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Mark Kos, PNNL PNNL-SA-92945

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

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Mark Kos, PNNL PNNL-SA-92945

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

  2. Overview • Status of CoGeNT • Latest results • Current understanding of backgrounds • Stepsto 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

  3. CoGeNT results and low-mass WIMPs Phys. Rev. Lett. 107 (2011) 141301 arXiv:1208.5737 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

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

  5. The background picture 68Ge arXiv:1208.5737 Background sum 65Zn 68Ga L-shell contribution 49V Resistor gammas ~324 events, ~16% of data 54Mn 56,57,58Co 55Fe 51Cr 73,74As Tritium b-decay 150 events, 7% of data Muon-induced neutrons 339 events, 16% of data Cavern neutrons (from radioactivity) 54 events, 3% of data • 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 • (a,n) neutrons from shielding material These backgrounds are tiny

  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)

  7. m- Muon-induced neutron simulation GEANT CoGeNT data MCNP-Polimi Mostly neutrons,~8% e- and g’s (simulation) Less than 16% neutron fraction in CoGeNT data after L-shell subtractions • 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 (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)

  8. Backgrounds from the front end electronics (2nd largest background) ILIAS database SNOLAB RESISTORS ARE HOT!

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

  10. Tritium production in germanium (3rd largest background) CoGeNT Data Tritium (simulation) • Cosmogenic production of tritium in Ge while detector at surface • Tritium b-decay endpoint at 18.6 keV • Half-life of 12.33 yrs • Tritium production rate: • 27.7 /kg-dayAstroparticle phys, 31, 417 (2009) • Based on IGEX dataPhysLett, 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)

  11. Neutrons from radioactivity in the cavern: (a,n) + fission Deep underground this background is higher than muon-induced neutrons Something that experiments pushing the zero-background limit need to address • 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

  12. Radioactivity in the CoGeNT shield SNOLAB assay of similar materials as used in CoGeNT 210Pb 210Bi 210Po 206Pb Ultra-low background leadaround CoGeNT: 0.02 Bq/kg 210Pb Source of (a,n) neutrons

  13. List of all backgrounds (we know about) Extensive simulations done at PNNL

  14. Backgrounds that Modulate: Radon CoGeNT data: Dec 3 2009 - March 6 2011 MINOS data: Averaged 2007-2011 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

  15. Backgrounds that Modulate: Muons Courtesy Alec T. Habig • 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 0.5-3 keVee energy range • The 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.

  16. Surface events and slow pulses Juan Collar (UC) 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

  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

  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 (a,n) from shielding Electroformed copper used to ease manufacture + has 10 X less background than OFHC Redesigned frontend will significantly reduce resistor background

  19. Expected Background Numbers

  20. The background picture for C4 arXiv:1210.6282 For C4 tritium may be the dominant background—but can be reduced by minimizing surface exposure of crystals

  21. What about a single 1 kg crystal? C4 1 kg C4 4 kg It we can use the same shield and maintain the predicted background rate things don’t look so bad:

  22. Implications for a low-mass dark matter search with C4 • 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 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)

  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

  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.

  25. Summary 25 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 data C4 will continue with this technology but increase target massand 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

  26. Modest neutron rejection with multiple scattering Neutron deposited energy distribution before coincidence cut After coincidence cut With 4 detectors we can remove ~40% of neutron energy depositions (multiple scattering)

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