Can We Detect Dark Matter With Bubble Chambers?

Andrew Sonnenschein

Fermilab

15 ft. Bubble Chamber

Cartoon of a Typical Galaxy

Invisible material of unknowncomposition

Stars and bright gas

• 20th century astrophysicists developed a bizarre model for galaxy mass distribution:

>85% of total mass

“Halo”

~ 200 kpc

Dark Matter in the Minimal Supersymmetric Standard Model

Standard Model

MSSM Higgs

Superpartnerflavor states

Superpartner mass states

u,d,s,c,t,b

squarks

e sleptons

e,,sneutrinos

W± charged winos ±2

H± charged higgsinos ±1

photino 04

Z0 zino 03

h h higgsino 02

H higgsino 01

A0

G gravitino

Charginos

Neutralinos

mixing

Lightest neutralino is dark matter?

Mass: 30-1000 GeV

Based on simple assumptions:• Particles are gravitationally bound to Halo, with Maxwellian

velocity distribution (Vrms=220 Km/s) and local density 0.3 GeV/cm3

• WIMPs are heavy particles, 10 GeV< MWIMP< 1 TeV.• Nuclear scattering can efficiently transfer energy to a nucleus, since Mnucleus~Mwimp.

The signal will be a nuclear recoil, with energy ~10 keV

• Scattering is non-relativistic.

• Shape of spectrum does not depend on particle physics inputs.

• Amplitude of spectrum depends onunknown supersymmetry parametersand some astrophysical uncertainties.

Spectrum of Wimps in a Detector on Earth

Germanium detector

Non- Relativistic Scattering of Wimps on Nuclei

• For v/c~0.001, the “vector”, “axial vector”, “scalar” couplings become simply spin-dependent and spin-independent.

• Interaction is coherent over nucleus, because de Broglie wavelength of Wimp is big:

• For spin-independent coupling, cross section is proportional to A2

(A = number of protons and neutrons).

• Spin-dependent cross section is suppressed by cancellation of pairs of opposite spin nuclei, so coupling is effectively to net total nuclear spin.

• So, it’s probably easier to discover Wimps via their spin-independent couplings, using target materials with heavy nuclei and taking advantage of A2 coherence factor.

h

mv

0.9 fm1

m /100 GeV

1

v / 220 km /s

Eve

nt R

ate

[eve

nts/

keV

-Kg-

day]

Energy [keV] WIMP Mass [GeV/c2]

WIM

P-n

ucle

on C

ross

Sec

tion

[cm

2 ]

Excluded RegionLargest compatible Signal for 1 TeV

Sensitivity is Limited by Background Radiation

Assume halo density 0.3 GeV/cm3

• Construction of exclusion plot is illustrated below with data from an early experiment.

Backgrounds from Radioactivity and Cosmic Rays

( figure from Brodzinski et al, Journal of Radioanalytical and Nuclear Chemistry, 193 (1) 1995 pp. 61-70)

Gammas & betasFrom primordial, cosmogenic, and manmadenuclei: (not an exhaustive list!)

238U, 232Th + daughters (incl. 222Rn)40K, 14C 85Kr, 137Cs, 3H - nuclear tests68Ge, 60Co - cosmogenic in detector setups

Cosmic Rays (p, , , e…)Can be reduced by going underground.The ’s penetrate to great depth.

NeutronsFrom spallation or (, n) reactionsin rocks, with alphas from U/Th chains. Canbe shielded with moderator at low energies.

• A long history of successful attempts toreduce by choosing special materials andshielding.

40 keV Ar in 40 Torr Ar 13 keV e- in 40 Torr Ar

• WIMPs interact with the nucleus, while most backgrounds are due to electron scattering by gamma and beta rays.

• The resulting spatial distributions of energy and charge are very different.

(Figures from DRIFT collaboration)

Discriminating Against Backgrounds

X [mm]X [mm]

Y [

mm

]

Y [

mm

]

Observables Which Could be Used to Separate WIMP-nucleus Events from Backgrounds

• Spatial distribution of charge deposited in a low-pressure drift chamber.

• Ratio of ionization to deposited energy in a cryogenic detector (CDMS).

• Pulse shape discrimination in scintillating materials (organic & inorganic).

• Ratio of ionization to scintillation in liquid noble gases.

• Ratio of scintillation to deposited energy in a cryogenic detector.

• Annual modulation due to motion of Earth around the Sun.

• Daily modulation in direction of ion tracks in a low-pressure drift chamber, due to rotation of the Earth.

• Efficiency for bubble formation in superheated liquids.

Recoil Energy [keV]

Cha

rge

Ene

rgy

[keV

] Co-60 rays.

Cf-252 neutrons.

Background D iscrimination With Simultaneous Charge and Energy Measurement in Cryogenic Detectors

+

Rays,neutrons,

&WIMPs

Semiconductor

-+

- -- -

- ++

+

phonons

electrons& holes

Temperature sensor

E

Charge collector

CDMS-I

CDMS Ge and Si ‘ZIP’ detectors

• Superconducting tungsten transition-edge sensors

• Simultaneously measure Ionization and athermal Phonon energy signals

• Pulse timing/parameters give sensitivity to event location – Z position – reduction of surface backgrounds

• The detectors are operated in dilution refrigerators at ~20mK

tungstenaluminum fins

Current CDMS-II Results (Sept. 2005)

• 6 Ge & 6 Si detectors x 75 days

• 10-100 keV energy window

• 1 candidate event with 0.4 expected in Ge

• No candidate events in Si

Nuclear recoil region

Passed all cuts Passed pulse timing/ shape cuts

CDMS Current Limits (Sept. 2005)

For spin- independent channel

Lots of Parameter Space Left to Explore

Assumptions in this plot include:• Spin-independent coupling.• Maxwellian halo velocity distrib.

Vrms= 220 km/s• dm=0.3 GeV/cm3

• MSSM Parameter Space fromGondalo et al., 1999.

CDMS, EDELWEISS, ZEPLINLimits

(Feb. 2005)

MSSM

A Selection of Past, Present, and Near-Future Experiments.Status Target Mass

Germanium Diode ExperimentsPNNL/UNC (Avignione et al.) 1986-1990 GeUCSB/LBL Oroville (Caldwell, Sadoulet,...) 1987-1992 Ge, Si ~ 1 KgCOSME-I, II Ongoing 230 gIGEX Ongoing Ge 2.1 KgHeidelberg-Moscow (Klapdor, Heusser,…) 90's Ge 2.7 Kg

HDMS Ongoing Ge 200 gGENIUS-TF Under construction 40 Kg

ScintillatorsDAMA Ongoing NaI, CaF2 87 KgLIBRA Under construction NaI 250 KgNAIAD (UK Collaboration) Ongoing NaI 50 KgANAIS Ongoing NaI 107 KgELEGANTS-V, VI Ongoing NaI 730 Kg

Cryogenic Detectors (<100 mK)CDMS-I, II Ongoing Ge, Si 3 Kg CRESST-I, II Ongoing Al2O3, CaWO4 Will reach ~10 KgEDELWEISS I, II Ongoing Ge 1.2 Kg (will reach ~10 Kg)LiF (Tokyo) Ongoing Li F 168 gORPHEUS (superheated grains) Ongoing Sn 500 g

Liquid XenonDAMA Ongoing? Xe 6 KgZEPLIN-I,II,III Ongoing Xe 4 Kg

Drift ChambersDRIFT Ongoing Xe ~ 100 g

Superheated Liquids SIMPLE Ongoing F (Freon) ~10 gPICASSO Ongoing F (Freon) ~10 gBubble Chamber (COUPP) F, I, Br 2 kg

Main Topic

Most Sensitiveas of Feb. 2006

Why Bubble Chambers for Dark Matter?1. Large target masses would be possible.

• Multi ton chambers were built in the 50’s- 80’s.

2. An exciting menu of available target nuclei.

No liquid that has been tested seriously has failed to work as a bubble chamber liquid (Glaser, 1960).

• Most common: Hydrogen, Propane

• But also “Heavy Liquids”: Xe, Ne, CF3Br, CH3I, and CCl2F2.

• Good targets for both spin- dependent and spin-independent scattering.

• Possible to “swap” liquids to check suspicious signals.

3. Low energy thresholds are easily obtained for nuclear recoils.

• < 10 keV easy to achieve according to standard nucleation theory.

4. Backgrounds due to environmental gamma and beta activity can be suppressed by running at low pressure.

• This effect used for superheated droplet detectors (SIMPLE, PICASSO).

Why Do Liquids Not Always Boil When They Pass into the “Vapor” Part of the Phase Diagram?

• In the “vapor” region, the equilibrium state is a vapor.

• But liquids have surface tension, so there is an energy cost to create a bubble.

• This energy barrier may be greater than kt.

a metastable (“superheated”) liquid state may continue to exist for some time.

• The liquid will boil violently once the energy barrier to the vapor phase is overcome.

Bubble Nucleation in Superheated Liquids by Radiation (Seitz, “Thermal Spike Model”, 1957)

Rc

Pvapor

Pexternal

particle

Surface tension

• Pressure inside bubble is equilibrium vapor pressure.• At critical radius Rc surface tension balances pressure.

• Bubbles bigger than the critical radius Rc will grow, while smaller bubbles will shrink to zero.• Boiling occurs when energy loss of throughgoing particle is enough to produce a bubble with radius > Rc

Rc

Ec

Radius

Work

RC 2

vaporP externalP

dE/dX Discrimination in a Small Propane Chamber

plateau

protons

electrons

Waters, Petroff, and Koski, IEEE Trans. Nuc. Sci. 16(1) 398-401 (1969)

Plot of event rate vs. “superheat pressure” (= vapor pressure - operating pressure)

(psi)

Bubble Chambers As WIMP Detectors

• The classical bubble chambers operated in the 50’s-80’s would not have been useful for dark matter detection, because they were only sensitive for a few msec at a time.

Example: Operating pressure cycle for SLAC 1-meter hydrogen chamber.

Ballam and Watt, 1977.

~ 10 msec

recompression decompression

Bubble Nucleation in Cracks

Liquid 0.1 m

Solid

nucleation sites• Trapped gas volumes in surface imperfections are now known to be the primary source of nucleation.

• Most (all?) construction materials have rough surfaces at scales below 1 m, but some materials much better than others.

• Historically, problem was overcome for high energy physics experiments by rapid cycling of chamber in sync with a pulsed beam. Bubbling at walls was tolerated because of finite speed of bubble growth.

• A few small “clean chambers” (~10 ml) were built in the 50’s and 60’s, with sensitive times ~1 minute.

Ways to preserve superheated state:

• Elimination of porous surfaces in contact with superheated liquid.

• Precision cleaning to eliminate particulates.

• Vacuum degassing.

•… a few other tricks borrowed from chemical engineers

Prototype High-Stability Bubble Chamber

3-way valve

Propylene glycol bufferliquid prevents evaporation of superheated liquid.

Acoustic sensor

pressuresensor

filter

gas

glycol

gas

Exhaust to Room

Compressed Air @ 140 psi

air

liquid

glycol

Quartz pressure vessel

Glass dewar with heat-exchange fluid

Piston

Camera(1 of 2)

Possible Target Liquids

Mass Fractions Density

Boiling

Point

@ 1 atm

Comments

CF3Br

8% C (Z=6)

38% F (Z=9)

54% Br (Z=35)

1.5 g/cc -58 CGood for spin-dependent

and spin- independent

couplings.

CF3I

6% C

29% F

65% I (Z=53)

2.1 g/cc -23 CSpin-dependent and

spin-independent

Non- ozone depleting

C3F8

19% C

81% F 1.4 g/cc -37 C Spin- dependent only.

Xe 100% Xe (Z=54) 3.0 g/cc -108 CPossible use in hybrid

scintillating bubble chamber.

High Speed Bubble Chamber Movie1000 frames/ second241Am-Be neutron source

QuickTime™ and aVideo decompressor

are needed to see this picture.

Bad surface Good surface

0

20

40

60

80

100

-30 -25 -20 -15 -10 -5 0

Bromine Nuclear Recoil Threshold (Seitz Theory)

En

erg

y T

hres

hold

[ke

V]

T [degrees C]

7 keV at -10 C

Calibration with Neutron and Gamma Sources

0

1

2

3

4

5

6

7

8

-30 -20 -10 0 10

CF3Br

pres

sure

at n

ucle

atio

n (a

tm)

operating temperature (°C)

full decompression

Br (Ethr

=530 keV)F (E

thr=2.1 MeV)

C (Ethr

=3.1 MeV)

Am/Be

88Y/BeBr (E

thr=7 keV)

F (Ethr

=29 keV)

C (Ethr

=43 keV)

88Y( sensitivity)

solid lines: RS modeldotted lines: Seitz model

• Isotope sources: 241AmBe, 88YBe (neutrons) and 88Y (gammas).

• Calibration based on maximum kinematically allowed nuclear recoil energy.

• Future: monoenergetic accelerator neutrons needed to cleanly measure efficiency.

2 mCi 88Y source

Position Reconstruction

• Bubble positions can be reconstructed in 3 dimensions by scanning images takenby two cameras offset by 90 degrees.

• Position resolution is currently 530 microns r.m.s. (approximately 1/4 bubble diameter)• Uniform spatial distribution of background events, consistent with cosmic ray neutrons.

163 background events (1.5 live days)

Background Rate

• 1 event/ (15 minutes) for 18 grams of CF3Br in sub-basement lab (~6 ft. depth).

• Observed rates are consistent with measured ambient neutron flux.

Neutron Multiple Scattering

• Multiple bubbles are present in approximately 4% of events in our “background” data set.

• These events can only be caused by multiple neutron scattering, since uniform size of bubbles implies simultaneous nucleation at multiple sites.

• Events such as this can be used to measure neutron backgrounds in- situ while searching for recoils due to WIMPs.

Background Due to Nuclear Recoils from Alpha Decay

• Alpha decay produces monoenergetic, low energy nuclear recoils.

For example, consider 210Po->206Pb:

206PbE = 5.407 MeV ER= 101 keV

• The recoiling nucleus will nucleate a bubble in any chamber that is sensitive to the lower energy (~10 keV) recoils expected from WIMP scattering.

• The 238U and 232Th decay series include many alpha emitters, including radon (222Rn) and its daughters.

• Radon is highly soluble in bubble chamber liquids.

Table of Isotopes

Iodine

Rare earth group

Uranium and Thorium Decay Chains

Fluorine

222Rn

prot

ons

neutrons

betas

Bromine

Counting Test Facility: A Prototype for Borexino Solar Experiment

Backgrounds for a Large Bubble Chamber Experiment

Assumptions Total Event Rate

neutrons Deep site with shielding <0.01/ton-day

Gammas & betas “Rejection factor” better than 10-9 <0.1/ton-day

Alphas Best Borexino CTF level

dominated by radon decay

~1/ton-day?

• Fluid handling techniques to be adapted from Borexino solar neutrino observatory.

• Background likely to be dominated by radon and its daughters.

• These levels are ~3 orders of magnitude lower than seen in current generation dark matter experiments.

test site~300 m.w.e.

at Fermilab

Neutron Flux Underground

Heusser, 1995.

NuMi Tunnel

U. Chicago Sub-basement lab

Dominant neutron sources:

• Direct cosmic ray neutrons.

• Neutrons from radioactivity in rock.

• Neutrons made by muons passing through high-Z materials around the detector.

COUPP: Chicagoland Observatory for Underground

Particle Physics(FNAL Test Beam Program T-945)

J. Collar, D. Nakazawa, B. Odom, A. Sonnenschein, K. O’SullivanKavli Institute for Cosmological Physics

The University of Chicago

P. Cooper, M. Crisler, J. Krider, K. Krempetz, C.M. Lei, H. Nguyen, E. Ramberg, R. Schmitt, A. Sonnenschein, R. Tesarek

Fermi National Accelerator Laboratory

Potential Sensitivity Of 1-Liter Chamber at Fermilab Site

Spin-independent Spin-dependent

Goal for this phase: reduce background to <1 event per liter per day

Design Concept for Large Chambers• Central design issue is how to avoid metal contact with superheated liquid.

• Fabrication of large quartz or glass pressure vessels is not practical, but industrial capability exists for thin-walled vessels up to ~ 1 m3 in volume.

Thin- walled quartz bell jar

Steel pressure vessel

Pressure balancing bellows

SUPERHEATEDLIQUID

VIEWPORT

VIEWPORT VIEWPORT

HEAT EXCHANGE FLUID

PISTON

Buffer fluid

Installation of 1 Liter Chamber At Fermilab NuMi Tunnel

• 100 times active mass of test tube chamber.

• Prototypes many design features required for chambers with volume ~1000 liters

Neutron Shielding and Muon Veto• Detector is surrounded by 30 cm of polyethylene neutron moderator.

Very effective for low energy neutrons coming from (,n) radioactivity in rock.Simulations show reduction in rate to < 1/month.

• Active muon veto: 150 plastic scintillator counters from KTEV.• Goal: < 0.1 -induced neutrons/ day in 1 liter chamber, requires >98% efficiency.

To be installed in spring ‘06.

Movie: One of the First Events

QuickTime™ and aPhoto - JPEG decompressor

are needed to see this picture.

160 msec of Video Buffer (20 msec/frame)

Muon Track @ 160 psi Superheat Pressure

Subtracted image

projection of subtracted image onto horizontal axis

projection with noise subtraction

Starting image Zoom In

Image Analysis Procedure

Spatial Distribution of Single Bubbles, First 42 Days

• Duty cycle of chamber allowed 33% live time (14 live days)

Space & Time Distribution of Single Bubbles

Rate in fiducial volume: 25.6 +/- 2.8 per live-day

QuickTime™ and aTIFF (LZW) decompressor

are needed to see this picture.

Number of Bubbles

Cou

nts

Multiple Bubble Analysis• Statistics of multiple bubble events suggests 10% of bulk singles (2.8 /day) are due to neutrons.

• These should be coincident with cosmic rays-- we’ll see when Muon veto is installed.

Rat

e [H

z/L

iter

]

Superheat Pressure [psi]

Goal (1/ day)

First NuMi data point

surface

basement (6 m.w.e.)

Event Rate at NuMi Compared to Surface Rate

Conclusions

We have demonstrated an operating mode for bubble chambers that in principle allows great sensitivity to WIMPs.

Have a mechanical design scalable to ~ton target masses at low cost.

Have built a fully working detector with active mass comparable to best competing experiments.

Demonstrated insensitivity to gamma ray backgrounds at 10-9 level.

Neutron backgrounds are under control, less than 0.1 event/day seems achievable with cosmic veto counters in NuMi tunnel.

Making progress on reducing alpha backgrounds.