0νββ Physics in WbLS. Andy Mastbaum University of Pennsylvania. WbLS Workshop LBNL 17 May 2014

Transcription

1 0νββ Physics in WbLS Andy Mastbaum University of Pennsylvania WbLS Workshop LBNL 17 May 2014

2 Requirements Future detectors must: Reach a sensitivity of 15 mev at the 3σ CL after years of running, according to the NSAC 0νββ report 1 Have a path forward to explore the nondegenerate normal hierarchy 1 See

3 Scintillation Detectors Pros: Straightforward scaling to very large masses External ( surface ) backgrounds removed by fiducialization Well-established detector technology (PMTs in a bucket) Cons: Energy resolution much poorer than solid-state detectors With large volume, 8 B solar ν scattering is a background You need a lot of isotope to reach m ββ 15 mev, so scalability is very important.

4 A Simple Detector Model PMTs Fiducial Volume r r FV Entire volume is filled with isotope-loaded scintillator For 0 kton H 2 O, r = 30 m, r FV 20 m

5 Choice of Isotope The ultimate irreducible background in a 0νββ search is 2νββ. The relative mass sensitivity based on 2νββ alone is approximated by 2 : ( ) 6 m ββ 2 7Q E Q G 2ν M 2ν 2 m e G 0ν M 0ν Xe wins, followed by 130 Te. Without enrichment, Te (34%) is better than Xe (8.9%) gram-per-gram, and is also much cheaper and more readily available in large quantities. 2 S. Elliot & P. Vogel, Ann. Rev. Nuc. Part. Sci., 52 (2002).

6 Signal and Backgrounds Signal Scales with isotope mass 2νββ Scales with energy resolution and isotope mass 8 B ν ES Scales with fiducial volume U/Th chain Reduced by analysis (e.g. coincidence tagging) External Reduced by fiducialization Cosmogenic Reduced by purification µ Reduced by depth Two key parameters: exposure (kg-years) and light yield (photoelectrons/mev)

7 Signal and Backgrounds Count rate (A.U.) νββ (200 mev) 2νββ 8 B ν ES Energy (MeV) Example spectrum with 300 pe/mev (σ E = 3.6%)

8 Scaling with Exposure (mev) ββ 3 nat 1% Te loading 300 hits/mev 90% CL 90% CL, 2νββ only 90% CL upper limit on m 2 Inverted, m 0 1 3σCL Normal, m Te exposure (kg y) 15 mev at 3σ in y 50 tons 130 Te, or 15 kton of scintillator loaded at 1% nat Te. 9

9 WbLS Prospects WbLS is attractive because of the possibility of scaling to very large isotope masses at low cost. New PMTs with fast timing, coupled with a slow scintillator, could allow use of the Cerenkov light: a means of reducing the 8 B solar ν background. Improved constraints on the 8 B shape parameters (from the same detector!) would reduce an important systematic uncertainty.

10 WbLS Prospects The main challenge is a much lower intrinsic light yield; you don t see many water Cherenkov 0νββ experiments. Simulated 0νββ in SNO

11 WbLS Prospects But, measurements at Penn 3 suggest a scintillation yield of 4 Cherenkov is possible with around a 1% scintillator fraction, and this can be tuned. Considering SNO detected about photoelectrons per MeV: 4 2 (coverage) 2 (QE) = 160 pe/mev Energy resolution of 5% is on par with current scintillator experiments. The fraction of Cherenkov light is 5 greater than in Te-loaded LAB, improving timing-based background analysis and making directionality much easier. 3 See S. Grullon s talk from this morning

12 Competitiveness T 1/2 = ln(2) N A M iso nfc 3σ (b) W ɛ t E f iso M iso b T 0ν 1/2 m ββ (%) (%) (tons) (cts/mev ton y) ( 26 y) (mev) SNO SNO CUORE CUORE WbLS WbLS t = y For scintillator, ɛ = 62.5%, W = 130 g/mol 4 S. Biller, TAUP CUORE Collaboration, arxiv:

13 Using Cherenkov Light Probability (A.U.) WbLS Cherenkov Scintillation Time residual (ns) PMT hit timing 1% pseudocumene WbLS timing (not optimized) and water optics 60% of photoelectrons with t < 4 ns are Cherenkov

14 Using Cherenkov Light: Scattering (degrees) θ Angular spread θ ms = 13.6 MeV βcp z ( )] x X 0 [ ln x X 0 z 1 x From NIST ESTAR g/cm 2 (PDG) X β Multiple Scattering ν Elastic Scattering β kinetic energy (MeV) 3 θ ms: J. Beringer et al. (PDG), Phys. Rev. D 86, 0001 (2012). ( 27.3) ν ES: Winter solar 8 B spectrum, Bahcall cross sections

15 Using Cherenkov Light Probability (A.U.) pe/mev 50 pe/mev 90 pe/mev cos θ 1 sun Angular distributions for early hits in 2.5 MeV r 8 B ν elastic scatters in 1%WbLS Vertical lines indicate 50% of 8 B cut

16 Caveats Can t really ignore U/Th chain backgrounds However, δ m ββ 4 δb R&D needed on TeWbLS Loading, optics, and timing Compatibility with other physics goals New backgrounds Optics changes WbLS cocktail re-optimization Phased approach? Potentially difficult to sell: The energy resolution of this approach tends to limit its potential.... If the background is very low, the technique can achieve a sensitive 0νββ bound. NSAC NLDBD report, Re: SNO+ and KamLAND-Zen

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18 A Very Brief 0νββ Summary The Big Questions Does ν = ν? Is L (or B L) a fundamental symmetry of the Standard Model? Is the mass hierarchy normal or inverted? What is the origin of ν mass? Experimental Approach Neutrinoless double beta decay, 0νββ, is currently the only practical experimental probe to search for this L violation. However, it is very rare (T 1/2 > 25 y) and only observable in a handful of isotopes.