Exploring Nature of Neutrinos:

Transcription

1 Exploring Nature of Neutrinos: Search for Neutrino-less double beta decay R. G. Pillay Department of Nuclear and Atomic Physics Tata Institute of Fundamental Research Mumbai, India

2 Neutrino A very elusive neutral spin-½ particle Theoretically postulated to be mass-less Essential to complete the family of particles Beta decay A weak radioactive process: age of Sun ~10 10 years Double Beta decay Doubly weak process ~10 20 years Neutrino-less Double Beta decay Unsolved Fundamental Physics Questions Mass and nature of the neutrino Technological Challenges in Experiments

3 What do we know about Atomic Nuclei Size of Atom m Size of Nucleus m m n = MeV m p = MeV m e = MeV Discovery of Radioactivity a, b, g Nucleus A = N + Z; strongly bound system not all N, Z combinations are stable The puzzle of continuum spectra in beta decay n p + e +? MeV (T 1/2 ~ 10 min) : n e (A,Z) (A,Z + 1) + e + n e (A,Z) (A,Z 1) + e + + n e

4 Beta Decay odd Mass and even Mass Isobars

5 Neutrino was introduced by W. Pauli in 1930 (small neutral particle) b-decay theory (weak interaction) by E. Fermi in 1933: (A,Z) (A,Z + 1) + e + ν e (A,Z) (A,Z 1) + e + + ν e

6 Sun Nuclear cycle p-p reaction p + p d + e + + ν e Solar neutrino

7 Parity Violation Experiment In 1956 by Madam C.S. Wu

8 Neutrino and Beta decay Radioactivity Weak interaction Massless neutrino Only left handed exist Parity violation

9 How many neutrinos are present? Relic Neutrinos from Big bang: 330 n/cm 3 ; E n ~ 4 X 10-4 ev Decoupled about 1 min. after the Big bang Neutrinos from Sun: Sun burns through nuclear Reaction 4p 4 He + 2e - +2n e +g E n ~ 0.1 ~ 20 MeV; Flux ~10 12 /cm 2 /s Neutrinos from the Earth Core: Radioactivity at the core of the earth: Power ~16 Terra Watts. Flux ~ 6 X 10 6 /cm 2 /sec.

10 Atmospheric neutrinos: Are neutrinos present? UHE ( ev )Neutrinos : AGN, GRBs Cosmic Ray sources ( > 5 X ev) Explosion of Star: Most of the binding energy released when a neutron star is born is emitted in the form of neutrinos E n ~10-30 MeV, T ~10 / sec

11 man made neutrinos Neutrinos from man made activities: Neutrinos produced by particle accelerators Reactor Neutrinos: E n ~ 4 MeV, Flux ~5 X /sec from a standard nuclear plant.

12 Are you bumping into neutrinos? Every second Your body receives billions neutrinos from the sun 50 billion neutrinos from the natural radioactivity of the earth billion neutrinos from nuclear plants all over the world Typically a neutrino has to zip through people before doing anything. Our body contains about 20 milligrams of 40 K which is radioactive. We emit about 340 millions neutrinos/day, travelling nearly at the speed of light.

13 Double Beta decay Second order weak process Neutrino-less Double Beta decay Neutrino mass Lepton number violation

14 The birth of double beta decay 1935 Maria Goeppert Mayer

15 Double Beta Decay 2 nd Order Weak Process

16 Properties of neutrino? Only left handed neutrino exists Violates parity How many types of neutrino exist? 3 families of leptons (e, ν e ), (μ, ν μ ), (τ, ν τ ) m n > 0? from neutrino oscillations Are n their own antiparticles? Neutrino-less double beta decay

17 Atmospheric neutrinos India connection ν μ /ν e = 2 Detection of muons produced by cosmic ray neutrinos deep underground C.V. Achar, M.G.K. Menon, V.S. Narasimham, P.V. Ramana Murthy and B.V. Sreekantan, Tata Institute of Fundamental Research, Colaba, Bombay K. Hinotani and S. Miyake, Osaka City University, Osaka, Japan D.R. Creed, J.L. Osborne, J.B.M. Pattison and A.W. Wolfendale, University of Durham, Durham Physics Letters 18 (196) 1965

18 What made solar neutrinos disappear? Neutrino oscillation?

19 19 For neutrinos flavor eigenstates can be different from mass eigenstates. In a weak decay one produces a definite flavor eigenstate Then at a later time t: Neutrino Oscillations n n n n n n cos sin sin cos e n n e (0) E L m L P E m E m m E E t E E t P f e f e sin 2 sin ) ; ( 2 2 ] ) ( 2 1 [ sin 2 sin ) ; ( n n n n

20 20

21 Puzzle solved n x e n x e n e d p p e n x d n x p n Neutrinos have mass!!

22 What we know so far? 3 neutrinos (n 1, n 2, n 3 ) n e n n t Neutrino oscillation data neutrino has mass Physics beyond STANDARD MODEL

23 INO site : Bodi West Hills N, E Pottipuram Village Theni District Tamil Nadu State

24 Underground Laboratory Layout The cavern is set under 1589 m peak with vertical rock cover of 1289 m. Accessible through a 2 km long tunnel Cavern - 1 will host 50 kt ICAL detector. Space available for additional 50 kt. Cavern - 2 & 3 available for other experiments.

25 Neutrino-less Double Beta decay Neutrino mass Left handed and Right handed states can be mixed Lepton number violation

26 n: n: LEFT RIGHT Dirac Majorana (1937)

27 Nuclear Double Beta Decay : early history n p +e + n ZA ZA Z 1A Z 1A Z 2A 2nbb : 2 nd order weak interaction normal beta decay (bn) suppressed by Q-value or J First suggested by Maria Goeppert- Mayer (1935) T 1/2 ~ yrs First geochemical observation of DBD T 1/2 ( 130 Te) ~1.2x yrs (Ingram & Reynolds, 1950) First DBD Experimental evidence in laboratory: 82 Se (Elliot et al. 1987)

28 (Z,A) (Z+2,A) + 2e +2n e (Z,A) (Z 2,A) + 2e + +2n e e + (Z,A) (Z 2,A) + e + +2n e 2e +(Z,A) (Z 2,A) +2n e (Z,A) (Z+2,A) + 2e (Z,A) (Z 2,A) + 2e + (b + b + ) e + (Z,A) (Z 2,A) + e + (b + EC) 2e +(Z,A) (Z 2,A) (ECEC) 2nbb Decay Conserves lepton number 2n Q 11 0nbb Decay Violates lepton number conservation 0n Q 5 G. Racah (1937) suggested testing Majorana theory with 0nbb 0nbb : Lepton number violating process occurs if neutrinos have mass and are their own antiparticles

29 Importance of 0nbb only experiment to test the true nature of neutrino whether it is a Dirac or a Majorana particle. enables the measurement of effective neutrino Majorana mass. may provide one of the most sensitive tests of physics beyond standard model implication for asymmetry of matter over anti-matter

30 0n2b [phase-space ( Q 5 )] [Nuclear ME] 2 m n 2 High Q 2b and abundance desirable } Simultaneous emission of two electrons Identification Constancy of the sum energy of the two emitted electrons experiments Emission of two electrons from the same point inside the source. Angular correlation of emitted electrons Sum energy peak High resolution Extremely low event rates very large sources and detector

31 Experimental Methods 2nbb & 0nbb 1. DBD nuclei integral part of detector (active source) 76 Ge, 128,130 Te Calorimetry (temperature, ionization, scintillation) E FWHM /E 0.5 % at 2 MeV 2. Source external to detector (passive source) Tracking of b (in magnetic field) + Vertex + Fast coincidence Poorer E FWHM /E 10 % Background reduction Underground location (reduce cosmic ray background) Careful choice of materials (detector & environs, shielding) Electronic rejection of background events For a conclusive proof, 0nbb measurement in several isotopes is essential

32 Background issues Radioactivity of the surrounding materials (Natural Decay Chains) T 1/2 : 238 U ~ 10 9 yrs, 235 U ~ 10 8 yrs, 232 Th ~ yrs 40 K radiative impurities DBD events T 1/ yrs, overwhelmed by natural background For t << T 1/2 N a = N A (M/A)i number of bb atoms T 1/2 ~ ln2 N a t e / N 0nbb t : measurement time e: detection efficiency N bkg = B(c/kev/t) E t T 1/2 ~ ln2 N A M i e t/ A(B Et) 1/2 N onbb ~ N bkg low energy neutrons induced by Uranium/Thorium activities in the surrounding rocks and concrete by the fission and (a,n) reactions. high energy neutrons interaction of muons in the rocks or with the heavy metal shielding around the detector The neutron background is quite difficult to suppress. Background reduction is very crucial

33 Present Status Many small scale experiments ~ kg T ½ ~ years, <m n > ~0.75 ev A few large scale experiments : Cuoricino, NEMO3 Some of these experiments have capability to upscale Improvement in sensitivity possible by background reduction in some cases Many new experiments are planned/proposed, R&D in progress 2nbb detected in about 12 nuclei, half life measured.

34 Has 0nbb been observed? Heidelberg- Moscow expt. 11 Kg 76 Ge ( ) Controversial!! Recent results from CUORICINO Bolometric detector operated at 8 mk 40.7 Kg 130 Te T 1/ x yrs, m n ~ ev Phys. Lett. B 586, 198 (2004) Mod. Phys. Lett. A21 (2006) 1547, Old data, new pulse shape analysis. t 1/2 = x y (6 σ), m n = 0.32 ± 0.03 ev n = 11±1.8 events T 1/2 1.8 x yrs, m n ev PRL 95, (2005) T 1/2 3 x yrs, m n ev PRC 78, (2008)

35 GERDA, EXO and KAMLAND ZEN 0υββ KamLANDZen + EXO : T 1/2 > y (90% C.L.), m bb < ev (PRL 110 (2013) ) 0υββ GERDA :T 1/2 > y (90% C.L.), m bb < ev (PRL 111 (2013) ) 0υββ KamLANDZen : T 1/2 > y (90% C.L.), m bb < ev (PRL 117 (2016) )

36 Major Upcoming Experiments CUORE ( 130 Te, cryogenic thermal detector) GERDA ( 76 Ge, HPGe detector) MAJORANA ( 76 Ge, HPGe detector) SuperNEMO ( 82 Se or 150 Nd, tracking detector) EXO ( 136 Xe, TPC + Ba + ) KamLAND-Xe ( 136 Xe, liquid scintillator) SNO+ ( 150 Nd, liquid scintillator)

37 Initiative for DBD experiment in India A multi-institutional effort Proposal for an experiment at site of the INO 124 Sn (Q = kev) Sn has T C ~ 3.7 K Electronic specific heat falls off exponentially below T C Only lattice specific heat (~T 3 ) present below ~ 500 mk Z=50 shell is closed Simple metallurgy 124 Sn: T 1/2 > ( ) x yrs Nucl. Phys. A 807, 269(2008) Strongly Multi-disciplinary project Nuclear Physics, Particle Physics, Low Temperature Physics, Material Science, Physical Chemistry

38 A bolometer is a calorimetric detector. Energy of particle Thermal energy in detector measurable temperature rise if net heat capacity is very low Bolometer Schematic Low Temperature Bolometry Incident Energy C D C S R 1 R 2 Sensor- contact T 0 Pulse decay time T S Resolution of Bolometer Limited by Thermodynamical fluctuation noise {de=(kt 2 C(T)) 1/2 } Depends only on operating temperature and specific heat Independent of incident Energy Challenge: to make measurements in time domain at mk temperature t

39 124 Sn bolometer G.Forster et al. Journal of low temp. phys. 93, 314 (1993) M.K. Bacrania et al. IEEE Trans. on Nucl. Science, 56, 2299 (2009) 0.95 x 0.95 x.25 mm 3 Sn sample Large Size Sn Bolometers have not yet been made.

40 How do we get mk temperature? 3 He- 4 He Dilution Refrigerator liquid 4 He ~ 4.2 K, 3 He ~ 3.2 K Pumping on liq. 4 He ~1.2 K, 3 He ~ 0.3 K Cooling through the continuous cycle of a 3 He / 4 He mixture for lower temperatures When a mixture of the two isotopes of helium is cooled below a critical temperature, it separates into two phases. The lighter "concentrated phase" is rich in 3 He and the heavier "dilute phase" is rich in 4 He. Since the enthalpies of the 3 He in the two phases are different, it is possible to obtain cooling by "evaporating" the 3 He from the concentrated phase into the dilute phase. Phase diagram for a mixture of 3 He / 4 He This process continues to work even at the lowest temperatures because the equilibrium concentration of 3 He in the dilute phase is still finite even as the temperature approaches absolute zero. Fraction of 3 He (originally proposed by H. London in 1951)

41 Cryogen free dilution refrigerator installed at TIFR Base Temp. ~7 mk cooling power 1.4

42 Diagnostic thermometry & wiring Stable within 0.5% of the base temperature (12.2±0.05 mk). V. Singh et. al. (Pramana) R&D on bolometer Choice of materials, mounting of sensors, Readout wires and making connections RF shielding & low temperature sensor readout, vibration issues Physics issues with SC bolometers (measurements in 10 mk to 4 K range)

43 Specific Heat Measurement of Tin Relaxation Calorimetry C = T G P = I 2. R = G. ΔT C = P T G = I2. R. T ΔT Sensor: Commercial Ruthenium Oxide chip resistor (Dale 1kΩ). Heater: NiCr chip resistor (100 Ω) Thermal links : NbTi wires. Current Source for heater: Keithley 220 programmable unit. Resistance readout : AVS-47B A.C Resistance Bridge Typical Decay Pulse

44 NTD Ge sensors widely used due to faster rise time Neutron Transmutation Doped more homogeneous than chemically doped (n,g) low, n-flux constant over sample uniform distribution of dopant 71 Ge, 75 Ge decay As, Se, Ga R (T) = R 0 exp( T 0 /T) 0.5 Ge wafers Neutron reactor BARC Sample Plane Thickness Resistivity (Ωcm) (mm) Sample 1 <100> 1.0 >35 Sample 2 <111> 0.4 >30 NTD Ge Neutron Flux/cm 2 Calculated carrier concentration/cm 3 NTD A 1.9 x x NTD B 1.6 x x10 17 NTD C 0.97 x x 10 17

45 Impurities in NTD Ge Impurity atom Energy (kev) T 1/2 days Sigma (barn) NTD A (ppt) NTD B Ge-8511 (ppt) NTD C Ge-8512 (ppt) 114 In (593) 1550(151) 50 Cr (60) 180(80) 965(455) 109 Ag (700) 64804(2793) 83708(2651) 45 Sc (62) 206(52) 304(54) 58 Fe (217) 1827(442) 3585(435) 64 Zn (7720) 83417(6737) 53672(7409) 59 Co (217) 3134(478) 8601(582) 182 Ta (661) 21502(614)

46 VI Characteristics of NTD Ge (30 to 300 K) Neutron dose~1.6x10 19 neutrons/cm 2, Carrier concentration ~4.94x10 17 /cm 3 Vacuum annealed at C for 6 hrs Sample dimension 2 x 5 x 0.4 mm 3 Ohmic contact pad: 2mm x 1mm x 150nm, Au-Ge alloy (88% - 12%); RTA at C for 2min. The contacts are found to be ohmic upto 30K

47 TiLES (Tifr Low background Experimental Setup) J-shaped cryostat C fiber end cap Plastic Scintillator 60 cm long cold finger C u Roman Pb Sensitivity of the setup: 2mBq/g for 40 K - 60 ppm 1mBq/g for 232 Th ppm A dedicated low background counting facility TiLES at TIFR Detector surrounded by OFHC Cu (5 cm), Pb (10 cm) ( 210 Pb < 0.3 Bq/kg). N 2 purging system and active muon veto (plastic scintillators) TiLES is used for material screening such as ETP Cu, INO site rock, etc.

48 Digital DAQ for TiLES A 14 bit, 100 MHz FPGA-based commercial CAEN N6724 digitizer Optimization of trapezoidal filter parameters for best energy resolution C++ based algorithm developed for offline event-by-event anticoincidence between Ge detector and plastic scintillators Gamma ray spectra of room background in TiLES

49 N 2 flushing in the TiLES Radon produced in the natural decay chains of U and Th get trapped in the volume of the detector. Boil-off nitrogen from LN 2 cylinder at a pressure of ~8 mbar flowed in the volume around the detector setup kept inside a sealed Perspex box. Gamma ray spectra of room background in TiLES (t = 7d) Decay scheme of 222 Rn (in 238 U)

50 DBD to excited state in 94 Zr Decay Scheme of 94 Zr Gamma ray spectra of nat Zr in TiLES for t = 7d The current best experimental limits are T 1/2 > 1.3 x y (68% C.L.) (Norman et al., Phys. Lett. B 195, 126 (1987)). 540 g of nat Zr (99.5% purity) counted in the TiLES, Double beta decay of 94 Zr to the 1 st excited state in 94 Mo T 1/2 > 2.0 x y 68% C.L, 6.12 x y at 90% C.L.. Systematic errors: efficiency (5%), isotopic abundance (1%) drifts in energy scale (1%) Background model fit parameters (14%) N. Dokania et al Manuscript submitted

51 n-induced background in 124 Sn 124 Sn (n, g) 125 Sn 125 Sn : β decay Q β = 2357 kev ( very close to Q ββ of 124 Sn = 2293 kev ) T 1/2 = 9.52m, 9.64 days 124 Sn (2n, g) 126 Sn 126 Sn undergoes β- decay to 126 Sb (T 1/2 = 2.3 X 10 5 years, Q β = 378 kev) 126 Sb undergoes β-decay to 126 Te (T 1/2 =12.35 days, Q β = 3673 kev) Simulation studies to estimate neutron flux at INO site based on rock composition are done Simulations for layered shield (Paraffin/rubber/water + lead) in progress

52 Neutron production rate from INO cavern Composition of INO cavern rock from SIMS and ICPMS methods Element Conc. (%) 23 Na 5 Element Level (ppb) 24,25,26 Mg 7 27 Al 25 28,29,30 Si U Th Ca 9.99 Neutron production rate (a, n) interactions on the elements present in the rock (R. Heaton et al., NIM A 276 (1989) 529) for INO cavern rock Spontaneous fission of 238 U and 232 Th following Watt spectrum: W a, b, E = C e ae sinh( be)

53 Estimation of neutron flux at INO cavern The energy integrated neutron flux (E n 15 MeV) is 2.76 (0.47) x 10-6 n cm -2 s -1 from the activity of INO rock. Spectra is parameterized by the function: N E = p0 exp (p1e 2 p2e) The finite size rock element approach reduces the computation time. The estimated neutron flux at center of an 6 m long, 4 m diameter underground tunnel at INO site. Comparable to other UG labs N. Dokania et al., J. of Instrumentation, 10 (2015) T12005

54 On 124 Sn & 124 Te Nuclear Structure Aspect Several nuclear models to calculate the NTME Shell-model and variants QRPA and extensions Alternative models The NTME (M 2ν ) is sensitive to details of the nuclear structure Spectroscopic properties of the initial and final nucleus Pairing and Deformation Observed physical properties of nuclei:test of nuclear models Uncertainty in estimated neutrino mass upto a factor of 10 due to uncertainty in NTMEs. Experiments to constrain NTME are essential A. Shrivastava et Nov.16

55 NDBD and neutrino mass hierarchy Degenerate: can be tested Inverted: can be tested by next generation of 2b experiments. Normal: inaccessible (new approach is needed)

56 Challenges in the 124 Sn NDBD experiment Make a natural Sn bolometric detector ~ Kg in progress Radiation background studies in progress Detector and background simulations in progress Thermalization issues between the electronic and phononic systems ballistic phonons single crystal vs polycrystalline Sn Sensor development in progress : first batch of sensors in use Reliable NTME calculations Enrichment of 124 Sn (AVLIS) Constraining NTME, GT measurement in NDBD nuclei

57 Summary Neutrino physics offers many exciting opportunities to explore new physics. Upcoming INO will address some of these key questions. Neutrinoless Double Beta Decay to test true nature of neutrino Majorana or Dirac? Several large scale experiments with increased sensitivity are proposed and some are underway Proposal for NDBD at INO lab: Feasibility study for NDBD in 124 Sn bolometer is underway.

58 Acknowledgement INO collaboration Tin.Tin Collaboration