Search for Sterile Neutrinos at RENO In-Sung Yeo May 22, 2017

Outline Introduction Motivation Experimental Set-up & Detector Data-taking & Analysis χ2 Fitting for Sterile Neutrino Search - Rate + Shape Analysis Results Summary

Introduction Since the past decades, great progress in the understanding of Pontecorvo- Maki-Nakagawa-Sakata matrix has been made. Three-neutrino mixing framework is well established, but many experiments results cannot be explained within the standard three-flavor framework. It seems to require an additional neutrino. “LSND anomaly” (The LSND experiment observed a small excess of νe events in νμ beam, and MiniBooNE experiment see the similar size excess.) “Gallium anomaly” ( GALEX and SAGE obtained that the ratio of the number of measured and predicted events, R, is 0.86±0.05 ) LSND νμ→νe search MiniBooNE νμ→νe search Gallium anomaly PRD 64, 112007 (2001) PRL 110, 161801 (2013) PRC 73, 045805 (2006)

-> Create additional oscillation modes Reactor antineutrino anomaly “Reactor antineutrino anomaly” ( R, is 0.940±0.027, ~6% deficit) - Antineutrino flux increase - Neutron lifetime decrease PRD 83, 073006 (2011) Daya Bay, Double Chooz, RENO experiments are also searching for sterile neutrinos. Light sterile states can be mixed with active states through the mass eigenstates. -> Create additional oscillation modes

Sterile Neutrinos Mixing Four-neutrino mixing The survival probability of electron antineutrinos is given by <θ13 & θ12 oscillation terms> <θ14 oscillation terms>

Motivation (reactor anomaly) < Results of short baseline experiments > CHOOZ Data  6% deficit !! KamLAND data RENO Near RENO Far (θ13 oscillation) T. Mueller et al., Phys. Rev. C 83, 054615 (2011)

RENO Collaboration Reactor Experiment for Neutrino Oscillation (9 institutions and 40 physicists) Chonnam National University Dongshin University GIST Gyeongsang National University Kyungpook National University Sejong University Seoul National University Seoyeong University Sungkyunkwan University Total cost : $10M Start of project : 2006 The first experiment running with both near & far detectors from Aug. 2011 YongGwang (靈光) : (New name : Hanbit )

RENO Experimental Set-up Far Detector Near Detector 1380m 290m 120 m.w.e. 16.5 GWth 450 m.w.e.

RENO Detector 354 ID +67 OD 10” PMTs Target : 16.5 ton Gd-LS, R=1.4m, H=3.2m Gamma Catcher : 30 ton LS, R=2.0m, H=4.4m Buffer : 65 ton mineral oil, R=2.7m, H=5.8m Veto : 350 ton water, R=4.2m, H=8.8m

RENO Data-taking Status Data taking began on Aug. 2011 with both near and far detectors. (DAQ efficiency : ~95%) A A (220 days) : First q13 result [11 Aug, 2011~26 Mar, 2012] PRL 108, 191802 (2012) B B (403 days) : Improved q13 result [11 Aug, 2011~13 Oct, 2012] NuTel 2013, TAUP 2013, WIN 2013 C (~500 days) : New result Shape+rate analysis (q13 and |Dmee2|) [11 Aug, 2011~21 Jan, 2013] PRL 116, 211801 (2016) → Sterile neutrino search in progress C 1500 days Total observed reactor neutrino events as of today (1500 days): ~ 1.5M (Near), ~ 0.15M (Far)

Detection of Reactor Antineutrinos (Prompt signal) (Delayed signal) ~200 ms + p  D + g (~2.2 MeV) ~30 ms (0.1% Gd) + Gd  Gd + g‘s (~8 MeV) Neutrino energy measurement Prompt signal 10-40 KeV 1.8 MeV Delayed signal ~8 MeV

Coincidence of Prompt and Delayed Signals (Prompt signal) (Delayed signal) Delayed signal Prompt signal n-Gd IBD ~30 ms ~8 MeV ~200 ms n-H IBD ~2.2 MeV Good agreement with data and MC.

Delayed Signals from Neutron Capture by Gd

Backgrounds m n m n m 9Li g p e n Accidental coincidence between prompt and delayed signals Fast neutrons produced by muons, from surrounding rocks and inside detector (n scattering : prompt, n capture : delayed) 9Li/8He b-n followers produced by cosmic muon spallation Accidentals Fast neutrons 9Li/8He b-n followers m n Gd m n m 9Li g p e Gd n Gd

IBD Candidates & Backgrounds Near Far DAQ live time (days) 458.49 489.93 IBD candidates 290,755 31,541 Total BKG rate (/day) 17.54± 0.83 3.14± 0.21 IBD rate (/day) after BKG subtraction 616.67± 1.44 61.24± 0.42

Observed Daily Averaged IBD Rate Good agreement with observed rate and prediction Accurate measurement of thermal power by reactor neutrinos

Expected IBD Templates <Oscillation parameter coordinate> Inputs IBD MC Template Oscillation parameters Signal MC with 4 isotopes Interaction fraction Expected # of IBD Detection efficiency Dead time · <Prompt spectrum> <Examples> Expected far to near ratio template |Dm241| = 0.1 eV2 Expected far to near ratio template sin22q14 = 0.1

χ2 Fitting for Sterile Neutrino Search - Rate + Shape Analysis Oscillation parameters are determined using far/near ratio of IBD prompt energy spectrum Minimize χ2 function Observed far/near ratio Expected far/near ratio (Detection efficiency, dead time, thermal power, fission fraction, energy scale....)

Results of Excluded Region 500 days of RENO data Consistent with standard 3-flavor neutrino oscillation model <Examples> Exclusion region full curves assumes Able to set limits in the region

Deviation (θ14) from θ13 Oscillated Spectrum <Unexcluded parameters> · · Normalized toθ13 oscillation · Exclusion region

Deviation (θ14) from θ13 Oscillated Spectrum <Excluded parameters> · · Exclusion region ·

Summary Summary We obtained an excluded parameter region for sterile neutrino oscillation due to sin2(2θ14) and |Δm241|, from 500 days of data sample. The far-to-near ratio measurement of reactor anti- neutrinos is used to reduce the flux and spectral uncertainties. We checked consistency with the standard three- flavor neutrino oscillation model.

Back up Evaluation of reactor neutrino flux: issues and uncertainties arXiv:1603.08990 235U=53.8%, 239Pu=32.8%, Pu241=5.6% and U238=7.8%, in fractions of fissions per isotope. Back up

Expected Reactor Antineutrino Fluxes Reactor neutrino flux - Pth : Reactor thermal power provided by the YG nuclear power plant - fi : Fission fraction of each isotope determined by reactor core simulation of Westinghouse ANC - fi(En) : Neutrino spectrum of each fission isotope [* P. Huber, Phys. Rev. C84, 024617 (2011) T. Mueller et al., Phys. Rev. C83, 054615 (2011)] - Ei : Energy released per fission [* V. Kopeikin et al., Phys. Atom. Nucl. 67, 1982 (2004)] * P.Huber PRD70, 053011 (2004)

박사 과정 한일 논문(주저자) - 4편 국내 구두 발표 국내 포스터 발표 한국물리학회 6회 한국물리학회 4회 해외 포스터 발표 " Study of Neutron Capture Events Produced by Cosmic Muons in the RENO”, New Physics, (2012) " Development of a Gadolinium-loaded LAB-based Aqueous Scintillator for Neutrino Detection”, JKPS, (2013) "Development of a Gadolinium-loaded Liquid Scintillator for the Hanaro Short Baseline Prototype Detector", JKPS, (2014) "Study of the Monte Carlo Simulation Environment by Using Radioactive Sources in the RENO", New Physics, (2014) 국내 구두 발표 국내 포스터 발표 한국물리학회 6회 한국물리학회 4회 해외 포스터 발표 3회 International Atomic Energy Agency, France, Dec. 2014 XXVII International Conference on Neutrino Physics and Astrophysics, (Neutrino 2016) 38th International Conference on High Energy Physics, (ICHEP 2016)

<포스터> <구두 발표> 2012년 봄 학술논문발표회 ->Study of neutron capture events produced by cosmic muons 2012년 가울 학술논문발표회 ->Study of the RENO detector stability with cosmic muon induced events 2013년 봄 학술논문발표회 ->Study of detection efficiency at RENO 2013년 가울 학술논문발표회 ->Production of liquid scintillator for SBL prototype detector 2014년 봄 학술논문발표회 ->Observed vs expected rates of reactor neutrinos at RENO 2014년 가울 학술논문발표회 ->Search for sterile neutrinos at RENO 주최기관 : International Atomic Energy Agency ->Search for sterile neutrinos at RENO (poster at France, 20141215) XXVII International Conference on Neutrino Physics and Astrophysics, (Neutrino 2016) ->Search for sterile neutrinos at RENO (poster at London, UK, 20160705) 38th International Conference on High Energy Physics, (ICHEP 2016) ->Search for sterile neutrinos at RENO (poster at Chicago, USA, 20160806) <구두 발표> 2015년 봄 (가을) 학술논문발표회 Search for sterile neutrinos at RENO 2016년 봄 (가을) 학술논문발표회

Introduction -> Create additional oscillation modes Great progress in the understanding of Pontecorvo-Maki-Nakagawa- Sakata matrix has been made. Three-neutrino mixing framework is well established, many experiments results cannot be explained within the standard three-flavor framework. It seems to require an additional neutrino with a mass. (“LSND anomaly”, “Gallium anomaly”, ‘reactor antineutrino anomaly’, “dark radiation and hot dark matter component”) Daya Bay, Double Chooz, RENO show a similar deficit in the measured electron antineutrino event rates. Light sterile states can be mixed with active states through the mass eigenstates. -> Create additional oscillation modes

Sterile neutrino search at RENO <θ13 &θ12 oscillation terms> <θ14 oscillation terms> Near : L~300m Near : L~1400m ~1400m <Ev>~4MeV ~300m Sensitive at Δm241 ~ 0.5×10-3eV2 Sensitive at Δm241 ~ 0.2×10-2eV2

Expected Antineutrino Events from Reactor Inverse Beta-Decay Number of expected anti-neutrinos from reactor Average cross section IBD cross section Prefactor κ ->From Beta-Decay of the free neutrons (926 s -> 881.4 s) G. Mention et al. The Reactor Anti-neutrino Anomaly, Phys. Rev. D 83, 073006 (2011).

Expected Antineutrino Events from Reactor 1. Antineutrino flux increase 235U (+2.5%), 239Pu(+3.1%), 238U(+9.8%), 241Pu(+3.7%) 2. Neutron lifetime decrease (926 s -> 881.4 s) (10-43 cm2/fission) Isotope Old New 235U 6.39±1.9% 6.61±2.11% 239Pu 4.19±2.4% 4.34±2.45% 238U 9.21±10% 10.10±8.15% 241Pu 5.73±2.1% 5.97±2.15% +3.4% +3.6% +9.6% +4.2% G. Mention et al. The Reactor Anti-neutrino Anomaly, Phys. Rev. D 83, 073006 (2011). Increase of the expected reactor antineutrino rate in 2011

Expected IBD Templates Sources of systematic uncertainties Inputs Sources of systematic uncertainties Uncertainties (%) Isotope fraction 0.7 Thermal power 0.5 Detection efficiency 0.2 Backgrounds (near) (far) 0.14 0.35 Energy Scale 0.15 IBD MC Template Oscillation parameters Signal MC with 4 isotopes Interaction fraction Expected # of IBD Detection efficiency Dead time Expected Far to Near ratio template |Dm241| = 0.1 eV2 Expected Far to Near ratio template sin22q14 = 0.1

Delayed Spectrum and Capture Time| Delayed signal peak: ~2.2 MeV Mean coincidence time: ~ 200 ms 32

Far/Near Shape Analysis for |Dmee2| Observed Far/Near Expected χ2 fitter Fit using far-to-near ratio (Work in progress) 1850 days of data Minimize Χ2 Function Energy-dependent disappearance of reactor antineutrionos

Results from Spectral Fit (Work in progress) Rate+shape results (± 8 %) (± 6 %)

RENO Results PRL 116, 211801 (2016) Submitted to PRD (arXiv:1610.04326) 500 days systematic errors are greatly reduced due to larger statistics and efforts on background reduction (Work in progress) New results (~1850 days)

F. P. An et al., Phys .Rev. Lett. 113, 141802 (2014)

Observation of an excess at 5 MeV ~2.5% ~2.5%

G. Mention et al. “The Reactor Anti-neutrino Anomaly”, Phys. Rev G. Mention et al. “The Reactor Anti-neutrino Anomaly”, Phys. Rev. D 83, 073006 (2011). Y. Declais et al., Nucl. Phys. B434, 503 (1995).

Make Template Inputs PhysRevLett. 116.211801 Oscillation Parameters : 14448개 Point Signal MC w/ 4 isotopes Vr.20150503 /data_terminalB/hkseo/mc/ntuple Interaction Fraction /scratch/public/expected_flux/ isotope_fraction/150814 Dead Time : Vr 20150814 IBD MC template Oscillation parameters Signal MC w/ 4 isotopes Interaction fraction Expected # of IBD Detection efficiency Dead time From RENO data PhysRevLett. 116.211801

Make Template Inputs sin2(2θ13)= 0.01~0.15 (dtheta 0.01) The value of sin2(2 θ 14), sin2(2θ13), and |Δm241| were unconstrained. Inputs Oscillation Parameters : 14448개 Point Signal MC w/ 4 isotopes Vr.20150503 /data_terminalB/hkseo/mc/ntuple Interaction Fraction /scratch/public/expected_flux/ isotope_fraction/150814 Dead Time : Vr 20150814 IBD MC template Oscillation parameters Signal MC w/ 4 isotopes Interaction fraction Expected # of IBD Detection efficiency Dead time sin2(2θ13)= 0.01~0.15 (dtheta 0.01) Before period : 216720 개 After period : 216720 개

Back up

Energy scale <Example>

It can’t apply different parameter values in batches. Energy scale I was a fitting function dividing the period or using complex functions. It can’t apply different parameter values in batches. So, it chosen 0.15% of energy scale difference by interpolator method in the code. Near1 Near2 Near3 Near4

Energy Scale Far2 Far1 Near6 Near5 Far3 Far4 Far5 Far6

Exclusion contour Exclusion region No.1 No.2 ① ② ③ ④ ⑥ ⑤ ①, ②, ③, ④ Reno - 95% C.L. Dayabay - exclusion ⑤, ⑥ Reno - exclusion Dayabay - 95% C.L. No.1 No.2

No.1 - ① No.1 - ②

Comparison of RENO and DayaBay <Theoretical calculation> RENO DayaBay It showed different trends by baseline difference. RENO DayaBay

1. Reactors by position reactor1 reactor2 reactor3 reactor4

1. Reactors by position reactor5 reactor6 Total

No.1 - ③ No.1 - ④

Comparison of RENO and DayaBay <Theoretical calculation> RENO DayaBay It showed different trends by baseline difference. RENO DayaBay

1. Reactors by position reactor1 reactor2 reactor3 reactor4

1. Reactors by position reactor5 reactor6 Total

No.2 - ⑤ No.2 - ⑥

1. Reactors by position reactor1 reactor2 reactor3 reactor4 reactor5

2. Theoretical calculation RENO DayaBay

F. P. An et al., Phys .Rev. Lett. 113, 141802 (2014)

Far prediction from near RENO data Dayabay data

RENO DayaBay Data It showed different trends by reno and dayabay data.

RENO data

RENO data

Survival Probability RENO data Mention (SBL)

Fixed sin2(2θ13) Exclusion contour 95% C.L. Exclusion region RENO data (input data)

<sin2(2θ13)=0.01> <sin2(2θ13)=0.02>

<sin2(2θ13)=0.03> <sin2(2θ13)=0.04>

<sin2(2θ13)=0.05> <sin2(2θ13)=0.06>

<sin2(2θ13)=0.08> <sin2(2θ13)=0.09>

<sin2(2θ13)=0.10> <sin2(2θ13)=0.11>

<sin2(2θ13)=0.12> <sin2(2θ13)=0.13> <sin2(2θ13)=0.14>

F. P. An et al., Phys .Rev. Lett. 113, 141802 (2014)

Far prediction from near Best-fit value

far # of events % near reactor1 4372.27 15.06 20663.25 7.21 reactor2 4973.06 17.13 47087.56 16.42 reactor3 4864.59 16.76 92366.50 32.21 reactor4 5523.63 19.03 82195.03 28.67 reactor5 4678.82 16.12 29388.57 10.25 reactor6 4619.2 15.91 15036.66 5.24 total 29031.57 100.00 286737.57 far # of events % near reactor1 4461.18 14.97 20777.10 7.40 reactor2 5100.56 17.12 46065.79 16.40 reactor3 5003.23 16.79 89852.15 31.98 reactor4 5685.62 19.08 79993.17 28.47 reactor5 4809.91 16.14 28997.15 10.32 reactor6 4732.59 15.88 15273.71 5.44 Total 29793.09 100.00 total 280959.07

Fitting Chi^2 function for shape analysis - rate + shape analysis Fixed theta13 Shape+Rate 0.20 MeV / bin Sin2(2θ14) (Χ 2 / d.o.f ) Stat. Error Sys. Error Total error Before cf 0.0313836 (26.1/32=0.81 ) + 0.016852 - 0.017220 + 0.006231 - 0.003328 + 0.017967 - 0.017539 After cf 0.04069123 (30/32=0.93 ) + 0.037688 - 0.035620 + 0.015158 - 0.017509 + 0.040623 - 0.039691 Total 0.02988635 (56.9/66=0.86 ) + 0.015365 - 0.015617 + 0.0055586 - 0.0057190 + 0.016340 - 0.0166314 Shape+Rate 0.20 MeV / bin Δm241 Stat. Error Sys. Error Total error Before cf 0.0070387 + 0.0005688 - 0.0006065 + 0.0002649 - 0.0002091 + 0.0006275 - 0.0006416 After cf 0.0079115 + 0.0015453 - 0.0014749 + -nan - 0.0003471 + 0.0015147 - 0.0015152 Total 0.0072050 + 0.0005623 - 0.0006000 + 0.0002517 - 0.0001892 + 0.0006160 - 0.0006291

Fitting Chi^2 function for shape analysis - rate + shape analysis Shape+rate 0.20 MeV / bin Sin2(2θ14)/(Χ 2 / d.o.f ) Stat. error Sys. error Total error Total 0.04599 (56/65=0.86) + 0.0156 - 0.0161 +0.00473 -0.00183 +0.01637-0.01620 Shape+rate 0.20 MeV / bin Δm241 Stat. error Sys. error Total error Total 0.007293 + 0.000395 - 0.000419 +0.00014-0.00010 +0.00042 -0.00043 The value of sin2(2 θ 14), sin2(2θ13), and |Δm241| were unconstrained. (Chi-square fitting with 3 parameters)

Sterile neutrino mixing Three-neutrino mixing UPMNS=R23(q23)R13(q13,d1)R12(q12) Four-neutrino mixing UF = R34(q34)R24(q24,d2)R14(q14)R23(q23)R13(q13,d1)R12(q12,d3) The survival probability of electron antineutrinos is given by <θ13 &θ12 oscillation terms> <θ14 oscillation terms>

<Oral presentation> <Poster> 2012년 봄 학술논문발표회 ->Study of neutron capture events produced by cosmic muons 2012년 가울 학술논문발표회 ->Study of the RENO detector stability with cosmic muon induced events 2013년 봄 학술논문발표회 ->Study of detection efficiency at RENO 2013년 가울 학술논문발표회 ->Production of liquid scintillator for SBL prototype detector 2014년 봄 학술논문발표회 ->Observed vs expected rates of reactor neutrinos at RENO 2014년 가울 학술논문발표회 ->Search for sterile neutrinos at RENO 주최기관 : International Atomic Energy Agency ->Search for sterile neutrinos at RENO (poster at France, 20141215) XXVII International Conference on Neutrino Physics and Astrophysics, (Neutrino 2016) ->Search for sterile neutrinos at RENO (poster at London, UK, 20160705) 38th International Conference on High Energy Physics, (ICHEP 2016) ->Search for sterile neutrinos at RENO (poster at Chicago, USA, 20160806) <Oral presentation> 2015년 봄 (가을) 학술논문발표회 Search for sterile neutrinos at RENO 2016년 봄 (가을) 학술논문발표회