1. Sterile Neutrinos at Borexino SOX G. Ranucci – INFN Milano On behalf of the Borexino Collaboration European Strategy for Neutrino Oscillation Physics - II CERN 15 May 2012 1

2. Borexino at Gran Sasso: real time detection of low energy neutrinos Water Tank:  and n shield  water Č detector 208 PMTs in water 2100 m 3 20 legs Carbon steel plates Scintillator: 270 t PC+PPO in a 150  m thick nylon vessel Stainless Steel Sphere: 2212 photomultipliers 1350 m 3 Nylon vessels: Inner: 4.25 m Outer: 5.50 m Design based on the principle of graded shielding Neutrino electron scattering e   e 2

3. The idea to use a neutrino source in Borexino and in other underground experiments dates back to at least 20 years – N.G.Basov,V.B.Rozanov, JETP 42 (1985) – Borexino proposal, 1991 (Sr90) Bx – J.N.Bahcall,P.I.Krastev,E.Lisi, Phys.Lett.B348:121-123,1995 – N.Ferrari,G.Fiorentini,B.Ricci, Phys. Lett B 387, 1996 (Cr51) Bx – 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) Bx – A.Ianni,D.Montanino,G.Scioscia, Eur. Phys. J C8, 1999 (Cr51 and Sr90) Bx – SAGE coll. PRC 59 (1999) 2246 Done (Cr51 and Ar37) – SAGE coll. PRC 73 (2006) 045805 – C.Grieb,J.Link,R.S.Raghavan, Phys.Rev.D75:093006,2007 – V.N.Gravrin et al., arXiv: nucl-ex:1006.2103 – C.Giunti,M.Laveder, Phys.Rev.D82:113009,2010 – C.Giunti,M.Laveder, arXiv:1012.4356 3

4. Source Experiment: Physics Case Probing Short Baseline Flavor Oscillations in disappearance Search for Neutrino Magnetic moment Probe neutrino-electron scattering at 1 MeV scale – Weinberg’s angle – g V and g A coupling (NSI) 4

5. Source location in Borexino A: underneath WT – D=825 cm – No change to present configuration B: inside WT – D = 700 cm – Need to remove shielding water C: center – Major change – Remove inner vessels – To be done at the end of solar Neutrino physics A B C 5

6. 6 Source position A

7. Sources  Activity: several 1000 evts within 1 year  E >250 keV ( 14 C background)  Half-life ≥1 month  Compact  Limited heat  Efficient shielding  Low impurities level 7

8. A similar option, but less viable, is 106 Ru– 106 Rh Neutrino source Anti-Neutrino sources 8

9. 51 Cr ~36 kg of Cr 38% enriched in 50 Cr 190 W/MCi from 320 keV  ’s 7  Sv/h (must be < 200) SAGE coll., PRC 59 (1999) 2246 Gallex coll., PL B 420 (1998) Done two times for Gallex at 35 MW reactor with effective thermal neutrons flux of ~5.4E13 cm -2 s -1 ~1.8 MCi Originally proposed by Raju Raghavan 9

10. 51 Cr source in Gallex shielding size dictated by  -emitting impurities 10

11. The case of e 51 Cr source in Borexino Window 0.250-0.700 MeV Background perfectly known : solar neutrinos + Bismuth210 Bismuth210 Source events CNO Be7 Detection as 7 Be solar neutrinos The uncorrelated nature of the measure forces the external deployement of the source: too much backg. from the shield for internal deployment 11

12. 90 Sr- 90 Y  Sr = 28.79 years  Y = 3.8 days 90 Sr 90 Y Inverse beta decay Product of nuclear fission Used in thermoelectric generators Known technology for 0.2 MCi sources 7.25 kg/MCi ~6700 W/MCi including Bremsstrahlung 7.25 kg/MCi ~6700 W/MCi including Bremsstrahlung =2±0.2MeV =7.2×10 -45 cm 2 source 12

13. 106 Ru- 106 Rh  Ru = 539 days  Rh =29.8 s 106 Ru 106 Rh Inverse beta decay Product of nuclear fission =2.5±0.2 MeV =89.2×10 -45 cm 2 source Similar option: 144 Ce– 144 Pr Advantage w.r.t. 90 Sr: lower activity affordable 13

14. Anti-nu Advantages Background free measure (delayed coincidence) -Higher counting rate due to the possibility to exploit the full volume, in this case the FV error can be ignored – the coincidence technique enables to fight efficiently the extra background added from the shield and makes it suited to be located in the center -> more events and less intensity required - Higher energy -> more events because of the quadratic dependence of the cross section from the energy - Same as geo-anti  measure in Borexino – bckg. totally negligible - Future scalability: in a post solar phase of the experiment the entire sphere can be filled with scintillator - Issues to be considered : heat dissipation, high energy gammas and bremmstralung background – shielding and “shadowing” around the center Th= 1.8 MeV 14

15. 51 Cr external Cannot be deployed internally because of background consideration – the test has zero impact on the apparatus and on the «solar» data taking - feasible within a couple of years Anti-nu source internal Internal deployment possible thanks to the coincidence measurement – but huge (and very pure) shield Require a major refurbishment of the detector for the support of the source Nylon vessel removed and the whole sphere converted into active volume Done by 2017 Staged two –phase approach 15

16. 51 Cr Source under the detector 16

17. 17

18. 18 At high  m 2 the fast wiggles are washed out when the resolution is included

19. Example of a simulation of the 51 Cr source externally positioned The fit allows also to determine precisely the oscillation parameters Oscillometry analysis: total rate + waveshape of the profile of the detected events 19

20. Reach of the sterile neutrino search with the 51 Cr source  2 analysis of the 51 Cr source outside BX activity=10MCi; Error on activity =1%; Error on FV=1%; Reactor anomaly Exclusion contours Sensitivity to the rate only FV error better than 1% already achieved in BX (calibration) Error of 1% on the source intensity is agressive – important effort to achieve it Sensitivity to the rate + waveshape Green region 90% CL excluded from Solar+KamLAND constraints accounting for the  13  0 value A. Palazzo - Phys. Rev. D 85, 077301 (2012) Rate + shape + additional handle: time decay of the source event rate to better discriminate against the background 20

21.  2 analysis of the 51 Cr source outside BX activity=10MCi; Error on activity =2%; Error on FV=1%; Reactor anomaly Exclusion curves Error of 2% on the source intensity as achieved in the framework of the Gallex calibration Reach of the sterile neutrino search with the 51 Cr source 21

22. Weinberg’s Angle @ 1 MeV 10 MCi source 5 MCi source  (sin 2  W ) = 2.6% 22

23. Neutrino Magnetic Moment Reactor anti-neutrinos: ~6×10 -11  B (90% CL) From Borexino (solar): ~5×10 -11  B (90% CL) 23

24. 144 Ce Source at the center of the detector 24

25. Waves from a source in the center Enhanced sensitivity due both to the pattern and the increased number of events Oscillation waves 25

26. Other simulations – 90 Sr at the center Good agreement with the analytical oscillation curves 26

27. Reach of the sterile neutrino search with the 144 Ce source Error of 1% on the source intensity is agressive – but the FV error could be omitted – included as safety margin 27 Adequate coverage of the region of interest of the oscillation parameter plane

28. EW couplings Standard Model – g V = -1/2+2sin 2  W = -0.038 – g A = -0.5 Use three-level cross-section Use 51 Cr and 144 Ce source 51 Cr 144 Pr CHARM II with  e ES 90 % C.L. 28

29. Status of the investigation 51 Cr 29 Enriched Cr used for Gallex still available at CEA Saclay Research reactor A) High thermal neutron flux throughout the entire target ideally 1E15 n/cm2/sec B) Enough space to accommodate the material C) Flexible enough to allow the reconfiguration of the core The Siloe’ reactor at Grenoble met this requirements, but it is no longer available, no other suitable reactors available in France Alternatives Petten reactor (Netherland) - promising, complete feasibility evaluation to be started soon Possibility in USA - the “Advanced Test Reactor” at Idaho National Laboratory, featuring neutron fluxes at the required level Opportunities in Russia are being investigated as well, a couple of reactors could be suited to do the irradiation

30. 30 More investigations required for the anti-  sources: 90 Sr can be available from the Companies who separate it from the other fissions products- Experience in Russia (heating equipments upto 1993) The same consideration apply to the 144 Ce source Joint (with potential supplier) feasibility study of the source preparation and delivery to be done Status of the investigation Anti-

31. Conclusions Borexino is well suited for a possible source based short baseline e disappearance test - performances and background perfectly known In a first step a totally non invasive measurement can be performed by deploying externally a 51 Cr source in the Tunnel underneath the Water Tank specifically prepared for this purpose during the construction of the detector, affording already an interesting sensitivity limit capable to address a sizable portion of the joint reactor and Gallium In the post solar phase scenario an anti-  source can be deployed in the center and the target volume increased achieving the ultimate sensitivity capable to cover a wide region of the oscillation parameter plane, thus fully addressing the reactor anomaly indication Investigations for the sources preparation and procurement in progress Opportunity for LNGS to maintain and strengthen the leadership role gained in the context of neutrino oscillation through the Gallex—GNO and Borexino results in the solar neutrino sector 31