Solar neutrino results of Super-Kamiokande IV Yuuki Nakano (ICRR)(ynakano@km.icrr.u-tokyo.ac.jp)

for the Super-Kamiokande Collaboration

Introduction & Physics motivations ◆Super-Kamiokande detector Super-Kamiokande (SK), a 50 kton water Cherenkov detector in Japan, observes 8B solar neutrinos with neutrino-electron elastic scattering. The fourth phase of SK (SK-IV) began data taking in September in 2008. ◆Main physics motivations The main motivation of solar neutrino measurements with SK-IV is to observe the MSW effect through a solar neutrino energy spectrum distortion induced by the matter in the sun, and through a Day/Night solar neutrino flux asymmetry induced by the matter in the Earth.

Day/Night asymmetry

Oscillation analysis results

Improvement and New data set during SK-IV

Summary

Energy spectrum

SK is scanning the transition region between vacuum dominated oscillations (lower energy solar neutrino) and the matter dominated oscillations (higher energy solar neutrino) leading to the distortion.

◆Day/Night asymmetry Due to the MSW effect, electron type solar neutrinos re-generate while they travel through the Earth’s matter.

𝐬𝐢𝐧𝟐 𝜽𝟏𝟐 = 𝟎.𝟑𝟏𝟒 ∆𝒎𝟐𝟏

𝟐 = 𝟒. 𝟗 × 𝟏𝟎−𝟓𝐞𝐕𝟐

SK phase 8B solar neutrino flux [×106/cm2/s]

SK-I 2.38±0.02(stat.)±0.08(sys.)

SK-II 2.41±0.05(stat.)±0.16(sys.)

SK-III 2.40±0.04(stat.)±0.05(sys.)

SK-IV 2.36±0.02(stat.)±0.04(sys.)

Combined 2.37±0.02(stat.)±0.04(sys.)

8B solar neutrino flux Consistent flux values within statistical error are observed among SK phase

◆Precise measurement of 8B solar neutrino energy spectrum SK obtained large statistical signals with lower background among SK phase. Add 3.5-4.0 MeV (kin) energy region in SK-IV. ◆Combined energy spectrum analysis Distorted shape is slightly favored (1 sigma level significance).

◆Asymmetry in Day/Night flux SK confirms a higher solar neutrino flux at night than during the day. ⇒The first indication for the solar neutrino Day/Night effect (2.7 sigma level) ⇒The first significant direct indication for matter enhanced neutrino oscillation.

𝐬𝐢𝐧𝟐 𝜽𝟏𝟐 = 𝟎. 𝟑𝟏𝟒 ∆𝒎𝟐𝟏

𝟐 = 𝟒. 𝟗 × 𝟏𝟎−𝟓𝐞𝐕𝟐

Expected

Statistical error only

SK phase Straight Asymmetry[%] Ampfit[%]

SK-I -2.1±2.0±1.3 -2.0±1.7±1.0

SK-II -5.5±4.2±3.7 -4.3±3.8±1.0

SK-III -5.9±3.2±1.3 -4.3±2.7±0.7

SK-IV -5.3±2.0±1.4 -3.4±1.8±0.6

Combined -4.2±1.2±0.8 -3.2±1.0±0.5

◆Day/Night asymmetry vs ∆𝒎𝟐𝟏𝟐

Day/Night asymmetry is sensitive to ∆𝒎𝟐𝟏𝟐.

Expected Day/Night asymmetry line agrees with the region of SK solar result.

𝐬𝐢𝐧𝟐 𝜽𝟏𝟐 = 𝟎. 𝟑𝟏𝟒 𝐬𝐢𝐧𝟐 𝜽𝟏𝟑 = 𝟎. 𝟎𝟐𝟓

◆Allowed region of neutrino oscillation parameters A global oscillation analysis with all SK phases, other solar experiments, and KamLAND reactor experiment has been carried out. A non-zero value of 𝐬𝐢𝐧𝟐𝜽𝟏𝟑 is preferred at a 2σ level significance, and consistent with reactor experiments results.

Filled region = 3σ

Filled region = 3σ

f8B=5.25×106/(cm2∙sec)

MC 8B Flux

◆Solar neutrino results of SK-IV are presented (2008/10-2012/12, 1306.3days) ◆All SK-phase combined result is also presented. ◆Energy spectrum result ⇒ distorted shape is slightly favored. ◆Day/Night asymmetry ⇒ first indication of solar neutrino matter effect. ◆Neutrino oscillation parameters are updated.

◆Hardware improvement ⇒Better water quality control -Lowered threshold (~3.5 MeV (kin.)) -Large statistics with lower background ⇒New electronics -Better timing determination -Better MC model of trigger efficiency ◆Analysis improvement ⇒Implementation of multiple scattering goodness (MSG) Although the 214Bi decay-electrons (majority of low energy BG) fluctuate up above 5.0 MeV, they truly have energy less than 3.3 MeV and should have more multiple scattering than true 5.0 MeV electrons. ⇒Reduced systematic error 1.7% for flux (SK-I : 3.2% , SK-III : 2.1%) ◆New data set during SK-IV New front-end electronics (Qbee) was installed/started data taking as SK-IV. New Data set: 2008/10-2012/12 (1306.3days)

3.5-4.0MeV

4.0-4.5MeV

MSG < 0.35 0.35<MSG<0.45

0.45<MSG

Nov. 11th-13th 2013 NNN13@Kavli IPMU