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Mark Kos, PNNL

PNNL-SA-92945

Searching for Dark Matter with the CoGeNT and C4 Detectors

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OverviewStatus of CoGeNT

Latest resultsCurrent understanding of backgrounds

Steps to help reduce/eliminate the major backgrounds we see in CoGeNT

Simulation of the C4 backgrounds

Expected WIMP (Weakly Interacting Massive Particle) sensitivity of C4

Implications for future low-mass dark matter searches

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CoGeNT results and low-mass WIMPs

Published CoGeNT analysis shows an excess of events at low energies that is inconsistent with known backgrounds, but hint at low mass WIMPs (Weakly Interacting Massive Particles)Also, a hint of annual modulation consistent with WIMP dark matterThe CoGeNT results have sparked an interest in low-mass WIMPsNeed multiple detectors with lower backgrounds and lower thresholds to test the CoGeNT results: C-4

Phys. Rev. Lett. 107 (2011) 141301

arXiv:1208.5737

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CoGeNT shield design

CoGeNT: 1 Ge crystal (440 g) at the Soudan mine (data taking since Dec 2009)

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The background picture

68Ge

68Ga

65Zn

73,74As

56,5

7,58C

o

55Fe54Mn

51Cr

49V

L-shell contribution

These backgrounds are tiny

Other sources of background simulated:U and Th chain backgrounds in surrounding material (copper)Muon-induced neutrons from the cavernU and Th chain backgrounds in lead shieldingSpontaneous fission neutrons from shielding material (a,n) neutrons from shielding material

Background sum

Resistor gammas ~324 events, ~16% of data

Muon-induced neutrons 339 events, 16% of data

Cavern neutrons (from radioactivity) 54 events, 3% of data

Tritium b-decay 150 events, 7% of data

arXiv:1208.5737

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Muon-induced neutrons (largest background)

1 cm panels do not allow muon-gamma separation

Veto operated at single photo-electron sensitivityGenerate ~12% dead time from spurious germanium detector-veto coincidences.

True coincidences are however observable and rate is in good agreement with Monte-carlo (next slide)

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Muon-induced neutron simulationTwo independent MC simulations used to

assess neutron contributionsmuon induced neutronnatural radioactivity in cavern

#1: GEANTSoudan muon flux, E, angular distribution

to generate (m,n) in full shield.Includes e- and g (8% of neutron contribution)

#2 MCNP-Polimi:Neutron generation in lead shielding(largest contributor)

Reasonable agreement between simulations(they use different inputs)339 +/- 68 events (GEANT)

m-

Mostly neutrons,~8% e- and g’s(simulation)

CoGeNT data

GEANT

MCNP-Polimi

Less than 16% neutron fraction in CoGeNT data after L-shell subtractions

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Backgrounds from the front end electronics (2nd largest background)

RESISTORS ARE HOT!

Description U-238(Bq/kg)

Th-232(Bq/kg)

K-40(Bq/kg)

Events in CoGeNT

Carbon film resistor

4.3 12.7 21.9 972 +/- 120

Metal film resistor 1

4.3 0.5 37.5 324 +/- 164

Metal film resistor 2

5.1 16.1 24.7 1208 +/- 160

Ceramic core resistor

5.9 4.6 34.3 644 +/- 131

Metal on ceramic resistor

28 40.7 25.7 4509 +/- 352

Ceramic 15.5 0.2 13.8 993 +/- 200

ILIAS database

SNOLAB

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Most beta-spectra and gammas are a flat background in the CoGeNT analysis region

Without an assay we cannot be sure the flat background is from the resistors, but typical resistor backgrounds can plausibly explain most of the CoGeNT flat background

This is expected from Compton scattering of high energy photons at these low energies

A background that can be reduced by having tightly packed detectors and rejecting multiples

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Tritium production in germanium (3rd largest background)

Cosmogenic production of tritium in Ge while detector at surfaceTritium b-decay endpoint at 18.6 keV

Half-life of 12.33 yrsTritium production rate:

27.7 /kg-dayAstroparticle phys, 31, 417 (2009)Based on IGEX dataPhys Lett, B432, 8 (2002)

Assuming a surface exposureof CoGeNT detector of 2 yrs:

150 events in 0.5 – 3.0 keVee(Geant4 simulation of 3H in CoGeNT)

CoGeNT Data

Tritium (simulation)

Years of surface exposure Tritium decays underground

1 299

2 583

3 850

4 1103

5 1342

6 1568

7 1782

8 1983

9 2174

10 2355

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Neutrons from radioactivity in the cavern: (a,n) + fission

Use Mei-Hime neutron flux:3.78 X 10-6 cm-2 s-1

(Phys Rev D 73 , 053004 (2006))

Use Monte-carlo neutron energy spectrum from Gran Sasso (worst case)Simulated background for CoGeNT:

54 events in the dataset

Deep underground this background is higher than muon-induced neutronsSomething that experiments pushing the zero-background limit need to address

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Material 238U (mBq/kg) 232Th (mBq/kg) 210Po (Bq/kg)

Lead sample1 0.41 +/- 0.17 0.08+/-0.08 93 +/- 19

Plastic lumber 121 +/- 4 68 +/- 3

Plastic lumber(recycled)

115 +/- 5 80 +/- 4

Plastic lumber McMaster-Carr

15 +/- 1 1.3 +/- 0.8

Aluminum plate 7.1 +/- 2.4 986 +/- 12

Aluminum framing pieces

42 +/- 8 1348 +/- 50

Radioactivity in the CoGeNT shield

210Pb 210Bi 210Po 206PbUltra-low background leadaround CoGeNT: 0.02 Bq/kg 210Pb

SNOLAB assay of similar materials as used in CoGeNT

Source of (a,n) neutrons

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List of all backgrounds (we know about)Source Events in CoGeNT dataset

(0.5 – 3 keVee)

Resistor backgrounds ~324

Muon induced events in shielding

339 +/- 68

Tritium b-decay <150

Cavern neutrons from radioactivity

<54

U and Th backgrounds in copper

<9

External cavern neutrons (muon-induced)

<1.4

Old lead (210Pb + daughters) <0.6

Spontaneous fission neutrons in lead

<0.5

SF neutrons in HDPE <0.2

HDPE (a,n) <0.038B solar neutrinos <0.014

Extensive simulations done at PNNL

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Backgrounds that Modulate: Radon

Radon levels modulate underground – Measured

Modulation out of phase!Inner shield is inside a sealed nitrogen purged box

So far it doesn’t look like radon

CoGeNT data: Dec 3 2009 - March 6 2011MINOS data: Averaged 2007-2011

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Backgrounds that Modulate: Muons

MINOS muon flux modulation measured in Soudan

Approximately +/-1.5%Peaks three months after best fit to present CoGeNT dataA 1.5% modulation of the estimated 339 +/- 68 muon-induced events in shielding predicts a modulation of 5 events in the 0.5-3 keVee energy rangeThe CoGeNT data set contains 2124 events in the 0.5-3 keVee energy range. A 5 event modulation of muon induced events could only produce a 0.2% modulation effect in the CoGeNT data set.

Courtesy Alec T. Habig

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Surface events and slow pulses

Surface events have degraded energy and pile up in the lowest energy bins (like WIMPs)Surface events (background dominated) on average have slower pulses than bulk events Rejection between bulk (fast pulses) and surface (slow pulses) gets worse at lower energiesWe can estimate the contribution of slow pulses in the data by fitting for the slow and fast pulse distributionsStill looks like there is an excess of events above the expected background

Juan Collar (UC)

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The next generation of CoGeNT, CoGeNT-4 (C4)

Four ~1 kg germanium detectors (unfortunately 4 detectors won’t be funded)2 inch thick veto panelsSoudan Underground LabNew DAQ with full energy range

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Reducing the Background Rate for C4

2 inch veto panels make the muon-induced neutron background negligibleThicker water shielding reduces the cavern neutron rate + reduces (a,n) from shieldingElectroformed copper used to ease manufacture + has 10 X less background than OFHCRedesigned frontend will significantly reduce resistor background

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Expected Background Numbers

Background CoGeNT (cnts/kg/day)

C-4 (cnts/kg/day)

Resistors 6.2 <1.4

Cu cryostat 5.0 X 10-2 7.0 X 10-3

Muon-induced neutrons in Pb shield

1.9 2.6 X 10-4

Cavern neutrons 3.0 X 10-1 1.3 X 10-3

(a,n) in shielding 2.2 X 10-4 1.3 X 10-4

Total ~10 ~2

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The background picture for C4

For C4 tritium may be the dominant background—but can be reduced by minimizing surface exposure of crystals

arXiv:1210.6282

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What about a single 1 kg crystal?

It we can use the same shield and maintain the predicted background rate things don’t look so bad:

C4 4 kg

C4 1 kg

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Implications for a low-mass dark matter search with C4

WIMP sensitivity prediction based on likelihood fit to background + WIMP signalUsing conservative background assumptions of some resistor background remaining and 2 years of surface exposure (tritium)

C4 WIMP sensitivity will be very competitive in the low-mass region and complement other experiments in excluding WIMP parameter spaceEven a modest lowering of the energy threshold can give a large increase in sensitivity at low masses

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Future of low-mass dark matter searches

C4 will compliment other low-mass dark matter experiments such as DAMIC, CDMSLite, MAJORANA in excluding parameter space at low massesFor all these experiments it is crucial to have a low threshold and minimize backgrounds

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Dark Matter analysis with C4

Use all possible information to get the most out of the data: PDFs for signal in energy and time dependence, PDFs for backgrounds in energy and time, constrain backgrounds with measurements outside the signal region, etc.

Summary

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We have done an extensive simulation of the radioactive and cosmogenic backgrounds in the CoGeNT detector arXiv:1208.5737 So far no explanation for excess at low energies and no candidate for the time dependence of the dataC4 will continue with this technology but increase target mass and reduce backgrounds The next generation, C4, will address many of the current concerns…2” thick veto panels, improved low-noise design (lower energy threshold), lower background cryostat C4 will be able to push the limit of sensitivity in the low-mass WIMP parameter space arXiv:1210.6282

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Modest neutron rejection with multiple scattering

With 4 detectors we can remove ~40% of neutron energy depositions (multiple scattering)

Neutron deposited energy distribution before coincidence cut

After coincidence cut