SuperCDMS SNOLAB: A G2 Dark Matter Search. Ben Loer, Fermilab Center for Particle Astrophysics On behalf of the SuperCDMS Collaboration

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1 SuperCDMS SNOLAB: A G2 Dark Matter Search Ben Loer, Fermilab Center for Particle Astrophysics On behalf of the SuperCDMS Collaboration

2 A bit of background Astronomical data at all scales indicates dark matter Weakly Interacting Massive Particles are a favored candidate ~GeV-TeV mass No direct coupling to photons Weak annihilation / scattering cross sections Assume a Boltzmann velocity distribution Need very low threshold, very low backgrounds, rejection of EM background 2

3 What is SuperCDMS SNOLAB? SuperCDMS will focus on unrivaled sensitivity to lowmass (<10 GeV) Weakly Interacting Massive Particles Current cross section limits for WIMPs <~3 GeV will be improved by 4 or more orders of magnitude Exact detector payload to be determined Ge/Si mix and total mass to be determined by funding At least one tower will be mixed Ge + Si CDMSlite (HV) mode Cryostat, infrastructure, and shielding are all capable of accommodating 400 kg of Ge Clear upgrade path to quickly confirm or deny potential signals from other experiments, possible collaboration with EURECA 3

4 SuperCDMS SNOLAB will be: Smarter! New technologies for better background rejection, ultra-low thresholds, and an active neutron veto Bigger! Bigger crystals, and more of them Deeper! North America s deepest underground laboratory Cleaner! Intensive material screening program; compact, efficient shield 4

5 SuperCDMS izips Ultrapure Ge and Si crystals operated at ~40 mk Read out athermal phonon and charge signals Phonons give total energy Ratio of charge/phonon discriminates bulk gamma and nuclear recoil events Outer charge and phonon rings remove outer surface events Interleaved 2-sided charge sensors remove face events SuperCDMS Soudan: 8 phonon, 4 charge chans SuperCDMS SNOLAB: 12 phonon, 4 charge -V 5

6 SuperCDMS izip surface rejection izip: Interleaved phonon and charge sensors on both sides Bulk events: charge collected on both izip faces e - Cross-section of electric field in izip Surface events: only detect charge on one face h + This layout gives both better surface background rejection and increased nuclear-recoil acceptance 6

7 Appl.Phys. Lett., 103, (2013) arxiv: SuperCDMS izips: Surface events are a thing of the past 2 izips at Soudan outfitted with Pb-210 sources to get high-statistics surface population 0 events leak into signal region out of >180k surface events with radial fiducial volume cut applied Leakage into WIMP signal band < 1.3x10-5 at 90% CL with 50% nuclear recoil acceptance (8-115 kevr) <0.2 expected leaked events in 5 years at SNOLAB! 7

8 CDMSlite low ionization threshold experiment Electrons/holes propagating in crystal reach terminal velocity Excess energy from bias field transferred to lattice as Luke phonons High field high gain charge measurement Can reach very low thresholds low WIMP masses BUT: phonon signals dominated by Luke part Intrinsic phonon signal buried, so lose NR/ER discrimination capability 8

9 CDMSlite run 1 at Soudan arxiv: evee baseline resolution; 170 evee (840 evnr) threshold World-leading low mass limit with only 6.3 kg-day exposure! CDMS II Si CoGeNT DAMA CRESST II XENON100 XENON10 (S2) CDMS II Ge CDMS II Ge LT EDELWEISS II TEXONO CDEX PICASSO CRESST II LT Performance will be even better at SNOLAB: 2x lower energy threshold from reading both detector sides 6x better phonon resolution and lower threshold has been demonstrated with lower- Tc detector 9

10 Bigger detectors, more of them 7.6cm X 1cm CDMS-II 5 towers, 30 ZIPs 4.75 kg Ge, 1.1 kg Si 7.6cm X 2.5cm SuperCDMS Soudan 5 towers, 15 izips 9 kg Ge 10cm X 3.8cm SuperCDMS SNOLAB Cryostat can hold up to 400 kg 10

11 Moving down (and south) to SNOLAB 100x reduction in muon flux at SNOLAB compared to Soudan 2 km 11

12 CDMS SNOLAB Shielding Outer layer of cm water or high density polyethylene Radon purge containment 23 cm lead 40 cm scintillator active veto Copper cryostat cans 3/8 to 1/2 thick 12

13 Efficient shielding needed for small ladder lab space! 13

14 CDMS SNOLAB Active Veto Modular acrylic tanks for easier assembly/access, low backgrounds Linear Alkylbenzene (LAB) scintillator (same as SNO+) With ~30% trimethyl borate (TMB) for enhanced neutron capture cross section, energy released as Li-7 + alpha Will double as in-situ monitor of background environment 14

15 CDMS SNOLAB Active Veto Read out with WLS fiber and ~1000 silicon PMs PMTs are too big and too radioactive! Will veto > 90% of events with neutrons scattering in the ZIPs (that would otherwise be indistinguishable from WIMPs) Still evaluating alternative designs (plastic modules, Gd loading) May be a phase II upgrade, since neutrons are not dominant background at low mass 15

16 CDMS Backgrounds at SNOLAB Environmental? Compact, efficient shielding Intrinsic? Comprehensive material screening program Gammas? Detector discrimination using charge/phonon ratio Surface events? Identified by sensor partitioning Cosmogenics? What cosmogenics? We re 2 km underground! Radiogenic neutrons? Active neutron veto 16

17 SuperCDMS SNOLAB Projected Sensitivity 17

18 Conclusions Detector fabrication expected to begin winter 2016; commissioning in summer 2018 Low energy thresholds and low backgrounds from new technology, a deeper, cleaner lab, and improved screening and shielding give unique discovery potential for M W < 10 GeV/c^2 CDMSlite detectors with high-gain, low-noise operation give extremely low energy thresholds and will lead the world in low mass WIMP sensitivity Most of the 1-10 GeV WIMP mass range will be covered by both Si and Ge HV detectors Infrastructure designed for easy upgrade path to significantly increased detector mass 18

19 Thank you! 19