low energy rare event searches with the cuore experiment

Low energy rare event searcheswith the CUORE experiment

S. Di DomizioINFN – Sezione di Genova

on behalf of the CUORE collaboration

RENCONTRES DU VIETNAM, QUY NHON, JULY 2012

2S. Di Domizio – Rencontres du Vietnam-- BSM – Quy Nhon, July 2012

Outline

● CUORE experiment and bolometric technique

● Low energy threshold trigger

● Potential for rare event searches at low energy in CUORE


CUORE: a Cryogenic Underground Observatory for Rare Events

● Primary goal: search for neutrinoless double beta decay in 130Te

● Tightly packed array of TeO2 bolometers

● Single bolometer mass: 0.75 kg

● Arranged in 19 towers

● Total mass: 741 kg

● 130Te mass: 200 kg

● Energy resolution: 5 keV FWHM

● bkg goal: 0.01 counts/(keV kg y)

● Sensitivity after 5 yrs: T1/2

> 1.6 x 1026 yrs

● mββ

< 40 – 94 meV

● Data taking will start in 2014

See S. Capelli's talk on Tuesday


Bolometers

Energy releases produce a measurable temperature rise of the absorber crystal: Δ T = EC

SensorNTD Ge Thermistor

AbsorberTeO

2 crystal

Weak thermal conductancePTFE supports

Heat bathOFHC copper

Working temperature: ~ 10 mK

● Absorber● M ~ 0.75 kg● C ~ 10-9 J/K● ΔT/ΔE ~ 100 μK/MeV

● Sensor● R = R0 exp[(T

0/T)1/2]

● R ~ 100 MΩ● ΔR/ΔE ~ 3 MΩ/MeV

● Output signal● ΔV/ΔE ~ 100 μV/MeV ● Signal bandwidth ~ 12 Hz● Signal duration ~ 5s


CUORE potential● The main goal of CUORE is to search for a

monochromatic peak in the MeV energy range

● However the experiment has features that make it suitable for other searches:

– Large mass

– Low background

– Excellent energy resolution

● The threshold of the trigger that was previously used in CUORE was of about 50 keV

● With a lower energy threshold CUORE could become sensitive to other interesting processes


Amplitude estimation: optimum filter

H (ω)=S∗(ω)N (ω)

exp(− jω tM)

Maximize the SNR based on the knowledge of signal and noise spectral shape

Transfer function:

Ideal detector response: evaluated by averaging many particle pulses

Noise power spectrum: average the noise power spectra of random waveform samplings not containing pulses

Raw pulseFiltered pulse

Raw NPSFiltered NPS

time of pulse maximum

Riv. Nuovo Cim. 9N1 (1986) 1-146


OF-based trigger algorithm

● Run a simple threshold trigger on data already processed with the optimum filter:

– Energy threshold is lowered

– Detector baseline fluctuations are suppressed

– Capability of discriminating non physical pulses

3 keV pulse

3 keV pulse

Electronicspike

Cryogenic noise

JINST 6 (2011) P02007


A test with CUORE-like crystals

● CUORE Crystals Validation Runs (CCVR): test of four crystals from each batch arriving at LNGS

● Test of the low energy threshold trigger with the CCVR2 data

● Data taken in June 2009 in the HallC R&D cryostat: 19.4 days of live time

● Each crystal had two thermistors attached on it

● Energy resolution: 3.5 keV FWHM at 2615 keVJournal of Crystal Growth 312 (2010) 2999Astropart. Phys 35 (2012) 839-849


Energy calibration

● Thoriated tungsten sources inserted between the cryostat and the external lead shields

● Calibration function: 3rd order polynomial without intercept

● The lowest energy calibration point is at 511 keV

● Check low energy calibration using Te metastable isotopes

Calibration points Check of residuals at low energy

Floating intercept

Intercept fixed to 0


Trigger threshold

● Each crystal had a Joule heater attached on it, that was used to inject a configurable amount of energy

● The trigger threshold was checked with a scan of heater pulses at low energy:

– The heater scan was used to validate a Monte Carlo simulation of the bolometer pulses

– The energy threshold was estimated by running the trigger on Monte Carlo-generated pulses of known amplitude

Heater scan

Standard trigger

Low energy trigger

Heater - dataHeater - simulationParticles - simulation


Pulse shape discrimination

● Filtered average pulse used as a fit function for the central part of the pulse

● Use χ2/ndf as a shape parameter to reject non physical pulses

Good pulse Bad pulse


Pulse shape discrimination

● Filtered average pulse used as a fit function for the central part of the pulse

● Use χ2/ndf as a shape parameter to reject non physical pulses

Elecronic spike

88 keV pulse


Background in CCVR2

Summed energy spectrum – 19.4 days of data taking – 3 crystals


Sensitivity to WIMPs

5 GeV

10 GeV

20 GeV

50 GeV

100 GeV

Amplitude of signal rate modulation in TeO2

for σ = 10-41 cm2

Assumptions:● ρ

W = 0.3 GeV / cm3

● v0= 220 km/s

● Quenching factor for nuclear recoils in TeO

2 is 1 (NIM A409 (1998) 451-453)


Solar axion searches● Axions have been postulated to solve the QCD “strong CP problem”

● A neutral pseudo-scalar particle, light and weakly coupled to matter

● Coupling to matter is inversely proportional to the axion energy scale fa:

– fa on the electro-weak scale has been excluded by experiments

– A possible window for fa is 105 -- 106 GeV

– Another window is in the range 1012 – 1013 GeV

● Two approaches for solar axion detection:

– Primakoff conversion

– Axio-electric effect

Ze ZeZe

axion

Ze

axion Free electron

PR

L 7

5 (

1995

), 3

222

Primakoff conversion Axio-electric effect

● Continuous spectrum● Detect via Bragg scattering in TeO

2

● Search for a daily modulation in the few keV region

● Search for a monochromatic line at 14.4 keV


Axio-electric effect

σae =αaxion2αEM ( ℏω

mec2 )

2

σ pe αaxion ≈8.3×10−8GeV 2

f a2 [GeV 2]

Φa (14.4keV ) = 1011 cm−2 s−1 ( 106GeVf a [GeV ] )

2

C 2(S )

57Fe M1 transition in the Sun has a small axion production branching fraction

Expected flux:

Two unknown parameters:● Axion energy scale f

a

● Flavor-singlet axial-vector matrix element S

Cross section:

S is expected to be in the range 0.15 – 0.55

CUORE axion detection rates for S=0.55


Supernova signal

● Neutrinos from Supernova explosion could be detected in CUORE by mean of neutral interaction coherent scattering on the detectors' nuclei

● Coherent scattering cross section increases for low momentum transfers

● Burst signal: about 10s

Number of neutrinos as a function of energy from the explosion of a Supernova at the galactic center (8.5 kpc)

Energy threshold [keV]

To

tal nu

mb

er

of

eve

nts

Te

O

Total

Total number of expected events for a Supernova distance of 8.5 kpc as a function of the detector energy threshold

Astropart. Phys 36 (2012) 151-155


Signal strengthE

ve

nts

/se

c

Event relative time [sec]

● Supernova occurrence probability is extremely small

● Signal trigger rate dominated by false positives

● Set a threshold on the signal strength corresponding to a rate of one false positive per week

PRELI

MIN

ARY

Monte Carlo simulation of a Supernova event seen by CUORE.

The event rate is fitted with an exponential function superimposed to a flat background

The amplitude of the exponential is used as an estimator for the signal strength


Conclusions

● Besides neutrinoless double beta decay of 130Te, the CUORE experiment has the potential to detect other rare processes at low energy

● This is made possible by a new trigger algorithm that can reach an energy threshold as low as few keV

● CUORE has the sensitivity to search for:

– Modulation signal from galactic WIMP interactions

– Solar axions

– Neutrinos from Supernova explosions

low energy rare event searches with the cuore experiment

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