Exploring Majorana landscape J.J. Gómez-Cadenas Instituto de Física Corpuscular (CSIC & UVEG) Florence, July, 2012 miércoles 11 de julio de 12
Double beta decay 10 Atomic Mass Difference (MeV) 136 I 8 136 Pr - � 6 + /EC � 4 136 La 136 136 Cs 136 2 Xe Ce - � + � /EC - - � � 0 136 Ba 52 53 54 55 56 57 58 59 60 Atomic Number Z About 10 isotopes, A ~70-150 Q bb ~2-3.5 MeV T 1/2 ~10 18 -10 20 yr. miércoles 11 de julio de 12
Neutrinoless Double Beta Decay T 1 / 2 ∼ 10 18 − 10 20 y If the neutrino is a Majorana ββ 2 ν particle the process called Neutrinoless Double Beta Decay may exist In bb0n, no neutrinos are emitted. T 1 / 2 > 10 25 y The sum of the energies of the two electrons equals the mass difference between mother and daughter nuclei (Qbb). ββ 0 ν The process requires an helicity flip, and therefore it becomes more likely as the neutrino mass increases. miércoles 11 de julio de 12
DBD and neutrino mass 1 / 2 ) − 1 = G 0 ν ( Q, Z ) | M 0 ν | 2 m 2 ( T 0 ν ββ (eV) Ν Μ Ν e Ν Τ Excluded by Cosmology 1 � � m sin 2 Θ 23 sin 2 Θ 12 3 2 sin 2 Θ 13 2 � m sol Degenerated neutrinos 2 1 � m atm -1 10 sin 2 Θ 12 2 � m atm 2 2 � m sol sin 2 Θ 23 Inverse hierarchy 1 3 sin 2 Θ 13 NORMAL INVERTED -2 10 Normal hierarchy EXO sets a limit of -3 10 -3 -4 -2 -1 10 10 10 10 1 T 1 / 2 ( Xe 136 ) = 1 . 6 × 10 25 yr (90% CL) m (eV) light Corresponding to m ββ ∼ 140 − 380 meV � � X m i U 2 m ββ = � � ei � � i miércoles 11 de julio de 12
NME Industry 8 GCM IBM 7 ISM QRPA(J) 6 QRPA(T) 5 M 0 ν 4 3 2 1 0 76 82 96 100 128 130 136 150 A Gomez-Cadenas et al. , JCAP 1106 (2011) 007 miércoles 11 de julio de 12
Current experimental situation (eV) 26 26 10 10 Excluded by Cosmology 0.2 1 0.3 � � 0.2 m 0.2 0.2 0.4 0.3 KK&K 68% CL 0.3 0.5 Degenerated neutrinos 0.3 Heidelberg- Ge (yr) 0.3 0.4 0.6 -1 10 Moscow 0.4 RQRPA-1 QRPA-2 0.7 90% CL GCM 25 25 0.5 10 10 0.4 0.4 NSM 0.8 76 0.5 90% CL 68% CL 1/2 0.9 0.6 Inverse hierarchy 0.5 0.5 T 1 0.7 IBM-2 KamLAND-ZEN 90% CL 0.6 0.6 EXO-200 (this work) 0.8 -2 10 0.9 0.7 0.7 1 0.8 0.9 Normal hierarchy 24 24 10 10 24 25 26 10 10 10 -3 10 -3 136 -4 -2 -1 T Xe (yr) 10 10 10 10 1 1/2 m (eV) light EXO result has excluded the claim of KK for all but one of the NME’s sets. It appears that KK claim will not hold water. GERDA should settle the matter soon. The region of m bb between 50-300 meV corresponds to the so-called degenerated hierarchy. EXO: T 1/2 ~10 25 y (m bb ~150 meV). To reach m bb ~50 meV needs T 12 ~10 26 y miércoles 11 de julio de 12
Ideal bb0nu experiment Get a large mass of double beta decay source (N = MtN A /A). Measure the energy of the emitted electrons. Select those with (T1+T2)/Qbb = 1 Count the number of events and calculate the corresponding half-life. T 1 / 2 = log 2 N A Mt A N ββ miércoles 11 de julio de 12
Energy resolution If detector resolution is not perfect, energy spike around Q becomes a Gaussian. Background events, both from bb2n process and from U & Th radioactive chains will leak into the Gaussian region (the ROI) Everything else (radiopurity, extra handles) being the same, experiments with superb energy resolution are preferred, to minimize the impact of background events. miércoles 11 de julio de 12
� Radioactive background Energy (keV) counts/channel/day Signal is in this region! channel Due to the radioactive chains of U and Th. Earth is a very radioactive planet. About 1 gr of U and 3 gr of Th per ton of rock. Lifetime of U-238 is 4.5 × 10 9 yr to be compared with a signal lifetime of ~10 26 (10 27 ) yr miércoles 11 de julio de 12
Radioactive background in Xenon A.U. Tl208 Bi-214 Bi214 80000 Signal Tl-208 70000 60000 Arbitrary normalization 50000 Assumed resolution: 1% 40000 Qbb 30000 20000 10000 0 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 Energy (MeV) Main backgrounds: high-energy gammas from Tl-208 and Bi-214 (natural radioactivity in detector materials). miércoles 11 de julio de 12
Signal & Background Imagine that signal is at a level of T1/2 ~10 25 y. How A.U. Tl208 Bi214 80000 Signal many events per year and kg? 70000 60000 50000 T 1 / 2 = log 2 N A Mt 40000 A N ββ 30000 20000 10000 N ββ ( Xe 136 ) = log 26 · 10 23 · 10 3 ( g ) · 1( y ) 0 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 ∼ 0 . 1 Energy (MeV) 10 25 · 136 Need a large mass to see the signal (100 kg yr for T 1/2 =10 25 y) Imagine that natural background for Bi-214 line is of the order of 1muBq. This is one count of background every 10 6 seconds. One year has 3 10 7 seconds. Thus 30 counts of Bi-214 go into the Bi-214 peak which overlaps at 50 % with signal peak. B/S ~1!!! miércoles 11 de julio de 12
Improving T is difficult M ⋅ t − 1 ∝ a ⋅ ε ⋅ T 1/2 Assume a, ε and Δ E Δ E ⋅ B constant. K ➝ isotope yield To improve T1/2 by 10 one ε ➝ detection efficiency needs to: a) Increase Mt by 10 2 or decrease B by 10 2 or M ➝ isotope mass increase Mt by 10 and decrease B by 10. t ➝ running time b ➝ background rate CHALLENGE: Build a Δ E ➝ energy resolution detector with 10 times the mass and 10 times less background. miércoles 11 de julio de 12
Improving m is VERY difficult ⇣ b ∆ E ⌘ 1 / 4 p m ββ = K 1 / ε Mt Today: 150 meV. Degenerated: 50 meV. Inverse: 20 meV First jump: Improve ( ) by 3 4 ~100 Second jump: Improve by 6 4 ~1000 EXO: ~100 kg yr: Fist jump: 10,000 kg yr, second jump: 100,000 kg yr No go, unless one reduces B at the same time by a factor 10 (100). miércoles 11 de julio de 12
Calorimeters/Bolometers M ⋅ t − 1 ∝ a ⋅ ε ⋅ T 1/2 Δ E ⋅ B Δ E ➝ 0 initial reach OK b ➝ S/V large, alpha particles. M ➝ expensive an modular (no scale) b ➝ Can improve by a factor 10 with advanced techniques. Main limitation: S/V and Mass. miércoles 11 de julio de 12
Low resolution calorimeters Enriched xenon dissolved in liquid scintillator. Poor resolution, 10% FWHM at the Q -value. Easy to pile up large mass Difficult to control backgrounds (K-ZEN initial run 10 2 larger than expected) miércoles 11 de julio de 12
Modular/non-homogenous Thin source foil (Se-82) within a tracking chamber surrounded by a calorimeter. Mediocre resolution, 4% FWHM at Q- value. Low efficiency (~30%). Extra handles (tracks) miércoles 11 de julio de 12
No-go technique Price & effort scales linearly Backgrounds (proportional to surfaces) scale linearly Not homogenous detector Not suited even for current modest scale Best feature of detector: Propaganda. miércoles 11 de julio de 12
The TPC detector Time Projection Chamber : invented by D. Nygren in the 1970’s. Can be seen as an electronic bubble chamber. requires a noble gas to operate E μ charged particles traversing TPC ionize gas leaving a track e - reado If track stops inside TPC then its energy is calorimetrically measured (with good resolution) Large volume possible (thus large mass) No surfaces in fiducial volume for background ions to attach to miércoles 11 de julio de 12
Imaging Xe chambers high-pressure gaseous Xenon (HPGXe) or LXe Ionization and Scintillation in Xenon can be recorded in Xe chambers miércoles 11 de julio de 12
Xe chambers are an homogenous detector M ⋅ t − 1 ∝ a ⋅ ε ⋅ T 1/2 Δ E ⋅ B Source = Detector Large TPC Mass goes with L 3 Large mass and good fiduciality Backgrounds scale with L 2 . Improve (doing nothing) as you make it larger. miércoles 11 de julio de 12
Easy to enrich to >90% M ⋅ t − 1 ∝ a ⋅ ε ⋅ T 1/2 and “cheap”. Δ E ⋅ B Isotope Natural Abundance (%) t 48 Ca 0.2 76 Ge 7.8 82 Se 9.2 96 Zr 2.8 100 Mo 9.6 110 Pd 11.8 116 Cd 7.5 124 Sn 5.6 130 Te 34.5 136 Xe 8.9 150 Nd 5.6 miércoles 11 de julio de 12
EXO-200 200 kg of liquid Xenon TPC ~4 % FWHM at Qbb 70% efficiency (hard fiducial cut needed for self-shielding) Bkgnd --> ~ 10 -3 c/(kg kev y) Large mass easy to achieve (liquid Xenon is very dense). Strong points: compact, self-shielding, mass, scalability. Weak points: mediocre resolution, low efficiency for effective shielding. miércoles 11 de julio de 12
EXO-Limitations Expects 4 events in 1 sigma. 35 MS 30 25 counts /20keV Observes 4. 20 15 10 Got lucky. 5 0 8 SS Hard to improve with 6 counts /20keV exposure (could get worse). 4 2 Limitations: energy resolution lack of extra 0 2000 2200 2400 2600 2800 3000 3200 energy (keV) handles, expensive self- shielding. miércoles 11 de julio de 12
GRAXE: A concept to improve LXe reach GraXe is an spherical TPC. Conceptually Cu container identical to EXO. ropes graphene balloon But EXE isolated from background by a inner volume buffer of pure NXE (no radioactive (enriched LXe) photosensors background) cryostat outer volume (natural LXe) EXE enclosed in a graphene baloon that lets UV light through (also perfect metallic conductor, for spherical TPC). 100 Sci Sci+Ion 20 tons of NXE will kill PMT radioactive 80 background (and make up for a nice DM experiment) 60 m ββ (meV) 1 ton extremely isolated EXE. 40 Sci only (mediocre due to poor resolution: KZEN is a no-go in the long run) 20 0 100 1000 10000 exposure (kg year) miércoles 11 de julio de 12
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