Dark matter detection with bubble chamber detectors Miguel Ardid Dark Matter from aeV to ZeV: 3rd IBS-MultiDark-IPPP Workshop 21-25 November 2016 Lumley Castle

DM detection with superheated liquids (bubble nucleation) Direct DM Basics already reviewed A bubble chamber is filled with a superheated fluid in meta-stable state. Energy deposition greater than Eth in a radius less than rc 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: pl pv σ Surface energy Latent heat

Bubble chambers as nuclear recoil detectors Thermodynamic parameters are chosen for sensitivity to nuclear recoils but not electron recoils. Better than 10-10 rejection of electron recoils (betas, gammas). Alphas are (were) a concern because bubble chambers are threshold detectors.

PICO fast compression bubble chambers Buffer fluid (water) Hydraulic fluid Target fluid (CF3I/C3F8) to hydraulic controller Synthetic silica jar Pressure expansion puts fluid (CF3I or C3F8) in superheated state I for spin-independent F for spin-dependent (many fluids possible) Particle interactions nucleate bubbles Cameras see bubbles, trigger Stereo reconstruction of bubble position, multiplicity Acoustics used to identify alphas Recompress to reset chamber

PICO bubble chambers COUPP-4: superheated fluid 4 kg of CF3I Observe bubbles with two cameras and piezo-acoustic sensors. Synchronized measurements of P, T, and control parameters

Measured gamma rejection Bubble nucleation probability from gamma interactions in C3F8 and CF3I 10-10 ~ 3 keV 10-10 ~ 11 keV 7

Acoustic discrimination 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 ~40 μm ~50 nm Daughter heavy nucleus (~100 keV) Helium nucleus (~5 MeV)

Calibration and efficiencies C3F8 In addition to understand the detector and background for Nuclear Recoil Calibration and Efficiency : Am-Be source, but as well Y-Be source and neutron beams for fluorine near threshold recoils Pion beams for Iodine recoils CF3I 9

Dark Matter Bubble Chamber Evolution PICO-60, PICO-40L PICO-500L 10

PICO-60 at SNOLAB

PICO-60 PICO-60 Run-1: June 2013 to May 2014. Filled with 36.8 kg of CF3I. Run-2 with C3F8 target in 2016.

PICO 60 – CF3I SNOLAB Phys. Rev. D, 93 (2016) 052014 Pattern suggests particulate contamination moving on convection currents Events are shifted systematically higher in AP than neutrons. They are systematically louder at larger expansion times. They are higher in the vessel at longer expansion times A background-free region can be identified By cutting on AP, Expansion Time, and Z 48.2% of total exposure Statistical penalties taken into account due to cut optimizations

Silica jar is an exact replica of COUPP-4 jar. PICO-2L Two liter active mass of C3F8. Same size as COUPP-4. Re-uses COUPP-4 location/water shield at SNOLAB. Better fluorine sensitivity than CF3I: Twice the F density. Lower threshold. Improved efficiency. More stable chemistry. Silica jar is an exact replica of COUPP-4 jar.

PICO-2L at SNOLAB Filled with 2.9 kg C3F8. Run 1 Oct 2013 - May 2014. Run 2 in 2015.

PICO-2L Run-1 results Phys. Rev. Lett. 114 (2015) 231302 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.

Low AP backgrounds in PICO 2L “collar” events Higher rate of “wall”, “collar”, and “surface” events ALL low AP candidates within 1000 sec of prior nucleation PICO-2L Surface bubbles low AP events 1 / 3 days Alpha events Z “wall” events R2

Background Source? Particulate Contamination? Activated by Steel Quartz Activated by U, Th contamination This seemed like the best candidate a Suppressed AP 218Po

Low-AP Backgrounds Four Step Program Assay analyze the contamination in the chambers Mitigate Control the particulates in the existing chambers Reproduce add contamination to test chambers reproduce the low-AP events Eliminate Revised Bubble Chamber Design

PICO-2L Run-2 flange First bubble February 27, 2015. Phys. Rev. D, 93 (2016) 061101 flange First bubble February 27, 2015. Physics run started June 12, 2015. Natural quartz inner vessel flange replaced with low-radioactivity fused silica. Assembled and filled with particular focus on minimizing particulate contamination. Technical improvements to cooling and piezo-acoustic sensors (0% sensor failure in Run-2). Bulk event rate consistent with expected neutron background. active camera cooling

PICO-2L Run-1 vs Run-2 (mitigation works) Run-1 3.2keV data 32 live days 9 events: much above the estimated neutron background Run-2 3.2keV data 66 live days 1 event: consistent with the expected neutron background

Present situation: SD WIMP-proton cross-section From LUX latest SD results: Phys. Rev. Lett. 116, 161302 (2016)

The Physics Case for 19F

Plan Going Forward PICO-60 Run 2 (with C3F8) Commissioning almost done Physics Data soon (blind strategy) PICO-40L (RSU: Right Side Up) 2017 Commissioning PICO-500L Proposals for funds submitted this year, decision in 2017 2018 Construction Start

PICO-60 Run 2 C3F8 new, all synthetic quartz vessel doubled the active volume, two extra cameras used Upgraded instrumentation Low Stress Seal Fluid Recirculation Commissioning Now Physics Data in Dec. Large Background Reduction

PICO-60 Run 2

PICO-60 Run 2: First Bubble

PICO-60 Run 2: Multiple Bubbles 25 bubble event during neutron calibration

PICO-40L PICO-60 (RSU) active volume hydraulic Fluid 15oC hydraulic H2O 15 oC “water piston” insulation active volume fused silica piston hydraulic fluid -5oC hydraulic Fluid 15oC C3F8 15oC hydraulic fluid -5oC

Right Side Up design virtues: Superheated/normal transition is maintained by a thermal gradient with no buffer fluid No buffer fluid means no surface tension effects No buffer fluid means no constraint on target fluid Works with any refrigerant, hydrocarbon, even xenon. Thermal gradient is naturally stable All metals at the bottom Cold zone, no boiling to liberate particulate No convection to move particulate up Geometry naturally lends itself to a recirculation loop

Summary PICO-2L and PICO-60 have already produced world-leading limits on Spin-Dependent WIMP-proton interactions with different target fluids. PICO with C3F8 also has excellent Spin-Independent limits on low-mass (few GeV) WIMPs. Also interesting CF3I for large masses . Both experiments observed a population of background events in Run-1 that were mitigated in Run-2. A very exciting plan forward with PICO-60, PICO-40L (RSU) and PICO-500L.

Extra slides

Nuclear Recoil Calibration Iodine recoil calibration is challenging fluorine cross-section dominates neutron scattering iodine largely invisible in neutron source spectrum Solution: 11 GeV pion elastic scattering Fluorine recoil calibration is challenging fluorine is light, typical source well above threshold Y-Be source – very low energy neutrons fluorine resonant scattering – Montreal accelerator

iodine recoil calibration in p- scattering Silicon pixel array 1 cm diameter bubble chamber 12 GeV/c p- test beam 1 mm wall thickness

calibration of iodine recoil energy

Mono-energetic neutron beam

Alpha acoustic calorimetry 222Rn 4 d 218Po 3 m 210Pb 22 y 214Bi 20 m 214Pb 26 m 214Po 164µs α (5.6MeV) α (6.1MeV) α (7.9MeV) β

PICO-2L Acoustic discrimination Acoustic cut 94% acceptance 3 keV threshold Preliminary No multiple bubble events in the low background data Two distinct alpha peaks, clearly separated from nuclear recoils Timing of events in high AP peaks consistent with radon chain alphas, and indicate that the higher energy 214Po alphas are significantly louder (a new effect not seen in CF3I) “Clean” alpha triplets 222Rn α(5.6 MeV) 218Po α(6.1 MeV) 214Po α(7.9 MeV) Preliminary 3 minutes 55 minutes

PICO-60 results (ArXiv 1510.07754) Best SD WIMP-proton cross section limit for large DM mass Interpretation of DAMA signal from nuclear recoils of I ruled out completely

SI Projection

The original right side up chamber Warm active region Waters, Petroff, Koski 1969 10cc Bubble Chamber COUPP turned this design upside-down and added the water buffer to build a larger scale device We need a better way to scale up from this design concept Thermal gradient Maintained in thin neck Cold plumbing region

PICO-500L – PICO-40L SNOLAB cage