- Email: [email protected]
Low Energy Challenges in Super-Kamiokande-III
Nuclear Physics B (Proc. Suppl.) 168 (2007) 118–121 www.elsevierphysics.com
Low Energy Challenges in Super-Kamiokande-III M. B. Smya a Department of Physics and Astronomy, 3117 Frederick Reines Hall, University of California, Irvine, USA
0920-5632/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2007.02.065
Δχ 2
x10
-4 1σ
Solar & KamLAND Zenith Seasonal Spectrum ν →ν e
μ/τ
95%C.L.
2
2
Super-Kamiokande (SK) and SNO have demonstrated flavor transformation of solar 8 B neutrinos [1]. These neutrino flavor transformations should be caused by neutrino oscillation, since reactor anti-neutrino oscillations and solar flavor transformations can be described by the same parameters (see Figure 1). While reactor anti-neutrinos oscillate simply with the vacuum frequency Δm2 /E (Δm2 is the difference in mass2 between two neutrino mass eigenstates and E the neutrino energy) the flavor transformation of solar neutrinos is more complex; it is described by the Mikheyev, Smirnov and Wolfenstein [4] (MSW) model which yields a flavor transformation probability cos2 θ (where θ is the mixing angle between two neutrinos) approximately independent of energy over a wide range of energy. For the currently valid oscillation parameters, low energy solar neutrinos (much below 3 MeV) oscillate very rapidly with the vacuum frequency, so effectively the transformation probability is 2 1 High energy solar neutrinos (much 2 sin 2θ. above 3 MeV) transform with the MSW probability. This is consistent with the observation of various different neutrino branches by different experiments [2]. However, the transition has not been observed directly. Only 8 B neutrinos cover the transition region near 3 MeV, so the measured effective energy spectrum of electron-neutrinos should be distorted near 3 MeV. SK observes elastic scattering of solar 8 B neutrinos with electrons. The recoiling electron energy is measured from the Cherenkov light intensity. The elastic
8 3σ 6 4 2σ 2
Δm in eV
1. Introduction
2
Strategies to lower the solar 8 B neutrino analysis energy threshold of Super-Kamiokande from 5 to 4 MeV are discussed and Super-Kamiokande-II data is presented.
1
1σ 2σ
3σ
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 2 4 6 8 2 2 sin (Θ) Δχ
Figure 1. Allowed Parameter Regions Describing Solar Neutrinos [1,2] (Blue) and Reactor AntiNeutrinos [3] (Green) at 95% C.L. (Solid Area) and 3σ (Lines). The top and side panel show χ2 differences after one parameter was minimized.
scattering cross section has no energy threshold, and the energy dependence of the flavor transformations causes a distortion in the recoil electron spectrum. During the first phase of the experiment, SK observed solar neutrinos above a total recoil electron energy of 4.5 MeV and no significant spectral distortion was found. In the third phase we will attempt to lower this threshold to 4 MeV and search for spectral distortion with improved sensitivity.
M.B. Smy / Nuclear Physics B (Proc. Suppl.) 168 (2007) 118–121 300
119
%
cm
100 LE Fit
250 95
Clusfit Hayai
200 90
BONSAI
150 85 100 80
50
0
5
7.5 10 12.5 Vertex Resolution
15
75
5 7.5 10 12.5 15 MeV Reconstruction Efficiency
Figure 2. Vertex Resolution and Reconstruction Efficiency of Various Vertex Reconstruction Programs.
2. Event Reconstruction and Background Reduction Due to the destruction of 55% of the photomultiplier tubes (PMT) of SK in 2001, the collected Cherenkov light intensity of the second phase of SK was only about half of that of the first phase (about 3 photo-electrons per MeV), so event reconstruction and background reduction was much harder. As a consequence we learned new techniques in event reconstruction and background reduction. The programs LE-Fit, Clusfit, and Hayai were vertex reconstruction programs used in SK-I. BONSAI was developed for SK-II. Figure 2 compares vertex resolution and the fraction of failed fits as a function of energy between 3 and 15 MeV in SK-I Monte Carlo. BONSAI has significantly better resolution, especially at low energy. BONSAI has also zero failed fits. The vertex resolution is important to reduce backgrounds arising from PMT radioactivity. Most of the very-low energy background is due to that, so defining a fiducial volume of being at least 2m away from any PMT reduces this background by several orders of magnitude. The direction from events from the resolution tail (i.e. recon-
Figure 3. Ratio of Recoil Electron Spectrum over Monte Carlo Expectation from a 8 B (hep) flux of 2.33 × 106 /cm2 s (15 × 103 /cm2 s) in SK-I (Left) and SK-II (Right).
structed further than 2m away from any PMT) often points inwards. The distance of the vertex to the “entry point” projected along the event direction is a useful variable to reduce background further. The performance of this “γ ray cut” is enhanced with better vertex resolution. Better vertex resolution also helps in tagging nuclear spallation products from previous cosmic ray muons. SK-II data taught us how to use differences of the vertex fit time-residuals and the azimuthal distribution around the event direction to distinguish good electron events from mis-reconstructed events or non-Cherenkov background. This “event goodness cut” reduces the background by another order of magnitude. Due to these techniques, it was possible to measure the recoil electron spectrum in SK-II with a threshold of 7 MeV (see Figure 3). The SK-II total rate is consistent with SK-I within statistical uncertainty. SK-II also measured the day/night asymmetry above 7.5 MeV to be D−N ADN = 0.5(D+N ) = −6.3 ± 4.2(stat)±3.5%(syst). The spectrum and time-variation data from SKII further limits solar neutrino oscillation parameters (see Figure 4) independent of the absolute neutrino fluxes.
M.B. Smy / Nuclear Physics B (Proc. Suppl.) 168 (2007) 118–121 10
-3
come from the PMT glass. In SK-III, this Rn emanation is reduced by the fiberglass/acrylic enclosures. In addition, a changed water flow pattern in SK will keep the remaining Rn near the bottom of the detector (ideally outside the fiducial volume), where it can be removed by vertex cuts.
SK 1496+791 Days
2
Δm in eV
2
120
10
10
-4
-5
3. Conclusion 10
10
10
-6
SK will attempt to lower the analysis energy threshold to 4 MeV to search for the transition between MSW and vacuum flavor transformation probability with 8 B solar neutrinos.
-7
-8
10
Zenith Seasonal Spectrum νe→νμ/τ (95%C.L.) -4
10
-3
10
-2
10
-1
REFERENCES 1
10 2 tan (Θ)
Figure 4. Exclusion Regions at 95%C.L. from Spectral and Time-Variation Data from SK-I (Red Area), SK-II (Black Lines) and SK-I+II (Blue Area) Independent of Neutrino Fluxes.
In addition to reducing background we also improved our understanding of the background, in particular the background due to the PMT radioactivity. A large component is 2.6MeV γ rays from 208 Tl. We build a 60kBq source using 200 camping lantern mantles. We placed this source in SK-II and SK-III to understand this background. For SK-III, the source was placed outside the inner detector behind the PMTs to model background from the fiberglass/acrylic PMT enclosures. Figure 5 shows the vertex distributions from BONSAI and Clusfit. We can clearly see this source, even outside the inner detector! Even if all external background can be eliminated, dissolved Rn in the water of SK will give significant background. Most of the SK-I Rn transport happened by convection rather than diffusion. The dominant sources were injection by the water system and emanation from the PMT glass. After most incoming Rn from the water system was removed, the remaining Rn should
1. J. Hosaka et al, Phys. Rev. D73 (2005) 112001; B. Aharmim et al, Phys. Rev. C72 (2005) 055502. 2. B. T. Cleveland et al., Astrophys. J. 496 (1998) 505; J. N. Abdurashitov et al, Venice 2005, Neutrino Telescopes 187; M. Altmann et al, Phys. Lett. B 616 (2005) 174. 3. T. Araki et al, Phys. Rev. Lett. 94 (2005) 081801. 4. S.P. Mikheyev and A.Y.Smirnov, Sov. Jour. Nucl. Phys. 42 (1985) 913; L. Wolfenstein, Phys. Rev. D17 (1978) 2369.
M.B. Smy / Nuclear Physics B (Proc. Suppl.) 168 (2007) 118–121
121
Figure 5. Reconstructed Vertex from ‘Lantern Mantle’ Events Outside the Inner Detector. BONSAI in red is compared against Clusfit in blue.
Low Energy Challenges in Super-Kamiokande-III M. B. Smya a Department of Physics and Astronomy, 3117 Frederick Reines Hall, University of California, Irvine, USA
0920-5632/$ – see front matter © 2007 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2007.02.065
Δχ 2
x10
-4 1σ
Solar & KamLAND Zenith Seasonal Spectrum ν →ν e
μ/τ
95%C.L.
2
2
Super-Kamiokande (SK) and SNO have demonstrated flavor transformation of solar 8 B neutrinos [1]. These neutrino flavor transformations should be caused by neutrino oscillation, since reactor anti-neutrino oscillations and solar flavor transformations can be described by the same parameters (see Figure 1). While reactor anti-neutrinos oscillate simply with the vacuum frequency Δm2 /E (Δm2 is the difference in mass2 between two neutrino mass eigenstates and E the neutrino energy) the flavor transformation of solar neutrinos is more complex; it is described by the Mikheyev, Smirnov and Wolfenstein [4] (MSW) model which yields a flavor transformation probability cos2 θ (where θ is the mixing angle between two neutrinos) approximately independent of energy over a wide range of energy. For the currently valid oscillation parameters, low energy solar neutrinos (much below 3 MeV) oscillate very rapidly with the vacuum frequency, so effectively the transformation probability is 2 1 High energy solar neutrinos (much 2 sin 2θ. above 3 MeV) transform with the MSW probability. This is consistent with the observation of various different neutrino branches by different experiments [2]. However, the transition has not been observed directly. Only 8 B neutrinos cover the transition region near 3 MeV, so the measured effective energy spectrum of electron-neutrinos should be distorted near 3 MeV. SK observes elastic scattering of solar 8 B neutrinos with electrons. The recoiling electron energy is measured from the Cherenkov light intensity. The elastic
8 3σ 6 4 2σ 2
Δm in eV
1. Introduction
2
Strategies to lower the solar 8 B neutrino analysis energy threshold of Super-Kamiokande from 5 to 4 MeV are discussed and Super-Kamiokande-II data is presented.
1
1σ 2σ
3σ
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 2 4 6 8 2 2 sin (Θ) Δχ
Figure 1. Allowed Parameter Regions Describing Solar Neutrinos [1,2] (Blue) and Reactor AntiNeutrinos [3] (Green) at 95% C.L. (Solid Area) and 3σ (Lines). The top and side panel show χ2 differences after one parameter was minimized.
scattering cross section has no energy threshold, and the energy dependence of the flavor transformations causes a distortion in the recoil electron spectrum. During the first phase of the experiment, SK observed solar neutrinos above a total recoil electron energy of 4.5 MeV and no significant spectral distortion was found. In the third phase we will attempt to lower this threshold to 4 MeV and search for spectral distortion with improved sensitivity.
M.B. Smy / Nuclear Physics B (Proc. Suppl.) 168 (2007) 118–121 300
119
%
cm
100 LE Fit
250 95
Clusfit Hayai
200 90
BONSAI
150 85 100 80
50
0
5
7.5 10 12.5 Vertex Resolution
15
75
5 7.5 10 12.5 15 MeV Reconstruction Efficiency
Figure 2. Vertex Resolution and Reconstruction Efficiency of Various Vertex Reconstruction Programs.
2. Event Reconstruction and Background Reduction Due to the destruction of 55% of the photomultiplier tubes (PMT) of SK in 2001, the collected Cherenkov light intensity of the second phase of SK was only about half of that of the first phase (about 3 photo-electrons per MeV), so event reconstruction and background reduction was much harder. As a consequence we learned new techniques in event reconstruction and background reduction. The programs LE-Fit, Clusfit, and Hayai were vertex reconstruction programs used in SK-I. BONSAI was developed for SK-II. Figure 2 compares vertex resolution and the fraction of failed fits as a function of energy between 3 and 15 MeV in SK-I Monte Carlo. BONSAI has significantly better resolution, especially at low energy. BONSAI has also zero failed fits. The vertex resolution is important to reduce backgrounds arising from PMT radioactivity. Most of the very-low energy background is due to that, so defining a fiducial volume of being at least 2m away from any PMT reduces this background by several orders of magnitude. The direction from events from the resolution tail (i.e. recon-
Figure 3. Ratio of Recoil Electron Spectrum over Monte Carlo Expectation from a 8 B (hep) flux of 2.33 × 106 /cm2 s (15 × 103 /cm2 s) in SK-I (Left) and SK-II (Right).
structed further than 2m away from any PMT) often points inwards. The distance of the vertex to the “entry point” projected along the event direction is a useful variable to reduce background further. The performance of this “γ ray cut” is enhanced with better vertex resolution. Better vertex resolution also helps in tagging nuclear spallation products from previous cosmic ray muons. SK-II data taught us how to use differences of the vertex fit time-residuals and the azimuthal distribution around the event direction to distinguish good electron events from mis-reconstructed events or non-Cherenkov background. This “event goodness cut” reduces the background by another order of magnitude. Due to these techniques, it was possible to measure the recoil electron spectrum in SK-II with a threshold of 7 MeV (see Figure 3). The SK-II total rate is consistent with SK-I within statistical uncertainty. SK-II also measured the day/night asymmetry above 7.5 MeV to be D−N ADN = 0.5(D+N ) = −6.3 ± 4.2(stat)±3.5%(syst). The spectrum and time-variation data from SKII further limits solar neutrino oscillation parameters (see Figure 4) independent of the absolute neutrino fluxes.
M.B. Smy / Nuclear Physics B (Proc. Suppl.) 168 (2007) 118–121 10
-3
come from the PMT glass. In SK-III, this Rn emanation is reduced by the fiberglass/acrylic enclosures. In addition, a changed water flow pattern in SK will keep the remaining Rn near the bottom of the detector (ideally outside the fiducial volume), where it can be removed by vertex cuts.
SK 1496+791 Days
2
Δm in eV
2
120
10
10
-4
-5
3. Conclusion 10
10
10
-6
SK will attempt to lower the analysis energy threshold to 4 MeV to search for the transition between MSW and vacuum flavor transformation probability with 8 B solar neutrinos.
-7
-8
10
Zenith Seasonal Spectrum νe→νμ/τ (95%C.L.) -4
10
-3
10
-2
10
-1
REFERENCES 1
10 2 tan (Θ)
Figure 4. Exclusion Regions at 95%C.L. from Spectral and Time-Variation Data from SK-I (Red Area), SK-II (Black Lines) and SK-I+II (Blue Area) Independent of Neutrino Fluxes.
In addition to reducing background we also improved our understanding of the background, in particular the background due to the PMT radioactivity. A large component is 2.6MeV γ rays from 208 Tl. We build a 60kBq source using 200 camping lantern mantles. We placed this source in SK-II and SK-III to understand this background. For SK-III, the source was placed outside the inner detector behind the PMTs to model background from the fiberglass/acrylic PMT enclosures. Figure 5 shows the vertex distributions from BONSAI and Clusfit. We can clearly see this source, even outside the inner detector! Even if all external background can be eliminated, dissolved Rn in the water of SK will give significant background. Most of the SK-I Rn transport happened by convection rather than diffusion. The dominant sources were injection by the water system and emanation from the PMT glass. After most incoming Rn from the water system was removed, the remaining Rn should
1. J. Hosaka et al, Phys. Rev. D73 (2005) 112001; B. Aharmim et al, Phys. Rev. C72 (2005) 055502. 2. B. T. Cleveland et al., Astrophys. J. 496 (1998) 505; J. N. Abdurashitov et al, Venice 2005, Neutrino Telescopes 187; M. Altmann et al, Phys. Lett. B 616 (2005) 174. 3. T. Araki et al, Phys. Rev. Lett. 94 (2005) 081801. 4. S.P. Mikheyev and A.Y.Smirnov, Sov. Jour. Nucl. Phys. 42 (1985) 913; L. Wolfenstein, Phys. Rev. D17 (1978) 2369.
M.B. Smy / Nuclear Physics B (Proc. Suppl.) 168 (2007) 118–121
121
Figure 5. Reconstructed Vertex from ‘Lantern Mantle’ Events Outside the Inner Detector. BONSAI in red is compared against Clusfit in blue.