Borexino and Neutrino Image Radiation Sources

Transcription

1 Short Distance Neutrino Oscillations with BoreXino Heidelberg Jul. 10 th, 2013 Marco Pallavicini on behalf of the Borexino Collaboration Dipartimento di Fisica - Università di Genova & INFN Sezione di Genova

2 Borexino experiment Mainly, a solar neutrino experiment ν + e- ν + e- in organic liquid scintillator Very low background obtained with selection, shielding e purifications Low energy threshold, good energy resolution, spatial reconstruction, pulse shape α/β identification but also Very good anti-neutrino detection (e.g. geo-neutrinos) Phys. Rev. Lett. 107, (2011) Phys. Rev. Lett. 108, (2012) sub-mev ν e detection: proved by 7 Be and pep sensitivity: as low as a few cpd/100 t pep: 3.1 ± 0.6(stat) ± 0.3(sys) cpd/100 t ν e detection: proved by geo-neutrinos total background: << 1 events / year in the whole volume 2

3 The detector Scintillator 270 t PC-PPO Stainless Steel Sphere ~1300 m 3 of liquid support for 2214 PMTs 13.7 m Principle: graded shielding. Nylon Vessels internal: R=4.25 m external: R=5.50 m Pure and pure materials toward the center of the detector 16.9 m 18 m PIT ~ 1 m 3 available at 8.25 cm from the center 8.25 m Tank 3300 m 3 of water 210 PMTs Cherenkov 3

4 Neutrino detection in Borexino ~ 3-5 mm Scintillation (dominant) Cherenkov (small) 57 Co 139 Ce 203 Hg 85 Sr Calibration with γ sources data Montecarlo Scintillation light detected by PMTs # of photons energy time of flight position pulse shape α/β β+/β- 54 Mn 65 Zn 40 K 60 Co α / β separation ( 214 Bi Po ) ENERGY RESOLUTION 200 kev 400 kev 1 MeV SPATIAL RESOLUTION kev kev 4

5 Rate 7 Be - I calibration points Experiment s historical goal Last paper improvements Accurate energy calibration Precise fiducial volume Big effort on Monte Carlo tuning Calibration source inside Borexino 5

6 Rate 7 Be - II Two quasi independent methods to check systematics Monte Carlo fit to the spectru, without α/β subtraction of the 210 Po peak Analytical fit of the spectrum after α/β subtraction of 210 Po peak Phys. Lett. B658: , 2008 Phys. Rev. Lett. 101, , 2008 Phys. Rev. Lett. 107, , 2011 Consistent results. Small difference included in systematic error. Final rate (100 t target): 46.0 ± 1.5 (stat) ± 1.5 (sys) c d -1 Source % Trigger efficiency and stability < 0.1 Live time 0.04 Scintillator density 0.05 Fiducial volume Fit method 2 Energy response 2.7 Cuts efficiency 0.1 Total

7 Day-night modulation of 7 Be rate Lack of modulation selects MSW-LMA Day--night spectra events / 5 kev R D Po = ± 7 Be = 0.04 ± Phys. Lett. B707:22 26, 2012 R D A dn =2 R 2 10 N 3 = ± ± cpd/100tr N + R D 0.57 cpd/100t A dn =2 R N 7 Be = 12 =0.001 cpd/100t ± ± R N + RNight D - Day spectrum No modulation observed MeV Counts /5 kev 130 tons Day spectrum Night spectrum MeV MeV events / 5 kev LOW LOW excluded by DN asymmetry MeV 7

8 First pep detection and CNO limit We have got the first direct evidence of pep neutrinos and set a strong upper limit on CNO Thanks to the low background and 11 C rejection techniques Tagging of 11 C with triple coincidence β + - β separation exploiting positronium formation Global multivariate analysis Triple concidence (TFC) μ + 12 C n + 11 C+ μ 236 μs No convection 11 C does not move Energy spectrum in FV data after TFC cut data no TFC cut 29.4 min Fast neutron thermalization and capture 11 C 11 B + e + + νe n + p D + γ (2.2 MeV) PHYSICAL REVIEW C 74, (2006) p.e. 8

9 PEP: β + tagging with positronium Orto-positronium ~ 50% (in PC) Signal is delayed by ~ 3 ns Different pulse shape! β - da 214 Bi- 214 Po β + da 11 C (TFC) Parameters measured in a dedicated setup Typical emission time Annihilation γs β+ scintillation light decay Boosted Decision Tree β- β ns Final selection base on Boosted Decision Tree (BDT) 9

10 PEP - CNO: final result Rate: 3.1 ± 0.6(stat) ± 0.3(sys) cpd/100 t No oscillations excluded at 97% c.l. PRL 108, (2012) Δχ 2 profile for ν pep SSM+ MSW-LMA SSM No Osc. No ν pep excluded at 98% Assuming MSW-LMA: Φ pep = 1.6 ± cm -2 s -1 Δχ 2 profile with free pep and CNO Borexino limit CNO limit assuming SSM CNO rate < 7.1 cpd/100 t (95% c.l.) 10

11 Phase I impact Before Borexino Borexino 2012 pp - all solar (w.o. BX) 8 B - all solar (Rad. + Cher. w.o. BX) Homestake MSW prediction 7 Be pep 8 B excluded by DN asymmetry 11

12 Borexino background today A significant purification effort done in 2010/2011 to improve purity further Very effective on 85 Kr, good on 210 Bi, excellent for 238 U and 232 Th about 3 months of data early 2012 arbitrary units pp ν region 210 Po peak decays in 3 y 210 Bi rate = 16±4 cpd/100tons 85 Kr rate = 7±5 cpd/100tons cpd/100 tons 85 Kr < 8.8 cpd / 100 t : 31.2 ± Bi sharp 7 Be shoulder days 18 ± 4 cpd / 100 t : 41.0 ± U < g/g 232 Th < g/g 85 Kr very reduced p.e. 12

13 SOX: Short distance ν e Oscillations with BoreXino Science Motivations Search for sterile neutrinos or other short distance effects on P ee Measurement of ϑ W at low energy (~ 1 MeV) Measurement of neutrino magnetic moment Check of g V e g A at low energy ERC Ideas approved Technology Neutrino source: 51 Cr Anti-neutrino source: 144 Ce Project SOX-A - 51 Cr external SOX-B Ce external SOX-C Ce internal 13

14 A long standing idea The idea to deploy a source in Borexino dates back to the beginning of the project Successfully implemented by Gallex (LNGS) and SAGE (Russia) Recently, revised and re-proposed by many authors to search for sterile neutrinos N.G. Basov, V. B. Rozanov, JETP 42 (1985) Borexino proposal, 1991 (Sr90) J.N.Bahcall,P.I.Krastev,E.Lisi, Phys.Lett.B348: ,1995 N.Ferrari,G.Fiorentini,B.Ricci, Phys. Lett B 387, 1996 (Cr51) I.R.Barabanov et al., Astrop. Phys. 8 (1997) Gallex coll. PL B 420 (1998) 114 Done (Cr51) A.Ianni,D.Montanino, Astrop. Phys. 10, 1999 (Cr51 and Sr90) A.Ianni,D.Montanino,G.Scioscia, Eur. Phys. J C8, 1999 (Cr51 and Sr90) SAGE coll. PRC 59 (1999) 2246 Done (Cr51 and Ar37) SAGE coll. PRC 73 (2006) C.Grieb,J.Link,R.S.Raghavan, Phys.Rev.D75:093006,2007 V.N.Gravrin et al., arxiv: nucl-ex: C.Giunti,M.Laveder, Phys.Rev.D82:113009,2010 C.Giunti,M.Laveder, arxiv: SOX proposal - ERC Feb approved Oct a very incomplete list! See White Paper and references therein: arxiv:

15 The Science case - I A few well known experimental results do not match the standard threeflavors scenario. In particular: LSND (Los Alamos) in 2001 measured a ν e excess using ν μ beam Apparently, a clear effect: 87.9 ± 22.4 ± 6.0 (3.8 σ) L/E NOT compatible with solari oscillations LSND region recently reduced by Icarus data, but not excluded Venice

16 The Science case - II Gallex and SAGE in the 90 s has made a calibration of their detector with an artificial neutrino source Strong enough to produce a detectable neutrino flux (about the Sun at 10 m) A portable Sun! Both experiments show a deficit w.r.t. expectations νe + 71 Ga 71 Ge + e - C. Giunti et al. arxiv: (hep-ph) <R>=0.85 ±0.05 ~ 3 σ effect 16

17 The Science case - III Reactor anomaly Many experiments at small L/E from reactors Supposedly better calculations of reactor neutrio fluxes released recently With these new calculations, neutrino deficit at small L/E is observed Credit: T. Lasserre 17

18 Comments on the Science case In my opinion, taken individually, each anomaly is weak: popular arguments, e.g. LSND region not clearly confirmed by Miniboone, allowed region shrinked significantly by Icarus Gallex and SAGE calibrated their detector with sources. Can we trust the efficiency so much to believe the anomaly? Can we trust the supposedly better reactor fluxes? Were previous measurements biased by older calculations? BUT All anomalies point consistently in the same direction, i.e. deficit at small L/E If any of them is true, new physics is mandatory High risk, high gain Methodologically, the only way to discard a wrong measurement is to do a better one We can t dismiss data based on theoretical prejudice 18

19 SOX: Three Phases Mission: test the existence of low L/E ν e and/or ν e anomalies by placing well known artificial sources close to or inside Borexino 144 Ce SOX-C SOX-A 51 Cr source in pit beneath detector 8.25 m from center [2015/2016] SOX-B 144 Ce- 144 Pr source in W.T. PPO everywhere to enhance sensitivity 7.15 m from center [2015/2016?] 144 Ce SOX-B SOX-C 144 Ce- 144 Pr source in the center Only after the end of solar program More effort and more time [>2016] 51 Cr SOX-A pit tunnel 19

20 Artificial neutrino sources Source Production τ (days) Decay mode Energy [MeV] Mass [kg/mci] Heat [W/kCi] 51 Cr νe Neutron irradiation of 50 Cr in reactor Φn cm -2 s EC γ 320 kev (10%) Ce- 144 Pr νe Chemical extraction from spent nuclear fuel 411 β- < Cr 144 Ce -144 Pr 144 Ce 144 Pr Detection threshold 20

21 The tunnel beneath the detector Steel Floor of the Water Tank 100 cm 21

22 Data analysis: two techniques Total counts: standard disappearance experiment Total number of events depends on θ 14 and (weakly) from Δm 2 14 Sensitivity depends on: Statistics (source activity) Error on activity (in particular) and on efficiency The relatively short life-time of 51 Cr yield useful time-events correlation The background is constant while the signal is not Spatial waves [C.. Grieb et al., Phys. Rev. D75: (2007)] With expected Δm 2 e and ~ 1 MeV energy, the wavelength is smaller than detector size (~11 m max) and bigger than resolution (~ 15 cm) The distribution of events as a function of distance to source shows waves Direct measurement of Δm 14 2 and θ 14 Very powerful and independent. Does not depend on knowledge of source activity. The two techniques can be combined in a single counts-waves fit 22

23 Geometry with external source Volume: V (l) =2 l 2 1 Flux and decay d 2 R 2 + l 2 2 dl source l d r R (l) = I 0 4 l 2 e td Oscillations (one sterile) 1 e t P ee = 1. sin 2 (2 s ) sin m 2 l E arbitrary units (l) = I 0 4 l 2 V(l) =2 l 2 1 d 2 R 2 + l 2 2dl The number of ν e -e - events at distance l from the source, with detection threshold T 1 and maximum recoil energy T 2 : V(l) (l) N 0 (l, T 1, T 2 )=n e (l) V(l) P ee (l, E) Z T2 d e (E, T) T 1 dt dt distance from external source (cm) N.B.: The distribution of events is not uniform even without oscillations 23

24 Example for SOX-A m 2 2 Δ m Reactor anomaly central value Waves may be detected in the distribution of events as a function of the distance from source With waves, both parameters can be measured Ideal curves Borexino Background - No fluctuations Unoscillated overall spectrum Oscillated overall spectrum 51 Cr ν 7 Be ν 210 Po Other bg Full Geant4 simulation - example Borexino Background Unoscillated overall spectrum Oscillated overall spectrum MC Data 51 Cr ν 7 Be ν 210 Po Other bg σ 3 σ sin 2 2θ 14 sin Distance from the source [m] Distance from the source [m] 24

25 Waves with ν e and space-energy correlation events m fiducial cut Space - Energy correlation Δm 2 = 1.0 ev 2 sin 2 (2ϑ s ) = 0.1 With the 144 Ce- 144 Pr source (both external SOX-B and internal SOX-C) global fit exploiting correlation between reconstructed event position and positron energy year distance from center (cm) events distance from center (cm) Δm 2 = 2.0 ev 2 sin 2 (2ϑ s ) = year distance distance from from the the source source (m) (m) 25 positron energy (MeV) positron energy (MeV

26 SOX-A sensitivity SOX-A: 2 Δm Reactor+Ga anomaly region 51 Cr source at 8.25 m from the center 10 MCi 1% precision in source activity RA: 95% C.L. RA: 99% C.L. 51 Cr: 95% C.L. 51 Cr: 99% C.L. Solar: 95% C.L. 1% in FV determination Solar: 99% C.L Phase I can happen any time during next solar neutrino phase 2015 is a realistic scenario sin 2 (2θ 14 ) 26

27 SOX-B sensitivity SOX-B Δm Reactor+Ga anomaly region 144 Ce- 144 Pr source at 7.15 m from the center 1 75 kci 1.5% precision in source activity RA: 95% C.L. RA: 99% C.L. 144 Ce (water): 95% C.L. 144 Ce (water): 99% C.L. Solar: 95% C.L. 2% bin-to-bin error to include all effects Solar: 99% C.L SOX-B can happen any time during next solar neutrino phase 2015 is a realistic scenario - 1 y of data taking sin 2 (2θ 14 ) 27

28 SOX-C sensitivity SOX-C: Δm Reactor+Ga anomaly region 144 Ce- 144 Pr source in the center ~50 kci 1 RA: 95% C.L. 1.5% precision in source activity 2% bin-to-bin error to include other systematics RA: 99% C.L. 144 Ce (center): 95% C.L. 144 Ce (center): 99% C.L. Solar: 95% C.L. Solar: 99% C.L SOX-C can happen only after the end of solar neutrino phase is a realistic scenario desicison to be taken after SOX-A and/or SOX-B results sin 2 (2θ 14 ) 28

29 Other low energy neutrino physics Weinberg angle: δ(sin 2 ϑw)=2.6% Magnetic moment 10 MCi Reactors μ < μb (90% CL) 5 MCi Borexino (solar) μ < μb (90% CL) SOX A+C With both sources (SOX-A and B or C) Independent measurement of gv e ga Test of SM EW running at very low energy Standard Model g V = -1/2 + 2 sin 2 ϑ W = g a = -1/2 = 0.5 CHARM II (1994) νμ ES su e- E ~ 10 GeV 29

30 Technology: 51Cr source Concept is the same as in Gallex 1994 ~36 kg, 50Cr enriched at 38% irradiated in a high neutron flux reactor (we may use more material) Candidate reactors: Russia (best), USA, Europe 190 W/MCi from photons ~few μsv/h on surface (required < 100) BUT: careful thermal design to handle 10 MCi (2 kw) 51Cr W Preliminary studies are encouraging External T must be acceptable Current value: T=90 C Gallex 1994 Internal T must be below syntherization (750 C) Current value: T=260 C Heidelberg - July. 10th,

31 The neutrino generator 31

32 Internal design 32

33 Final assembly without cooling fins with cooling fins 33

34 Thermal studies Bulk temperatures Surface temperatures 34

35 Technology: location for 51Cr source Heidelberg - July. 10th,

36 Logistics at the Lab The neutrino generator will enter underground directly It will stay in Hall C 4-6 months icarus pit entrance rail 36

37 calorimetry T out% The neutrino generator will actually stay within a calorimeter for precise measurement of the activity W,#Ni,#Fe#Alloy# steel# thermal#insula4on# copper## T in% 37

38 SOX-C: 144 Ce source inside detector Very massive source ~ 4 t of shielding Source: spent nuclear fuel from Russia DENSIMET (W) shielding plus ultra-pure copper layer to reduce background W is very dirty for Borexino γ background is a problem if rate too high random coincidences make background Source deployment to be studied Either from the top or from the bottom PPO everywhere in the SSS to enlarge active volume (active radius up to 5.5 m) New anti-neutrino trigger Trigger on singles would be too hard, but this is not a problem > No schedule yet. 38

39 Summary We plan to perform an extensive search of sterile neutrinos with neutrino and anti-neutrino sources 10 SOX-A 51 Cr neutrino source (external) Tentative schedule: 2015/2016 SOX-B 144 Ce anti-neutrino source (external) Tentative schedule: (TBD) SOX-C 144 Ce anti-neutrino source (internal) No schedule (>2016) Δm Unoscillated overall spectrum Oscillated overall spectrum MC Data 51 Cr ν 51 Be ν 210 Po Other bg RA: 95% C.L. RA: 99% C.L. 51 Cr: 95% C.L. 51 Cr: 99% C.L. 144 Ce (water): 95% C.L. 144 Ce (water): 99% C.L. 144 Ce (center): 95% C.L. 144 Ce (center): 99% C.L. Solar: 95% C.L. Solar: 99% C.L SOX-A Distance from the source [m] sin from the source (m) distance from the source (m) 2 (2θ 14 ) SOX-C positron energy (MeV) energy (MeV) 39