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)

Outline  PICASSO principle of detection.  Experimental setup & Data taking.  Data analysis and Results.  Summary. B.Beltran, TAUP 09 Rome 2-jul-09 2

Why Dark Matter? Figure: NASA  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. B.Beltran, TAUP 09 Rome 2-jul-09 3

Dark Matter spin-dependent interaction � M χ M A � 2 Enhancement σ A = 4 G 2 Neutralino ( ) interaction C A χ F factor with matter: M χ + M A ( is the Fermi coupling constant ) G F { Spin-independent interaction C A ∝ A 2 Depending on the type of target nucleus and the neutralino composition C A = 8 π (a p < S p > +a n < S n > ) 2 J + 1 Spin-dependent J ( =eff. WIMP-nucleon coupling, a p , n λ 2 Isotope Spin Unpaired λ 7 Li 3/2 p 0.1100 S p , n =expectation value of the 19 F 1/2 p 0.8630 nucleon spin) 23 Na 3/2 p 0.0110 29 Si 1/2 n 0.0840 73 Ge 9/2 n 0.0026 Fluorine has one of the biggest 127 I 5/2 p 0.0026 enhancement factors ! 131 Xe 3/2 n 0.0147 B.Beltran, TAUP 09 Rome 2-jul-09 4

PICASSO principle of detection Gas Bubble C 4 F 10 Superheated ~x10 2 the droplet volume of the 14cm droplet ~mm 4.5 l acrylic container Nuclear C 4 F 10 WIMP 228 recoil 40cm droplets Signal read by a piezoelectric 100 transducer Piezoelectric 50 Voltage sensor 0 Stainless steel -50 frame -100 0 2000 4000 6000 8000 10000 Time ( μ s) B.Beltran, TAUP 09 Rome 2-jul-09 5

Energy threshold 6  Threshold depends on pressure and temperature. Neutron threshold energy (keV) 5  Minimum threshold of 5 keV achieved at 45 °C 4 and 1.2 atm (=underground pressure). α particles 3  Insensitive to , and . e − γ µ WIMPs  Relevant backgrounds: 2 γ , μ , e -  α particles from U/Th contamination in the 1 gel → very stringent detector fabrication 0 requirements.  fast neutrons → 30 cm of water shielding. − 1 0 5 10 15 20 25 30 35 40 45 50 55 60 Temperature ( ° C) Legend: ◆ test beam measurements ⎯ prediction B.Beltran, TAUP 09 Rome 2-jul-09 6

Detector response 226 Ra spiked detector (5.6 1.275 MeV γ from a 22 Na MeV α particles) data points calibration source fit with fit with a continuous line a dashed line Threshold Energy (keV) 2 -1 10 10 10 1 Detector Response 1 0.8 0.6 0.4 0.2 0 20 40 60 0 Temperature ( C) fast neutrons from a AmBe expected response from calibration source (points) compared a 50 GeV/c 2 WIMP to MC a simulation (dotted line) B.Beltran, TAUP 09 Rome 2-jul-09 7

PICASSO international collaboration Czech Technical Université de Montréal Indiana University CAPP, at Saha University South Bend Institute for in Prague Nuclear Physics University of Alberta Laurentian University Queens University ~35 participants from 4 different countries SNOLAB B.Beltran, TAUP 09 Rome 2-jul-09 8

Experimental setup at SNOLAB  Located in Sudbury, Canada.  2070m deep, ~6000m.w.e. PICASSO at  0.29 muons m -2 d -1 SNOLAB  ~4000 fast neutrons m -2 d -1  30.5cm edge-length  32 detectors water cubes serve as neutron moderator and underground. shielding  Installation completed ~2.5m  10 -5 c/h/g neutron end 2008. counts, two orders of  2 kg of 19 F in total. magnitude below the alpha background rate. B.Beltran, TAUP 09 Rome 2-jul-09 9

Data taking summary  By applying pressure we can recover the Pressure evaporated droplets  40h of data taking are followed by 15h of 6bar pressure runs. 228  8 thermally and acoustically insulated boxes 56cm hold 4 detectors each.  Temperature control units with ±0.1 °C accuracy. m c 5 6 65cm B.Beltran, TAUP 09 Rome 2-jul-09 10

Data analysis (pvar) Power Power Cumulative Sum Cumulative Sum Absolute Difference Absolute Difference 3 6 6 × 10 × 10 × 10 200 150 2000 150 100 100 1000 50 50 0 0 0 0 2000 4000 6000 8000 0 2000 4000 6000 8000 0 2000 4000 6000 8000 Time ( µ s) Time ( µ s) Time( µ s) 600 0 41 pvar= ln(integral of 500 the difference 400 histogram) 300 Excellent separation between physics 200 events and acoustic/ 100 electronic 0 background events 9 9.5 10 10.5 11 11.5 pvar B.Beltran, TAUP 09 Rome 2-jul-09 11

Pvar: temperature dependent cut 1.2 Normalized Counts det 71: y=7.134664 + (0.115096) x + (-0.000860)x^2 11 0 0 0 0 0 det 72: y=6.920494 + (0.160071) x + (-0.001473)x^2 0 35 20 25 30 38 41 Detector 72 1 0.8 PVar 0.6 10 Detector 71 0.4 0.2 9.1 0 9 9.5 10 10.5 11 20 25 30 35 38 41 pvar Temperature °C B.Beltran, TAUP 09 Rome 2-jul-09 12

Data analysis (fvar)  Extra information in the Fourier Transform of the wave B A  Discrimination variable by adding the power of the signal in regions A and B. B.Beltran, TAUP 09 Rome 2-jul-09 13

Data analysis: fvar vs pvar Pvar(T) cut Physics events 3 fvar 2.5 Acoustic/ Electric noise 2 1.5 B A Fvar cut 1 0.5 B C Legend: + neutron calibration 0 0 0.5 1 1.5 2 2.5 alpha events pvar B.Beltran, TAUP 09 Rome 2-jul-09 14

Data analysis summary Detector 71 Detector 72 Live time (days) 101.5 103.5 Active Mass 65.06±3.2 68.97±3.5 ( 19 F grams) Exposure (kgd) 6.60 7.14 Total number of 7322 5784 events before cuts Total number of 1555 496 events after cuts B.Beltran, TAUP 09 Rome 2-jul-09 15

Count rate from detectors underground ) -1 Reference detector used h -1 Event Rate (g to define the response to 0.08 the alpha background 0.06 0.04 0.02 Detector 71 and 72 used in this analysis 20 25 30 35 40 45 0 Temperature ( C) ) -1 h 0.015 -1 Detector 71 null Event Rate (g hypothesis test χ 2 /ndf=1.15 0.01 Detector 72 null WIMP response ( σ p= 1pb) to: 0.005 hypothesis test 10 Gevc -2 WIMP χ 2 /ndf=1.25 30 Gevc -2 WIMP 20 25 30 35 40 45 100 Gevc -2 WIMP 0 Temperature ( C) B.Beltran, TAUP 09 Rome 2-jul-09 16

Data analysis: systematic errors Uncertainty (1 σ ) σ p uncertainty Systematic source Active mass 5% 5% Energy resolution 20% 3% parameter Atmospheric pressure 3% 1% changes Pressure gradient ±2% ±1% within a detector Energy scale 3% 2% Temperature ±0.1°C 0% 2.5% acceptance Cuts 3% 1.5% definition B.Beltran, TAUP 09 Rome 2-jul-09 17

PICASSO 09 results: cross section to proton 2 10 XENON10(08) ) 5 0 ( O S S A C I P COUPP(08) 10 ) 5 CDMS(08) 0 ZEPLINIII(09) ( E L P M ) 5 I 0 S ( D A I A N 1 sensitivity with 2 detectors ) 7 0 ( S M and a total exposure of I K (pb) 13.75 kgd DAMA/LIBRA(08) PICASSO(09) -1 σ p =0.16 pb (90% C.L.) 10 SD p at 24 GeVc -2 � -2 10 SUPERK(04) -3 10 ICECUBE-HARD(09) arXiv:0907.0307 -4 10 3 2 10 10 10 2 WIMP mass (GeV/c ) B.Beltran, TAUP 09 Rome 2-jul-09 18

PICASSO 09 results: a n -a p plot 4 0 CDMS-II 1 N EDELWEISS O 3 N E X 2 DAMA/LIBRA 1 COUPP n 0 a KIMS 9 ) 0 ( O S S A C I P -1 N A I A D -2 ZEPLIN-III (Bonn CD) ZEPLIN-II -3 -4 -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 a p B.Beltran, TAUP 09 Rome 2-jul-09 19

Discovery of alpha n-recoil discrimination effect Recoils 0.006 Detector 93 0.025 Neutron counts/50 mV h –1 g –1 Alpha counts/50 mV h –1 g –1 0.005 Alphas 0.020 0.004  PICASSO has discovered a significant 0.015 0.003 difference in amplitudes between alpha- 0.010 0.002 particles and nuclear recoils. 0.005 0.001  This proves that signals carry information 0 0 about the first moments of bubble formation. 0 500 1000 1500 2000 Amplitude (mV)  This effect can significantly increase the sensitivity for experiments based on Nuclear recoils have very superheated liquid technique. short tracks: one nucleation site within the  Feature not exploited in the present analysis droplet due to saturation effects. Alphas can have more nucleation sites, each one contributes to the total amplitude. NJP 10 (2008) 103017. B.Beltran, TAUP 09 Rome 2-jul-09 20

Summary  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 σ p =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 21

Picasso in the news B.Beltran, TAUP 09 Rome 2-jul-09 22