Towards One Tonne WIMP Direct Detectors: Have we got what it takes?

Transcription

1 Towards One Tonne WIMP Direct Detectors: Have we got what it takes? Center for Particle Astrophysics UC Berkeley source at Gaitskell

2 Future Reach / Lower limit on quark σ Genino 100 kg Ge in 5 m tank LN2 Genius 100 kg Ge in 14 m tank LN2 Shaded regions: Unconstrained MSSM ~1 event/kg/d CDMS Latest Feb 2000 Now limited by shallow site CDMS II 5 kg Ge at Soudan ZEPLIN 6 kg Xe at Boulby ~1 event/kg/yr 0 < M 3 < 1000 GeV msugra & Naturalness 95 < m 0 < 1000 GeV Mandic, Gondolo, < A 0 < 3000 GeV Murayama, Pierce 1.8 < tan(β) < 25 sign of µ < Ω h 2 < 1 CryoArray 1 tonne event by event discrim. ~ 1 event/100 kg/yr see also Ellis et al. hep-ph

3 Direct Detection: History & Future 90% CL Limit on Cross section for 60 GeV WIMP (scalar coupling) Homestake (87) Oroville (88) [m=20 GeV] ~1 event kg -1 day -1 Edelweiss (98) Heidelberg-Moscow (94) UKDMC (96) [m=100 GeV] DAMA (98/00) Heidelberg-Moscow (98) DAMA (96) CDMS SUF (99) CDMS SUF (00) NOW CDMS SUF (T) Ge NaI Cryodet Liq Xe (T) Target Signal ~1 event kg -1 yr -1 Different Colours Indicate Different Technologies GENINO (T) 100 kg Ge Diode CDMS Soudan (T) 7 kg Ge+Si Cryodet [m =?? GeV - if significantly better limit obtained at LHC ~1 event 100 kg -1 yr -1 different mass] GENIUS (T) 100 kg Ge Diode CryoArray (T) tonne Cryodet Not meant to be a complete list - see rjg

4 Achieving Detector Mass Scale up to 1 tonne (not accounting for gain/loss due to Form Factor- threshold relationship) LiqXe 300 litres fiducial PMT coverage for 3-6 m 2 Ge Diode ~400 x 2.5 kg Ge Semiconductor [~$6M/tonne (unenriched)] Cryogenic Typical crystals kg; detectors Industrial Fab cost yet to be established (CUORE proposal) Gas TPC 10,000 m 3 gas 1 m 40 torr is 2.3 g x RMM (e.g. Ar is 92 g/m 3 )

5 Problem: Performance -> Production Mass Recent challenge has been to demonstrate discrimination techniques Now it is time to solve some of problems of scale up Production issues (scales of collaboration) Good experience of running technology underground

6 Misidentified events Inverted Rare Event Search Inverted approach: apply rare event search techniques to background Looking for anomalous events becomes like a rare event search (Dave Wark, SNO) in amongst signal Importance of having as many handles on events as we can muster Finite ranges for ratios of coincident Large separation Not decay times (event by event difficult) Events must stay above noise in all channels Tendency to confuse events Tell-tale signal for DM Ann. mod - tough <5% mass, systematics require lower background Directionality - tough to achieve mass in gas Kinematics (Recoil spectra in different target material) We need a number of experiments!!

7 Misidentified events Inverted Rare Event Search (2) It is clear that we will need to go through a period of seeing nothing Before we are ready to see something Ladder - we need to make transition to extended periods of seeing nothing before can justify building a bigger version. Challenge: Publication of paper with no events. (Offer of a keg of beer to experimentalists who set new (best) limit using no events) I sometimes envy the theorists 15 orders of magnitude on scale Improvements in experimental reach (2 orders of magnitude has been hard fort)

8 2nd Kind of Limit on Threshold of Discrimination Background (e.g. Gamma or Surface Betas) Lateral spread of lines simulates noise x 2 x 2 For 1st case - (see CDMS talks on surface electron confusion with nuclear recoils) 1st Kind Signal (Nuclear Recoil) 2nd Kind In 2nd case - will single event in x 1 only be background free? Effective threshold raised due to event-by-event discrimination confused by noise x 1 In this region x 2 channel for signal events are buried in noise: Example where 2nd limit on discrimination may be relevant: CRESST x1(phonon) x2(photon) CaWO 4 Gamma: x1=50 kevee x2~350 evdet [based on current # s - see CRESST II talk] Neutron: x1=50 kevee&r x2~50 evdet (close to noise threshold) Assumes 0.7% energy of electron recoil signal detected in photons for gamma event and QF (established for O nuclei recoils relative to gammas) is 14% x 1

9 Influence of Coherence & Form Factor (q=0) Rate~A Xe, 127 I (σ= cm 2, m=300 GeV) 73 Ge Rate kg -1 day Ar,~ 32 S Ge=15 kevee (30%QF) Xe=10 kevee (20%QF) I=4.5 kevee (9%QF) Solid line is integrated rate above threshold (dotted is differential rate) Recoil Energy Threshold [kevr]

10 SUSY/Weak Scale Note that LHC is unlikely to give LSP mass directly Can argue that DM direct search can be used to fix this parameter (Tovey) DarkSUSY Clear that channels all Annihilation Rate (Ω m error < factor 2 (?)) However, particular channel branching ratio may have many orders of magnitude range Direct - Cross-section on quarks Indirect - Decay BR -> ν, γ, e & Model of density enhancements Incredible range of experimental techniques that are now/on the verge producing new limits Look outside SUSY? WIMPs - weak scale (framework?) Mass prediction 10 TeV (unitarity)

11 Projected Sensitivity CDMS II Background table Background rejection & Statistical subtraction Based on exposure (counts) 2500 kg-days Assumptions in forecasts formally outlined e.g. Will include projected limit plots posted at & Show same exercise for 1 tonne Extend statistics

12 CDMS II Soudan (2070 mwe depth) Goal: 0.01 evt/kg/day= evt/kg/kev/day Currently Deep site should give this Background source Shielded Muon After detector Veto rejection γ s, external radioactivity Background subtracted Systematics γ s, cosmics in shield Currently 0.05 in best detector Continue to push on surface contamination γ s, internal single scatters % γ rejection Total γ s ~20 kevts ~100 evts subtraction ~16 evts β s, surface contamination % β rejection n s, external radioactivity n s, cosmics in shield n s, cosmics in rock Total neutrons Total background /kg /day Special shield against high energy neutrons Tn = MeV are important ~1 per 0.25 kg detector per year Units: /kg/kev/day at 15 kevr (5kg Ge, 2kg Si kg.days in Ge) rjg

13 Neutrons from Muons Neutron production ~ Muon Flux E µ ~Depth 0.47 means Soudan to x15 (negligible for other ratios) Soudan Site Relative Muon Flux Soudan/WIPP x 20 Boulby x 2 Gran Sasso x 1 Frejus x 1/2 Aglietta et.al. Nuove Cimento 12, N4, page 467

14 Neutron Penetration (Water) Attenuation of High Energy Neutron Flux in water shield Monte Carlo Simulations Performed by Thushara Perera, CWRU using GEANT/MICAP/FLUKA 300 MeV 10 MeV 1 MeV Energy x10 atten x100 atten [MeV] [cm] [cm] (10 events) Typical multiplicity of 1-few Water 300 MeV neutron injected in +z direction

15 Neutron Penetration (Fe vs Water) Summary Fe (KE MeV) 100 cm x10 atten (>1 MeV) 200 cm x10 atten (>10 kev) 300 MeV 100 kev 10 kev (only 1 event shown for clarity) Multiplicity of neutrons generated per event is higher (~20) Fe (10 events) Typical multiplicity of 1-few Water 300 MeV 10 MeV 1 MeV

16 Shielding Eff x Flux - Balanced in range MeV Neutron production (fluxes for Soudan) Energy Wall Flux Pentration Flux inside [MeV] cm wate[/m2/yr] % % % % 0.9

17 Soudan - outer Pb and poly shielding Base sections of lead and polyethylene shields test assembly at UCSB who built shield. Note Poly-Pb-Poly layers

18 Neutron Subtraction WIMP /Neutron Kinematics/Cross-section e.g. Mixed Ge & Si targets Multiple Scattering Position / timing (ns) resolution Neutron Capture (e.g. MACHe3 3H) in bulk

19 CDMS II Soudan (2070 mwe depth) Goal: 0.01 evt/kg/day= evt/kg/kev/day Currently Deep site should give this Background source Shielded Muon After detector Veto rejection γ s, external radioactivity Background subtracted Systematics γ s, cosmics in shield Currently 0.05 in best detector Continue to push on surface contamination γ s, internal single scatters % γ rejection Total γ s ~20 kevts ~100 evts subtraction ~16 evts β s, surface contamination % β rejection n s, external radioactivity n s, cosmics in shield n s, cosmics in rock Total neutrons Total background /kg /day Special shield against high energy neutrons Tn = MeV are important ~1 per 0.25 kg detector per year Units: /kg/kev/day at 15 kevr (5kg Ge, 2kg Si kg.days in Ge) rjg

20 Nuclear Recoil Discrimination - Event by Event Nuclear recoils arise from WIMPs Neutrons NOT A SIMULATION! 1334 gamma events, 616 neutron events Gammas (external source) Electron Recoils arise from photons electrons alphas (Typical Background) Neutrons (external source) Ionization yield ionization/recoil energy strongly dependent on type of recoil Recoil energy Phonons give full recoil energy Phonon Trigger Threshold

21 CryoArray (Sensitivity <1 per 100 kg-yr, s~10-46 cm -2 ) Scale up to 1 tonne detector with target (90%CL) <1 evt per 100 kg-yr (improvement relative to CDMS II figures) Reduce γ/β backgrounds by factor 20 γ > cts/kevee -1 kg -1 day -1 (This compares to kevee -1 kg -1 day kev for HMDS) β > cts/kevee -1 kg -1 day -1 (Challenge to survey surfaces to this sensitivity) Improve γ/β rejection by factor 10 γ 99.5% -> 99.95% (1 in 2000) [Currently showing 99.9%] β 95% -> 99.5% (1 in 200) [Currently showing >95%] For comparison: Without discrimination needs ~10 4 reduction in background from present (HM) levels

22 Cosmogenic Tritium Created ~200 3 H kg -1 day -1 at sea level Equilibrium concentration 1.4x10 6 atoms kg -1 Avignone et al. Nucl. Phys. B (Proc. Suppl.) 28A (92) 280 Example: GENIUS Target: 10 days at SL+3 years of cooling 1700 atoms 3 H 11 kev-19 kevee 15 evts kg.yr -1 Would dominate and create effective WIMP threshold 60 kevr Compare with 3x10-2 kev -1 kg.yr -1 Surface contamination 3H less of an issue for p-type contacts (dead regions) cover most of det. surface Counts /kev/day/10 5 atoms 15.5 cts/day/10 5 atoms

23 kevee (Electron Equivalent) -> kevr (Nucl Recoil) DAMA ROM2F/2000/01 Preprint Figure 1 2 kevee 100 kg NaI == 22 kevr 1 count kevee -1 kg -1 day -1 == 0.09 cnt kevr -1 Heidelberg-Moscow 2.7 kg HP Ge CDMS 0.1 count kevee -1 kg -1 day -1 == 0.03 cnt kevr count kev -1 kg -1 day -1 == 0.1 cnt kevr kevee == 40 kevr Already Recoil Energy

24 rjg

25 Direct Detection: History & Future 90% CL Limit on Cross section for 60 GeV WIMP (scalar coupling) Homestake (87) Oroville (88) [m=20 GeV] ~1 event kg -1 day -1 Edelweiss (98) Heidelberg-Moscow (94) UKDMC (96) [m=100 GeV] DAMA (98/00) Heidelberg-Moscow (98) DAMA (96) CDMS SUF (99) CDMS SUF (00) NOW CDMS SUF (T) Ge NaI Cryodet Liq Xe (T) Target Signal ~1 event kg -1 yr -1 Different Colours Indicate Different Technologies GENINO (T) 100 kg Ge Diode CDMS Soudan (T) 7 kg Ge+Si Cryodet [m =?? GeV - if significantly better limit obtained at LHC ~1 event 100 kg -1 yr -1 different mass] GENIUS (T) 100 kg Ge Diode CryoArray (T) tonne Cryodet Not meant to be a complete list - see rjg