We study a strategy to reduce veto-time in the search for neutrino-less double-beta decay (0υββ) with CANDLES-III system. We develop a new likelihood analysis and apply it to our new Run010 data. We show that we can increase the un-vetoed live-time by 11.8%. Thanks to this improvements, We expect to increase a limit on the life-time of 0υββ by a factor of three by analyzing both Run009 and Run010 data

Fushimi, Kenichi

Harada, Takaaki

Hashimoto, Akihiro

Hazama, Ryuta

Hiraiwa, Tasuku

Iga, Tomoki

Iida, Takashi

Kawashima, Yusuke

Kishida, Yumiko

Kishimoto, Tadafumi

Matsuoka, Kenji

Minami, Yuto

Miyoshi, Go

Muramatsu, Yoshiki

Nakajima, Kyohei

Ogawa, Izumi

Rittirong, Anawat

Sakai, Toshiki

Sakaue, Akitoshi

Suzuki, Kohtaku

Takihira, Yukichi

Tamagawa, Yoichi

Tozawa, Masashi

Umehara, Saori

Yamamoto, Asahi

Yoshida, Sei

Yoshida, Tomomi

Yoshioka, Atsushi

English

Tokushima University Institutional Repository

Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing Ltd17th International Conference on Topics in Astroparticle and Underground PhysicsJournal of Physics: Conference Series 2156 (2022) 012145IOP Publishingdoi:10.1088/1742-6596/2156/1/0121451Status of 48Ca double beta decay search in CANDLESYuto Minami1, Kenichi Fushimi2, Takaaki Harada3, AkihiroHashimoto4, Ryuta Hazama5, Tasuku Hiraiwa4, Tomoki Iga3,Takashi Iida6, Yusuke Kawashima4, Yumiko Kishida7, TadafumiKishimoto1, Kenji Matsuoka1, Go Miyoshi3, Yoshiki Muramatsu4,Kyohei Nakajima4, Izumi Ogawa4, Anawat Rittirong5, ToshikiSakai3, Akitoshi Sakaue8, Kohtaku Suzuki9, Yukichi Takihira1, YoichiTamagawa4, Masashi Tozawa4, Saori Umehara1, Asahi Yamamoto3,Sei Yoshida3, Tomomi Yoshida3, and Atsushi Yoshioka3,1Research Center for Nuclear Physics (RCNP), Osaka University, Ibaraki, Osaka 567-0047,Japan2Graduate School of Technology, Industrial and Social Sciences, Tokushima University, 2-1,Minami Josanajima-cho, Tokushima city, Tokushima, 770-8506, Japan3Graduate School of Science, Osaka University, Toyonaka, Osaka 560-0043, Japan4Graduate School of Engineering, University of Fukui, Fukui, 910-8507, Japan5Department of Environmental Science and Technology, Osaka Sangyo University, Daito,Osaka 574-8530, Japan6Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571,Japan7Graduate School of Advanced Technology and Science, Tokushima University, 2-1, MinamiJosanajima-cho, Tokushima city, Tokushima , 770-8506, Japan8Faculty of Science and Technology, Tokushima University, 2-1, Minami Josanajima-cho,Tokushima city, Tokushima , 770-8506, Japan9The Wakasa-wan Energy Research Center, Tsuruga, Fukui 914-0192, JapanE-mail: yminami@rcnp.osaka-u.ac.jpAbstract. We study a strategy to reduce veto-time in the search for neutrino-less double-betadecay (0νββ) with CANDLES-III system. We develop a new likelihood analysis and apply it toour new Run010 data. We show that we can increase the un-vetoed live-time by 11.8%. Thanksto this improvements, We expect to increase a limit on the life-time of 0νββ by a factor of threeby analyzing both Run009 and Run010 data.1. IntroductionThe origin of neutrino masses is one of the unsolved puzzles in particle physics. One possibilityis that neutrinos have Majorana masses [1]. We can test the Majorana nature of neutrinos bysearching for neutrino-less double-beta decay (0νββ) events [2]. CANDLES (CAlcium fluoridefor the study of Neutrinos and Dark matters by Low Energy Spectrometer) is a project whichtargets 0νββ events from 48Ca using its high Qββ = 4.27 MeV [3]. We developed a CANDLES-III system with 96 CaF2 scintillation crystals with natural Ca isotope, which corresponds to350 g of 48Ca [4]. The key to search for 0νββ with the CANDLES-III is to identify and removethe β decay background events from impurities in CaF2 crystals.17th International Conference on Topics in Astroparticle and Underground PhysicsJournal of Physics: Conference Series 2156 (2022) 012145IOP Publishingdoi:10.1088/1742-6596/2156/1/0121452Ref. [4] shows a results using data with 130.4 days of live-time, which we call Run009. There,β decay events from 208Tl with Qβ-value of 5.0 MeV are dominant background events.In this proceeding, we introduce a new likelihood analysis to efficiently veto β decaybackgrounds from 208Tl. After the Run009, we took data with 652.0 days of live-time in total,which we call Run010. We apply the new likelihood analysis to the Run010 data.The rest of the proceeding is organized as follows. In Sect. 2, we explain our CANDLES-III system. In Sect. 3, we review the results of Run009 analysis. In Sect. 4, we describe themethodology of the likelihood analysis. We report our results in Sect. 5 and conclude in Sect. 6.2. CANDLES-III systemCANDLES-III system is equipped with undoped CaF2 scintillation crystals1. CANDLES-III isinstalled at the Kamioka Underground Laboratory in the Institute of Cosmic Ray Research ofthe University of Tokyo. The system consists of the following five main components:• CaF2 scintillator with 3.2 kg × 96 CaF2 crystals which contains 350 g of 48Ca.• Liquid scintillator as a 4π active shields.• 62 photo multiplier tubes (PMTs) to observe scintillation lights from CaF2 and LS.• Surrounding shields of leads with thickness of 10 − 12 cm for the reduction of γ-raybackgrounds.• Surrounding shields of B4C sheets with thickness of 0.5 cm for the reduction of thermalneutron backgrounds.3. Review of Run009We review the Run009 with CANDLES-III system [4]. There are following five main backgroundcandidates which we veto and estimate:• α-ray backgrounds: we use pulse shape discrimination (PSD) technique [4], which use theinformation of pulse shape, to distinguish β-ray events from α-ray backgrounds.• External γ backgrounds: we also use PSD technique to remove environmental γ-ray eventswhich deposit energy in the surrounding LS.• (n, γ) process backgrounds: we reduced the background events by Pb and B4C shields andestimated the number of events by Monte Carlo (MC) simulations [6].• 212Bi→ 212Po sequential decay: since the life time of 212Po’s α decay, 0.299 µs, is short weobserve the sum of the energy of the both 212Bi ’s β decay and 212Po’s α decay. we canidentify this background events using a rising pulse shape of both events [4].• 208Tl backgrounds: 208Tl’s decay produces β- and γ-rays. We can only measure sum ofenergies of the two rays, which has cut-off energy of 5.001 MeV. We can veto this backgroundusing delayed coincidence method by tagging parent 212Bi’s α decay, whose quenched energyis 1.63 MeV.After the appropriate veto and estimation of background events, Ref. [4] set a half-life limitof 0νββ as 5.6× 1022 yrs using pure 21 crystals among 96 crystals. Since the dominantbackgrounds are 208Tl β decay events [4], in this proceeding, we concentrate on the reductionof the background events using a likelihood analysis.1 See Ref. [5], for the conceptual design of the CANDLES detector.17th International Conference on Topics in Astroparticle and Underground PhysicsJournal of Physics: Conference Series 2156 (2022) 012145IOP Publishingdoi:10.1088/1742-6596/2156/1/01214531200 1400 1600 1800 2000 2200Energy (keV)100101102103104Events/10 keVEpFigure 1. Energy distribution ofsignal Ep.0 1000 2000 3000 4000 5000Energy (keV)100101102Events/20 keVEdFigure 2. Energy distribution ofsignal Ed.4. Likelihood analysisFrom here, the signal refers to 212Bi → 208Tl events and the backgrounds refers to all theother events including accidental coincidence of 212Bi decay and 208Tl decay. With probabilitydistribution function (PDF) of a variables ~x of signal, P S(x), and of backgrounds, PB(x), weconstruct a likelihood function asL(~x) =P S(~x)P S(~x) + PB(~x). (1)Our strategy is to set a likelihood threshold, `, and identify all the events with L(~x) ≥ ` assignal events. In this proceeding, we use following three variables as elements of ~x:• Ep: energy of prompt events which are candidates of 212Bi’s α-decay.• Ed: energy of delayed events which are candidates of 208Tl’s β-decay.• ∆T : time difference between the prompt and the delayed events.4.1. Construction of signal PDFSince Ep, Ed, and ∆T are independent of each other, we construct a signal PDF of all variablesfrom products of each variable PDF asP S(Ep, Ed,∆T ) = PS(Ep) · P S(Ed) · P S(∆T ). (2)We use public code Geant3 [7] to estimate P S(Ep) and PS(Ed). We estimate PS(∆T )analytically assuming the exponential decay with life-time of 3.08 min.We show the histograms of Ep and Ed in Fig. 1 and Fig. 2 respectively. We normalize thesehistograms to unity and use them as PDFs.4.2. Construction of backgrounds PDFSimilar to signal, since Ep, Ed, and ∆T are independent of each other, we construct a backgroundPDF of all variables from products of each variable PDF asPB(Ep, Ed,∆T ) = PB(Ep) · PB(Ed) · PB(∆T ). (3)We take all the β-tagged events in Run010 data with the PSD technique as PB(Ed). We take allthe events in Run010 data without any tag as PB(Ep). Since backgrounds events do not dependon time, we use flat distribution for ∆T . We show a histogram of Ep of crystal-01 in Fig. 3 asa example. We normalize these histograms to unity and use them as PDFs.17th International Conference on Topics in Astroparticle and Underground PhysicsJournal of Physics: Conference Series 2156 (2022) 012145IOP Publishingdoi:10.1088/1742-6596/2156/1/01214541000 2000 3000 4000 5000Energy (keV)100101102103104Events/10 keVEd of BGsFigure 3. Energy distribution ofbackground Ep.Figure 4. Likelihood distributionin Ep vs ∆T plane with Ed ∈[3.6, 3.87] MeV.Figure 5. Likelihood which excessthe threshold, `, in Ep vs ∆T planewith Ed ∈ [3.6, 3.87] MeV.4.3. Construction of likelihood functionTo remove statistical uncertainty, we set bin width of the histograms for background PDFs tomake the number of events in each bin larger than 100. We then construct a likelihood withEq. (1) (Fig. 4).We set the likelihood threshold, `, to make 208Tl efficiency, εRun010 to be the same as that ofRun009, εRun009. The efficiencies are calculated asεRun009 = εα−tag ·∫ ∞MeV3.5MeVdEd∫ 1.85MeV1.45MeVdEp∫ 1080 s0 sd∆T PS(Ed) · PS(Ep) · PS(∆T ), (4)εRun010 = εα−tag ·∫ ∞MeV3.5MeVdEd∫ ∞MeV0.5MeVdEp∫ 1080 s0 sd∆T PS(Ed) · PS(Ep) · PS(∆T )· P (L(Ep, Ed,∆T ) ≥ `),(5)where εα−tag is the tag efficiency of α-ray events by PSD technique used in Run009. We showthe region where likelihood excess the likelihood threshold in Fig. 5.17th International Conference on Topics in Astroparticle and Underground PhysicsJournal of Physics: Conference Series 2156 (2022) 012145IOP Publishingdoi:10.1088/1742-6596/2156/1/01214555. ResultsWe apply our likelihood analysis based veto (Run010 veto) to the Run010 data. Sincethe likelihood-based cut depends on Ed, we use maximum ∆T with L ≥ ` among the allEd ≥ 3.5 MeV. As a comparison, we also apply veto similar to Run009 (Run009 veto) tothe Run010 data.We define the un-vetoed live-time over live-time as a live-time efficiency. We show the resultedlive-time efficiencies of the two vetoes in Table 1.Table 1. Live-time efficiency with 93 crystals.live-time efficiencyRun009 veto 70.9%Run010 veto 78.9%We find that we can increase the un-vetoed live-time by 11.3% using the likelihood analysis.6. Conclusions and discussionsIn this proceeding, we study a likelihood method to reduce the veto-time in the search for 0νββwith CANDLS-III system. Applying the likelihood method to Run010 data, we find that wecan increase the un-vetoed live-time by 11.3%.A limit on a half-life of 0νββ decay, T 1/2, is proportional to observation time, T obs, oversquare root of a number of backgrounds of 0νββ search,√NBG, as,T 1/2 ∝ Tobs√NBG. (6)We can increase the T obs by 11.3%; We can increase the NBG by a factor of six when we useRun010 data in addition to Run009 data. Thus, we expect to give a limit on T 1/2 as 1.5× 1023 yr.This is a improvement of a factor of three from Run009 result [4].Since we find that we can increase the number of events by introducing the likelihood analysis,we can tighten the event selection to achieve more pure measurements with the same live-timeefficiency. For example, we plan to veto events what 212Bi-α and 208Tl-β deposit energy indifferent crystals. This could increase the number of pure crystals with which we give a limit onT 1/2.AcknowledgmentsThis work was supported by Japan Society for the Promotion of Science (JSPS) KAKENHI,Grants No. 19H05804, 19H05809, 26104003, 16H00870, 24224007, and 26105513. This workwas supported by the research project of Research Center for Nuclear Physics (RCNP), OsakaUniversity. This work was also supported by the joint research program of the Institute ofCosmic Ray Research (ICRR), the University of Tokyo. The Kamioka Mining and SmeltingCompany has provided service for activities in the mine.References[1] Majorana E 1937 Nuovo Cim. 14 171–184[2] Furry W H 1939 Phys. Rev. 56 1184–1193[3] Kwiatkowski A A et al. 2014 Phys. Rev. C89 045502[4] Ajimura S et al. (CANDLES) 2021 Phys. Rev. D 103 092008 (Preprint 2008.09288)[5] Yoshida S et al. 2009 Nucl. Instr. and Meth A601 282–293[6] Nakajima K et al. 2018 Astropart. Phys. 100 54–60[7] Brun R, Bruyant F, Maire M, McPherson A C and Zanarini P 1987

Status of 48Ca double beta decay search in CANDLES

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Status of 48Ca double beta decay search in CANDLES

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