SK-Gd - PowerPoint PPT Presentation

超新星背景ニュートリノ探索 SK-Gd 実験における 中島 康博（東大宇宙線研究所） 2020 年 1 月 6 日 新学術「地下宇宙」 第６回超新星ニュートリノ研究会

� � � � � � � � � � Contents • SK-Gd project • Di ff use Supernova Neutrino Backgrounds • Status of SK-Gd • Towards first observation of DSNB 2

Super-Kamiokande • 50-kton water Cherenkov detector located at Kamioka, Japan • Overburden: 2700 mwe • Inner Detector covered by > 11000 20-inch PMTs 41.4 m • Can detect neutrinos for wide energy rage ~ MeV • Solar neutrinos • Supernova neutrinos • Atmospheric/Accelerator neutrinos ~ GeV • Operational since 1996 39.3 m 3

SK-Gd project • Dissolving Gd to Super-Kamiokande to significantly enhance n ̅ e ν detection capability of neutrons from ν interactions p Gd e + • Idea first proposed in: γ 8 MeV J. F . Beacom and M. R. Vagins, Phys. Rev. Lett. 93 (2004) 17110 Δ T~30µs, Vertices within 50cm Goals of SK-Gd: ν e + p → e + +n • First observation of Di ff use Supernova Neutrino | + x Gd → x+1 Gd + γ (s) (8 MeV) Background (DSNB) | + H → D + γ (2.2 MeV) 100 • Improve pointing accuracy for galactic supernova 90 Capture on gadolinium [%] 80 • Precursor of nearby supernova by Si-burning neutrinos 70 60 • Reduce proton decay background 50 40 • Neutrino/anti-neutrino discrimination (Long-baseline and 30 atmospheric neutrinos) 20 10 • Reactor neutrinos 0 0.001 0.01 0.1 0 0 0.02 0.2 Gadolinium concentration [%] Gadolinium sulfate concentration [%] 4

Improving pointing accuracy for supernova burst neutrinos • By tagging IBD events with Gd (which does not have directional information), extract ν +e elastic scattering events from SN burst • Pointing accuracy for SN at 10 kpc: 4~5 o → 3 o (90%CL) • Helps finding coincidence with optical observations Simula'on of SN at 10kp n ̅ e +p (IBD) Without Gd SK-GD (80% n-tagging eff.) n +e scat. 5

Pre-supernova signals • Precursor signal from Si-burning can also be detectable for nearby SN bursts. • eg. Betelgeuse at ~200 pc • KamLAND warning system have been implemented and running KamLAND, Astrophys. J. 818 , 91 (2016) • SK-Gd will also have sensitivity C. Simpson et al [Super-Kamiokande Collaboration] Astrophys. J. 885 , 133 (2019) Figure from Odrzywolek & Heger, 2010 SK-Gd SN at 200 pc 6 (b) DC expected signal rate

Diffuse Supernova Neutrino Background • Di ff use Supernova Neutrino Background (DSNB): Neutrinos produced from the past SN bursts and di ff used in the current universe. ~ a few SN explosions every second O(10 18 ) SNe so far in this universe • Can study history of SN bursts with neutrinos • SN rate problem : Observed SN burst rate lower H. Horiuchi et al, Astrophys. J. 738 , 154 (2011) than prediction from cosmic star formation rate P rediction from cosmic SFR 10 10 • Invisible dim supernova? -3 ] s t n -1 Mpc e m e r u s a e m R N S c i m -4 yr s o C • Black-hole formation? SNR [10 1 1 Li et al. (2011a) • Something blocking optical light? Cappellaro et al. (1999) Botticella et al. (2008) Cappellaro et al. (2005) mean local SFR Bazin et al. (2009) (see Fi gu re 2) Dahlen et al. (2004) DSNB signal will help resolving the puzzle 0.1 0.1 0 0 0.2 0.2 0.4 0.4 0.6 0.6 0.8 0.8 1.0 1.0 Redshift z 7

Current status of DSNB searches [Super-Kamiokande Collaboration] Phys. Rev. D 85 , 052007 (2012) • Current most stringent limit by SK within 9 e + candidates>16MeV/22.5kton year SK 1497+794+562 Days 8 one order of magnitude from most of the Excluded (E>16MeV) 7 ν e → e + (90 % C.L.) models. 6 5 • Experimental sensitivity limited by 4 HMA backgrounds LMA 3 6 MeV 2 • Reducing backgrounds is the key for the CE FS 4 MeV 1 CGI first observation of DSNB 0 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 T ν in MeV → Neutron tagging in SK-Gd K. Nakazato et al, Astrophys. J. 804 , 75 (2015) n � ̅ e ν � p Gd � e + � γ � � � � � 8 MeV � � � � � Δ T~30µs, Vertices within 50cm � � � � � � • First observation within reach of SK-Gd � � � � � � � � � � Predictions by � � � � � � Nakazato et al � � � � � � 8 � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � � �

Towards first Gd loading to SK • As the first step, we will load 10 tons of Gd 2 (SO 4 ) 3 Final goal (0.01% Gd concentration) in 2020 (1/10 of the final 100 90 goal) Capture on gadolinium [%] 80 Initial loading 70 • 50% of neutron would be captured by Gd (2020) 60 50 40 • x2 - x3 enhancement of n-tagging e ffi ciency 30 20 • Many preparation works towards the first Gd 10 0 loading: 0 0.001 0.01 0.1 0 0.02 0.2 Gadolinium concentration [%] Gadolinium sulfate concentration [%] ν e + p → e + +n • Fixing leak from the SK tank | + x Gd → x+1 Gd + γ (s) (8 MeV) | + H → D + γ (2.2 MeV) • Production of ultra-pure Gd 2 (SO 4 ) 3 powder n-Gd detection e ffi ciency: ~0.9 n-H detection e ffi ciency: ~0.25 • Construction of a dedicated purification/ recirculation system for Gd-water Details of the preparation works in the next talk by Ito-san 9

Timeline Step 1: Preparation of the new Gd-water system Step 2: SK pure water recirculation w/ the new water system ( Started on Dec. 24, 2019 ) Step 3: Gd loading to 0.01% ( This spring! ) 10

DSNB search at SK(-Gd) - Current and Future - 11

DSNB signal and backgrounds (after requiring neutrons) Signal 9 Li (from cosmic Atmospheric neutrinos muon spallation) ic ν µ CC e − ν e n ν µ n n p µ + (T < 50 MeV) e + e + ic ν e CC ν e n + Accidental coincidence 
 p (mostly spallation products + fake-neutrons) e + NC(QE) + Reactor neutrons n ν x / ν x γ 16 O ν x / ν x 12

DSNB search with the full SK-IV data • Analysis using the full SK-IV data (2970 live days) in progress Observation 15 • Utilize neutron tag with H-capture ATMNU (CCQE-like+NC non-QE) ATMNU (nu-NCQE) ATMNU (nubar-NCQE) • Signal e ffi ciency: ~10% (dominated by Li9 y Reactor poor n-tag e ffi ciency) r a n i m Accidental fake coincidence i l e r p SRN (Nakazato et al.) • Major backgrounds: Preliminary • Accidental coincidence • 9 Li • Atmospheric neutrino NCQE and CC interactions • Sensitivity limited by backgrounds Total backgrounds: ~50 evts/22.5kton/2970days (> 8 MeV) → ~6 evts/22.5kton/year 13

What we expect with SK-Gd SRN flux; Expected change from Horiuchi, Beacom and Dwek, the latest analysis w/ n-tag PRD, 79, 083013 (2009) • Signal e ffi ciency: SK-Gd • Increase w/ n-tag e ffi ciency: 
 9 Li and atm NC Atm CC ~20% → ~50% (0.01% Gd) 
 → >70%?( >0.03% Gd) • Backgrounds • Accidental: significantly reduced • Shorter neutron capture time 10 12 14 16 18 20 22 24 26 28 • Less fake neutron signal due to larger Position Energy (MeV) n-capture signal (8 MeV vs 2 MeV) • SK-Gd : FV = 22.5 kton, 10 year observation, 0.1%Gd 9 Li and atmospheric neutrino backgrounds expected to remain similar to SK-IV n-tag 10-16MeV 16-28MeV Total Model analysis HBD 8MeV 11.3 19.9 31.2 • Increase w/ n-tag e ffi ciency HBD 6MeV 11.3 13.5 24.8 HBD 4MeV 7.7 4.8 12.5 • 9 Li reduction w/ re-optimization of HBD SN1987a 5.1 6.8 11.9 spallation neutron cut BG 10 24 34 • Atmospheric event reduction w/ n-multiplicity cut 14

Impact of backgrounds SK-Gd : FV = 22.5 kton, 10 year observation, 0.1%Gd • Current background uncertainty: >40% 10-16MeV 16-28MeV Total Model • Toy sensitivity estimation: HBD 8MeV 11.3 19.9 31.2 • ε prompt = 64% (based on current SK-IV HBD 6MeV 11.3 13.5 24.8 analysis) HBD 4MeV 7.7 4.8 12.5 • ε neutron = 50% (first two years) 
 HBD SN1987a 5.1 6.8 11.9 70% (From the third year) BG 10 24 34 • Background sys error = 40% Background ~ 3.4 events / 22.5 kton / year • Significance = N sig / σ bkg (stat+sys) Assumed DSNB rate = 4 int/year • Can only reach 1-1.5 σ for DSNB at 4 int/22.5kton/ (before detection e ffi ciency) 4 (stat + sys) year 3.5 • Reducing backgrounds and its systematic — Bkg. = 3.4 ± 1.4 evt/year bkg 3 uncertainty are both critical for DSNB observation σ / sig Significance = N 2.5 Most problematic background: 
 2 Not an o ffi cial SK-Gd sensitivity Atmospheric ν NCQE interactions • Spectrum shape similar to DSNB 1.5 • Su ff ered by large uncertainty from 1 • Atmospheric nu flux (~15%) • NCQE cross-section (~30%) 0.5 • Neutron multiplicity (~40%) 0 0 1 2 3 4 5 6 7 8 9 10 Exposure [22.5 kt x Year] → Many e ff orts to tackle NCQE backgrounds ongoing 15