PoS(HQL2018)054

- 0νββ

The The EXO

https://pos.sissa.it/ yrs at 90% C.L., C.L., 90% at yrs

class class experiment - 25 observation observation of ×10

search. The most recent

0νββ yrs yrs by using 5 tonne enriched

28 200 and nEXO and 200 -

process process becomes a quite hot topic after the

he he region of inverted hierarchy. Lots of R&D ) ) 0νββ ND 4.0). 200 has been improved to 3.7 to improved been has 200 - - life sensitivity of 10

- NC - half beta beta decay ( 0νββ

Xe from EXO from Xe Decay with EXO with Decay

136 reach

zero zero mass by oscillation experiments, as the

-

200 and nEXOcollaboration and 200 - 0νββ

NoDerivatives 4.0 International License (CC BY License 4.0 International NoDerivatives - 200, 200, aiming to -

life sensitivity of sensitivity life 1 - @ihep.ac.cn a running detector based on liquid TPC, which is the first 100 kg

half June 1, 2018 1, June

- NonCommercial - activities in nEXO are being carried out to enhance the TPC performance. In this proceeding, part part proceeding, this In performance. TPC the enhance to out carried being are nEXO in activities highlighted. be will them of 200 is producing results and well demonstrating the key technologies in 0νββ by combining the data set collected in phase I and phase II. nEXO experiment is from extrapolated EXO liquid xenon TPC, which can entirely cover t The search for neutrinoless double double for The neutrinoless search establishment of non process would indicate the discovery of the new beyond the standard model. mail: caogf mail: Copyright owned by the author(s) under the terms of the Creative Commons Creative terms the author(s) of under the the by owned Copyright -

Speaker

1

Yamagata Terrsa, Yamagata,Japan May 27 May (HQL2018) Leptons and Quarks Heavy on Conference International XIV On behalf of EXO of behalf On Guofu Cao Guofu Physics Instituteof High Energy China 100049, Beijing District, Shijingshan Road, Yuquan 19B E Search for for Search © Attribution

PoS(HQL2018)054

0νββ 200 and - life in the Guofu Cao Guofu -

which is proposed proposed is which decays decays of different ), β), β life are set to the order the to set are life 2], with half with 2], zero zero neutrino mass has - -

- 0νβ 0νβ since only two are rocess. The products of the ckground and improve the energy

7], 7], which also increased the interest in - decay decay would the establish Majorana nature β feature.

0νβ 2

decay decay carry all energy of the decay which equals to

osed osed and built to search for β scale experiments are proposed, aiming to improve the - n isotopes have been observed [1 observed been have isotopes n 0νβ EXO is a proposed 5 tonne detector, and will take full process process is allowed in the framework of the standard model. lation experiments [5 decay can only occur when the neutrino is a Majorana particle a Majorana particle is when decay can occur only neutrino the ) consist of two electrons, two antineutrinos and one β β β class class experiment to produce results and has demonstrated key - 200 and nEXO and 200 - 2νβ 0νβ (2νβ

search [8]. n process is a lepton number violation process, process, numberviolation lepton a is process years. years.

β

process process worldwide, because it would indicate the discovery of new physics 22 β 0νβ 0νββ decays of about a doze a about of decays 2 orders. For experiments relying solely on the measurement of total event energy, energy, event total of measurement the on solely relying experiments For orders. 2 β - Decay with EXO with Decay 200 and nEXOexperiments and 200 0νβ to 10

-

18 2νβ 200 is a running detector based on a liquid xenon time projection chamber (LXe TPC), TPC), (LXe chamber projection time xenon liquid a on based detector running a is 200 - 0νββ EXO Introduction

years so far. Recently, several ton several Recently, so far. years

neutrino and demonstrate the lepton number is not conserved. In addition, the rate of of rate the addition, In conserved. not is number lepton the demonstrate and neutrino value, the spectrum of total observed energy should have a narrow peak at this value. In the the In value. this at peak narrow a have should energy observed total of spectrum the value,

EXO Since the two electrons from the Fig. Fig. 1: Feynman diagrams of the ordinary double beta decay (left) and the neutrinoless The The double beta decay is the rarest known nuclear weak p - 25 ch for for ch Wendell H. Furry in 1939 [3] after the establishment of the Majorana theory of neutrino [4].

f 10 ouble beta decay (right). decay beta ouble technologies for resolution. A more powerful detector should provide more power to reject background and such show which detectors typical two nEXO)are which which is also the first 100 kg simultanously fit the signal and background. The Enriched Xenon Observatory (EXO 2. sensitivity by 1 by sensitivity the only way to enhance the sensitivity is to reduce the ba level of hundreds of kilograms or smaller. Most stringent limits on the half the on limits stringentMost smaller. orkilograms of hundreds of level o isotopes by using various detection techniques.The masses target of these experiments are at the d Q the past, a dozen experiments have been prop decay can also provide the information about the absolute neutrino mass. neutrino absolute the about information the provide also can decay the search for beyond the The standard model. observation of the of and the mass of neutrino is not zero. In the past several decades, the non been established by neutrino oscil indicates that the The emitted from the decay. range of 10 ( decay beta double neutrinoless is decay beta double the of type Another by The Feynman diagrams of the two double different beta decays are shown in Fig. 1. The Fig 1. ordinary ordinary double beta decay daughter nucleus. In this process, baryon number and lepton number are conserved respectively. now, to Up 1. Sear PoS(HQL2018)054

-

life -

shielding - Guofu Cao Guofu monstrated monstrated the full data set nner nner part of LXe ly collecting the and light signals from signals light and value value (2.458 MeV) of

- years years and entirely cover the

is extracted and found to be

28 Xe 136 life of 10 - APDs APDs has been upgraded to reduce the -

site site (SS). As shown in Fig. 3, the SS/MS

200 -

h sigma of 1.2% at the Q 3 - a Maximum Likelihood fit. The fitting results are

life sensitivity of - tween and scintillation signals, the data are r are collected in two phases. Phase I started in 2011 and 2011 in started I Phase phases. two in collected are r half 200 can reac - 0νββ site site (MS) in EXO. This feature is quite different with that of 200 and nEXO and 200 - - 200 detecto 200 resultsfrom EXO - correlation correlation be - 0νββ 0νββ Xe enrichment is easier and safer, the LXe detector can be easily scaled to tonne to scaled easily be can detector LXe the safer, and easier is enrichment Xe s excellent background rejection capabilities. The major background in the region the backgroundmajorThein capabilities. backgroundexcellent rejection s Decay with EXO with Decay

Th calibration source. Right: The energy resolution versus energy for SS events. SS for energy versus resolution energy The Right: source. calibration Th correlated. correlated. The energy resolution can be improved by combining the two signals, 136

- value. Instead of mutiple segmented detectors, a monolithic detector will be built for for built be will detector monolithic a detectors, segmented mutiple of Instead value. yrs yrs at 90% C.L. is derived from noise, noise, a system has been implemented to suppress the radon background and the -

228 ost recent recent ost 0νββ 25 and yrs yrs at 90% C.L., and no significant excess is The observed. lower limit on the half in TPC, called multi

M

200.

- 25 The data taken by EXO by taken data The Fig. Fig. 2 Left: The anti ch for for ch Xe decay, as shown in Fig. 2. The designed goal of energy resolution for nEXO is better than than better is nEXO for resolution energy of goal designed The 2. Fig. in shown as Xe decay,

×10 3.7 electronics electronics end Bythe of 2018. in combining finish to and ongoing expected still is II phase of of ×10 1.8 upgraded upgraded detector, in which, the electronics of LA of taking data The I. phase the to compared 50% by raised been has on operating from phase I and phase II, the stopped in 2014 After due two accidents.to WIPP years, the phase II data taking started with an 3. in EXO measured and identified by the multiple Compton scattering, which by can and measured the lead Compton identified scattering, to multiple energy multiple deposits electrons, which de is well is this and backgrounds, gamma reject dominated to tool powerful very a is by discrimination the single taken from clean, since the attenuation length of a 2.4 MeV gamma is about 8.7 cm in LXe. This self This LXe. in cm 8.7 about is gamma MeV 2.4 a of length attenuation the since clean, is more when efficient On the the detector other hand, becomes the gamma larger. rays can be of interest is caused by gamma rays emitted from sources external to the xenon. On one hand, the the hand, one On xenon. the to external sources from emitted rays gamma by caused is interest of outer part of LXe in TPC can effectively shield the background and keep the i 1% at the Q the at 1% it to nEXO due LXe LXe are anti and the energy resolution in EXO 136 parameter space of inverted hierarchy [9]. Since xenon can as and [9]. serve both the source detector hierarchy space of inverted parameter medium, be can xenon because negligible, is LXe the inside originating background radioactive The class. continously purified. The LXe TPC measures ionizationandscintillationconservation,signals.energy Duecharge to the the energy by simultaneous Sear advantage of LXe TPC to reach the sensitivity of half PoS(HQL2018)054

200 200 - -

Guofu Cao Guofu

like charge readout nning nning of EXO - detector system based based system detector -

4 e TPC, respectively. A photo A respectively. TPC, e

Th calibration data. calibration Th 228 200 and nEXO and 200 -

going, while only part of them are presented in this proceeding. this in presented are them of part only while going, Decay with EXO with Decay

ighlights of R&D activities in nEXO 0νββ H

Fig. 4: Best fit to the SS energy spectrum for Phase I (top) and Phase II (bottom). II Phase and (top) I Phase for spectrum energy SS the to fit Best 4: Fig. Even Even though lots of experiences are gained from the successful ru ch for for ch ta, the lower two plots are from

rings, used to detect ~175nm scintillation lights on fromare activities liquid xenon. In nEXO, lots of R&D tile aretilelocated theatbottom and thetop ofth shaping field the behind TPC of barrel the in installed be will SiPMs sensitive VUV area large on performance. TPC the enhance to order in nEXO, in required still are optimizations some detector, TPC The and in of cathode nEXO. a design Fig 5. pad shows the conceptual 4. Fig. Fig. 3: Demonstration of SS/MS The discrimination. upper two plots are from low background da collaboration [10]. collaboration Sear presented in Fig. 4. More details can be found in a recent published paper by the EXO PoS(HQL2018)054

-

vity vity

Guofu Cao Guofu IHEP/IME in refractrive index refractrive detected by the SiPMs the by detected - layer on the top of FBK of top the on layer

2 the surface of theWith the wafter.

film on top is also measured, which

2 m SiO µ

5

the importance of the photon detection efficiency for

collecting collecting strips, 3 mm wide, on a 10 cm × 10 cmfused - like charge readout tile The is tileunder is study. produced by - detector system. The overall efficiency is determined by photon photon by determined is efficiency overall The system. detector - 200 and nEXO and 200 - LF device from FBK is promising and this device is one of candidates candidates of one is device this and promising is FBK from device LF - , and the results show no influences on SiPM characterizations [12]. HD - ed box indicates the region that can meet the requirements of nEXO. The modular and pad and modular

Decay with EXO with Decay

0νββ efficiency of the photo the of efficiency

and Si. In principle, the reflected photons have the chance to be re be to chance the have photons reflected the principle, In Si. and

Fig. 5 Left: Conceptual design of nEXO detector. Right: Detailed structure of TPC. of structure Detailed Right: nEXOdetector. of design Conceptual Left: 5 Fig. 2 One of the main factors contributing to the energy resolution of nEXO is the overall photon photon overall the is nEXO of resolution energy the to contributing factors main the of One In nEXO, a ch for for ch depositing depositing 60 orthogonal metal charge silica wafer, shown in the left picture in Fig 7. The prototype has been made by

Fig. Fig. 6 Left: PDE versus correlated avalanches measured for three different devices from FBK. vacuum. in measured wavelength versus reflectivity specular The Right: been carefully studied carefully been shows the higher reflectivity because of no extra structure on electrical external high the to exposed be will them of part TPC, in SiPMs of arrangement current has strength field electric difference in operating SiPM of performance The kV/cm. 20 to up field, measurements made in vacuum. The oscillation is caused by the thin SiO thin the by caused is oscillation The vacuum. in made measurements SiPM. For a comparison, a silicon wafer with 1.5 performance of NUV of performance that we are interested in. The right plot in Fig. 6 presents the results of specular reflecti the energy resolution, the R&D efforts in nEXO is focused on optimization of the PDE and the reflectivity of SiPMs. The left plot in Fig 6. shows the SiPM PDE measurements performed by nEXO [11]. The r because because of the lower transmittance. Given detection detection efficiency (PDE) of SiPMs and photon transport efficiency (PTE) in TPC. More than of mismatch the to due SiPMs, by reflected be can (~175nm) photons VUV 50% SiO of decrease will SiPM of PDE the However, PTE. higher the means reflectivity higher the so TPC, in detection Sear PoS(HQL2018)054

life - half discovery discovery Guofu Cao Guofu 3σ 0νββ yrs yrs at 90% C.L.

25 ht: The comparison comparison The ht: r, the

200 has demonstrated the key - years years in nEXO. The exclusion

discovery potential as a function function a as potential discovery 27 3σ Bi source. Bi

207 life sensitivity of 3.7 ×10 - 200, 200, and combined with the better energy -

half 6 years. years. Under the assumption of a light Majorana in Fig. 8, with 10 years data taking, a

27 0νββ decay decay is observed. EXO

0νββ life is expected to be 5.7 × 10 - ry if 200 and nEXO and 200 - as as a function of the lightest neutrino mass for normal and inverted

half ββ

0νββ search and achieved

Xe Xe 136 0νββ Decay with EXO with Decay

ange, the effective Majorana neutrino mass is computed with various nuclear matrix matrix nuclear various with computed is mass neutrino Majorana effective the ange, 0νββ Summary nEXOsensitivity

Fig. 8 Left: nEXO median sensitivity at 90% C.L. and and C.L. 90% at sensitivity median nEXO Left: 8 Fig. It It will be a great discove Based Based on experiences gained from EXO Fig. 7 Left: the picture of the 10 cm x 10 cm prototype of charge tile. Rig tile. charge of prototype cm 10 x cm 10 the of picture the Left: 7 Fig. ch for for ch

technologies in

of of the experiment livetime. Right: Exclusion mass m neutrino Majorana sensitivity at 90% C.L. reach to hierarchy. mass neutrino the effective 6.

potential potential for the sensitivity at 90% C.L. can reach 9.2×10 exch neutrino hierarchy. inverted of wholeregion the cover whichcan choices, elements details details are reported Asin the shown Ref [9]. resolution and more powerful SS/MS sensitivity discrimination has been carefully in estimated. A detailed detector nEXO simulation software has detecto also been measuredbackgroundradioassayMoretheinputs.themodelwith build implemented to usedand 5. between data and simulation, the data are taken with with taken are data the simulation, and data between [13],shown inthe right plot inFig.7. Sear of the The tile can characterization be China fully and understood tested at University. Stanford PoS(HQL2018)054

− Guofu Cao Guofu yrs, which which yrs,

28

p + p + e + p + → p d +

e ν life of 10 of life - Antineutrino Antineutrino -

sensitive Siliconsensitive are are ongoing in nEXO, in - half

decay,” Physical Review C, vol. vol. C, Review Physical decay,” 1567. – 0νββ β - Beta Decay with the Upgraded Upgraded the with Decay Beta -

Beta Decay”, Hindawi Publishing Publishing Hindawi Decay”, Beta -

disintegration,” Physical Review, vol. 56, 56, vol. Review, Physical disintegration,” - Sudbury Neutrino Observatory". Physical Physical Observatory". Neutrino Sudbury

neutrino double neutrino ss Doubless -

7 Decay, Phys. Rev. C 97 (2018) 065503. (2018) 97 C Rev. Phys. Decay, β

-

Sun, T. Tolba, G.F. Cao et al., “Study of Silicon Photomultiplier Photomultiplier Silicon of “Study al., et Cao G.F. Tolba, T. Sun,

life values for two -

200 and nEXO and 200 - 200 detector, part I: Detector design and construction. J. Instrum. 7, 7, Instrum. J. construction. and design Detector I: part detector, 200 - Kamiokande Collaboration) (24 August 1998). "Evidence for Oscillation Oscillation for "Evidence 1998). August (24 Collaboration) Kamiokande -

ay Collaboration) (2012). "Observation of Electron of "Observation (2012). Collaboration) ay

after after phase II running, which is one of the most competitive experiments in 1193, 1939.

– 25 ollaboration, J. B. Albert et al., Sensitivity and Discovery Potential of the Proposed Proposed the of Potential Discovery and Sensitivity al., et Albert B. J. ollaboration, Decay with EXO with Decay 200 Collaboration, “Search for Neutrinoless Double Neutrinoless for “Search Collaboration, 200

- ana, “Teoria simmetrica dell’elettrone e del positrone,” Il Nuovo Cimento, vol. 14, no. 4, 4, no. 14, Nuovo vol. Cimento, Il positrone,” del e dell’elettrone simmetrica “Teoria ana,

184, 1937. 184, – 200 Detector”, Phys. Rev. Lett. 120, 072701 (2018). 072701 120, Lett. Rev. Phys. Detector”, 200 0νββ - O Xe Xe by combining the data sets of phase I and phase II. The sensitivity can be further The nEXO collaboration, X.L. collaboration, nEXO The arXiv:1807.03007 Fields”, Electric External in Performance for Tile Readout Ionization an of “Characterization al., et Jewell, M. collaboration, nEXO The (2018). P01006 13 nEXO”,JINST nEXO Experiment to Neutrinoless Double Neutrinoless to nEXOExperiment EXO The EX “UV al., et Hufschmidt P. Ziegler, T. Jamil, A. collaboration, nEXO The arXiv:1806.02220 nEXO”, in Detection Light Scintillation Xenon for Photomultipliers F. P. An; et al.171803. (17): 108 Letters. Review Physical Bay". (DayaDaya at Disappearance B EXO The al. et M. Auger, (2012). P05010 c nEXO The Ahmad, Q. R.; et al. (SNO Collaboration) (2001). "Measurement of the Rate of of Rate the of "Measurement (2001). Collaboration) (SNO al. et R.; Q. Ahmad, InteractionsProduced by 8B Solar atthe (7). 87 Letters. Review Y. Fukudae;1562 (8): 81 Letters. Review Physical Neutrinos". Atmospheric of et al. (Super W. H. Furry, “On1184 pp. no.12, transition probabilities Major E. in double beta pp.171 A. S. Barabash, “Precise half “Precise Barabash, S. A. 2010. 035501, ID Article 3, no. 81, “Neutrinole Poves, Alfredo and Giuliani Andrea 857016. ID Article 2012, Volume Physics, Energy High in Advances Corporation,

ch for for ch 136 roved to 5 ×10 [8] [9] [7] [6] [5] [3] [4] [1] [2] [12] [13] [10] [11] imp reachTPC,aiming to LXe tonne proposed5 a nEXO field.is the can entirely cover the inverted hierarchy region. Lots of R&D work TPCperformance. the enhance to order References Sear for