Sensitivities for coherent elastic scattering of solar and supernova neutrinos with future NaITl dark matter search detectors of COSINE-2001T

2025-05-03 0 0 931.36KB 23 页 10玖币

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Sensitivities for coherent elastic scattering of solar and

supernova neutrinos with future NaI(Tl) dark matter

search detectors of COSINE-200/1T

Young Ju Koa, Hyun Su Leea,b

aCenter for Underground Physics, Institute for Basic Science (IBS), Daejeon 34126,

Republic of Korea

bIBS School, University of Science and Technology (UST), Deajeon 34113, Republic of

Korea

Abstract

We investigate the prospects for measuring the coherent elastic neutrino–nucleus

scattering of solar and supernova neutrinos in future NaI(Tl) dark matter detec-

tion experiments. Considering the reduced background and improved light yield

of the recently developed NaI(Tl) crystals, more than 3σobservation sensitivi-

ties of the supernova neutrino within the Milky Way are demonstrated. In the

case of the solar neutrino, approximately 3σobservations are marginal with a

1 ton NaI(Tl) experiment assuming an order of magnitude reduced background,

ﬁve photoelectron thresholds, and 5-year data exposure.

Keywords: NaI(Tl) crystal, coherent elastic neutrino–nucleus scattering, solar

neutrino, supernova neutrino

1. Introduction

A claim of dark matter observations of the DAMA/LIBRA experiment [1, 2]

from an annual modulation of the rate of low-energy events in an array of

NaI(Tl) crystals has triggered independent eﬀorts worldwide to reproduce the

annual modulation signals with these NaI(Tl) crystals [3, 4, 5, 6, 7, 8, 9, 10,

11]. Ongoing NaI(Tl) experiments, such as COSINE-100 and ANAIS-112, have

Email addresses: yjko@ibs.re.kr (Young Ju Ko), hyunsulee@ibs.re.kr (Hyun Su Lee)

Preprint submitted to Astroparticle Physics August 29, 2023

arXiv:2210.01386v2 [hep-ph] 28 Aug 2023

achieved background levels of approximately 2–4 counts/kg/day/keV at the 1–

6 keV region of interest (ROI). These experiments are run with approximately

100 kg, although DAMA/LIBRA used a 250 kg NaI(Tl) array with a back-

ground level of less than 1 counts/kg/day/keV in the ROI. Further eﬀorts to

reduce the background level, aiming for compatibility with DAMA/LIBRA,

have been realized using high-light yields [12, 13] and low-background NaI(Tl)

detectors [14, 15, 16]. Based on these eﬀorts, the COSINE-200 experiment [17]

will begin at the end of 2023 with a 200-kg NaI(Tl) detector with reduced back-

ground levels, to realize full-size and low-background NaI(Tl) crystals.

With a large number of NaI(Tl) detectors, these detectors can also be used

to detect solar and supernova neutrinos via coherent elastic neutrino–nucleus

scattering (CEνNS) [18, 19, 20]. Ton-scale dark matter detectors will soon

encounter the background caused by CEνNS of solar neutrino [21], the so-called

neutrino ﬂoor. This will be a hurdle to the dark matter searches, however, it

will be also an opportunity to study new physics in the neutrino sector such

as non-standard interactions [22] and neutrino magnetic moment [23]. In this

study, we investigate the feasibility of future NaI(Tl) dark matter detectors as

neutrino telescopes using the CEνNS process.

2. Future NaI experiments

2.1. 200 kg NaI(Tl) experiment (COSINE-200)

Eﬀorts to upgrade the ongoing COSINE-100 experiment to the next-phase

COSINE-200 have resulted in the production of NaI(Tl) crystals with reduced

internal backgrounds from 40K and 210Pb [24, 25, 26], as well as an increased

light yield of 22 photoelectrons (NPE) per unit kiloelectron volts electron-

equivalent energy (keVee) [13, 27]. The recrystallization method has achieved

chemical puriﬁcation of raw NaI powder with suﬃcient reduction of K and Pb

contamination [24, 25]. A dedicated Kyropoulos grower for small test crystals

produces low-background NaI(Tl) crystals with reduced 40K and 210Pb values

of less than 20 ppb and 0.5 mBq/kg, respectively, corresponding to background

levels of less than 1 counts/kg/day/keVee at the 1–6 keVee ROI [26]. A full-

size Kyropoulos grower was built for the 100 kg-size crystal ingot to provide

approximately 200 kg of low-background NaI(Tl) detectors for the COSINE-200

experiment. The expected background levels of these crystals were less than

0.5 counts/kg/day/keVee in the ROI [17]. This estimation was based on the

measured background levels of the small test crystals reported in Ref. [26].

An increased light yield of the NaI(Tl) crystal is essential to reduce the en-

ergy threshold below a keVee. With an optimized concentration of thallium

doping in the crystal, we achieved a high light yield of 17.1±0.5 NPE/keVee,

which is slightly larger than that of the COSINE-100 crystal (approximately

15 NPE/keVee). A further increase in the light-collection eﬃciency by ∼50% in

the NaI(Tl) crystal was achieved by an improved encapsulation scheme, as de-

scribed in Ref. [13]. In this scheme, the crystal and photomultiplier tube (PMT)

are directly connected without an intermediate quartz window, for which a

22 NPE/keVee light yield is achieved. A similar crystal encapsulation technique

was applied to the reactor CEνNS search experiment NEON, and approximately

22 NPE/keVee light yields were measured [27].

The typical trigger requirement of the COSINE-100 experiment is satisﬁed

by coincident photoelectrons in two PMTs attached to each side of the crystal at

approximately 0.13 keVee. However, PMT-induced noise events are dominantly

triggered at energies below a few keVees. The multivariable boosted decision

tree (BDT) provided a 1 keVee analysis threshold with less than 0.1% noise con-

tamination and above 80% selection eﬃciency [28]. A key variable in the BDT

is the likelihood parameter using the event shapes of scintillation-like and PMT-

induced noise-like events. Further improvement of the low-energy event selection

is ongoing using the COSINE-100 data by developing new parameters for the

BDT and employing a machine learning technique that uses raw waveforms di-

rectly. COSINE-200 targets an analysis threshold of 5 NPE (0.2 keVee) [17],

which is similar to the energy threshold that has already been achieved by the

COHERENT experiment with CsI(Na) crystals [29] and the target threshold of

the NEON CEνNS search experiment with NaI(Tl) crystals [27].

The COSINE-200 experiment can be realized in a 4×4 array of 12.5 kg

NaI(Tl) modules by replacing the crystals inside the COSINE-100 shield [5].

The COSINE-200 experiment will run for at least 3 years for an unambigu-

ous test of DAMA/LIBRA annual modulation signals [30]. In addition to the

veriﬁcation of the DAMA/LIBRA experiment, this experiment can achieve the

best sensitivity for low-mass dark matter searches, especially for spin-dependent

WIMP–proton interactions [17]. Sensitivities on CEνNS from the solar and

supernova neutrinos have background-level assumptions, as shown in Fig. 1,

22 NPE/keVee high-light yield [27], and 5-year data.

2.2. 1000 kg NaI experiment (COSINE-1T)

Assuming the successful operation of the 200 kg NaI detectors and achiev-

ing the best sensitivities for low-mass dark matter of the WIMP-proton spin-

dependent interaction, it is normal to consider a large-scale NaI(Tl) dark mat-

ter search experiment corresponding to COSINE-1T (1000 kg NaI). Multiple

developments in high-quality NaI(Tl) detectors have been extensively investi-

gated to reduce the background and increase the light yield. The responses

of the NaI(Tl) crystal at −35◦C showed an increased light yield of approx-

imately 5% in electron-equivalent energy and an additional 10% increase in

α-induced events [31]. Silicon photomultipliers (SiPMs) can replace conven-

tional PMTs owing to their increased quantum eﬃciency and reduced radioac-

tive background [32, 33]. If we use SiPMs, pure NaI without thallium doping

can be optional and operated at liquid nitrogen temperature, and this option is

expected to provide improved high light yield [34, 35, 36].

In the estimation of the COSINE-1T sensitivity for the CEνNS, we consider

an improved light yield of 30 NPE/keVee and reduced background. Assuming

an improvement in the internal background reduction on raw powder puriﬁca-

tion and crystallization, as well as removing external backgrounds dominated

by attached PMTs, our target background is an order of magnitude reduction

compared to the COSINE-200 crystal in the ROI. However, we also evaluate

the sensitivities assuming the same background of the COSINE-200 considering

Visible Energy (keVee)

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2

Counts /kg/day/keV

3−

2−

1−

External

+Cosmogenic

+Surface

+Internal (Total)

COSINE-1T

Figure 1: The black solid line is the expected background spectra of the COSINE-200 exper-

iment while the black dotted line is the target background of the COSINE-1T experiment.

The expected spectrum of COSINE-200 is based on the developed low-background NaI(Tl)

crystal [26]. The red line denote external backgrounds due to the radiations from outside the

crystal such as PMT and acrylic table. The green line is the summation of external and cos-

mogenic components by 3H, 113Sn and 109Cd, etc. The blue line shows spectrum adding the

surface background to the green line, which is caused by 210Pb contaminated on the crystal

surface. The solid black line representing the total background is the addition of the internal

background to the blue line, which is caused by radioactivity inside the crystal contaminated

with U/Th/K and 210Pb.

ambiguity of the background reduction for the COSINE-1T experiment. The

threshold and exposure are used for sensitivity estimation with the same values

as for the 200-kg NaI experiment (2–8 NPE threshold and 5-year data).

3. Neutrino sources

Since its prediction in 1974 [37], searching for the CEνNS from various neu-

trino sources was conducted without success until the ﬁrst observation in 2017

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SensitivitiesforcoherentelasticscatteringofsolarandsupernovaneutrinoswithfutureNaI(Tl)darkmattersearchdetectorsofCOSINE-200/1TYoungJuKoa,HyunSuLeea,baCenterforUndergroundPhysics,InstituteforBasicScience(IBS),Daejeon34126,RepublicofKoreabIBSSchool,UniversityofScienceandTechnology(UST),Deajeon34113,Re...

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Sensitivities for coherent elastic scattering of solar and supernova neutrinos with future NaITl dark matter search detectors of COSINE-2001T

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