Search for Sterile Neutrinos with Reactor Antineutrinos
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Search for Sterile Neutrinos with Reactor Antineutrinos. An Opportunity for NIST?. 50 Years of Reactor Neutrino Physics. ?. Daya Bay. Double Chooz Reno. Neutrino Physics at Reactors. Next - Discovery and precision measurement of θ 13.

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?

Daya Bay

Double Chooz

Reno

Neutrino Physics at Reactors

Next - Discovery and precision measurement of θ13

2008 - Precision measurement of Δm122 . Evidence for oscillation

2003 - First observation of reactor antineutrino disappearance

1995 - Nobel Prize to Fred Reines at UC Irvine

KamLAND

1980s & 1990s - Reactor neutrino flux measurements in U.S. and Europe

Past Reactor Experiments

Hanford

Savannah River

ILL, France

Bugey, France

Rovno, Russia

Goesgen, Switzerland

Krasnoyark, Russia

Palo Verde

Chooz, France

1956- First observation of (anti)neutrinos

Chooz

Chooz

55 years of liquid scintillator detectors

a story of varying baselines...

Savannah River


Discovery of the neutrino

inverse beta decay

νe + p → e+ + n

Discovery of the Neutrino

1956 - “Observation of the Free Antineutrino” by Reines and Cowan


Antineutrino detection
Antineutrino Detection

inverse beta decay

νe + p → e+ + n

coincidence signature

prompt e+ and delayed neutron capture

powerful background suppression technique

10-100 keV

1.805 MeV

Eνe ≅ Ee+ + En + (Mn-Mp) + me+

including E from e+ annihilation, Eprompt=Eν - 0.8 MeV


νe from n-rich fission products

Reactor Antineutrinos

observed spectrum

threshold: neutrinos with E < 1.8 MeV are not detected

calculated reactor spectrum

~ 200 MeV per fission

~ 6 νe per fission

~ 2 x 1020νe/GWth-sec

mean energy of νe: 3.6 MeV

only disappearance experiments possible

cross-section accurate to +/-0.2%

time-dependent rate and spectrum

only ~ 1.5 νe/fission can be detected

rule of thumb:

~ 1 event per day per ton of LS per GWth at 1km


νe from n-rich fission products

Reactor Antineutrinos

time-dependent rate and spectrum

observed spectrum

threshold: neutrinos with E < 1.8 MeV are not detected

calculated reactor spectrum

threshold: neutrinos with E < 1.8 MeV are not detected

mean energy of νe: 3.6 MeV

only disappearance experiments possible

only ~ 1.5 νe/fission are detected

cross-section accurate to +/-0.2%

only ~ 1.5 νe/fission can be detected

rule of thumb:

~ 1 event per day per ton of LS per GWth at 1km


Oscillation experiments with reactors

L/E oscillation effect provides measurement of Δm2

amplitude of oscillation provides measurement of θ

Oscillation Experiments with Reactors

Looking for non-1/r2 behavior of νe interaction rate

for 3 active neutrinos, can study oscillation with two different oscillation length scales: Δm212, Δm213

baseline

intermediate

short

Δm212 ~ 8 x 10-5 eV2

Δm213 ~ 2.5 x 10-3 eV2

L ~ 60 km

L ~ 1.8 km

Only disappearance experiments are possible since mean antineutrino energy is 3.6 MeV.


Oscillation Experiments with Reactors

P(νe→νe)≈1-sin2(2θ)sin2(Δm2L/4E)

Disappearance at Different Baselines

Δm2atm

Δm2sol

past experiments

θ13

θ12

L ~ 1-2 km

L ~ 60 km


νe

νe

νe

νe

νe

νe

νe

Oscillation Searches with Reactor Antineutrinos

1980-1990s

Chooz

8.5GW power

1 km baseline

~3000 events

335 days

5 ton target

νe + p → e+ + n

absolute measurement with 1 detector, σdet ~ 2.7%

time with reactors off critical for background studies

No evidence for oscillation


Japan

Kamioka

Measuring Reactor Antineutrinos with KamLAND

KamLAND

Reactors in Japan

Kashiwazaki

Takahama

Ohi

νe + p → e+ + n

through inverse β-decay

55 reactors

235U:238U:239Pu:241Pu = 0.570: 0.078: 0.0295: 0.057

reactor ν flux at KamLAND

~ 6 x 106/cm2/sec


Kamland 2003 evidence for reactor e disappearance

PRL 90:021802 (2003)

Observed νe 54 events

No-Oscillation 86.8 ± 5.6 events

Background 1 ± 1 events

Livetime: 162.1 ton-yr

KamLAND 2003: Evidence for Reactor νe Disappearance

Reactor Neutrino Physics 1956-2003

mean, flux-weighted reactor distance ~ 180km


Kamland 2010 precision measurement of oscillation
KamLAND 2010: Precision Measurement of Oscillation

Prompt Energy Spectrum

L/E Dependence

L0=180km

reactor phase of KamLAND is completed

KamLAND → KamLAND-Zen in July 2011

continues to record antineutrino data

KamLAND, Phys.Rev. D83 (2011) 052002


Neutrino Oscillation

Mixing Angles & Mass Splittings

UMNSP Matrix

Maki, Nakagawa, Sakata, Pontecorvo

reactor and accelerator

atmospheric, K2K

SNO, solar SK, KamLAND

0νββ

Schwetz et al

arXiv:0808.2016


Neutrino Oscillation

Mixing Angles

UMNSP Matrix

Maki, Nakagawa, Sakata, Pontecorvo

reactor and accelerator

atmospheric, K2K

SNO, solar SK, KamLAND

0νββ

large, but not maximal!

maximal?

small? zero?

because of small sin22θ13, solar & atmospheric ν oscillations almost decouple


νe

νe,x

νe,x

θ13

Δm213≈Δm223

νe/MeV/fisson

Neutrino Energy (MeV)

Energy (MeV)

detector 2

detector 1

Measuring θ13 with Reactor Experiments

Near-Far Concept

distance L ~ 1.5 km

near

far


νe

νe,x

νe,x

θ13

Δm213≈Δm223

Neutrino Energy (MeV)

detector 2

detector 1

Measuring θ13 with Reactor Experiments

Near-Far Concept

distance L ~ 1.5 km

near

far

νe/MeV/fisson

Energy (MeV)

a clean measurement of θ13 at optimized distances

without parameter degeneracies


Reactor Anomaly

and 4th Neutrino Hypothesis


Reactor evolution codes

Reactor data

Thermal power, δPth <1%

Fraction of fissions from isotope k, δαk=few %

Nuclear databases

ν spectrum per fission

E released per fissions of isotope k, δEk≈0.3%

Antineutrino Spectrum Emitted by a Reactor

The prediction of reactor ν spectrum is the dominant source of systematic error for single detector reactor neutrino experiments



Reactor anomaly
Reactor Anomaly

Comparison of measured antineutrino fluxes and spectra with predictions.

Recent re-evaluation of all reactor neutrino experiments.

Average = 0.943 ± 0.023 (χ2=19.6/19)

Phys. Rev. D83, 073006, 2011

G. Mention, M. Fechner, T. Lasserre, M. Cribier, Th. Mueller D. Lhuillier, A. Letourneau


Reactor anomaly1
Reactor Anomaly

A Deficit in the Reactor Antineutrino Flux

Laserre, HEP2011

Lindner, TAUP2011


A Deficit at Short Baselines

Oscillation into New Neutrino State at Short Baselines?

L ~ 1-2 km

L ~ 60 km

Δm2atm

Δm2sol

L ~ 10-100m

θ13

θ12


4th neutrino hypothesis
4th Neutrino Hypothesis

Combine all rate measurements, no spectral-shape information

Fit to anti-νe disappearance hypothesis

Absence of oscillations disfavored at 98.6% C.L.

allowed

excluded

area

data suggest neutrino with Δm2 ~ 1 eV2



Indications in cosmology
Indications in Cosmology

Current precision cosmological data show a preference for extra relativistic degrees of freedom (beyond 3 neutrinos).

from Y. Wong


4th neutrino hypothesis1
4th Neutrino Hypothesis

What does the hypothesis of 4th (or sterile) neutrino imply?

neutrino mass spectrum

3 active neutrinos in color

sterile neutrinos in white

Δm2 ~ 1 eV2

new mass splitting, Δm2, defined by sterile neutrino



Short baseline searches for sterile neutrinos
Short Baseline Searches for Sterile Neutrinos

What’s Next?

  • LSND, MiniBoone, reactor data and indications of Neff>3 in cosmology require definitive laboratory experiment to confirm or refute anomalies

  • Oscillation signature is critical for unambiguous evidence of 4th neutrino. Oscillation signatures include

    • baseline dependence of rate

    • spectral distortion as a signature of energy dependence

spectrum

oscillatory measurement

3-4m oscillation cycle

E (MeV)

length (m)


Short baseline searches for sterile neutrinos1
Short Baseline Searches for Sterile Neutrinos

What’s Next?

  • LSND, MiniBoone, reactor data and indications of Neff>3 in cosmology require definitive laboratory experiment to confirm or refute anomalies

  • Oscillation signature is critical for unambiguous evidence of 4th neutrino. Oscillation signatures include

    • baseline dependence of rate

    • spectral distortion as a signature of energy dependence

  • What do we need?

  • powerful “point source” of antineutrinos (extended source washes out neutrino oscillation effect)

  • access to range of baselines from source, L ~ O(1-20m)

  • antineutrino detector at different baselines

  • background suppression through active and/or passive shielding


Baselines and reactor core dimensions
Baselines and Reactor Core Dimensions

  • extended neutrino source washes out neutrino oscillation effect

From: N. Bowden, LLNL


Portable antineutrino detectors
Portable Antineutrino Detectors

Antineutrino Detectors for Reactor Monitoring & Non-Proliferation

Hawaii

Saclay

LLNL


Search for sterile with reactor neutrinos
Search for Sterile ν with Reactor Neutrinos

SCRAAM: The Southern California Reactor Antineutrino Anomaly Monitor

core ∅: ~3m, baseline: 24m

Only one baseline, extended core

proposal in preparation

Bowden, SNAC 2011


Search for sterile with reactor neutrinos1
Search for Sterile ν with Reactor Neutrinos

NUCIFER at Osiris

core: σ~0.3m

baseline: 7m

Only one baseline, small core

Lhuillier, AAP2011


Search for Sterile ν with Reactor Neutrinos

An Opportunity for NIST?

- Close access to reactor core. Closest distance to reactor core compared to other experimental efforts.

- Relatively compact core. Burnup well-understood. Detailed predictions for antineutrino flux possible.

- Reactor-off periods would allow detailed studies of backgrounds. Not possible at San Onofre, or other commercial reactors.

- Place m3-sized antineutrino detector at multiple baselines


A m 3 sized antineutrino detector at nist
A m3 sized Antineutrino Detector at NIST?

May need to add muon veto.

For a 1.5-ton Gd-doped liquid scintillator target mass with 40% efficiency,

~ 610 signal events/day at 5m distance. 

< 1% stat error in 1 month of data taking.


A m 3 sized antineutrino detector at nist1
A m3 sized Antineutrino Detector at NIST?

May need to add muon veto.

L~14m

If detector movable over 3-4m, we can map out oscillation cycle.

Can build multiple modules at different baselines and map out oscillation

2

6

10

14

distance from core (m)


Summary
Summary

  • Long-standing anomalies in accelerator and reactor neutrino experiments suggest possible existence of 4th (sterile) neutrino with Δm2 ~ O(1-2 eV2)

  • Definitive oscillation experiment needed. For reactor antineutrinos with <E> ~ 3-4 MeV we need baselines of 1-10m. Oscillation length L ~3 m

  • Significant progress in reactor monitoring with antineutrinos over past decade. Several m3-sized detectors deployed and taking data. Detectors on or near surface under little overburden.

  • Small but powerful reactor core, close distance to core, and variable baseline key. Reactor-off periods critical for measuring backgrounds. For typical parameters data taking time 1-2 months per baseline.

    NIST may have unique space for such an experiment.


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