in total (background), respectively. The increase in background is from the change of cut tuning unrelated to spallation.
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Figure 26
The time variation of event rates in the 2970-day final data sample. The vertical dashed line shows the threshold change from 34 hits to 31 hits. The events below 5.49 MeV during convection periods in 2018 are removed.
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Figure 27
Reconstructed electron kinetic energy distribution of the observed data after each reduction step. The square, triangle, rhombus, cross, and circle correspond to after 1st reduction, spallation cut, ambient cut, external cut, and tight fiducial volume cut, respectively. The dotted lines correspond to the SK-IV 1664-day final data sample in the previous analysis [25] and can be compared to the circle markers.
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Figure 28
Signal efficiency after each reduction step as a function of the reconstructed electron kinetic energy. The filled-square corresponds to the number of events after the trigger cut in the 22.5 kt standard fiducial volume. Other markers are the same as Fig. 27.
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Figure 29
The energy-correlated systematic uncertainties in SK-IV 2970-day dataset. The gray, light-gray, and cross-pattern show the systematic uncertainty of the ${}^{8}B$ spectrum shape, energy scale, and energy resolution uncertainties, respectively.
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Figure 30
Solar angle distribution for the energy range of 3.49–19.49 MeV using SK-IV 2970-day dataset. The black points show the observed data, the dark and light gray shades show the background and the best-fit signal, respectively.
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Figure 31
Comparison of ${}^{8}B$ solar neutrino flux among different SK phases. The thick red (thin blue) bars illustrate the statistical uncertainty (total uncertainty). The gray band shows the combined value.
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Figure 32
Comparison of ${}^{8}B$ solar neutrino flux among solar neutrino experiments. The measured flux of other experiments are obtained from the three phases of SNO [60, 61, 62, 63], Borexino [64], KamLAND [65], and SNO+ (pure water) [66]. The left (right) vertical axis shows the measured flux without neutrino oscillation (the ratio of the measured flux to the SNO’s neutral current flux [42]).
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Figure 33
Solar angle distribution for the energy range of 3.49–3.99 MeV using SK-IV 2970 days data sample. The definitions are the same as in Fig. 30.
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Figure 34
Yearly solar neutrino flux measured by SK. The red-filled circle points show the SK data with statistical uncertainty and the gray striped area shows the systematic uncertainty for each phase. The horizontal black solid line (red shaded area) shows the combined value of measured flux (its combined uncertainty). The black-blank circle points show the sunspot numbers from 1996 to 2018. The sunspot numbers are taken from Ref. [67].
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Figure 35
Solar zenith angle dependence of the measured ${}^{8}B$ solar neutrino flux in SK-IV. The vertical bars show the statistical uncertainty.
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Figure 36
Energy spectrum of the solar neutrino signal using the SK-IV 2970-day dataset. The blue circle markers show the observed event rate as a function of the recoil electron energy. The red triangle markers show the expected rate from the ${}^{8}B + h e p$ MC simulation without the oscillation effect. The signal efficiency shown in Fig. 28 is corrected.
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Figure 37
The measured energy spectrum in SK-IV. The blue points and bars show the observed rate divided by the expected event rate assuming no neutrino oscillation, with statistical and energy-uncorrelated uncertainties. The red bars and gray bands show the energy-uncorrelated and energy-correlated systematic uncertainties.
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Figure 38
Top: the recoil electron energy spectrum taken during day (night) time in SK-IV. The red filled circle (blue filled square) points show the day spectrum (night spectrum). To improve visibility, the night spectrum is shifted by $+ 0.1 MeV$ . Bottom: the straight day/night asymmetry (black cross) as a function of the recoil electron energy. The red shaded area shows the statistical average of the day/night asymmetry, which is shown in Sec. 6c.
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Figure 39
SK-IV constraints on solar oscillation parameters. The shaded region displays the $3 σ$ confidence level. Also shown are $1 - 5 σ$ allowed areas.
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Figure 40
Comparison of the recoil electron spectra of SK-IV (blue solid) with SK-I (red dashed), SK-II (black dotted), and SK-III (green dot-dashed).
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Figure 41
SK constraints on solar oscillation parameters with [blue (medium gray)] and without [green (light gray)] using the absolute elastic scattering rate. The shaded regions are for $2 σ$ confidence level. Also shown are $1 σ$ and $3 σ$ allowed areas. The dotted (dashed) contours are for $4 σ (5 σ)$ C.L.
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Figure 42
Oscillation parameters allowed by SK. Green (light gray) area is SK contour ( $3 σ$ ), blue (medium gray) area is KamLAND contour ( $3 σ$ ) and red (dark gray) area is $SK + KamLAND$ combined ( $3 σ$ ). Green (light gray) solid lines are SK global contours ( $1 - 5 σ$ C.L.), blue (medium gray) dashed line: KamLAND contours ( $1 - 3 σ$ C.L.), and red (dark gray) dotted line: $SK + KamLAND$ contours ( $1 - 3 σ$ C.L.). A ${}^{8}B$ (hep) flux constraint of $(1 \pm 0.0381) \times 5.25 \times 10^{6} {cm}^{- 2} s^{- 1}$ ( $(1 \pm 2) \times 7.88 \times 10^{3} {cm}^{- 2} s^{- 1}$ ) is applied.
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Figure 43
Comparison of the recoil electron spectra of SK-I (top left), SK-II (top right), SK-III (bottom left), and SK-IV (bottom right) with MSW predictions. The green(light gray) [blue(medium gray)] histograms are for the solar ( $solar + KamLAND$ ) best-fit $Δ m_{21}^{2}$ value. The thick (thin) lines are distorted (not distorted) by adjusting the energy scale, resolution, and neutrino spectrum within uncertainties.
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Figure 44
Statistical combination of the recoil electron spectra of SK-I, II, III, and IV overlaid with the corresponding combination of the MSW predictions. The green (light gray) [blue(medium gray)] histograms are for the solar ( $solar + KamLAND$ ) best-fit $Δ m_{21}^{2}$ value. The thick (thin) lines are distorted (not distorted) by adjusting the energy scale, resolution, and neutrino spectrum within uncertainties.
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Figure 45
Oscillation parameters allowed by SK [green (light gray)] and SNO [blue (medium gray)] data. The shaded regions are allowed at $3 σ$ confidence level. Also shown are $1 σ$ , $2 σ$ , $4 σ$ and $5 σ$ contours. The $SK + SNO$ combined analysis is shown in red (dark gray).
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Figure 46
Borexino spectrum fit parameter pair correlations (lower left from Ref. [77]) emulated with a Gaussian covariance (upper right).
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Figure 47
$θ_{12}$ and $Δ m_{21}^{2}$ allowed by the global analysis. The green (light gray) area is the solar global contour ( $3 σ$ ), the blue (medium gray) area is the KamLAND contour ( $3 σ$ ) and the red (dark gray) area is the $Solar + KamLAND$ combined ( $3 σ$ ). Green (light gray) solid lines are solar global contours ( $1 - 5 σ$ C.L.), blue (medium gray) dashed line: KamLAND contours ( $1 - 3 σ$ C.L.), and red (dark gray) dotted line: $Solar + KamLAND$ contours ( $1 - 3 σ$ C.L.). The dashed-dotted contours are $SK + SNO$ contours for comparison.
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Figure 48
$θ_{12}$ and $θ_{13}$ allowed by the global analysis. The green (light gray) area is the solar global contour ( $3 σ$ ), the blue (medium gray) area is the KamLAND contour ( $3 σ$ ) and the red (dark gray) area is the $Solar + KamLAND$ combined ( $3 σ$ ). Green (light gray) solid lines are solar global contours ( $1 - 5 σ$ C.L.), blue (medium gray) dashed line: KamLAND contours ( $1 - 4 σ$ C.L.), and red (dark gray) dotted line: $Solar + KamLAND$ contours ( $1 - 5 σ$ C.L.).
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Figure 49
SK-IV day/night amplitude fit dependence on $Δ m_{21}^{2}$ . The black line (gray band) shows the best-fit value of day/night asymmetry (its uncertainty). The red solid curve shows the expected day/night asymmetry. The green solid (blue dashed) box shows the $1 σ$ range allowed by solar experiments (solar experiments and KamLAND).
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Figure 50
SK-IV day/night amplitude fit dependence on energy. The gray band shows the combined value.
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Figure 51
Day/night asymmetries from amplitude fits by the four different SK phases. The black thick (red thin) bar shows the statistical (total) uncertainty and the gray band shows the combined value.
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Figure 52
SK-I-IV combined day/night amplitude fit dependence on $Δ m_{21}^{2}$ . The definitions are the same as Fig. 49.
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Figure 53
The electron neutrino survival probability as a function of neutrino energy obtained from all SK data. The blue (medium gray) [red (dark gray)] region obtained form the $P_{e e, quad} (E_{ν})$ [ $P_{e e, \exp} (E_{ν})$ ] function. The green (light gray) region is from the $P_{e e, cubic} (E_{ν})$ function. The thick blue (medium gray) line is the $P_{e e} (E_{ν})$ distribution expected from neutrino oscillation at all $solar + KamLAND$ best-fit parameter set, and the thick green (light gray) line is the $P_{e e} (E_{ν})$ distribution expected from neutrino oscillation at all solar best-fit parameter set.
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Figure 54
The electron neutrino survival probability as a function of neutrino energy. The green (light gray) [blue (medium gray)] region is obtained from the $P_{e e, quad} (E_{ν})$ function with SK (SNO) spectrum data. The red (dark gray) region is obtained from the same function, but with $SK + SNO$ data. The thick blue (medium gray) and green (light gray) lines are the same as Fig. 53.
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Figure 55
The electron neutrino survival probability as a function of neutrino energy and solar neutrino measurements. The red region is obtained from the $P_{e e, quad} (E_{ν})$ function with $SK + SNO$ spectrum data. The thick blue and green lines are the same as Fig. 53. The light blue (filled triangle) and gold (open triangle) data points are the average $p p$ and CNO neutrino survival probabilities inferred from the radio-chemical solar neutrino data as well as $SK + SNO$ and Borexino ${}^{7}{Be}$ measurements, respectively. The dark blue (filled square) and dark red (open circle) data points represent the average $p p$ and ${}^{8}B$ neutrino survival probabilities, respectively. The green (open square) and turquoise (filled circle) data points represent the ${}^{7}{Be}$ and pep neutrino survival probabilities from Borexino data, respectively.
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Figure 56
Relative size of effective number of hits without (left) and with (right) the correction of the gain drift. Different colored points represent PMTs for different production years [From bottom to top: 1992–1995 (red), 1996–1997 (green), 2003 (blue), 2004 (yellow), 2005 (purple)]. The gap between adjacent horizontal lines corresponds to a 5% deviation.
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Figure 57
Values of $S (θ, ϕ)$ for the barrel (upper panel) and top/bottom (lower panel) PMTs.
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Figure 58
Solar angle distribution for the MSG sub-groups below 5.49 MeV. Below 4.99 MeV, the tight fiducial volume cut is applied.
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Figure 59
Solar angle distribution for the MSG sub-groups above 5.49 MeV and below 7.49 MeV.
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Figure 60
Solar angle distribution for the energy region above 7.49 MeV and below 12.49 MeV. In this energy region, categorization by the MSG parameter is not used.
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Figure 61
Solar angle distribution for the energy region above 12.49 MeV. In this energy region, categorization by the MSG parameter is not used.
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