# Presentation: 626232

## Status of Cold Dark Matter SearchesDan Bauer, Fermilab

- Introduction
- Scientific case compelling for cold dark matter; WIMPs are a likely candidate
- Take the experimental approach; let’s see what’s out there...
- Direct Detection of WIMPS
- How does one go about this?
- A ‘typical’ experiment - Cryogenic Dark Matter Search (CDMS)
- First results from CDMS at Soudan
- The future of direct detection
- Sensitivity of CDMS at Soudan
- The competition
- Summary

## Standard Model of Astrophysics

_**Many CDM candidates**_
**SUSY neutralinos**
**Axions**
**Q balls**
**Kaluza-Klein states**
**...****.**
**Most natural candidate:**
**Weakly-interacting**
**Massive Particle**
**(WIMP)**

## Direct Detection of WIMPs

- WIMPs elastically scatter off nuclei in targets, producing nuclear recoils, with ****n**** related roughly by crossing to ****A(~10-38 cm2)
- Slow velocities  large de Broglie   coherent interaction with all nucleons
- Spin-independent interaction A2
- Spin-dependent needs target with net spin
- Loss of coherence minimizes advantage of largest-A targets
- Energy spectrum & rate depend on WIMP distribution in Dark Matter Halo
- Standard assumptions: isothermal and spherical, Maxwell-Boltzmann velocity distribution
- V0= 230 km/s, vesc= 650 km/s,
****** = 0.3 GeV / cm3

- Energy spectrum of recoils is featureless exponential with E ~ 50 keV
- Rate (based on ****n****  and ******) is smaller than 1 event per kg material per day

**Notes:**
We can potentially detect WIMPs directly when they elastically scatter off the nuclei in our terrestrial targets, producing nuclear recoils.  Because WIMPs move at slow, Galactic velocities, they tend to interact coherently with the entire nucleus.  Spin-independent interactions therefore scale as the square of the atomic mass of the target and hence dominate for most models over spin-dependent interactions.  The expected rate and spectrum of recoil energies depends on how often and with what velocities WIMPs pass through the terrestrial detectors, which in turn depends on how WIMPs are distributed in the galaxy's dark matter halo.  Under standard “spherical cow” assumptions for this WIMP distribution, the expected energy spectrum is exponential with typical energies of 50 keV or less, and the expected rate, dependent on the wimp-nucleon cross-section and the local wimp number density, is fewer than 1 event per kilogram of target material per day. (2:00)

## Experimental Challenges for Direct Detection of WIMPs

- keV energy threshold
- Sensitivity to low mass WIMPs
- Low radioactive contamination
- Screening/purification of materials
- Clean surfaces
- Dust (U/Th/K)
- Radon (daughter implantation)
- Background suppression
- Deep sites (reduced cosmic ray flux)
- Passive/active shielding
- Residual background rejection
- Active nuclear recoil discrimination
- Sensitivity improves:
- Linearly with target mass and exposure time if no background
- As 1/√MT by statistical subtraction of background
- No further improvement if systematics of background subtraction dominate

- Signal Features
- Location and type of interaction
- 	Surface versus bulk
- Backgrounds preferentially on surfaces, WIMPS interact anywhere
- Electron versus nuclear recoil
- Backgrounds cause electron recoils, WIMPS cause nuclear recoils
- Multiple versus single scatter
- Backgrounds multiple-scatter; WIMPS don’t
- Annual modulation
- Surfing the WIMP “wind”
- Diurnal modulation
- Detect recoil direction
- Scale to large target mass
- Maximize # of WIMP interactions
- Different target nuclei
- Determine if possible signal from WIMPs or backgrounds

## A ‘typical’ experiment - CDMS

- Dark Matter Search
- Goal is direct detection of WIMP halo that holds our galaxy together
- Cryogenic detectors
- Cool very pure Ge and Si crystals to < 50 mK using dilution refrigerator
- Active Background Rejection
- Detect heat and charge
- WIMPS, neutrons => nuclear recoils
- Charge/Heat ~ 1/3
- EM backgrounds => electron recoils
- Charge/Heat = 1
- Reject Neutrons
- Neutrons multiple scatter, WIMPS don’t
- 	Look for single scatters
- WIMP cross sections x5 higher in Ge than in Si but neutron cross sections similar.
- 	Look for Ge recoils, not Si

- Shielding
- Layered shielding (Pb, polyethylene, Cu) against radioactive backgrounds and active scintillator veto (>99.9% efficient against cosmic rays).

- Detector Tower

- Dilution
- Refrigerator

- Shield/Muon Veto

- Electronics and Data Acquisition

- Deep Underground
- Reduce fast neutrons
- from cosmic rays interacting
- in rock (< 1 /kg/year at Soudan)

## Really Cool Detectors: ZIPs

**1**  **tungsten**

**380** **x** **60** **aluminum fins**

- 250 g Ge or 100 g Si crystal
- 1 cm thick x 7.5 cm diameter
- Photolithographic patterning
- Collect athermal phonons:
- XY position imaging
- Surface (Z) event veto based on pulse shape risetime

- Measure ionization in low-field (~volts/cm) with segmented contacts to allow rejection of
- events near outer edge

_***Z***_-sensitive _***I***_onization and _***P***_honon-mediated

- Qouter

- Qinner

**Notes:**
Our detectors provide additional information to allow us to reject these surface events. Because each detector collects athermal phonons with sensors divided into 4 quadrants, (0:15)

## Demonstration of ZIP Position Sensitivity 

- Demonstration of ZIP Position Sensitivity

- Cd109 :
-  22 kev
- i.c. electr 63, 84 KeV

**Delay Plot**

**A**

**D**

**C**

**B**

**Notes:**
The results indicate position resolution of a few millimeters?, and allow us to show off the detector’s excellent energy resolution by showing the different energy spectra corresponding to the different sources and geometries. (0:30)

## CDMS II Background Discrimination

- Ionization Yield (ionization energy per unit recoil energy) depends strongly on type of recoil
- Most background sources (photons, electrons, alphas) produce electron recoils

- 47k Photons (external source)

**Notes:**
Our experiment is founded on using detectors that discriminate against background interactions by measuring both the ionization energy and the recoil energy of each event.  The ratio of these quantities, called the ionization yield, depends strongly on the type of recoil that occurred. Most background sources, such as photons, electrons, and alpha-particles, interact with the charged electrons, producing electron recoils, with ionization yield about one, (0:30)

## CDMS II Background Discrimination

- Ionization Yield (ionization energy per unit recoil energy) depends strongly on type of recoil
- Most background sources (photons, electrons, alphas) produce electron recoils
- WIMPs (and neutrons) produce nuclear recoils

- Neutrons from external source

- Photons from external source

**Notes:**
while WIMPs, as well as neutrons, interact with an atom's nucleus, producing nuclear recoils, with lower ionization yield.  (0:10)

## Detectors provide near-perfect event-by-event discrimination against otherwise dominant bulk electron-recoil backgrounds

- Detectors provide near-perfect event-by-event discrimination against otherwise dominant bulk electron-recoil backgrounds

- CDMS II Background Discrimination

- Ionization Yield (ionization energy per unit recoil energy) depends strongly on type of recoil
- Most background sources (photons, electrons, alphas) produce electron recoils
- WIMPs (and neutrons) produce nuclear recoils

- Particles (electrons) that interact in surface “dead layer” of detector result in reduced ionization yield

**Notes:**
Discrimination is not quite this perfect for electron-recoil events that occur near the detector surfaces.  For electron recoils caused by particles (such as low-energy electrons) that interact near the detector surfaces, the ionization yield is slightly less than that expected for an electron recoil in the bulk. (0:20)[1:20]

## The ‘Z’ in ZIPs: Electron Risetime Discrimination

- 3 populations
- Electron recoils in bulk
- Surface events
- Nuclear recoils in bulk
- Surface events produce faster phonons: 2nd discrimination parameter
- Pulse risetime and delay from charge pulse

**Important second handle on electron backgrounds with ‘tail’ in charge yield**

**Notes:**
Neutrons from 252Cf source
Surface-electron recoils (selected  via nearest-neighbor multiple scatters from 60Co source)
(Single-scatter) photons from 60Co Source

## First Data from CDMS II at Soudan

- October 2003- January 2004 run of “Tower 1”
- 4 Ge (0.85 kg) and 2 Si (0.17 kg) ZIPs
- 62 “raw” livedays, 53 livedays after cutting times of poor noise, etc.
- Expect all background sources combined to contribute < 1 event
- Set cuts while “blinded,” opened box on March 20
- Detailed checks since then
- Preprint released on Monday, May 3

- Tower 2

- Tower 1

**Notes:**
This past summer, while preparing the deep site for running, we completed a run at the shallow site at Stanford.  This run provided a complete system test of the first 6 detectors that we will soon run at Soudan.  We found that the radioactive contamination levels of the detectors were acceptable even at the shallow site, and the detector performance exceeds our specifications.  The primary physics goal of this run was to confirm whether most of the nuclear recoil events we observe are due to neutrons.  We added an internal polyethylene shield, which reduced the external neutron background by over a factor of 2, as predicted.  We also measured the neutron background simultaneously in both germanium and silicon, and found consistent results. (1:30)

## Excellent live time efficiency(for a first run)

- Collected 52.6 live days during 92 calendar days
- Efficiency nearly 85% for last six weeks
- Gaps were calibration runs with minimal cryo-lapses

## 252Cf Neutron & Gamma calibration data

- Upper red dashed line are +/- 2  gamma band
- Lower red dashed line are +/- 2  nuclear recoil band
- Phonon non-uniformity corrected with high statistics gamma calibrations
- Bands and cuts determined with calibration data as was the analysis threshold energy

- Recoil Energy (keV)

- Ionization Yield = Ionization Energy/Recoil Energy

## Phonon timing + yield

- Rejection of surface electrons (low energy betas)
- Use phonon risetime and charge-to-phonon delay
- “Blind” Analysis
- Cuts and energy threshold based on *calibration data*
- WIMP-search data blinded until analysis ‘fixed’
- Simplest possible cuts
- NOT optimized
- We already can do better on both background rejection and nuclear recoil acceptance.

- gammas

- neutrons (**WIMP**-signal region)

- “ejectrons”

- Charge yield

- Phonon arrival time

- 252Cf neutron & 133Ba gamma  calibrations

## WIMP search data with Ge detectors

- Recoil energy (keV)

- Charge yield

- Exposure
- 92 days (October 11, 2003 to  January 11, 2004)
- 52.6 live days
- 20 kg-d net (after cuts)
- 53% nuclear recoil acceptance
- Data: Yield vs Energy
- Timing cut off
- Timing cut on
- Yellow points from neutron calibration

## WIMP search data with Ge detectors

- Exposure
- 92 days (October 11, 2003 to  January 11, 2004)
- 52.6 live days
- 20 kg-d net (after cuts)
- Data: Yield vs Energy
- Timing cut off
- Timing cut on
- Yellow points from neutron calibration

- Recoil energy (keV)

- Charge yield

## WIMP search data with Ge detectors

- Exposure
- 92 days (October 11, 2003 to  January 11, 2004)
- 52.6 live days
- 20 kg-d net (after cuts)
- Data: Yield vs Energy
- Timing cut off
- Timing cut on
- Yellow points from neutron calibration

- Recoil energy (keV)

- Charge yield

**No nuclear-recoil candidates**

## WIMP search data with Ge detectors

- Exposure
- 92 days (October 11, 2003 to  January 11, 2004)
- 52.6 live days
- 20 kg-d net (after cuts)
- Data: Yield vs Energy
- Timing cut off
- Timing cut on
- Yellow points from neutron calibration

- Recoil energy (keV)

- Charge yield

**Well, maybe 1....**

- Expected beta background = 0.7 +/- 0.3 events

## NEW CDMS limit from Soudan

*NEW* CDMS limit from Soudan

- Exposure after cuts of 52.6 kg-d raw exposure with Ge ≈ 20 kg-days for 60 GeV/c2 WIMP
- No nuclear-recoil candidates
- Expect ~1 mis-identified beta
- Second non-blind analysis has 1 candidate (dashed limit curve show effect of this)
- Expect 0.1 unvetoed neutrons (1.0 muon coincident neutron)
- New limit ~4x (x10) better than EDELWEISS (CDMS SUF) at a WIMP mass of 60 GeV/c2
- Really hard to accommodate DAMA annual modulation effect as a WIMP signal!
- Starting to seriously constrain MSSM models

- MSSM models

## What’s next for CDMS?

- Improve our analysis
- Reduce analysis threshold to 5 keV (better low-mass WIMP sensitivity)
- Improved cuts to reject betas, improve nuclear recoil efficiency (but no longer blind)
- Expected exposure: 100 kg-d	Limit: 90% CL : 1.5 x 10-43 cm2
- Towers 1 and 2 are currently taking data -> July 04
- 50% more Ge, 100% more Si than in first run
- Use blind analysis for this independent sample, based on best version of Tower 1 analysis
- Install 3 additional detector towers in September 04
- Total of 4.5 kg Ge, 1.2 kg Si
- New towers have improved handling, hopefully lower beta backgrounds
- Run all 5 Towers January-December 31, 2005
- Exposure: 1,200 kg-d	 90% CL upper limit: 2 x 10-44 cm2
- If we’re lucky, a WIMP signal begins to appear!
- Reduce beta and neutron backgrounds even further
- Detector optimization, Beta screening of materials, additional scintillator veto
- Construct two more towers and run through 2008 (CDMS III)
- Exposure: 4,800 kg-d	 90% CL upper limit: < 7 x 10-45 cm2
- Would allow us to explore any signal which starts to appear in CDMS II

## Comparison with Competition

- DAMA - NaI
- Very limited discrimination; must rely on 2% annual modulation effect
- Systematic effects near energy threshold hard to control
- Only WIMP signal reported thus far (6 years of annual modulation data)
- EDELWEISS - Ge thermal and ionization
- Slower thermal detector technology (no additional rejection from timing)
- Very deep site, but no Si detectors to measure neutrons when they appear
- Begin running in 2005 with substantial target mass.
- CRESST - Ca2WO4 thermal and scintillation
- Very low threshold but no light for W & O nuclear recoils
- Have problems with phonon-only signals from alphas
- Need additional shielding/veto; begin running again in 2005?
- ZEPLIN, XENON and XMASS - Xe ionization and scintillation
- Must demonstrate sufficiently low energy threshold (time scale is 2006)
- Light for nuclear recoils not yet demonstrated
- Clearest path towards large target mass in the long-term
- DRIFT - CS2 low pressure gas TPC
- Only technology capable of determining event direction
- Difficult to instrument 5-10 kg of target in near future
- Heavy Liquid Bubble Chamber (Collar, Sonnenschein - Univ. Chicago)
- Very impressive discrimination against backgrounds in small prototype
- Larger prototype may be tested at Soudan in 2005

## Summary and Projections

- WIMPs
- Looking for 23% of the universe!
- New particle physics (SUSY neutralino)
- Sensitive to 10-10000 GeV masses
- Challenging MSSM models
- Broad range of experimental approaches/efforts
- CDMS II at Soudan leads the chase
- Significant competition by next year
- Expansion to ton-scale detector mass
- Several approaches possible, none demonstrated
- Results from next few years will decide
- Growing scale of experiments
- Room for increased HEP participation
- DUSEL - Deep Underground Science and Engineering Laboratory needed by 2010
- Shorter-term requires low background screening facility - proposal at Soudan

- 90% CL upper limits assuming standard halo, *A*2 scaling

- MSSM models

**2003**

**2006**

**2010**

- DAMA

- CDMS II @ Soudan
- Edelweiss II

- CryoArray, Xenon, Genius

## CDMS Collaboration

**UC Berkeley, Stanford, LBNL, UC Santa Barbara,****Case Western Reserve U, FNAL, Santa Clara U, **
**NIST, U Colorado Denver, Brown U, U Minnesota,**
**U. Florida, Princeton**