Direct Detection of Dark Matter

Direct Detection of Dark Matter

Natallia Karpenka

Experimental Techniques in Particle Astrophysics

October 27, 2010

Outline

1

Background

2

Experiments

3

General principles

4

Nuclear recoil detectors

5

XENON

6

Summary

1

Background

2

Experiments

3

General principles

4

Nuclear recoil detectors

5

XENON

6

Summary

Evidence for Dark Matter at different scales

Spiral galaxies rotation curves Clusters and Superclusters

gravitational lensing Large scale structure

structure formation

CMB anisotropy: WMAP

(Incomplete) List of DM candidates

Neutrinos Axions

Lightest Supersymmetric particle (LSP) - neutralino, sneutrino, axino Lightest Kaluza-Klein Particle (LKP) Heavy photon in Little Higgs Models Solitons (Q-balls, B-balls)

Black Hole remnants

Hidden-sector technipions

...

WIMPs are the most popular candidates

1

Background

2

Experiments

3

General principles

4

Nuclear recoil detectors

5

XENON

6

Summary

Experiments

DAMA-LIBRA/Nal CDMS

EDELWEISS-II XENON ZEPLIN II WARP COUPP

PICASSO

KIMS

ANAIS

ROSEBUD

CRESST

EURECA

CUORE

1

Background

2

Experiments

3

General principles

4

Nuclear recoil detectors

5

XENON

6

Summary

3 ways to detect recoils

”Direct detection” usually means looking for elastic scattering of WIMPs

producing a nuclear recoil

Recoil rates

dN dE

= σρ

2µ

m

F

Z

v_min

f (v ) v dv

N = number of scatterings Er = nuclear recoil energy σ = WIMP-nucleus cross-section ρ = WIMP density µ = WIMP-nucleus reduced mass mχ = WIMP mass F = nuclear form factor f(v) = WIMP velocity distribution v = WIMP velocity

vmin = minimum v to produce recoil Er vesc = halo escape velocity

Recoil rate is degenerate in unknowns WIMP mass

local WIMP density

halo velocity distribution

WIMP-nucleus cross-section

Difficulties with direct detection experiments

1

The recoil energy is small ( tens of keV)

Detector needs to have as low energy threshold as possible

2

The interaction rate is low

Need high target masses in order to gain sufficient statistics in a reasonable life-time of the experiment

3

Extremely high background

To be discussed later

Cosmic-ray muons

Cosmic rays create high-energy muons when they interact with the atmosphere.

Muons release high-energy neutrons when they collide near an experiment.

Neutron sources

detector components ultra-pure materials

rock

passive shielding

radon

radon removal, gas-tight sealing and ventilation with radon-free air

Some more problems

Differentiating nuclear collisions from electron collisions

Separating WIMP collisions from neutron

collisions

Daily and yearly modulation

Due to Earth’s proper motion in the galaxy we expect to have annual and

day-night modulation

1

Background

2

Experiments

3

General principles

4

Nuclear recoil detectors

5

XENON

6

Summary

Cryogenic detectors

Scintillation + phonons

CRESST Rosebud

Ionisation + phonons

CDMS

Edelweiss

Cryogenic detectors

The cryostat at CDMS uses six nested layers to cool the detectors to less then

50 mK. This reduces the background vibrations of the detector’s atoms and

makes them more sensitive to individual particle collisions.

Cryogenic detectors

Different layers of shielding around the icebox

Dilution refrigerator

Dilution refrigerator

A dilution refrigerator is a cryogenic device which uses a mixture of helium-3 and

helium-4. When cooled below about 870 millikelvin, the mixture undergoes a

spontaneous phase separation to form a He

-rich phase and a He

-poor phase.

Bolometer

Bolometers are widely used as particle detectors. They consist of a crystal used as energy absorber and a temperature sensor thermally coupled to the crystal and to a thermal bath. The interaction of a particle in the absorber produces a small temperature rise ∆T = E /C , where E is the energy deposited in the absorber and C is the heat capacity of the bolometer. For rare events experiments, the absorbers are usually chosen among dielectric and

diamagnetic materials and are operated at very low temperatures to guarantee

a measurable ∆T .

Semiconductor detectors

Ionisation detectors Examples : IGEX, HDMS

Detect electrons after ionisation in the semiconductor

Originally designed to look for neutrinoless ββ decay.

Crystal scintillator detectors

DAMA

Particle DM seach with highly radiopure scintilators at Gran Sasso

9 scintillating thallium-doped sodium iodide (NaI) crystals of 9.7 kg each

electrons and nuclei recoiling after a collision

cause emmission of photons that are detected

using photomultiplier tubes

Liquid noble gas detectors

Scintillation + ionisation detector Examples: XENON100, WARP, XMASS, ZEPELIN II

Liquid scintillators, high yield

Easily scaled up to large mass

Bubble chamber search

Why bubble chambers for dark matter?

Large target masses would be possible.

An exciting menu of available target nuclei.

Low energy thresholds are easily obtained for nuclear recoils.

Backgrounds due to environmental

gamma and beta activity can be

suppressed by running at low

1- liter detector at Fermilab

Triple neutron scatter

1- liter detector at Fermilab

Muon Track at superheat pressure

Axion detectors

1

Background

2

Experiments

3

General principles

4

Nuclear recoil detectors

5

XENON

6

Summary

The XENON roadmap

past (2005-2007)

current (2008-2010)

future

(2011-2015)

Why Xenon?

Large mass number (A=131): high rate for SI interactions if nuclear recoil threshold is low

Excellent stopping power: active volume is shelf-shielding

Excellent scintillator and Ionizer: highest yield among noble liquids Intrinsically pure: no long-lived radioactive isotopes; Kr/Xe reduction to ppt level with established methods

Nuclear recoil discrimination with simultaneous measurement of scin- tillation and ionization

Scalability: relatively inexpensive for very large detector

The XENON two-phase TPC

XENON-detector

Use lessons/technologies from XENON10 to build a detector with x 10 more fiducial mass and x 100 less background

170 kg of LXe: the active target (65 kg) is surrounded on all sides by a 105 kg of LXe active veto

TPC size: 30 cm drift x 30 cm diameter viewed by two arrays of PMTs with less then 1 mBq (U/Th) and 30 procent QE (bottom array)

Background from internal components reduced by: a) materials screening and selection; b) cryocooler and FTs outside shield; c) cryogenic distillation to reduce Kr/Xe contamination

Background from external sources reduced by: a) active LXe veto; b) improved shield with 5 cm Cu lining of Poly and with water outside Pb

XENON results

1

Background

2

Experiments

3

General principles

4

Nuclear recoil detectors

5

XENON

6

Summary

Summary

The identity of Dark Matter remains a mystery today but potential for breakthrough in the coming decade very likely

Direct detection experiments have made significant progress in recent years, driven in part by an aggressive competition worldwide.

Several 100 kg scale experiments in operation underground or under construction.

On the other hand, if cross-section is at the 10-8 pb as in some favored SUSY models, we will start to see a handful of WIMP events and that is very exciting!

Equally important is that for the first time a low background, massive target, other than NaI, can probe annual modulation

A direct detection signal, from either or both SI and SD interactions, needs to be

validated with more than one target and concept: current zoo of experiments

vital for field. Directional experiments advancing at good pace. Will provide the

ultimate smoking gun for DM signal

Questions