1. Scintillating Bubble Chambers for Direct Dark Matter Detection Jeremy Mock On behalf of the UAlbany and Northwestern Groups 1

2. People Northwestern University: – Eric Dahl, Jianjie Zhang, Miaotianzi Jin, and Dan Baxter University at Albany – Matthew Szydagis, Sean Fallon, and Steven Young 2

3. Motivation Dark Matter Search experiments rely on the ability of the detector technology to discriminate between ER and NR Typical two-phase scintillation detector can measure energy deposited but has discrimination ~ 99.5% (5x10 -3 ) Bubble chamber has 10 -10 discrimination against ER, but has no measure of energy deposited Solution: Combine the technologies 3

4. How a bubble chamber works Metastable fluid, in liquid phase below the vapor pressure If incident particle deposits sufficient energy, bubble formed Threshold detector: – Critical energy – Critical stopping power – Thresholds a function of temperature, pressure Thresholds applied such that detector is insensitive to ER NR events induce bubbles Target liquid is swappable 4 Superheated liquid gas

5. Gammas and Betas 10 -10 Discrimination against electrons! 5 Dahl - March 6, 2015

6. How a noble scintillating detector works Energy deposited in 3 channels – Heat, prominent with NR reduces light and charge more than ER Organic scintillators produce scintillation, but no charge – No organic scintillator TPC Larger for NR Both lead to scintillation Excitation 6

7. Combine the two technologies Have BOTH energy reconstruction AND fantastic rejection of electromagnetic backgrounds Consider: Xenon – 10 7 improvement in discrimination Solves the 85 Kr and neutrino-electron scattering backgrounds – No drift field necessary! Argon – pulse shape discrimination at higher energies, can achieve low thresholds with coupling to bubble technology Fluorinated-organic scintillators – More sensitive to spin-dependent interactions 7

8. Consider a Xenon Chamber A xenon bubble chamber was attempted successfully in the past, but was thought not to work originally, because energy lost into scintillation instead of bubbling Scintillation was initially quenched to see high energy gammas But for dark matter detection, not seeing gammas is a strength! Nuclear recoils will make bubbles without quenching additives PhysRev.102.586 Tracks of high energy gammas from a xenon bubble chamber with 2% ethylene, circa. 1956 IONIZATION or CHARGE SCINTILLATION OR LIGHT PHONONS OR HEAT LUX CDMS CRESST Phys. Rev. Lett. 112, 091303 (2014) Phys. Rev. Lett. 112, 241302 (2014) Initially, use the scintillation and heat channels for energy reconstruction and discrimination 8

9. But what if the ionization could be measured as well? – Unprecedented attempt at capturing all 3 dimensions of discrimination. 9 IONIZATION or CHARGE SCINTILLATION OR LIGHT PHONONS OR HEAT LUX CDMS CRESST Xe bubble chamber Phys. Rev. Lett. 112, 091303 (2014) Phys. Rev. Lett. 112, 241302 (2014)

10. Another Possible Advantage Bubble chamber turns “disadvantage” of Xe into an advantage: decreasing NR S1 + S2 at low energies means more energy is going into heat And heat means bubbles, in a superheated system 10 Excitation LUX LUX DD Dashed – NEST 1.0 Dashed – NEST mod Manzur 2010 Plante 2011

11. Useful as a Test Apparatus At University at Albany: – Chamber is operated at -40C, very different from other experiments – New temperature / pressure profiles can be studied and might have better energy resolution and discrimination – In theory, almost no limit to energy threshold: keep lowering pressure and raising temperature – at the cost of worse ER misidentification, but one can start at orders of magnitude better than LUX/LZ: T = -110 C T = -30 C (FWHM) Bolotnikov 1997 and Nygren 2009 11 NEST

12. Useful as a Test Apparatus PICO bubble chambers currently background limited (arXiv:1503.00008, in press at PRL)arXiv:1503.00008 – Possibly due to alpha particles straggling out of particulate – Other possibilities include chemistry effects, micro- droplets of buffer fluid, … Scintillation signal will identify and discriminate against these backgrounds – Feedback will help standard bubble chambers remove background sources 12

13. Conclusion Scintillating bubble chamber has potential for 10 -10 discrimination against ER at low thresholds, with energy information Have the ability to exchange targets: Everything turns into a bubble chamber if you go low enough in pressure and/or hot enough Useful to test TPC and traditional bubble chamber ideas, even at small scale First prototypes now underway (next talk) 13