Results from the PICASSO experiment Berta Beltran, University of - - PowerPoint PPT Presentation

Results from the PICASSO experiment

Berta Beltran, University of Alberta (for the PICASSO collaboration)

B.Beltran, TAUP 09 Rome 2-jul-09

Outline

 PICASSO principle of detection.  Experimental setup & Data taking.  Data analysis and Results.  Summary.

2

B.Beltran, TAUP 09 Rome 2-jul-09

Why Dark Matter?

3  Dark Matter (DM) is needed to explain

gravitational observations taken at many different scales.

 Supersymmetric WIMPs are one possible

DM candidate.

 WIMP particles from our galactic halo

would elastic-scatter with the atoms nuclei, creating a signal that can be detected.

Figure: NASA

GF

σA = 4G2

F

MχMA Mχ + MA 2 CA

B.Beltran, TAUP 09 Rome 2-jul-09

Dark Matter spin-dependent interaction

4

{

CA = 8 π (ap < Sp > +an < Sn >)2 J + 1 J

CA ∝ A2

λ

Isotope Spin Unpaired λ2

7Li

3/2 p 0.1100

19F

1/2 p 0.8630

23Na

3/2 p 0.0110

29Si

1/2 n 0.0840

73Ge

9/2 n 0.0026

127I

5/2 p 0.0026

131Xe

3/2 n 0.0147

ap,n

Sp,n

Enhancement factor Depending on the type of target nucleus and the neutralino composition Spin-independent interaction Spin-dependent

Fluorine has one of the biggest enhancement factors !

is the Fermi coupling constant ) ( =eff. WIMP-nucleon coupling, ( =expectation value of the nucleon spin) Neutralino ( ) interaction with matter:

χ

B.Beltran, TAUP 09 Rome 2-jul-09

PICASSO principle of detection

5

2000 4000 6000 8000 10000

• 100
• 50

50 100

Signal read by a piezoelectric transducer C4F10 Superheated droplet Gas Bubble ~x102 the volume of the droplet 228 Nuclear recoil WIMP ~mm Time (μs) Voltage 40cm 14cm Stainless steel frame Piezoelectric sensor 4.5 l acrylic container C4F10 droplets

B.Beltran, TAUP 09 Rome 2-jul-09

Energy threshold

6  Threshold depends on pressure and

temperature.

 Minimum threshold of 5 keV achieved at 45 °C

and 1.2 atm (=underground pressure).

 Insensitive to , and .  Relevant backgrounds:  α particles from U/Th contamination in the

gel →very stringent detector fabrication requirements.

 fast neutrons →30 cm of water shielding.

γ µ

e−

5 10 15 20 25 30 35 40 45 50 55 60 −1 1 2 3 4 5 6

Temperature (°C) Neutron threshold energy (keV)

α particles WIMPs γ, μ, e- Legend: ◆ test beam measurements ⎯ prediction

B.Beltran, TAUP 09 Rome 2-jul-09

Detector response

7 C) Temperature ( 20 40 60 Detector Response 0.2 0.4 0.6 0.8 1 Threshold Energy (keV)

• 1

10 1 10

2

10

226Ra spiked detector (5.6

MeV α particles) data points fit with a continuous line fast neutrons from a AmBe calibration source (points) compared to MC a simulation (dotted line) expected response from a 50 GeV/c2 WIMP 1.275 MeV γ from a 22Na calibration source fit with a dashed line

B.Beltran, TAUP 09 Rome 2-jul-09

PICASSO international collaboration

8

University of Alberta Laurentian University SNOLAB Queens University Université de Montréal Indiana University South Bend Czech Technical University in Prague ~35 participants from 4 different countries CAPP, at Saha Institute for Nuclear Physics

B.Beltran, TAUP 09 Rome 2-jul-09

Experimental setup at SNOLAB

9  32 detectors

underground.

 Installation completed

end 2008.

 2 kg of 19F in total.

~2.5m PICASSO at SNOLAB

 Located in Sudbury, Canada.  2070m deep, ~6000m.w.e.  0.29 muons m-2d-1  ~4000 fast neutrons m-2d-1  30.5cm edge-length

water cubes serve as neutron moderator and shielding

 10-5 c/h/g neutron

counts, two orders of magnitude below the alpha background rate.

B.Beltran, TAUP 09 Rome 2-jul-09

Data taking summary

10

228 Pressure

 By applying pressure we can recover the

evaporated droplets

 40h of data taking are followed by 15h of 6bar

pressure runs.

 8 thermally and acoustically insulated boxes

hold 4 detectors each.

 Temperature control units with ±0.1 °C

accuracy. 56cm 65cm 6 5 c m

B.Beltran, TAUP 09 Rome 2-jul-09

Data analysis (pvar)

11

s) µ Time ( 2000 4000 6000 8000 50 100 150 200

6

10 ×

Cumulative Sum Cumulative Sum

s) µ Time( 2000 4000 6000 8000 50 100 150

6

10 ×

Absolute Difference Absolute Difference

s) µ Time ( 2000 4000 6000 8000 1000 2000

3

10 ×

Power Power

pvar 9 9.5 10 10.5 11 11.5 100 200 300 400 500 600

41

Excellent separation between physics events and acoustic/ electronic background events pvar= ln(integral of the difference histogram)

B.Beltran, TAUP 09 Rome 2-jul-09

Pvar: temperature dependent cut

12

pvar 9 9.5 10 10.5 11 Normalized Counts 0.2 0.4 0.6 0.8 1 1.2

20 25 30 35 38 41

det 71: y=7.134664 + (0.115096) x + (-0.000860)x^2 det 72: y=6.920494 + (0.160071) x + (-0.001473)x^2

Detector 72 Detector 71 Temperature °C PVar 20 25 30 35 38 41 11 10 9.1

B.Beltran, TAUP 09 Rome 2-jul-09

Data analysis (fvar)

13

A B

 Extra information in the Fourier Transform

• f the wave

 Discrimination variable by adding the

power of the signal in regions A and B.

B.Beltran, TAUP 09 Rome 2-jul-09

Data analysis: fvar vs pvar

14

pvar 0.5 1 1.5 2 2.5 fvar 0.5 1 1.5 2 2.5 3

A B

B C

Physics events Acoustic/ Electric noise Fvar cut Pvar(T) cut Legend: + neutron calibration alpha events

B.Beltran, TAUP 09 Rome 2-jul-09

Data analysis summary

15

Detector 71 Detector 72 Live time (days) 101.5 103.5 Active Mass (19F grams) 65.06±3.2 68.97±3.5 Exposure (kgd) 6.60 7.14 Total number of events before cuts 7322 5784 Total number of events after cuts 1555 496

C) Temperature ( 20 25 30 35 40 45 )

• 1

h

• 1

Event Rate (g 0.005 0.01 0.015

B.Beltran, TAUP 09 Rome 2-jul-09

Count rate from detectors underground

16

Detector 71 and 72 used in this analysis Reference detector used to define the response to the alpha background Detector 71 null hypothesis test χ2/ndf=1.15 Detector 72 null hypothesis test χ2/ndf=1.25

C) Temperature ( 20 25 30 35 40 45 )

• 1

h

• 1

Event Rate (g 0.02 0.04 0.06 0.08

WIMP response (σp=1pb) to: 10 Gevc-2 WIMP 30 Gevc-2 WIMP 100 Gevc-2 WIMP

B.Beltran, TAUP 09 Rome 2-jul-09

Data analysis: systematic errors

17

Systematic source Uncertainty (1σ) σp uncertainty Active mass 5% 5% Energy resolution parameter 20% 3% Atmospheric pressure changes 3% 1% Pressure gradient within a detector ±2% ±1% Energy scale 3% 2% Temperature ±0.1°C 0% Cuts 2.5% acceptance 1.5% definition 3%

B.Beltran, TAUP 09 Rome 2-jul-09 18

)

2

WIMP mass (GeV/c 10

2

10

3

10 (pb)

p SD

• 4

10

• 3

10

• 2

10

• 1

10 1 10

2

10

PICASSO(09)

P I C A S S O ( 5 ) K I M S ( 7 ) N A I A D ( 5 ) COUPP(08) DAMA/LIBRA(08) CDMS(08) ZEPLINIII(09) XENON10(08) S I M P L E ( 5 ) SUPERK(04) ICECUBE-HARD(09)

PICASSO 09 results: cross section to proton

sensitivity with 2 detectors and a total exposure of 13.75 kgd σp=0.16 pb (90% C.L.) at 24 GeVc-2 arXiv:0907.0307

B.Beltran, TAUP 09 Rome 2-jul-09

PICASSO 09 results: an-ap plot

19

p

a

• 2
• 1.5
• 1
• 0.5

0.5 1 1.5 2

n

a

• 4
• 3
• 2
• 1

1 2 3 4

P I C A S S O ( 9 )

DAMA/LIBRA EDELWEISS N A I A D ZEPLIN-II COUPP KIMS CDMS-II X E N O N 1 ZEPLIN-III (Bonn CD)

B.Beltran, TAUP 09 Rome 2-jul-09

Discovery of alpha n-recoil discrimination effect

20  PICASSO has discovered a significant

difference in amplitudes between alpha- particles and nuclear recoils.

 This proves that signals carry information

about the first moments of bubble formation.

 This effect can significantly increase the

sensitivity for experiments based on superheated liquid technique.

 Feature not exploited in the present analysis

due to saturation effects.

Amplitude (mV) 500 1000 1500 2000 Neutron counts/50 mV h–1 g–1 0.005 0.010 0.015 0.020 0.025 Alpha counts/50 mV h–1 g–1 0.001 0.002 0.003 0.004 0.005 0.006

Detector 93

Nuclear recoils have very short tracks: one nucleation site within the droplet Alphas can have more nucleation sites, each one contributes to the total amplitude. Alphas Recoils NJP 10 (2008) 103017.

B.Beltran, TAUP 09 Rome 2-jul-09

Summary

21  All 32 detectors are up and running underground since beginning 2009.  A set of minimun 4 detectors have been continuously taking data since mid-07.  We have analyzed the data collected for 2 years with two detectors, 71 and 72.  With a total exposure of 13.75 kgd, our current limit sets at σp=0.16 pb (90%

C.L) for a WIMP mass of 24 GeVc-2.

 The presented new PICASSO results excludes almost completely the most

recent DAMA/Libra no-channeling allowed region.

 A new discrimination feature has been discovered between the alpha

background and nuclear recoils.

 This effect can significantly increase the sensitivity for experiments based on

superheated liquid technique.

 Currently we are exploring how to upgrade our hardware to fully exploit this

effect.

B.Beltran, TAUP 09 Rome 2-jul-09

Picasso in the news

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