Neutrinoless double beta decay with 76 Ge Bernhard Schwingenheuer Max-Planck-Institut für Kernphysik, Heidelberg
Standard Model no new physics found at the LHC so far → SM could be valid up to Planck scale? BUT - no dark matter candidate - baryon asymmetry of the universe not explained - dark energy not understood - origin of (tiny) neutrino mass unknown, Majorana particle? IIHE Brussels, 17 Feb 2017 Schwingenheuer, 0νββ with 76Ge 2
Neutrino mass: non-SM effect? possible neutrino mass terms ( ν has no electric charge) C + m R (̄ C ν R + h.c. L Yuk = m D ̄ ν L ν R + m L ̄ ν L (ν L ) ν R ) H H H (ν L ) C (ν R ) C ν R ν L ν L ν R ∆ L=0 ∆ L=2 ∆ L=2 ν L couples to Standard Model W,Z bosons, ν R does not (SM singlet) m D ~ normal Dirac mass term m L , m R new physics eigen vector N ∼ ν R + (ν R ) C ν ~ ν L + ( ν L ) C Majorana particles 2 / m R mass (m L ~0) m R m D in general: expect ν to be Majorana particles → L violation IIHE Brussels, 17 Feb 2017 Schwingenheuer, 0νββ with 76Ge 3
N mass range possible N mass ranges ( little guidance on scale available !) 10 9 – 10 14 GeV: motivated by GUT, can explain baryon asymmetry (lepton asymmetry by CP violation converted via sphaleron to BAU), see-saw: light neutrino mass ~ m D 2 / M R 0.1-few TeV: can explain baryon asymmetry, no hierarchy problem (see below), accessible by LHC GeV: can explain baryon asymmetry if <5 GeV observation e.g. D → Ν µ X with Ν → µ π by SHIP (200 MCHF) 10 keV: (warm+cold) dark matter candidate, N → γ ν decay ~ U 2 m R 5 hint for 3.5 keV line ?? (arXiv:1402.2301, arXiv:1402.4119) eV range: LSND oscillation signal, reactor anomaly, … → SOX, Stereo, … contribute to number of relativistic neutrinos measured by PLANCK neutrino minimal SM ( ν MSM): 1x 10 keV N for DM and 2x ~GeV N for baryon asymmetry, minimal extension of SM IIHE Brussels, 17 Feb 2017 Schwingenheuer, 0νββ with 76Ge 4
How to observe ∆ L=2: 0νββ Look for a process which can only occur if neutrino is Majorana particle 3 2 m i m ββ = ∑ U ei coupling strength ~ i = 1 function of 1 e ,R = e 2 1 5 = - neutrino mixing parameters - lightest neutrino mass 3 ∑ i ,h = 1 m i - 2 Majorana phases U ei E i , h =− 1 i = 1 also possible: heavy N exchange e ,L = 1 2 1 − 5 e = 3 2 / M i → coupling strength ~ ∑ V ei 3 U ei i , h =− 1 m i ∑ E i ,h = 1 i = 1 i = 1 h=helicity IIHE Brussels, 17 Feb 2017 Schwingenheuer, 0νββ with 76Ge 5
Neutrinoless double beta decay masses of A=76 nuclei experimental signature for ββ sum electron energies / Q ββ ”single” beta decay not allowed only ”double beta decay” Note: similar process in principle also (A,Z) (A,Z+2) + 2 e - + 2 ν ∆ L=0 observable at accelerator or reactor or ... but for light Majorana neutrino: (A,Z) (A,Z+2) + 2 e - ∆ L=2 - background too high - flux too low compared to Avogadro N A 0νββ : search for a line at Q value of decay IIHE Brussels, 17 Feb 2017 Schwingenheuer, 0νββ with 76Ge 6
Light Majorana neutrino exchange 2 , ∆ m sol 2 , m min , θ atm , θ sol , θ 13 , 2 Majorana Φ) scan of m ββ ( ∆ m atm 10 -4 .. 10 -3 according to measurements or random (2 Maj. phases) 10 28 yr 76 Ge arXiv:1510.01089 6 10 27 yr 136 Xe NME=latest SM 10 -3 .. 10 -2 including cosmological bound Σ = (22 ± 62) meV 1 10 -2 .. 10 -1 1 true for flat Λ CDM only unless Majorana phases are ”aligned” high m ββ values are more likely to occur IIHE Brussels, 17 Feb 2017 Schwingenheuer, 0νββ with 76Ge 7
LHC vs 0νββ: other mechanisms extensions of SM → other contributions to 0νββ possible, example LRSM LHC might find W R and/or ∆ L=2 process LHC arXiv1508.07286 LHC arXiv:1509.00423 LHC SHIP current 0νββ T 1/2 0νββ 10 27 yr 76 Ge ∆ L=2 at LHC best case: find s.th. at LHC and 0νββ and lepton flavor violation µ → e γ IIHE Brussels, 17 Feb 2017 Schwingenheuer, 0νββ with 76Ge 8
From T 1/2 to m ββ selected 0 νββ isotopes from PRD 83 (2011) 113010 2 Isotope G 0ν [10 -14 y] Q[keV] nat. abund.[%] 2 〈 m ββ 〉 1 4 G 0 ν ∣ M 0 ν ∣ 0 ν = g A 2 T 1 / 2 m e 48 Ca 2.5 4273.7 0.187 76 Ge 0.23 2039.1 7.8 0 T 1 / 2 = measured experimentally 82 Se 1.0 2995.5 9.2 100 Mo 1.6 3035.0 9.6 g A = axial vector coupl. = 1.25 130 Te 1.4 2530.3 34.5 0 = phase space factor ~ Q 5 G 136 Xe 1.5 2461.9 8.9 0 M 150 Nd 6.6 3367.3 5.6 = nuclear matrix element m e enrichment required except for 130 Te, = electron mass not (yet) possible for all, costs differ need M 0ν to understand physics mechanism 0 = ln2 N A N bkg = M ⋅ t ⋅ B ⋅ E and Experiment observes N A ⋅ a ⋅ ⋅ M ⋅ t / T 1 / 2 Experimental sensitivity M = mass of detector T 1 / 2 90 % CL { t = measurement time N A ln 2 A = isotope mass per mole for N bkg = 0 A a ⋅⋅ M ⋅ t N A = Avogadro constant 2.3 a = fraction of 0νββ isotope N A ln 2 M ⋅ t for large N bkg A a ⋅ ε = detection efficiency 1.64 B ⋅ E B = background index in units cnt/(keV kg y) ∆ E = energy resolution = energy window size IIHE Brussels, 17 Feb 2017 Schwingenheuer, 0νββ with 76Ge 9
Expected T 1/2 for different matrix elements newer EDF newer calculations 76 Ge more favorable need all possible experimental and theoretical inputs to reduce uncertainty newer SM newer QRPA 10 28 yr for 20 meV effective mass 0.6 76 Ge decays per t*yr exposure 0.3 136 Xe decays per t*yr exposure (before enrichment fraction & cuts) → background free conditions required No favored isotope arXiv:1610.06548 considering spread of nuclear matrix elements IIHE Brussels, 17 Feb 2017 Schwingenheuer, 0νββ with 76Ge 10
How to reduce background sources: cosmic rays (p,n, µ,γ ) → underground like LNGS neutrons from ( α ,n) and spallation induced by µ α,β,γ from radioactive decay chains 238 U, 232 Th → avoid contamination → screen & select materials like cables, holders → shield (external) radioactivity → example 232 Th activities [ µ Bq/kg] 1000 - steel, <1 - Cu, <1 - water, ~0 liquid argon / org. scintillator → identify background events (multi-dim. selection) → localize interactions (surface events, multiple interactions) identify particle type ( α versus β/γ ) 'measure' all energy depositions (active veto) IIHE Brussels, 17 Feb 2017 Schwingenheuer, 0νββ with 76Ge 11
GERDA: Ge in LAr @ Gran Sasso Phase I (2011-13): lock & glove box 0 ν > 2.1 ⋅ 10 25 yr (90% C.L.) for string insertion T 1 / 2 76 Ge 0νββ decay, PRL 111 122503 Ge detectors Phase II: ( 76 Ge ~ 86%) 2x Ge mass (30 BEGe det.) p + 64 m 3 LAr n + LAr scint. light readout 590 m 3 pure water / Cherenkov veto started end 2015 EPJ C73 (2013) 2330 IIHE Brussels, 17 Feb 2017 Schwingenheuer, 0νββ with 76Ge 12
Phase II start December 2015 7 strings in 9 top PMT nylon cylinder 810 fibers read out by 90 SiPM → 15 ch 7 bottom PMT all Ge + LAr veto ch. 'working' !!! IIHE Brussels, 17 Feb 2017 Schwingenheuer, 0νββ with 76Ge 13
Background reduction: argon veto line at 1525 keV from 42K: deposits up to 2 MeV in LAr → factor ~5 suppression 600-1300 keV: ~97% of events are 2νββ after LAr veto → almost clean sample at Q ββ ~ factor ~2 background reduction (depends on bkg composition, location, ...) IIHE Brussels, 17 Feb 2017 Schwingenheuer, 0νββ with 76Ge 14
Background red.: det. pulse shape BEGe detectors use time profile of detector signal to → identify signal-like evt, proxies = 2νββ & Double Escape Peak of 2615 keV γ pulse shape parameter ( γ + A→ e + e - with 2x511 keV escape) α all α (surface) events removed γ lines suppressed by factor ~6 efficiency γ lines DEP (87.3 ± 0.2 ± 0.8) % 2νββ (85.4 ± 0.8 ± 1.7) % in fit energy window 1930-2190 keV: 1 evt remains + 1.2 x 10 -3 cnt/(keV kg yr) 0.7 − 0.5 bkg ~ ~10x lower than other exp. reach our background goal! IIHE Brussels, 17 Feb 2017 Schwingenheuer, 0νββ with 76Ge 15
New limit (blind analysis) 0 ν > 5.2 ⋅ 10 25 yr (90% C.L.) T 1 / 2 sensitivity = 4.0 10 25 y eventually >1 10 26 yr first background-free experiment in field (<1 evt in FWHM until design exposure for Phase II coaxial + BEGe detectors) IIHE Brussels, 17 Feb 2017 Schwingenheuer, 0νββ with 76Ge 16
Majorana Demonstrator @ SURF 29 kg 76 Ge detectors (87% enr) in conventional copper/lead shield (+15 kg nat Ge detectors) point-contact detectors → rejection surface evt + multiple int. ultra-clean copper (”home made”) + cables + … goal: prove design for ton scale proto-type module: 10 detectors, 2014-2015 Module 1 29 detectors, 2015 first installation running since Jan 2016 Module 2: 29 detectors, running since July 2016 IIHE Brussels, 17 Feb 2017 Schwingenheuer, 0νββ with 76Ge 17
SNO+ default: 0.5% loading → 3900 kg nat Te / 1300 kg 130 Te 780 ton LAB+PPO in Ø12 m acrylic vessel 9500 PMT 7000 ton water IIHE Brussels, 17 Feb 2017 Schwingenheuer, 0νββ with 76Ge 18
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