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 Neutrino generators such as 51 Cr (753 kev) and 37 Ar (814 kev) have been used in the past Monoenergetic Require measurement of vertex position only for L/E Detection in LS via elastic scattering off electrons must be very strong (5-10 MCi) to overcome solar neutrino background > too low in energy for WbLS detector? Antineutrino generators are detected in LS detected via inverse beta decay (IBD) Antineutrino energy > 1.8 MeV (IBD threshold) Lifetime > 1 month to allow time for production and transport Requires nuclei with high Q β and long lifetime No single nucleus satisfies this condition Pairs of beta decay nuclei needed: the first one with low Q β and long lifetime followed by the second one with high Q β and short lifetime Jelena Maricic, University of Hawaii "5
144 Ce 133 Pr antineutrino generator Nuclei are in equilibrium Jelena Maricic, University of Hawaii 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 "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 2σ 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 2θ = 0.1 and m 2 = 1 ev 2 Jelena Maricic, University of Hawaii "9
Anti-neutrino spectrum sin 2 2θ = 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 osc 2.4 MeV unosc osc 2.8 MeV unosc osc 132,300 125,800 88,500 84,200 27,700 26,400 177,300 168,600 118,600 112,800 37,100 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] Jelena Maricic, University of Hawaii Courtesy of T. Lasserre "12
Oscillated vs Unoscillated Spectrum sin 2 2θ = 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 2θ = 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 resolution effects 13%, 24 cm 6.5%, 12 cm 26%, 48 cm sin 2 2θ = 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
KamLAND -- 1 kt sphere ( JUNO 20 kt squat cylinder LENA 50 kt long cylinder 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 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