Dark Ma'er Search Results from PICO- 60 and PICO- 2L

Transcription

1 Dark Ma'er Search Results from PICO- 60 and PICO- 2L Russell Neilson, Drexel University TAUP 2015, Torino, September

2 PICO bubble chambers COUPP- 4: superheated fluid 4 kg of CF 3 I Observe bubbles with two cameras and piezo- acoustic sensors. TAUP, September 7, 2015 Russell Neilson 2

3 Bubble chambers as nuclear recoil Thermodynamic parameters are chosen for sensitivity to nuclear recoils but not electron recoils. Better than rejection of electron recoils (betas, gammas). Alphas are (were) a concern because bubble chambers are threshold detectors. detectors TAUP, September 7, 2015 Russell Neilson 3

4 Gamma background rejection Bubble nucleation probability from gamma interactions in C 3 F 8 and CF 3 I 10 2 Gamma Rejection Preliminary CF 3 I (various) CF 3 I fit C 3 F 8 various C F fit 3 8 Probability Threshold (kev) TAUP, September 7, 2015 Russell Neilson 4

5 Discovery of acoustic discrimination against alphas (Aubin et al., New J. Phys. 10:103017, 2008) Alphas deposit their energy over tens of microns. Nuclear recoils deposit theirs over tens of nanometers. In PICO bubble chambers alphas are several times louder. Observable bubble ~mm Acoustic discrimination ~40 μm ~50 nm Daughter heavy nucleus (~100 kev) Helium nucleus (~5 MeV) TAUP, September 7, 2015 Russell Neilson 5

6 Three new results PICO- 60 (experiment formerly known as COUPP- 60) preliminary DM results. PICO- 2L Run DM result. PICO- 2L Run- 2 Status report from an ongoing run. TAUP, September 7, 2015 Russell Neilson 6

7 PICO- 60 Filled with 36.8 kg of CF 3 I. PICO- 60 Run- 1: June 2013 to May Run- 2 with C 3 F 8 target in TAUP, September 7, 2015 Russell Neilson 7

8 PICO- 60 at SNOLAB TAUP, September 7, 2015 Russell Neilson 8

9 PICO- 60 results Counts Unknown background Alphas AmBe source data WIMP search data WIMP search data show ~2000 background events of unknown origin, with mean Acoustic Parameter (AP) slightly above nuclear recoil peak ln(ap high ) TAUP, September 7, 2015 Russell Neilson 9

10 The unknown background 10 2 Unknown background Alphas Alphas Unknown background Rate [counts/day] Expansion time [s] The unknown background distributions are markedly different from alphas (and Dark Matter). TAUP, September 7, 2015 Russell Neilson 10

11 Combined PICO- 60 cuts AP bin (equal exposure) Expansion time bin (equal exposure) Z [mm] R 2 /R jar [mm] 48.2% combined efficiency, and no surviving background events TAUP, September 7, 2015 Russell Neilson 11

12 PICO- 60 preliminary limits SI WIMP nucleon cross section [cm 2 ] CDMS II COUPP 4 (2012) DarkSide 50 PICO 60 (prelim.) XENON100 LUX WIMP mass [GeV/c 2 ] SD WIMP proton cross section [cm 2 ] CMS PICO 2L ATLAS PICASSO (2012) XENON100 SK (soft) SIMPLE cmssm WIMP mass [GeV/c 2 ] COUPP 4 (2012) SK (hard) IceCube (χχ bb) PICO 60 (prelim.) (χχ W + W ) Statistical penalty for setting cuts on data is calculated with Monte Carlo (similar to Optimum Interval method: S. Yellin, Phys.Rev. D66 (2002) ) We re currently upgrading PICO- 60 for a new run with C 3 F 8 target. TAUP, September 7, 2015 Russell Neilson 12

13 PICO- 2L Two liter active mass of C 3 F 8. Same size as COUPP- 4. Re- uses COUPP- 4 location/water shield at SNOLAB. Better fluorine sensitivity than CF 3 I: Twice the F density. Lower threshold. Improved efficiency. More stable chemistry. Silica jar is an exact replica of COUPP- 4 jar. Bellows Acoustic Sensors Fused Silica Jar Pressure Vessel To Hydraulic Cart Water (buffer) C 3 F 8 (target) Cameras Mineral Oil (hydraulic fluid) TAUP, September 7, 2015 Russell Neilson 13

14 PICO- 2L at SNOLAB Filled with 2.9 kg C 3 F 8. Run 1 Oct 2013 to May Run 2 started Feb TAUP, September 7, 2015 Russell Neilson 14

15 PICO- 2L Run- 1 results No expected neutron background (<1 event at 3.2keV), and no multiple bubble events observed, so these are not neutron events. There are timing correlations of events at 3.2keV with previous events, therefore not DM or neutrons. We set limits by applying a timing cut with MC- derived statistical penalty, similar to PICO- 60. TAUP, September 7, 2015 Russell Neilson 15

16 PICO- 2L limits SI WIMP nucleon cross section [cm 2 ] DAMA CDMS lite LUX CoGent PICASSO 2012 CDMS Si PICO 2L CRESST II SuperCDMS XENON10/ WIMP mass [GeV/c 2 ] SD WIMP proton cross section [cm 2 ] PICO 2L XENON100 SIMPLE CMS cmssm ATLAS PICASSO (2012) COUPP 4 (2012) SK (soft) SK (hard) (χχ bb) PICO 60 (prelim.) IceCube (χχ W + W ) WIMP mass [GeV/c 2 ] C. Amole et al., Phys. Rev. LeV. 114, (2015) TAUP, September 7, 2015 Russell Neilson 16

17 Background hypothesis Neutron events ParWally contained alpha decays Partially contained alpha decays are slightly louder than nuclear recoil events in droplet detectors. Similar effect expected in a monolithic bubble chamber for alpha decays originating in suspended particulate. Droplet detector: S. Archambault et al.; New J. Phys. 13(2011) Program to test this hypothesis by assaying the DM chambers and reproducing the effect in small test chambers. TAUP, September 7, 2015 Russell Neilson 17

18 Background studies To look for particulate impurities, fluids in PICO- 60 and PICO- 2L were filtered as they were emptied. Filter sample from PICO- 2L buffer fluid. Silica and steel particles found TAUP, September 7, 2015 Russell Neilson 18

19 Background studies Filter sample from PICO- 60 buffer fluid Quartz particle TAUP, September 7, 2015 Russell Neilson 19

20 PICO- 2L Run- 2 First bubble February 27, flange Physics run started June 12, Natural quartz inner vessel flange replaced with low- radioactivity fused silica. Assembled and filled with particular focus on minimizing particulate contamination. active camera cooling Technical improvements to cooling and piezo- acoustic sensors (0% sensor failure in Run- 2). Bulk event rate consistent with expected neutron background. TAUP, September 7, 2015 Russell Neilson 20

21 PICO- 2L Run- 1 vs Run edge events high AP low AP 120 edge events high AP low AP Z (mm) 20 0 Z (mm) R (mm) Run keV data 32 live days Est. neutron bkgd events R (mm) Run keV prelim. data ~51 live days and counwng Est. neutron bkgd events TAUP, September 7, 2015 Russell Neilson 21

22 Next steps PICO- 60 with C 3 F 8 C 3 F 8 target fluid New vessel with fused silica flange Coated bellows to eliminated steel particulate Active fluid recirculation with filtering Right- side- up bubble chamber with no buffer fluid Stable against convection currents Simplified fluid handling/ recirculation. Wider range of possible target fluids SD WIMP proton cross section [cm 2 ] PICO 2L CMS ATLAS PICASSO (2012) COUPP 4 (2012) XENON100 SIMPLE SK (soft) cmssm SK (hard) IceCube (χχ bb) PICO 60 (prelim.) (χχ W + W ) PICO 60 C3F8 (proj.) WIMP mass [GeV/c 2 ] TAUP, September 7, 2015 Russell Neilson 22

23 Summary PICO- 2L and PICO- 60 have produced world- leading limits on Spin- Dependent WIMP- proton interactions with different target fluids. PICO- 2L also has excellent Spin- Independent limits on low- mass (few GeV) WIMPs. Both experiments observed a population of background events we believe is due to particulate contamination of the target fluids. After efforts to reduce the background, Run- 2 of PICO- 2L now shows an event rate consistent with neutron background. PICO- 60 is being prepared for a second run with similar improvements and C 3 F 8 target fluid. TAUP, September 7, 2015 Russell Neilson 23

24 E. Vázquez-Jáuregui I. Lawson C. Amole, M. Besnier, G. Caria, G. Giroux, A. Kamaha, A. Noble M. Ardid, M. Bou-Cabo, I. Felis D.M. Asner, J. Hall D. Baxter, C.E. Dahl, M. Jin, J. Zhang S. Fallows, C. Krauss, P. Mitra P. Bhattacharjee. M. Das, S. Seth E. Behnke, H. Borsodi, O. Harris, R. Neilson A. LeClair, I. Levine, E. Mann, J. Wells R. Filgas, I. Stekl F. Debris, M. Fines-Neuschild, C.M. Jackson, M. Lafrenière, J.I. Collar, M. Laurin, J.-P. Martin, A.E. Robinson A. Plante, N. Starinski, V. Zacek S.J. Brice, D. Broemmelsiek, P.S. Cooper, M. Crisler, W.H. Lippincott, E. Ramberg, M.K. Ruschman, A. Sonnenschein D. Maurya, S. Priya K. Clark J. Farine, F. Girard, A. Le Blanc, R. Podviyanuk, O. Scallon, U. Wichoski

25 Extra slides TAUP, September 7, 2015 Russell Neilson 25

26 Particle detection with bubble chambers A bubble chamber is filled a superheated fluid in meta- stable state. Energy deposition greater than E th in radius less than r c from particle interaction will result in expanding bubble (Seitz Hot- Spike Model). A smaller or more diffuse energy deposit will create a bubble that immediately collapses. Classical Thermodynamics says- p l σ p v University of Maryland, November 5th, 2014 Surface energy Russell Neilson Latent heat 26

27 Alpha acoustic calorimetry 222 Rn 4 d α (5.6MeV) Po 3 m 214 Pb 26 m α (6.1MeV) β 214 Bi 20 m β 214 Po 164µs 1st in chain 2nd in chain 3rd in chain α (7.9MeV) AP 210 Pb 22 y khz TAUP, September 7, 2015 Russell Neilson 27

28 Mono- energetic neutron beam F recoil Threshold C3F8 with water C3F8 with LAB G4 sim with SRIM Bubble Rate (normalized) Max Fluorine Recoil: 11.6 kev TAUP, September 7, 2015 Russell Neilson Seitz Threshold (kev) 28

29 Bubble chamber operation Expand the chamber to the superheated state (10sec). Cameras see the bubble Trigger Stereoscopic position information Recompress the chamber (100msec) and wait 30sec after every bubble. Synthetic silica jar Hydraulic fluid to hydraulic controller Buffer fluid (water) Target fluid (CF 3 I/C 3 F 8 ) TAUP, September 7, 2015 Russell Neilson 29

30 Proposed 250- liter active volume bubble chamber. Designed for CF 3 I or C 3 F 8 target. PICO- 250L TAUP, September 7, 2015 Russell Neilson 30