Current status and plans of the Gerda experiment R. Mingazheva for the Gerda collaboration University of Zurich 23 Aug 2016 Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 1/12
Motivation for 0 νββ searches. Double beta decay u u A(Z, N) → A(Z+2, N) + 2e- + 2¯ ν d d u d e- W - second order weak SM process can be observed for even-even ¯ ν nuclei if β -decay is forbidden ¯ ν W - e- u d d d u u Odd-odd has been observed for 11 M(A, Z) nuclei Even-even β β + 76 Ge: β + β + ββ T 2 νββ 1 / 2 =(1.926 ± 0.095) · 10 21 yr Q ββ [Eur. Phys.J. C75 (2015), no.9, 416] Z-2 Z-1 Z Z+1 Z+2 Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 2/12
Motivation for 0 νββ searches u u d d u d W - e- ¯ ν R ν L e- W - u d d d u u A(Z, N) → A(Z+2, N) + 2e- Hypothetical non-SM process, ∆L=2 ν = ν , m ν � = 0 ¯ e.g. light Majorana neutrino exchange Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 3/12
Motivation for 0 νββ searches <m ββ > = | � 3 i =1 U 2 ei m i | Ca Zr Nd u u Te Se Cd d d u d Te 1 Mo W - Ge e- Xe ¯ ν R (eV) current bound 1 10 − Xe) ν L e- W - m IH 2 u 10 − d d d u u NH 3 10 − A(Z, N) → A(Z+2, N) + 2e- 3 4 2 1 10 10 10 10 − − − − 50 100 150 m (eV) A lightest Hypothetical non-SM It would shed light on... process, ∆L=2 ν = ν , m ν � = 0 ¯ absolute neutrino mass scale e.g. light Majorana neutrino neutrino nature e.g. Majorana exchange vs. Dirac Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 3/12
How to claim a discovery? Signal/background 2 νββ N sig ≈ Mt · α · ǫ · ln (2) T 0 ν m iso 1 / 2 Events N bg ≈ BI · ∆ E · Mt 0 νββ We claim a discovery with 99.7% CL when: ∆ E N sig ≥ 3 σ bg , where σ bg ≈ � N bg Energy Q ββ � � Mt T 0 ν � 1 / 2 ∝ ǫ 76 Ge detectors � BI · ∆ E High intrinsic purity Background-free case: Q ββ = 2039 keV T 0 ν 1 / 2 ∝ ǫ · Mt Best energy resolution (3-4 keV at Q ββ ) Mt - exposure α - enrichment factor 86% enrichment of 76 Ge ǫ - detection efficiency BI - backgr. index Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 4/12
How to claim a discovery? Signal/background 2 νββ N sig ≈ Mt · α · ǫ · ln (2) T 0 ν m iso We need large scale and background free experiments! 1 / 2 Events N bg ≈ BI · ∆ E · Mt 0 νββ We claim a discovery with 99.7% CL when: ∆ E N sig ≥ 3 σ bg , where σ bg ≈ � N bg Energy Q ββ � � Mt T 0 ν � 1 / 2 ∝ ǫ 76 Ge detectors � BI · ∆ E High intrinsic purity Background-free case: Q ββ = 2039 keV T 0 ν 1 / 2 ∝ ǫ · Mt Best energy resolution (3-4 keV at Q ββ ) Mt - exposure α - enrichment factor 86% enrichment of 76 Ge ǫ - detection efficiency BI - backgr. index Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 4/12
GERmanium Detector Array ( Gerda ) LNGS underground laboratory: covered by 1400m rock reduced muon flux ( ≈ 1 · m − 2 h − 1 ) Background rejection Active veto: Water tank: ⊘ = 10 m LAr cryostat: ⊘ = 4 m Ultra-clean materials High-pure 76 Ge detectors Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 5/12
76 Ge detectors in Gerda Coaxial detectors (from HdM, IGEX) Enriched: 7 detectors, 15 kg total mass 7 strings Natural: 3 detectors, 7 kg total mass Coaxial BEGe Active Active volume volume A p + contact n + contact BEGe (Broad Energy Germanium) detectors (produced by Canberra) 30 detectors, 20 kg total mass higher energy resolution better background events discrimination Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 6/12
Gerda stability control Weekly calibration using 228 Th source Monitoring of energy scale stability [G. Benato. Doctoral thesis] Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 7/12
Gerda stability control U n d e r Z u t h r i e c Weekly calibration using 228 Th h r e g s r p o o u n p source s i b i l i t y o Monitoring of energy scale f t h e stability During the calibration each source is lowered to the required position During the physics data taking sources are kept [G. Benato. Doctoral thesis] shielded in the position above 76 Ge strings Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 7/12
Gerda Phase I. Results Nov 2011 - May 2013 15 kg of 76 Ge Exposure: 21.6 kg · yr ROI: Q ββ ± 5keV Blind analysis N exp = 2.0 ± 0.3 N obs = 3 Profile LL: N 0 ν =0 [Phys. Rev. Lett. 111, 122503 (2013)] No 0 νββ observation BI: 1 · 10 − 2 cts/(keV · kg · yr) T 0 ν 1 / 2 > 2 . 1 · 10 25 yr - world best limit for 76 Ge Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 8/12
Transition to Phase II Upgrades Increased mass of BEGe detectors Reduce close background sources: cleaner materials LAr veto to reject external background: 16 PMT SiPM and optics fiber read out Phase II goals Background < 10 − 3 cts/(keV · kg · yr) Exposure ≥ 100 kg · yr Sensitivity T 0 ν 1 / 2 > 10 26 yr Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 9/12
Phase II. First unblinding BEGe Dec. 2015 - May 2016 exposure: 5.8 kg · yr in the Q ββ ± 25 keV: N bg exp =0.3 N obs =0 BI = 7 +11 − 5 · 10 − 4 Coaxial exposure: 5.0 kg · yr In the Q ββ ± 25 keV: N bg exp =0.8 N obs =2 BI = 35 +21 − 15 · 10 − 4 BI is shown in the units FWHM at Q ββ = 3 – 4 keV cts/(keV · kg · yr) Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 10/12
Conclusion & outlook Achieved lowest ever background: BEGe: − 5 · 10 − 4 cts/(keV · kg · yr) 7 +11 Coax: − 15 · 10 − 4 cts/(keV · kg · yr) 35 +21 No evidence for 0 νββ found 1 / 2 > 5.2 · 10 25 yr (90% C.L.) T 0 ν | m ee | < [160, 260] meV (90% C.L.) Goal: total exposure of 100 kg · yr and sensitivity T 0 ν 1 / 2 > 10 26 yr [Talk at Neutrino 2016] Follow us: it will be published soon! Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 11/12
Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 12/12
BackUp: Neutrino Double Beta Decay Postulated in 1935 First observation in 1980s Can occure if single beta decay is forbidden due to spin-coupling, seen by the pairing term in the semi-empirical mass formula. ≈ (10 18 − − 10 24 ) yr T 2 νββ 1 / 2 Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 1/11
Backup: Signal and background events topology Signal Localized energy deposition within ≈ 1mm in one detector (Single Side Events [SSE]) Background Multiple energy deposition in one detector (Multi Side Events, removed by Pulse Shape Discrimintaion [PSD]) Events with coincident energy deposition in the LAr (active veto) Surface events fast ( p + ) and slow ( n + ) rising signals (PSD) Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 2/11
BackUp: Isotopes DBD Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 3/11
BackUp: Other mechanism of the DBD Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 4/11
BackUp: 3+1 scenario and 0 νββ decay Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 5/11
BackUp: From 0 νββ to neutrino mass Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 6/11
BackUp: NME M 0 ν = <f || O K || i> O K operator, which creates two protons and annihilate two neutrons. Depends on the distance between nucleons, and on their quantum numbers ISM : the interacting shell model QRPA : the quasiparticle random-phase approximation IBM-2 : the interacting boson model EDF : and the energy density functional method Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 7/11
BackUp: LAr background suppression. Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 8/11
BackUp: LAr background suppression. Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 9/11
BackUp: muon flux Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 10/11
BackUp: From mass eigenstates to effective majorana neutrino mass � � 3 � � � U 2 | m ββ | = ei m i , � � � � � � i =1 c jk ( s jk ) = cosθ jk (sin θ jk ). 13 m 2 e iα + s 2 13 m 3 e iβ � � = � � c 2 12 c 2 13 m 1 + s 2 12 c 2 | m ββ | = � �� c 2 12 c 2 13 m 1 + s 2 12 c 2 13 m 2 cos α + s 2 � = 13 m 3 cos β + � s 2 12 c 2 13 m 2 sin α + s 2 �� + i 13 m 3 sin β , � �� � 2 + c 2 12 c 2 13 m 1 + s 2 12 c 2 13 m 2 cos α + s 2 | m ββ | = 13 m 3 cos β � 2 � s 2 12 c 2 13 m 2 sin α + s 2 + 13 m 3 sin β . θ 12 and θ 13 , m 1 , m 2 and m 3 , the two Majorana phases α and β Rizalina Mingazheva — Aug 23 rd , 2016 The Gerda experiment 11/11
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