Search for 0νββ decay with EXO-200

and nEXO

Guofu Cao

Institute of High Energy Physics, Beijing, China

On behalf of the EXO-200 and nEXO Collaborations

XIV International Conference on Heavy Quarks and Leptons

May 27 – June 1, 2018, Yamagata, Japan

Neutrinoless Double Beta Decays

Two types of double beta decay Ordinary double beta decay (2νββ, with

two anti-neutrinos)

Neutrinoless double beta decay (0νββ), takes place between two even-even nuclei.

Observation of 0νββ decay = Discovery of NEW PHYSICS Antineutrino and neutrino are the same

particle – Majorana particle

Lepton number violation

Constrain neutrino mass scale

2

2νββ decay 0νββ

Single β decay is forbidden

Requirements of 0νββ Detection 3

2νββ

0νββ(A,Z) (A,Z+2) + 2e–

(A,Z) (A,Z+2) + 2e– + 2ne

For a one dimensional 0νββ search by using energy, the only way to enhance the sensitivity is to reduce the background and improve the energy resolution. This is the case for a small detector.

However, a more powerful detector should provide more parameters used to reject background and simultaneously fit signal and background.

Use liquid Xenon TPC to search for 0νββ 4

- 8kVCharge collection

e-

e-

e-e-

e-e-

e-e-

e-

e-

e-

e-

e-

Ionization Scintillation

Example of TPC schematics (EXO-200)

Advantage of Xenon: Xenon is used both as the source

and detection medium.

136Xe enrichment is easier and safer. Easily scale to tonne scale. Low background -- No long lived

radioactive isotopes and can be continuously purified.

Advantage of liquid Xenon TPC: Simultaneous collection of both

ionization and scintillation signals. Full 3D reconstruction of all energy

depositions in LXe.Monolithic detector structure with

excellent background rejection capabilities.

Combine all parameters, processing a simultaneously fitting for signal and background.

EXO-200 is a running LXe detector,

the first 100 kg-class experiment to

produce results and has demonstrated

key technologies for 0νββ search.

nEXO is a proposed 5 tonne detector,

will take full advantage of LXe TPC to

reach T1/2 ~ 1028 yrs and entirely cover

the Inverted Hierarchy.

Energy measurement

Combine light and ionization to enhance energy resolution

(E.Conti et al. Phys Rev B 68 (2003) 054201)

EXO-200 has achieved ~1.23% energy resolution at the Q value.

nEXO will reach resolution < 1%, sufficient to suppress background from 2νββ.

5

228Th source

Again, LXe TPC IS NOT A PURE CALORIMETER, it can use optimally more than just the energy. Event multiplicity (SS/MS in EXO-200)Distance from the TPC surface Particle ID (α-electron)

Event multiplicity information 6

Low

bac

kgro

un

dd

ata

228Th

cal

ibra

tio

nso

urc

e

2νββ

Low

bac

kgro

un

dd

ata

228Th

cal

ibra

tio

nso

urc

e

γ γ

multiple site events (MS)

2νββ

single site events (SS)

SS/MS discrimination is a very powerful tool to reject gamma backgrounds, because Compton scattering results in multiple energy deposits. This is well demonstrated in EXO-200.

EXO-200 Liquid 136Xe TPC 7

EXO-200 Most Recent 0νββ Results 8

“Blind” analysis (box opened on July 1st, 2017)

Background model + data maximum likelihood fit

Combine Phase I + Phase II profiles

No statistically significant excess: combined p-value ~ 1.5σ

From EXO-200 to nEXO (~200kg to 5000 kg) 9

EXO-200

Detector

From EXO-200 to nEXO (~200kg to 5000 kg) 10

EXO-200

Detector

From EXO-200 to nEXO (~200kg to 5000 kg) 11

EXO-200 Detector

nEXO Detector

1.3 m

A 5000 kg enriched LXe TPC, directly extrapolated from EXO-200.

A monolithic detector is essential!!

Monolithic Detectors 12

LXe mass (kg) Diam. or length (cm)

5000 130

150 40

5 13

5kg

150kg

5000kg

2.5MeV gamma ray attenuation

length 8.5 cm =

Monolithic detector is essential for background rejection:

• Rejection of surface background

• Self-shielding, containment of Compton scattering

• Inner fiducial volume extremely clean

EXO-200nEXO

View of conceptual design of nEXO 13

Ø 13 m

14 m

14m

6,000 m.w.e. depth sufficient to shield cosmogenic background.

SNOLAB’s cryopit

Optimizations from EXO-200 to nEXO 14

nEXO TPC Conceptual Design 15

Cathode is located at the bottom of TPC.

A pad-like charge readout tile is on top of TPC.

Photo-sensors are behind the field shaping rings and will operate in a high field region.

Photo-detector System in nEXO 16

VUV sensitive SiPMsThe overall photon detection efficiency is crucial for nEXO, which is determined by PDE of SiPM and light transport efficiency in TPC.

Copper vessel24 staves, 4-5m2

R&D items:

SiPM characterization Reflectivity on SiPM (>50% VUV lights

reflected) SiPM performance in external high E

field Silicon/quartz interposer Large area test …

Highlights of part of R&D progresses 17

FBK SiPM

PreliminarynEXO

goal

Preliminary

Preliminary

Preliminary

In Vacuum

Charge Readout Tile 18

Prototype charge readout tile

In nEXO, a modular and pad-like charge collection scheme is under study.

A 10cm x 10cm prototype has been made by IHEP/IME in China.

The prototype is tested at Stanford.

JINST 13 P01006 (2018)

nEXO Sensitivity 19

Summary

It will be a great discovery, if 0νββ decay is observed.

EXO-200 achieved 0νββ half-life sensitivity of 3.7×1025 yrs (90% CL), and will further improve to 5×1025 yrs after Phase II running, which is one of most competitive experiments in the field.

nEXO is extrapolated from EXO-200, and aiming to reach 0νββhalf-life sensitivity of 1028 yrs by using 5 tonne liquid 136Xe TPC, which can entirely cover inverted hierarchy region.

Lots of R&D work is ongoing in nEXO, in order to enhance TPC performance.

We are ready to enter the world of ton scale.

20

University of Alabama, Tuscaloosa AL, USA —M Hughes, O Nusair, I Ostrovskiy, A Piepke, AK Soma, V VeeraraghavanUniversity of Bern, Switzerland — J-L VuilleumierUniversity of California, Irvine, Irvine CA, USA — M MoeCalifornia Institute of Technology, Pasadena CA, USA — P VogelCarleton University, Ottawa ON, Canada — I Badhrees, R Gornea, C Jessiman, T Koffas, D Sinclair, B Veenstra, J WatkinsColorado State University, Fort Collins CO, USA —C Chambers, A Craycraft, D Fairbank, W Fairbank Jr, A Iverson, J ToddDrexel University, Philadelphia PA, USA —MJ Dolinski, P Gautam, EV Hansen, YH Lin, Y-R YenDuke University, Durham NC, USA — PS BarbeauFriedrich-Alexander-University Erlangen, Nuremberg, Germany —G Anton, J Hoessl, P Hufschmidt, T Michel, M Wagenpfeil, S Schmidt, G Wrede, T ZieglerIBS Center for Underground Physics, Daejeon, South Korea —DS LeonardIHEP Beijing, People’s Republic of China —G Cao, W Cen, T Tolba, L Wen, J ZhaoITEP Moscow, Russia —V Belov, A Burenkov, M Danilov, A Dolgolenko, A Karelin, A Kuchenkov, V Stekhanov, O Zeldovich

University of Illinois, Urbana-Champaign IL, USA —D Beck, M Coon, J Echevers, S Li, L YangIndiana University, Bloomington IN, USA — JB Albert, SJ DaughertyLaurentian University, Sudbury ON, Canada —B Cleveland, A Der Mesrobian-Kabakian, J Farine, C Licciardi, A Robinson, U WichoskiUniversity of Maryland, College Park MD, USA — C HallUniversity of Massachusetts, Amherst MA, USA — S Feyzbakhsh, A Pocar, M TarkaMcGill University, Montreal QC, Canada — T Brunner, L Darroch, K MurrayUniversity of North Carolina, Wilmington NC, USA — T DanielsSLAC National Accelerator Laboratory, Menlo Park CA, USA —M Breidenbach, R Conley, J Davis, S Delaquis, A Johnson, LJ Kaufman,

B Mong, A Odian, CY Prescott, PC Rowson, JJ Russell, K Skarpaas, A Waite, M WittgenUniversity of South Dakota, Vermillion SD, USA — A Larson, R MacLellanStanford University, Stanford CA, USA — J Dalmasson, R DeVoe, D Fudenberg, G Gratta, M Jewell, S Kravitz, G Li, A Schubert, M Weber, S WuStony Brook University, SUNY, Stony Brook, NY, USA — K Kumar, O NjoyaTechnical University of Munich, Garching, Germany — W Feldmeier, P Fierlinger, M MarinoTRIUMF, Vancouver BC, Canada — J Dilling, R Krücken, Y Lan, F Retière, V StricklandYale University, New Haven CT, USA —A Jamil, Z Li, D Moore, Q Xia

The EXO-200 Collaboration

22

23

Backup

EXO-200 Detector 24

25

The power of homogeneous detector.

The background is constrained by outer part of detector, and keeping inner part clean.

26

Background Assumption 27

Background index 28