Short Distance Neutrino Oscillations with BoreXino SOX
Borexino experiment Borexino experiment • Mainly, a solar neutrino experiment • ν + e - → ν + e - in organic liquid scintillator • Very low background obtained with material screening, shielding and 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, 141302 (2011) new Phys. Rev. Lett. 108, 051302 (2012) Neutel Venice 2013 • ν e detection: proved by geo- neutrinos • sub-MeV ν e detection: proved by 7 Be and pep • total background: • << 1 events / year in the whole • sensitivity: as low as a few cpd/100 t Milano 4-4- 2013 2 volume • pep: 3.1 ± 0.6(stat) ± 0.3(sys) cpd/100 t
The detector The detector Stainless Steel Sphere ~1300 m 3 of liquid Scintillator support for 2214 PMTs 270 t PC-PPO Principle: “graded shielding”. 13.7 m Nylon Pure and pure Vessels materials toward internal: R=4.25 the center of the m external: R=5.50 detector m 16.9 m 18 m Tank 8.25 m PIT 3300 m 3 of water 210 PMTs Cherenkov ~ 1 m 3 available at 8.25 cm from the center Milano 4-4- 2013 3
Borexino background today 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 • 85 Kr arbitrary units cpd/100 tons • < 8.8 cpd / 100 t pp ν • 2007-2010: 31.2 ± 5 region 210 Po peak • 210 Bi decays • 18 ± 4 cpd / 100 t days sharp 7 Be • 2007-2010: 41.0 ± shoulder 2.8 • 238 U • < 9.7 10 -19 g/g • 232 Th • < 2.9 10 -18 g/g 85 Kr very p.e. reduced Milano 4-4- 2013 4
ν e hort distance ν SOX: S Short distance O Oscillations with Bore scillations with BoreX Xino ino SOX: e • Science • Motivations • Search for sterile neutrinos or other short distance effects on P ee and also • Measurement of ϑ W at low energy (~ 1 MeV) Funded by the European • Improved limit on neutrino magnetic moment Research Council with an • Measurement of g V e g A at low energy ERC advanced grant • Constraints on neutrino NSI amounting to 3.5 Meuros • Technology • Neutrino source: 51 Cr SOX is the “European” • Anti-neutrino source: 144 Ce official project acronym • Project • SOX-A - 51 Cr external • SOX-B - 144 Ce external • SOX-C - 144 Ce internal Milano 4-4- 2013 5
A long standing idea 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 (Baksan- Russia) • Recently, revised and re-proposed by many authors to search for sterile neutrinos a very incomplete list! • N.G. Basov, V. B. Rozanov, JETP 42 (1985) See White Borexino proposal, 1991 (Sr90) Paper and 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) references I.R.Barabanov et al., Astrop. Phys. 8 (1997) therein: Gallex coll. PL B 420 (1998) 114 Done (Cr51) arxiv:1204.537 A.Ianni,D.Montanino, Astrop. Phys. 10, 1999 (Cr51 and Sr90) 9 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) 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 SOX proposal - ERC 320873 - Feb. 2012 - approved Oct. 2012 Milano 4-4- 2013 6
SOX: the Science case SOX: the Science case Eur. Phys. J. C (2013) 73:2345 Reactor Anomaly Credit: T. Lasserre C. Giunti et al. arxiv:1210.5715 (hep-ph) Gallium Anomaly Milano 4-4- 2013 7
SOX: Three Phases SOX: Three Phases • Mission: test the existence of low L/E ν e and/or ν e anomalies by placing 144 Ce well known artificial sources close to SOX-C or inside Borexino • SOX-A • 51 Cr source in pit beneath detector • 8.25 m from center [2014/2015] • SOX-B • 144 Ce- 144 Pr source in W.T. 144 Ce • PPO everywhere to enhance sensitivity SOX-B • 7.15 m from center [2015/2016] • SOX-C • 144 Ce- 144 Pr source in the center • Only after the end of solar program tunnel 51 Cr • More effort and more time pit SOX-A [>2017] Milano 4-4- 2013 8
Artificial neutrino sources Artificial neutrino sources Source Production τ Decay Energy Mass Heat (days) mode [MeV] [kg/MCi] [W/kCi] 51 Cr Neutron irradiation of 40 EC 0.746 1.1 0.19 ν e 50 Cr in reactor γ 320 keV Φ n ≳ 5. 10 14 cm -2 s -1 (10%) 144 Ce- 144 Pr Chemical extraction 411 β- <2.997 0.314 7.6 ν e from spent nuclear fuel 5 51 Cr 144 Ce Detection 144 Pr threshold 144 Ce -144 Pr Milano 4-4- 2013 9
The tunnel beneath the detector The tunnel beneath the detector Steel Floor of the Water Tank 100 cm Milano 4-4- 2013 10
Data analysis: two techniques Data analysis: two techniques • Total counts: standard “disappearance” experiment • T otal number of events depends on θ 14 and (weakly) from Δm 214 • Sensitivity depends on: • Statistics (source activity and active volume) • 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: 093006 (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 • Oscillometry analysis →Direct measurement of Δm 142 and θ 14 • Very powerful and independent. Does not depend on knowledge of source activity. Milano 4-4- 2013 11
Geometry with external source Geometry with external source • Volume: • Flux and decay • Oscillations (one sterile) • The number of ν e -e - events at distance l from the source, with detection threshold T 1 and N.B.: The distribution of events is not maximum recoil energy T 2 : uniform even without oscillations Milano 4-4- 2013 12
Example for SOX-A Example for SOX-A Reactor anomaly central value • Waves may be detected in the distribution of events as a function of the distance from source 1 σ 3 σ • With waves, both parameters can be measured Ideal curves Full Geant4 simulation - example Borexino Background - No fluctuations Borexino Background Milano 4-4- 2013 13
Waves with ν e and space-energy correlation Waves with ν e and space-energy correlation 1 m fiducial cut • Space - Energy correlation • With the 144 Ce- 144 Pr source (both external SOX-B and internal SOX-C ) global fit exploiting Δm 2 = 1.0 eV 2 correlation between sin 2 (2ϑ s ) = 0.1 reconstructed event position 1 year and positron energy External Δm 2 = 2.0 eV 2 ) V e positron energy (MeV) M sin 2 (2ϑ s ) = 0.05 ( y g r 1 year e n e d i n s t d a i s n o t c a e n f c r e r o f m r t o t m h i e t s h s o e u s r o o c u e r ( c m e p ) ( m ) Milano 4-4- 2013 14
More examples of space-energy patterns for 144 Ce- More examples of space-energy patterns for 144 Ce- Pr 144 Pr 144 2.2 2.2 positron energy (MeV) positron energy (MeV) 0 0 d i s t a n c e d f i External s r 1 t o 15 a m n c t e h f e r s o o m u r t h c e e 1 s ( m o u ) r c e 15 ( m ) ∆ m 2 =2eV 2 ∆ m 2 =6eV 2 sin 2 2 θ =0.1 sin 2 2 θ =0.1 2.2 2.2 positron energy (MeV) positron energy (MeV) 0 0 d i s d t a 1 i s n t a c e n c f e r o Center f r 1 m o m t h e t h 10 s o e u s o r u c r e c ( e m 10 ( m ) ) Milano 4-4- 2013 15
SOX-A sensitivity SOX-A sensitivity Reactor+Ga anomaly region • SOX-A: • 51 Cr source at 8.25 m from the center • 10 MCi • 1% precision in source activity • 1% in FV determination • Phase A can happen any time during next solar neutrino phase Milano 4-4- 2013 16 • 2014/15 is realistic – 3 months of data taking
SOX-B sensitivity SOX-B sensitivity Reactor+Ga anomaly region • SOX-B • 144 Ce- 144 Pr source at 7.15 m from the center (in water tank) • 75 kCi • 1.5% precision in source activity • 2% bin-to-bin error to include all effects • SOX-B can happen any time during next solar neutrino phase Milano 4-4- 2013 17 • 2015/16 is a realistic scenario – 1.5 years of data taking
SOX-C sensitivity SOX-C sensitivity Reactor+Ga anomaly region • SOX-C: • 144 Ce- 144 Pr source in the center • 75 kCi • 1.5% precision in source activity • 2% bin-to-bin error to include other systematics • SOX-C can happen only after the end of solar neutrino phase • 2017-2018 is in principle possible – 1.5 years of data taking Milano 4-4- 2013 18 •
T echnology: 51 Cr source T echnology: 51 Cr source 51 Cr W Gallex External T must be 1994 acceptable Current value: T=90°C Internal T must be below syntherization (750°C) Current value: T=260 °C Milano 4-4- 2013 19
T echnology: location for 51 Cr source T echnology: location for 51 Cr source Milano 4-4- 2013 20
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