COUPP: First ResultsDark Matter Limits From a Bubble

Chamber

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• Non-baryonic Galactic Dark Matter close to a paradigm

(certainly in the minds of many), but yet to be detected.

• ~20-30% Cold (non-relativistic) DM presently favored

(we don’t seem to be able to explain large scale structure

of the universe without WIMPs–Weakly Interacting

Massive particles, relics of early stages)

• Cautious strategy: start by looking first for non-ad hoc

particle candidates, i.e., those already invoked by particle

theories (e.g., neutralino <-> MSSM, axions <-> strong CP problem)

• WIMPs: dominant interaction via low-energy nuclear elastic

scattering, expected rates << 1 per kg of target per day in keV

region.

(local ~0.3-0.4 GeV/cm3, <v>~2-300 km/s, ~< 10-42 cm2).

Supersymmetric WIMPS can have rates as low as 1 recoil/ton/yr!

• The challenge: build cost-effective ton or multi-ton detectors

sensitive exclusively to WIMP-induced nuclear recoils (down to

one a year) and nothing else--not even neutron recoils.

• The scale of things: a 1 kg Ge detector fires in this room at the

rate of rate of ~1 kHz, so we certainly have our work cut out for

us…

Direct Detection of Cold Dark Matter: The Challenge Ahead

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• Detection of single bubbles induced by high – dE /dxnuclear recoils in heavy liquid bubble chambers

• > 1010 rejection factor for MIPs. INTRINSIC (no data cuts necessary)

• Scalability: large masses easily monitored (built-in “amplification”). Choice of three triggers: pressure, acoustic (ultrasound), motion sensing (video)

The COUPP Approach to WIMP Detection:

Threshold also in stopping power,allows for efficient, INTRINSICMIP background rejection

Threshold in deposited energy

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Normally this wouldcause boiling!

Seitz model of bubble nucleation(classical BC theory):

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Conventional BC operation(high superheat, MIP sensitive)

Low degree of superheat, sensitive to nuclear recoils only

Neutron(s) WIMP behaviorMuon 60°C 40°C 40°C

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neutron-induced nucleation in 20 cm3 CF3Br (0.1 s real-time span)Movie available from http://cfcp.uchicago.edu/~collar/bubble.mov

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Stereo view of a typical event in the 2 kg chamber

Spontaneous bulk nucleation rate = exp(-2.5e5)/(kg*day)!! (Tc= 122°C, but we run at ~30-40°C)

The COUPP Approach to WIMP Detection:

Liquid 0.1 mm

Solid

nucleation sites

Surface nucleations are produced by gas-filled voids, but we learned how to control them (cleaning, outgassing, buffer liquid, etc. – see astro-ph/0503398)

• Revisit an old detector technology with improvements leading to extended (unlimited?) stability (ultra-clean BC: not your daddy’s BC)

• Moderate cost, moderate temperature operation, safe chemistry (fire-extinguishing industrial refrigerants), moderate pressure (180 psig)

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The COUPP Approach to WIMP Detection: An old precept: an attack on both fronts

• Excellent sensitivity to both SD and SI couplings (CF3I)

• Target fluid can be replaced (e.g., C3F8, C4F10, CF3Br). Useful for separating WIMP- from n-induced recoils and pinpointing WIMP in SUSY parameter space.

• Short mean free path for n’s => additional n-rejection mechanism

• Single concentration: reducing -emitters in fluids to levels already achieved elsewhere (~10-17) will lead to a complete probing of so-called “natural” SUSY models

Fluorine is best target for SD

Iodine has X3.5 reach of Ge for SI

Spatial distribution of bubbles (~1 mm resolution)

SD SUSY space harder to get to, but there are more robust predictions there (astro-ph/0001511, 0509269, and refs. therein)

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Fluorine is best target for SD(p)

Iodine has 2x reach of Ge for SI at 1 keV

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UC HEP seminar J.I.

Collar Feb. 25

FNAL J.I. Collar Sept

20

test site @~300 mwe

2 kg (1 L) CF3I engineering prototype chamber built at Uchicago, installed May ’05

COUPP @

(Fermilab Test Beam Proposal T945,now experiment E961) Concept similar

to 1st Hydrogen BC

~80% livetime

evolution! …..

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Neutron and Gamma Calibrations in situ:137Cs (13 mCi) Best MIP rejection

factor measured anywhere(<10-10 INTRINSIC, no data cuts)

Blind absolute comparison with expectations(~30% uncertainty in those)

SwitchableAm/Be source (4.9 n/s)

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Low-energy WIMP-likerecoil energy signal used in these calibrations

First and foremost, a DM search needs to separate nuclear recoils from electron recoils.

~10-13! (preliminary)

Other experimentsas references:XENON ~ 10-3

CDMS ~ 10-5

WARP ~ 10-8

• Surface (alpha) rate consistent with measured 50 ppb U and 30 ppb Th in standard quartz

• Tell-tale pressure sensitivity onset (’s)

• Can be rejected, but must be reduced by >10 to allow for >60% live-time in ~50 kg (20 kg data shows promise)

• Addressed via modified etch during vessel manufacture and use of synthetic silica (few ppt)

A look at initial data: Rn-domination

• Rn sources present: viton o-ring, thoriated weld lines.

•Time correlations of bulk events are consistent with

3.1 minute half-life of Po-218. Max likelihood analysis

favors 100% Rn and 100% efficiency to it.

• Addressed by use of metallic gaskets, lanthanated tips for flange welding, custom-made bellows (electron beam welded) and SNO water (~1E-15 g/g U,Th).

Surface Events

Bulk Events

when life gives youlemons…

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First COUPP Limits The bubble chamber is back

New limits exclude the low-mass region favored by a SD interpretation of the DAMA NaI signal(last bastion for a conventional WIMP explanation for DAMA)

First COUPP results…

…coming up…

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Science, Vol. 319, Feb. 15, 2008, pg. 933

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A peek at our promising future (which is here already)

• New run of 1-liter chamber, started on July 30, 2007, with Rn countermeasures in place, has produced better-looking data.

• Some radon introduced during filling -- but has long since decayed to equilibrium

• 2009-10 goals: exploring SD “favored” region for the first time, and producing more competitive SI limits.

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(muon veto coincidence timing at work)

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COUPP (20kg, 2009), muon veto running, 300 mwe (expected sensitivity)

COUPP (2kg, 2007)

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dedicated chamber

• Demonstrated by PICASSOcollaboration using Superheated Droplet Detectors (arXiv:0807.1536).

• Effect arises from much larger number of acoustic sources (protobubbles) along longer ranges: expected to be more dramatic in bulk superheated liquid (bubble chambers).

• Ongoing R&D to expand frequency range and sensitivity of piezo transducers used.

• Could result into a DM detector exclusively sensitive to WIMPs.

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test site~300 m.w.e.

Next step: 20 kg @ 330 mwe, 100 kg target mass at SNOlab

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Next step: 20 kg @ 330 mwe, 100 kg target mass at SNOlab

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Commissioning to take place first in the NUMI tunnel, prior to later deployment even deeper underground

Under construction at FNAL: 60 kg Chamber

Completed water tank veto/shieldfor use with 60 kg device

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Winter 2009 20 kg windowless prototype installation

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Winter 2009 20 kg windowless prototype installation

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Summary:• Easily scalable, room-T

approach to DM detection: 20, 60 kg chambers coming online. 500 kg design to follow.

• ~10-13 intrinsic rejection of MIP backgrounds while fully sensitive to low-E recoils.

• Best SD sensitivity, quickly approaching forefront in SI.

• Promising acoustic /n discrimination method to be investigated

• Deep underground deployment planed for 2010 (Soudan or SNOlab)

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Extra Transparencies

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Physics Reach at Fermilab Site and Beyond

Spin-independent Spin-dependent

~ 1c/kg-d expected from simulated (n). ~ 0.1c/kg-d is for 90% efficient veto.A further reduction to ~0.03 c/kg-d is possible (simulated gallery n’spercolate through 30 cm polyethylene shielding at that level).By then better than 10-15 U,Th needed (World’s best is KAMLAND @ ~10-18).

Background goal for E-961: << 1 events per kg per day

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The First Results (Science 15 February 2008: Vol. 319, no. 5865: pp. 933 – 936.

Buzz-phrase: the bubble chamber is back.)

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Template For the Future:

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• Detection of single bubbles induced by high – dE /dx

nuclear recoils in heavy liquid bubble chambers

• < 10-10 rejection factor for MIPs. INTRINSIC (no data

cuts)

• Scalability: large masses easily monitored (built-in

“amplification”). Choice of three triggers: pressure, acoustic (ultrasound), motion sensing (video)

• Revisit an old detector technology with

improvements leading to extended (unlimited?) stability (ultra-clean BC)

• Excellent sensitivity to both SD and SI couplings

(CF3I)

• Target fluid can be replaced (e.g., C3F8, C4F10,

CF3Br). Useful for separating WIMP- from n-induced

recoils and pinpointing WIMP in SUSY parameter space.

• High spatial granularity = additional n rejection

mechanism

• <$500/kg, room temperature operation, safe

chemistry (fire-extinguishing industrial refrigerants), moderate pressure (<200 psig)

• Single concentration: reducing -emitters in fluids to

levels already achieved elsewhere (~10-17) will lead to a complete probing of “natural” SUSY models

The COUPP Approach to WIMP Detection:

Rates measured inCF3I and C3F8 (vertical bands) tightly constrain responsible SUSY parameter space and type of WIMP(LSP vs. LKKP)

Neutrons on the other hand produce essentially thesame rates in both(n for F and I are very similar)

Can take one’s pick of a fluid – they all superheat!

G. Bertone, D.G. Cerdeno, J.I. Collar, and B. Odom, Phys. Rev. Lett. 99, 151301 (2007).

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The COUPP Approach to WIMP Detection:

• Detection of single bubbles induced by high – dE /dx

nuclear recoils in heavy liquid bubble chambers

• < 10-10 rejection factor for MIPs. INTRINSIC (no data

cuts)

• Scalability: large masses easily monitored (built-in

“amplification”). Choice of three triggers: pressure, acoustic (ultrasound), motion sensing (video)

• Revisit an old detector technology with

improvements leading to extended (unlimited?) stability (ultra-clean BC)

• Excellent sensitivity to both SD and SI couplings

(CF3I)

• Target fluid can be replaced (e.g., C3F8, C4F10,

CF3Br). Useful for separating WIMP- from n-induced

recoils and pinpointing WIMP in SUSY parameter space.

• High spatial granularity = additional n rejection

mechanism

• <$500/kg, room temperature operation, safe

chemistry (fire-extinguishing industrial refrigerants), moderate pressure (<200 psig)

• Single concentration: reducing -emitters in fluids

to levels already achieved elsewhere (~10-17) will lead to a complete probing of “natural” SUSY models

Larger chambers will be “self-shielding”

WIMPsignature:homogeneousdistribution of singles(n-inducedaccumulatetowards theexterior)

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KTeVpaddles to be reused as COUPP-veto

COUPP @NuMI Tunnel(Fermilab Experiment E961)

n flux already dominated by rock radioactivity in a site this deep. Muon veto and 30 cm of polyethylene allow to reach ~0.03 c/kg-day (= CDMS-II sensitivity)

Nucleation rate at surface and shallow UC site (6 m.w.e.) in good agreement with environmental n’s(a first “calibration”) > 100 improvement achieved at NuMI

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LEGEND:green is carbon,red is fluorine,blue is iodine,black is argon.

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WIMP-nucleon SI cross-section of 1 pb

1000 GeV WIMP.

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