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Mark Kos, PNNL PNNL-SA-92945 Searching for Dark Matter with the CoGeNT and C4 Detectors 1.

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Presentation on theme: "Mark Kos, PNNL PNNL-SA-92945 Searching for Dark Matter with the CoGeNT and C4 Detectors 1."— Presentation transcript:

1 Mark Kos, PNNL PNNL-SA Searching for Dark Matter with the CoGeNT and C4 Detectors 1

2 Overview Status of CoGeNT Latest results Current 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 2

3 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 matter The CoGeNT results have sparked an interest in low-mass WIMPs Need multiple detectors with lower backgrounds and lower thresholds to test the CoGeNT results: C-4 3 Phys. Rev. Lett. 107 (2011) arXiv:

4 CoGeNT shield design 4 CoGeNT: 1 Ge crystal (440 g) at the Soudan mine (data taking since Dec 2009)

5 The background picture 68 Ge 68 Ga 65 Zn 73,74 As 55 Fe 5 54 Mn 51 Cr 49 V 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 cavern  U and Th chain backgrounds in lead shielding  Spontaneous fission neutrons from shielding material  ( ,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  -decay 150 events, 7% of data arXiv:

6 Muon-induced neutrons (largest background) 1 cm panels do not allow muon-gamma separation Veto operated at single photo- electron sensitivity Generate ~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) 6

7 Muon-induced neutron simulation Two independent MC simulations used to assess neutron contributions muon induced neutron natural radioactivity in cavern #1: GEANT Soudan muon flux, E, angular distribution to generate ( ,n) in full shield. Includes e - and  (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) 7 -- Mostly neutrons, ~8% e- and  ’s (simulation) CoGeNT data GEANT MCNP-Polimi Less than 16% neutron fraction in CoGeNT data after L-shell subtractions

8 Backgrounds from the front end electronics (2 nd largest background) RESISTORS ARE HOT! 8 DescriptionU-238 (Bq/kg) Th-232 (Bq/kg) K-40 (Bq/kg) Events in CoGeNT Carbon film resistor /- 120 Metal film resistor /- 164 Metal film resistor /- 160 Ceramic core resistor /- 131 Metal on ceramic resistor /- 352 Ceramic /- 200 ILIAS database SNOLAB

9 Most beta-spectra and gammas are a flat background in the CoGeNT analysis region 9 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

10 Tritium production in germanium (3 rd largest background) Cosmogenic production of tritium in Ge while detector at surface Tritium  -decay endpoint at 18.6 keV Half-life of yrs Tritium production rate: 27.7 /kg-day Astroparticle phys, 31, 417 (2009) Based on IGEX data Phys Lett, B432, 8 (2002) Assuming a surface exposure of CoGeNT detector of 2 yrs: 150 events in 0.5 – 3.0 keVee (Geant4 simulation of 3 H in CoGeNT) CoGeNT Data Tritium (simulation) 10 Years of surface exposureTritium decays underground

11 Neutrons from radioactivity in the cavern: ( ,n) + fission Use Mei-Hime neutron flux: 3.78 X cm -2 s -1 (Phys Rev D 73, (2006)) Use Monte-carlo neutron energy spectrum from Gran Sasso (worst case) Simulated background for CoGeNT: 54 events in the dataset 11 Deep underground this background is higher than muon-induced neutrons Something that experiments pushing the zero-background limit need to address

12 Material 238 U (mBq/kg) 232 Th (mBq/kg) 210 Po (Bq/kg) Lead sample / / /- 19 Plastic lumber121 +/ /- 3 Plastic lumber (recycled) 115 +/ /- 4 Plastic lumber McMaster-Carr 15 +/ /- 0.8 Aluminum plate7.1 +/ /- 12 Aluminum framing pieces 42 +/ /- 50 Radioactivity in the CoGeNT shield 210 Pb  210 Bi  210 Po  206 Pb Ultra-low background lead around CoGeNT: 0.02 Bq/kg 210 Pb SNOLAB assay of similar materials as used in CoGeNT 12 Source of ( ,n) neutrons

13 List of all backgrounds (we know about) SourceEvents in CoGeNT dataset (0.5 – 3 keVee) Resistor backgrounds~324 Muon induced events in shielding 339 +/- 68 Tritium  -decay <150 Cavern neutrons from radioactivity <54 U and Th backgrounds in copper <9 External cavern neutrons (muon-induced) <1.4 Old lead ( 210 Pb + daughters)<0.6 Spontaneous fission neutrons in lead <0.5 SF neutrons in HDPE<0.2 HDPE ( ,n) < B solar neutrinos< Extensive simulations done at PNNL

14 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 14 CoGeNT data: Dec March MINOS data: Averaged

15 Backgrounds that Modulate: Muons MINOS muon flux modulation measured in Soudan Approximately +/-1.5% Peaks three months after best fit to present CoGeNT data A 1.5% modulation of the estimated 339 +/- 68 muon- induced events in shielding predicts a modulation of 5 events in the keVee energy range The CoGeNT data set contains 2124 events in the keVee energy range. A 5 event modulation of muon induced events could only produce a 0.2% modulation effect in the CoGeNT data set. 15 Courtesy Alec T. Habig

16 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 energies We can estimate the contribution of slow pulses in the data by fitting for the slow and fast pulse distributions Still looks like there is an excess of events above the expected background 16 Juan Collar (UC)

17 The next generation of CoGeNT, CoGeNT-4 (C4) Four ~1 kg germanium detectors (unfortunately 4 detectors won’t be funded) 2 inch thick veto panels Soudan Underground Lab New DAQ with full energy range 17

18 Reducing the Background Rate for C4 2 inch veto panels make the muon- induced neutron background negligible Thicker water shielding reduces the cavern neutron rate + reduces ( ,n) from shielding Electroformed copper used to ease manufacture + has 10 X less background than OFHC Redesigned frontend will significantly reduce resistor background 18

19 Expected Background Numbers 19 BackgroundCoGeNT (cnts/kg/day) C-4 (cnts/kg/day) Resistors6.2<1.4 Cu cryostat5.0 X X Muon-induced neutrons in Pb shield X Cavern neutrons3.0 X X ( ,n) in shielding 2.2 X X Total~10~2

20 The background picture for C4 For C4 tritium may be the dominant background— but can be reduced by minimizing surface exposure of crystals 20 arXiv:

21 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: 21 C4 4 kg C4 1 kg

22 Implications for a low-mass dark matter search with C4 22 WIMP sensitivity prediction based on likelihood fit to background + WIMP signal Using 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 space Even a modest lowering of the energy threshold can give a large increase in sensitivity at low masses

23 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 masses For all these experiments it is crucial to have a low threshold and minimize backgrounds 23

24 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. 24

25 Summary 25 We have done an extensive simulation of the radioactive and cosmogenic backgrounds in the CoGeNT detector arXiv: arXiv: So far no explanation for excess at low energies and no candidate for the time dependence of the data C4 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: arXiv:

26 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 26


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