Daniel Lenz, University of Wisconsin, Madison11/05/2010 - APS DNP

Cryogenic search for neutrinoless double beta decay

Daniel Lenz on behalf of the CUORE collaboration

Daniel Lenz, University of Wisconsin, Madison11/05/2010 - APS DNP

Motivation – 0νββ

ν ≠ ν ν = ν

• 0νββ is only experimentally feasible way to distinguish Dirac vs. Majorana neutrinos

• if 0νββ observed:

?

• lepton number violationΔL = 2

• Schechter-Valle theoremν = ν

• hint for seesaw type 1

mν = mD2 / MR << mD

• possible to determine absolute ν mass scale and ν hierarchy

n p

n

ν = ν e-

e-

p

0νββ-decay

0νββ2νββ

n

e-

e-

p

n p

νν

2νββ-decay

1

Daniel Lenz, University of Wisconsin, Madison11/05/2010 - APS DNP

Bolometric Approach

• rise time: ~ 50 ms

• total pulse length several seconds

GE

• decay constant: ~ 200 ms

• several techniques employed to search for 0νββ

• here: bolometric approach

• several isotopes can be used

• large masses

• all energy measured in form of heat

• source = detector

• no event topology

2

typical pulse shape parameters for large bolometers:

ΔT = E/C

• pulse shapes help identifying fake signals, e.g. electronics noise

Daniel Lenz, University of Wisconsin, Madison

CUORE Experiment

3m

80cm

• Cryogenic Underground Observatory for Rare Events

LNGS

• it is all about reducing background:

• only radiopure material copper, teflon

• goal: reach a flat bkg of 0.01 cts/(keV kg y) in ROI

• at least 36cm lead shields

• array of 988 bolometers19 towers, 13 planes4 bolometers /plane /tower

3

• Borated polyethylene neutron shield

• located in Hall A of Gran Sasso National Lab (LNGS)

• overburden of ~3100 m.w.e. (relative to flat overburden) Phys.Rev. D73 053004

• total detector mass 741kg M(130Te) ~200 kg (130Te: 0νββ candidate isotope)

300 K

40 K

4 K

lead shield

0.7 K70 mK

10 mK

lead shield

+ external lead shield(not shown)

lead

sh

ield

cryogen-free dilution refrigerator

11/05/2010 - APS DNP

• demonstrated within factor 2 - 4

Daniel Lenz, University of Wisconsin, Madison11/05/2010 - APS DNP

• large bolometers 5 x 5 x 5 cm

• made from natural Te02

• 130Te abundance ~34 % (no enrichment)

• FWHM ~ 5 keV @2528 keV

CUORE Bolometers

Temperature sensor:

• NTD Ge thermistor

• resistance change converted into voltage pulse

• non-linear energy response

4

Resolution:

high purity copper & Teflon holder structure

• Q-value (130Te) = 2528 keV

511 keVe+e-

583 keV208Tl

911 keV228Ac

965 + 969 keV 228Ac

1588228Ac

+ 1593 keV DEP 2104 keV

SEP

2615 keV208Tl

2528 keV0νββ

Phys. Rev. C 80 025501Phys. Rev. Lett. 102, 212502

Daniel Lenz, University of Wisconsin, Madison11/05/2010 - APS DNP

CUORE Calibration System see also poster 123 by Ian Guinn

• 12 source strings move under own weight 300K to 10mK without shifting detector base temperature

300 K

40 K

4 K

lead shield

0.7 K

70 mK

10 mK

lead shield

5

• heat load requirements very strict: sources need to be at 4K

• dedicated thermalization mechanism currently tested

Kevlar string

copper capsuleswith Telfon

solenoid linear

actuator

pushing blade

iso view

• calibration is big challenge• slow detector response requires low activity sources (~100mBq)• large shielding & good self shielding requires sources inside detector array

• for bkg reasons sources have to be stored outside the cryostat

• each string has 30 copper crimps housing 232Th wire, calibrating 500keV - 3000keV

kevlar stringPTFE copper

capsule

thoriated tungsten wire

Daniel Lenz, University of Wisconsin, Madison11/05/2010 - APS DNP

Cuoricino

6

• T0v1/2 (130Te) > 2.8 x 1024 y (90% C.L.)

(presented @ Neutrino 2010)

• mββ < 300 -700 meV (depending on choice of NME)

• M(Te02) = 40.7 kg => 130Te = 11.3 kg

44 crystals: 5cm x 5cm x 5cm (CUORE-type)

• Total exposure 19.75 kg • y

18 crystals: 3cm x 3cm x 6cm

• located at LNGS Hall A smaller cryostat, single tower

• Cryostat will be further used for CUORE-like tower assembly tests: CUORE-0

• b(ROI) = 0.169 ± 0.01 cts / (keV kg y)

• predecessor of CUORE using same experimental technique

4 are enriched

FWHM ~6 keV FWHM ~10 keV FWHM ~14 keV

• two independent analyses, US and Italy: (very similar, agreeing, results)

0νββ expected

Daniel Lenz, University of Wisconsin, Madison

CUORE Experiment - Status

• CUORE experimental hut finished

• cryostat main support in place

• 450 bolometers ready and stored underground at LNGS; tests ongoing

• 12/2010 outer cryostat vessels delivery

• US bolometer production started

• early 2011 cryostat 4K tests

• 12/2010 partial calibration system delivery

7

• 2013 CUORE data taking

11/05/2010 - APS DNP

• fall 2011 CUORE-0 data taking

CUORE hut

cryostat main support

now:

• mid 2011 delivery dilution unit

bolometerspackaged

Daniel Lenz, University of Wisconsin, Madison11/05/2010 - APS DNP

Projected Sensitivity

8

mββ = 41 – 96 meV (depending on NME)

• start probing inverted hierachyT0ν

1/2 = 1.5 x 1026 y at 1σCUORE Sensitivity at 1σ:

• 5 years live time and bkg of b = 0.01 cts/ (keV kg y)

Cuoricino 1σ result

CUORE-0 – 0.06 cts/(keV kg y)

Combined Cuoricino + CUORE-0CUORE – 0.01 cts/(keV kg y)

Daniel Lenz, University of Wisconsin, Madison11/05/2010 - APS DNP

Summary

• 0νββ is only experimentally feasible way to distinguish Dirac vs. Majorana neutrinos

• CUORE uses bolometers made of natural Te02 to search for 0νββ of 130Te

• Goal of background in ROI of 0.01 cts/(keV kg y); demonstrated within factor 2-4

• Good progress being made:

• CUORE-0 data-taking starts fall 2011• CUORE data-taking 2013

• CUORE sensitivity at 1σ T0ν1/2 = 1.5 × 1026 y => mββ = 41-96 meV

start probing inverted hierarchy

• calibration is challenging: 12 strings with 30 sources brought into cryostat from 300K to 10mK, without changing detector base temperature

9