Sterile Neutrinos with WbLS detector Jelena Maricic University of - - PowerPoint PPT Presentation

Jelena Maricic University of Hawaii at Manoa May 17, 2014

Sterile Neutrinos with WbLS detector

Jelena Maricic, University of Hawaii

Outline

• Physics motivation for the very short baseline neutrino
• scillations search
• Concept of the antineutrino generator experiment
• 144Ce-144Pr 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

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Motivation for the short baseline antineutrino search

• There may be 4th neutrino flavor living at a very short baseline
• Unexplored area at reactor neutrino (MeV) energies

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Dash line: 3 ν’s Solid line: 3+ 1 ν states with ∆m2 = 1 eV2

• G. Mention et al. Phys.Rev.D83:073006,2011

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Testing short baseline oscillation

• If the 4th neutrino is present and oscillates distance-dependent flux

from the source will demonstrate it at the distances of the order of

• scillation length from the neutrino source
• In case of sterile neutrino Δ m2 ~ 1-2 eV2, oscillation distance of

interest is of the order of couple of meters.

• Large detectors with low energy threshold favorable for checking this

hypothesis

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Neutrino and antineutrino generators

• Neutrino generators such as
• 51Cr (753 keV) and
• 37Ar (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 Qβ and

long lifetime

• No single nucleus satisfies this

condition

• Pairs of beta decay nuclei needed: the

first one with low Qβ and long lifetime followed by the second one with high Qβ and short lifetime

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144Ce – 133Pr antineutrino generator

• Nuclei are in equilibrium
• Decay rate completely driven by

144Ce

• Up to 150 kCi production

capability (~5 PBq)

• Antineutrino emitted in 144Ce

decay below IBD threshold 1.8 MeV

• Antineutrinos above 1.8 MeV

emitted in 144Pr 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

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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

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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

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Expected rate

• 140 kCi source for 18 months and t1/2 = 285 days for 144Ce
• 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

sin2 2θ = 0.1 and ∆m2 = 1 eV2

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Anti-neutrino spectrum

• sin2 2θ = 0.1 and
• ∆m2 = 1 eV2
• 10 kton detector
• Source 18 m from the

center

• Spectrum peaked toward

high energy, BUT most difference between

• scillated vs. unoscillated

spectrum in the peak region below 2.8 MeV

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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

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10 kton 20 kton 1.8 MeV unosc

• sc
• 132,300

125,800

• 177,300

168,600 2.4 MeV unosc

• sc
• 88,500

84,200

• 118,600

112,800 2.8 MeV unosc

• sc
• 27,700

26,400

• 37,100

35,300

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Illustration of the statistics effect

• Example from 144Ce 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

12 Courtesy of T. Lasserre

arXiv:1312.0896 [physics.ins-det]

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Oscillated vs Unoscillated Spectrum

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• sin2 2θ = 0.1 and
• ∆m2 = 1 eV2
• Oscillation pattern

much less pronounced farther from the source.

Cumulative rate vs distance

• sin2 2θ = 0.1 and
• ∆m2 = 1 eV2
• 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 ∆m2

• Larger detector increases

sensitivity to smaller ∆m2 due to longer baseline

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Energy and Vertex resolution effects

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6.5%, 12 cm 13%, 24 cm 26%, 48 cm

• sin2 2θ = 0.1 and
• ∆m2 = 1 eV2

Energy Resolution effect

• 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

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arXiv:1312.0896 [physics.ins-det]

Vertex resolution effect

• Vertex resolution varied between 5 cm and 50 cm
• Larger mixing masses more affected; effect significan

in the shape only analysis

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arXiv:1312.0896 [physics.ins-det]

Antineutrino source - detector distance effect

• 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)

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arXiv:1312.0896 [physics.ins-det]

Decay At Rest Source 8Li

• 8Li decay produces antineutrino flux with higher energy,

weakening energy threshold/detection efficiency requirement

• 8Li produced from 7Li by exposure to copious neutron flux

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KamLAND -- 1 kT sphere (

• JUNO – 20 kT squat cylinder
• LENA – 50 kT long cylinder

* Reactor anomaly – νe disappearance is a direct test of the signal * LSND/MB -- If CPT is a good symmetry, then νe disappearance limits exclude νe signals Dependences on: geometry, distance to detector, aspect ratio of detector

• Slide from Matt Toups

regarding IsoDAR

IsoDAR

IsoDAR for WATCHMAN

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Slide from Matt Toups regarding IsoDAR

IsoDAR and possible alternatives for 8Li

• 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 ∆m2

• Alternatives:
• other sources of copious neutrons - d-t neutron generators with 1014 n/s

yield exists —> gets the DAR 8Li source closer to detector

• cheaper technology than accelerator
• use of heavy water to moderate neutrons efficiently (expensive)
• better purify 7Li, although difficult to go beyond current 99.99% 7Li

purity (expensive)

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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 8Li source, close to the detector with

knowledge of the absolute antineutrino production rate at the level of 1-2%

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