First results on neutrinoless double beta decay of 82 Se with CUPID-0

Transcription

1 First results on neutrinoless double beta decay of 82 Se with CUPID-0 Lorenzo Pagnanini on behalf of the CUPID-0 collaboration 30 th Rencontres de Blois

2 CUPID: a next generation experiment CUPID (CUORE Upgrade with Particle IDentification) is a proposed tonne-scale experiment based on criogenic calorimeters which aims at a sensitivity to the Effective Majorana Mass on the order of mev. INCREASE THE SOURCE MASS Three candidates: 30 Te - 82 Se - 0 Mo REDUCE THE BACKGROUND ISOTOPIC ENRICHMENT ALPHA REJECTION IMPROVED MATERIAL SELECTION MUON VETO BETTER ENERGY RESOLUTION 2

3 CUPID: a next generation experiment CUPID (CUORE Upgrade with Particle IDentification) CUPID-0 is the first array of enriched scintillating cryogenic calorimeter based on Zn 82 Se crystals: the first demonstrator towards CUPID. INCREASE THE SOURCE MASS Three candidates: 30 Te - 82 Se - 0 Mo REDUCE THE BACKGROUND ISOTOPIC ENRICHMENT ALPHA REJECTION IMPROVED MATERIAL SELECTION MUON VETO BETTER ENERGY RESOLUTION 3

4 Scintillating cryogenic calorimeters Modular design allows for large scalability α Q-value > 2.6 MeV 20 LYα LYβ/γ Particle ID ce Excellent energy resolution (<%) 60 ur Grown from different ββ emitters 80 ZnSe: Light Vs Energy JINST 8 (203) P0502 bu lk Reflective foil Detected light [kev] Scintillating Crystal (ZnSe) ββ emitter embedded ε > 80% Thermal Sensors 0νββ Signal: monochromatic peak at the Q-value of the reaction. so Weak Thermal Link (PTFE) dα Thermal Sensors ea re Heat Sink (Copper) rce ou s n o r s nnt eut e v m e o γfr β/γβs/ sm Light Detector (Ge) Scintillating crystals operated at ~ mk Particle interaction T increasing Energy [kevee]

5 The CUPID-0 detector a a. Copper Frame Detector read-out c. c a. b. c. d. Ge-LD Column PTFE Ge-LD d. d Zn82Se PTFE Zn82Se b. b Single module Top view CUPID-0 array Cryostat 24 95%-enriched Zn82Se crystals + 2 natural ones 3 Ge light detectors Reflective foil 3M Vikuiti Total Mass:.5 kg (ZnSe) Mass of 82Se: 5.32 kg Goal: Qββ ~-3 counts/(kev kg y) Qββ = ( ± 0.3) kev 5

6 The CUPID-0 assembly All activities were carried out in an underground Rn-suppressed clean room Assembly started on October, 206 Complex assembly: crystals have all different shapes and heights Single module assembly Ge-NTD thermal sensor Si-heater for gain drift corrections Zn 82 Se crystal Reflecting foil gold wires for the sensor read-out 6

7 Data-taking efficiency The data taking has started on March 207. Data collected in the first months used for system debugging and improvement First data release: June December 207 Maintenance of cryogenic system Characterization of the β/γ shape parameters at the RoI Energy calibration Physics run (0νββ search) Total exposure = 3.44 kg yr ZnSe 7 Data from June to December are divided in 4 dataset

8 Detector performances Counts / 3.2 kev 400 Entries CUPID calibration Th calibration spectrum - Dataset h : 94.6 h Mean RMS Tl Integral 2.687e Ac Bi 208 Tl Tl DE 208 Tl SE Energy Baseline resolution: 5 kev FWHM Resolution at 2.6 MeV ~ 20 kev (deteriorated by crystals quality) Threshold is channel dependent and ranges between and kev Further details in Eur.Phys.J. C78 (208) no.5, 428 CUPID-0 detector paper 8

9 Response function Residuals Events / (.85 kev ) Double Gaussian FWHM ~ MeV multi-compton flat background Energy response of ZnSe to γs is not gaussian. Double-Gaussian provides the best line-shape fit. For each dataset the 265 kev-line is used as benchmark. 9

10 Energy resolution The energy resolution at the Q-value (2998 kev) is extrapolated by linear regression of the FWHM of the calibration peaks For each dataset the resolution at the Q-value is calculated and exposure weighted dataset exposure FWHM [kev] Se Q-value 5 ΔE = E 0 + ae totale exposure FWHM(Q-value) = (23.0 ± 0.6) kev [0.77 %]

11 Total spectrum [counts/(kev kg y)] 2 65 Zn 40 K 208 Tl Tl Non-particle events are rejected by Pulse Shape Analysis on Heat pulses (ZnSe) [counts/(kev kg y)] ROI kev Anti-coincidence cut ( t = 20 ms) removes a large fraction of the events due to multi-compton and muon showers. Counting rate ~ -2 counts/(kev kg y) is dominated by α-particles and γs form 208 Tl decays

12 65Zn Zn 2 65 α-particles are removed exploiting the different pulse shape of the light signal 40KK TlTl 208 Se 2νββ 47Sm [counts/(kev kg y)] ROI kev 8 6 αs β/γs Acceptance threshold optimized on multisite events due to muon induced showers (i.e. pure sample of β/γs) ROI 82 Light Shape Parameter [a.u.] [counts/(kev kg y)] Data selection: β/γ P(Light Shapeα < 6) = 5-8

13 Data selection: delayed veto [counts/(kev kg y)] Zn 40 K 82 Se 2νββ 208 Tl Tl Qα = 6207 kev BR = 35.9 % 208 Tl 22 Bi T/2 = m Qβ = 2254 kev BR = 64. % 22 Po T/2 = 3.05 m T/2 = 299 ns [counts/(kev kg y)] ROI kev Qβ = 500 kev BR = 0 % 208 Pb stable Qα = 8954 kev BR = 0 % 22 Bi α-events are selected in the energy The short life of its daughter, 24 Po, causes range (2-6.5) MeV to include also surface contaminations For each 22 Bi candidate, the crystal where the the decay occurred is disabled for 9.5 minutes (=3 t/2).

14 Data selection [counts/(kev kg y)] Zn 40 K 82 Se 2νββ 208 Tl Tl Background index in the ROI, after all the selection criteria are applied is ( ) -3 counts/(kev kg y). -.4 [counts/(kev kg y)] ROI kev An unprecedented level for a detector based on cryogenic calorimeter. This result is due to the excellent α-rejection achieved by the scintillating calorimeter technique

15 CUPID-0 results: T/2 and mββ No signal evidence in 3.44 kg y of exposure (ZnSe) was found, being able to set the following lower limit on the half-life of 82 Se 0νββ decay: T 0ν > y (90% C.I.) which corresponds to an upper bound to Effective Majorana Mass of mββ < mev Result published today in Phys. Rev. Lett. 20, (208) Flat background + Hypothetical 0νββ signal corresponding to an half-life of y [counts/(kev kg yr)] 2 3 Heat Spectrum + α Rejection + Delayed Coincidences Veto 0ν T/2 = [yr] ; BKG level = [counts/(kev kg yr)]

16 Conclusions ) CUPID-0 is the first array of enriched scintillating calorimeters. 2) Data taking is stably ongoing since March ) α-background rejection was fully demonstrated, reaching for a bolometric experiment an unprecedented background level of 4) The analysis of the first data released allows to set the best limit on 82 Se 0νββ half-life (paper published Phys. Rev. Lett. 20, (208)) BI = ( ) 3 counts/(kev kg y).4 T 0ν > y (90% C.I.) 5) We plan to reach an exposure of kg y of ZnSe, collecting enough data to obtain a reliable background model in for CUPID. 6

17 Conclusions Effective Majorana Mass [ev] GERDA 76 Ge 0. KamLAND-Zen 36 Xe Allowed IH region (Δm 2 <0) I.H CUPID-0 82 Se NH (Δm 2 >0) Allowed region N.H m ev Lightest Neutrino Mass [ev] CUORE 30 Te CUPID Parameters space Adapted from Phys.Rev. D90 (204) no. 7

18 CUPID-0 collaboration 8