Sterile Neutrinos with WbLS � detector Jelena Maricic � University of Hawaii at Manoa � May 17, 2014
Outline ● Physics motivation for the very short baseline neutrino oscillations search � ● Concept of the antineutrino generator experiment � ● 144 Ce- 144 Pr PBq antineutrino generator (IsoDAR briefly mentioned) � ● Statistics with 10-50 kton size WbLS detector � ● Effects from energy threshold � ● Effects from energy resolution � ● Effects from vertex resolution � ● Summary Jelena Maricic, University of Hawaii � 2
Motivation for the short baseline antineutrino search G. Mention et al. Phys.Rev.D83:073006,2011 ? Dash line: 3 ν ’s � Solid line: 3+ 1 ν states with ∆ m 2 = 1 eV 2 ● There may be 4th neutrino flavor living at a very short baseline � ● Unexplored area at reactor neutrino (MeV) energies � 3
Testing short baseline oscillation ● If the 4 th neutrino is present and oscillates � distance-dependent flux from the source will demonstrate it at the distances of the order of oscillation length from the neutrino source � � � � ● In case of sterile neutrino Δ m 2 ~ 1-2 eV 2 , oscillation distance of interest is of the order of couple of meters. � � ● Large detectors with low energy threshold favorable for checking this hypothesis Jelena Maricic, University of Hawaii � 4
Neutrino and antineutrino generators ● Antineutrino generators are ● Neutrino generators such as � detected in LS detected via inverse ● 51 Cr (753 keV) and � beta decay (IBD) � ● 37 Ar (814 keV) have been used in ● Antineutrino energy > 1.8 MeV the past � (IBD threshold) � ● Monoenergetic � ● Lifetime > 1 month to allow time ● Require measurement of vertex for production and transport � position only for L/E � ● Requires nuclei with high Q β and � long lifetime � ● Detection in LS via elastic ● No single nucleus satisfies this scattering off electrons � condition � � must be very strong (5-10 � MCi) to overcome solar neutrino ● Pairs of beta decay nuclei needed: the background � first one with low Q β and long � lifetime followed by the second one —> too low in energy for WbLS with high Q β and short lifetime detector? Jelena Maricic, University of Hawaii � 5
144 Ce – 133 Pr antineutrino generator � ● Nuclei are in equilibrium � ● Decay rate completely driven by 144 Ce � ● Up to 150 kCi production capability (~5 PBq) � � ● Antineutrino emitted in 144 Ce decay below IBD threshold 1.8 MeV � ● Antineutrinos above 1.8 MeV emitted in 144 Pr undergo IBD � � ● Main intrinsic background comes from 2.185 keV gamma with 0.7% branching ratio � similar energy as 2.2 MeV deexcitation gamma from neutron capture on hydrogen Jelena Maricic, University of Hawaii � 6
Antineutrino generator outside of the detector � ● Advantages: safe, simpler to deploy; almost point like source; baseline as low as 3 – 4 m � ● •Disadvantages: lot of neutrinos lost due to partial solid angle coverage Jelena Maricic, University of Hawaii � 7
Potential of the currently existing detectors ● Current generation of LS detectors has the ability to probe the reactor antineutrino anomaly at 2 σ level � ● Scientific interest for a more decisive measurement especially in the case of possible positive signals � 8
Expected rate ● 140 kCi source for 18 months and t 1/2 = 285 days for 144 Ce � ● Assume that the source can be placed at 4 m distance from the target volume edge � ● ~177,300 (132,300) interactions in no oscillation scenario for 20 (10) kton detector � ● Using � � � � ● We get ~168,600 (125,800) interactions for sin 2 2 θ = 0.1 and ∆ m 2 = 1 eV 2 Jelena Maricic, University of Hawaii � 9
Anti-neutrino spectrum ● sin 2 2 θ = 0.1 and � ● ∆ m 2 = 1 eV 2 � ● 10 kton detector � ● Source 18 m from the center � ● Spectrum peaked toward high energy, BUT most difference between oscillated vs. unoscillated spectrum in the peak region below 2.8 MeV � 10
Effect of Energy Threshold Ability to distinguish between oscillated and unoscillated spectrum strongly dependent on the energy threshold. � Rate for a 10 kton detector comparable to 1 kton LS detector with 1.8 MeV threshold � Detection efficiency NOT included —> further affect the signal statistics 10 kton 20 kton � � 1.8 MeV � unosc � 132,300 � 177,300 � osc 125,800 168,600 � � 2.4 MeV � unosc � 88,500 � 118,600 � osc 84,200 112,800 � � 2.8 MeV � unosc � 27,700 � 37,100 � osc 26,400 35,300 � 11
Illustration of the statistics effect ● Example from 144 Ce in KamLAND � ● General shape of the sensitivity curves does not change with roughly twice as many events, BUT increased sensitivity to smaller mixing angles and masses � ● Note the importance of knowing the absolute rate for larger masses arXiv:1312.0896 [physics.ins-det] Courtesy of T. Lasserre Jelena Maricic, University of Hawaii � 12
Oscillated vs Unoscillated Spectrum ● sin 2 2 θ = 0.1 and � ● ∆ m 2 = 1 eV 2 � � Oscillation pattern � much less � pronounced farther � from � the source. Jelena Maricic, University of Hawaii � 13
Cumulative rate vs distance ● sin 2 2 θ = 0.1 and � ● ∆ m 2 = 1 eV 2 � ● Without energy and vertex resolution effects � ● 10 kton detector � ● Source 18 m from the center � ● Oscillation effects more pronounced closer to the source � ● Important to bring source as close to target volume as possible to probe larger ∆ m 2 � ● Larger detector increases sensitivity to smaller ∆ m 2 due to longer baseline � 14
Energy and Vertex 13%, 24 cm resolution effects 6.5%, 12 cm 26%, 48 cm ● sin 2 2 θ = 0.1 and � ● ∆ m 2 = 1 eV 2 � � 15
Energy Resolution effect arXiv:1312.0896 [physics.ins-det] ● Energy resolution varied between 2.5% and 15% flat in 1kton LS detector � ● Effects more pronounced in shape only analysis � ● Overall, weak sensitivity on energy resolution � 16
Vertex resolution effect arXiv:1312.0896 [physics.ins-det] ● Vertex resolution varied between 5 cm and 50 cm � ● Larger mixing masses more affected; effect significan in the shape only analysis � 17
Antineutrino source - detector distance effect arXiv:1312.0896 [physics.ins-det] ● Keeping the distance between the source and detector as short as possible is critical � ● Especially important in the shape only analysis (some of the effect is due to reduced statistics) � 18
Decay At Rest Source 8 Li ● 8 Li decay produces antineutrino flux with higher energy, weakening energy threshold/detection efficiency requirement � ● 8 Li produced from 7 Li by exposure to copious neutron flux � 19
IsoDAR * Reactor anomaly – ν e disappearance is a direct test of the signal * LSND/MB -- If CPT is a good symmetry, then ν e disappearance limits exclude ν e signals KamLAND -- 1 kT sphere ( � JUNO – 20 kT squat cylinder � LENA – 50 kT long cylinder Dependences on: geometry, distance to detector, aspect ratio of detector � � � Slide from Matt Toups regarding IsoDAR � 20
IsoDAR for WATCHMAN Slide from Matt Toups regarding IsoDAR � 21
IsoDAR and possible alternatives for 8 Li ● Issues with IsoDAR: � ● compact accelerator under development � ● expensive technology and significant power/space/shielding requirement � ● long distance to the detector (7 m to detector edge) affects sensitivity to large ∆ m 2 � ● Alternatives: � ● other sources of copious neutrons - d-t neutron generators with 10 14 n/s yield exists —> gets the DAR 8 Li source closer to detector � ● cheaper technology than accelerator � ● use of heavy water to moderate neutrons efficiently (expensive) � ● better purify 7 Li, although difficult to go beyond current 99.99% 7 Li purity (expensive) � 22
Summary ● High sensitivity test of the sterile neutrino hypothesis with large WbLS detector seems feasible � � ● Measurement prospect very dependent on energy threshold, statistics, source-detector distance and knowledge of the absolute antineutrino rate � � ● Retaining low energy threshold (bellow 2.5 MeV) is more critical then going to larger detector size � ● Optimized cylindrical shape is better than spherical (average source-detector distance smaller) � ● Requirements are moderately stringent for energy (15%) and vertex resolution (25-50 cm) � � ● Ideal solution for WbLS detector: DAR 8 Li source, close to the detector with knowledge of the absolute antineutrino production rate at the level of 1-2% � 23
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