Starting Points (Collaboration ; Laboratories ; Research Program)

1. Ultra-Low-Energy HPGe Detector for WIMP Searches • Starting Points (Collaboration ; Laboratories ; Research Program) • Physics & Requirements for ULE-HPGe - focus on WIMP Searches • R&D on ULE-HPGe Prototypes • Status & Plans Henry T. Wong /王子敬 Academia Sinica /中央研究院 @

2. TEXONO Collaboration • Collaboration : Taiwan (AS, INER, KSNPS, NTU) ; China (IHEP, CIAE, THU, NKU, NJU) ; Turkey (METU) ; India (BHU) • Close Partnership with :Korea (KIMS) • Facilities : Kuo-Sheng Reactor Lab (Taiwan) ; Yang-Yang Underground Lab (Korea) ; CIAE Neutron Beam (China) • Program : Low Energy Neutrino & Astroparticle Physics • Goals :Develop O[100 eV threhold1 kg mass1 cpd detector]for neutrino physics and dark matter searches

3. Kuo Sheng Reactor Neutrino Laboratory : NPS-IV Different detectors for different physics 28 m from core#1 @ 2.9 GW

4. quality Detector requirements mass Reactor Neutrino Interaction Cross-Sections 1 counts / kg-keV-day On-Going Data Taking & Analysis • SM s(ne) • T > 2 MeV • R&D : • Coh. (nN) • T < 1 keV Threshold ~ 100 eV Completed Results : • mn(ne) : T~1-100 keV [Lin ST’s talk] • Axions : T ~ keV-MeV [Chang HM’s talk ]

5. Yang-Yang Underground Laboratory • Operated by KIMS Collaboration, 700 m of rock overburden in east Korea • flagship program on CsI(Tl) for CDM searches [SK Kim’s talk] • TEXONO  Install 5 g ULB-ULEGe at Y2L ; Study background and feasibility for CDM searches ; may evolve into a full-scale O(1 kg) CDM experiment Y2L

6. “Ultra-Low-Energy” HPGe Detectors • ULEGe – developed for soft X-rays detection ; easy & inexpensive & robust operation • Prototypes built and studied : • 5 g @ Y2L • 4 X 5 g @ KS/Y2L • 10 g @ AS/CIAE • segmented 180 g @ AS/KS • Being built : 500 g single element • Physics for O[100 eV threhold1 kg mass1 cpd detector]: • nN coherent scattering • Low-mass WIMP searchesThis Talk • Improve sensitivities on mn • Implications on reactor operation monitoring • Open new detector window & detection channel available for surprises

7. Sensitivity Plot for CDM-WIMP direct search • Low (<10 GeV) WIMP Mass / Sub-keV Recoil Energy : • Not favored by the most-explored specific models on galactic-bound SUSY-neutralinos as CDM ; still allowed by generic SUSY • Solar-system bound WIMPs require lower recoil energy detection • Other candidates favoring low recoils exist: e.g. non-pointlike SUSY Q-balls. • Less explored experimentally

8. ULEGe-Prototype built & being studied : 5 g 10 g 4 X 5 g Segmented 180 g with dual readout

9. R&D Program towards Realistic O(1 kg) Size Experiments (both nN & CDM) : • measure & study background at sub-keV range at KS & Y2L ; design of active & passive shielding based on this. • compare performance and devise event-ID (PSD & coincidence) strategies of various prototypes • devise calibration & efficiency evaluation schemes applicable to sub-keV range • measure quenching factor of Ge with neutron beam • study scale-up options ULEGe-detector • Keep other detector options open

10. A PSD Scheme to differentiate Physics Vs Noise Events PSD Cut Threshold ~ 100 eV • Calibrations by keV lines & “0” from random trigger • Achieved threshold < 100 eV with Fe55 source : lowest for bulk radiation detectors ! • Threshold depends on background level

11. Single Readout Event ID – correlate two channels with different gains & shaping times e.g. 4 X 5 g Sampling of Specific Range for non-trigger-Channel 2 – i.e. look for +ve fluctuations at specific and known times Energy as defined by trigger-Channel 1 Noise Signal Ch #1 : Ch #2 :

12. Evaluation of Selection Efficiency: • Select clean sample ofphysicsevents with cosmic-ray and anti-Compton tags • Study survival probabilities of these with the independent selection cuts on Ge-signals • Good efficiency > 200 eV for low background KS data with 4X5 g

13. Dual Readout Event ID – correlate anode/cathodes in amplitude & timing e.g. Peak Position Correlations between Electrodes Seg. 180 g Noise Signal Cathode : Anode :

14. Sub-keV Background Measurements & Comparisons 1 kg 5 g 4 X 5 g • Similar background at KS & Y2L for same detector • Apparent difference between 5 g & 1 kg at T> 5 keV due to scaling with surface area instead, reproduced in simulations • Best Background with 4X5 g comparable to CRESST-1 after corrections due to quenching factor • Intensive studies on background understanding under way

15. WIMP Spin-Independent Couplings : Limits & Sensitivities Standard conservative analysis – WIMP rates cannot be higher than total events measured

16. Quenching Factor Measurement for Ge at CIAE’s Neutron Facilities: With 13 MV Tandem • Goals for Oct 07 Runs : • Use actual ULEGe 100-eV detector • Use lower energy neutron beam with a smaller tandem

17. p S1 S0 Detector Scale-up Plans: • 500-g, single-element, modified coaxial HPGe design, inspired by successful demonstration of Chicago group (nucl-ex/0701012) • Dual-electrode readout and ULB specification • Being built, available first-half 2008.

18. Summary & Outlook (Score Card) • An O[100 eV threshold1 kg mass1 cpd detector] has interesting applications in neutrino and dark matter physics, also in reactor monitoring • Open new detector window & detection channel : potentials for surprise • Mass Scale-Up: recent demonstration of realistic design • Threshold– ~300 eV at hardware level, intensive studies on software techniques to aim at ~100 eV, & on their stabilities and universalities • Prototype data at reactor already provide competitive sensitivities for WIMP search at mass<10 GeV . • Sub-keV Background understanding and suppression – under intensive studies