Search for Sterile NeutrinosSearch for Sterile Neutrinos

Kim SiyeonChung‐Ang University  

2014 SpringYonsei Workshop on Particle Physics

April 11

Search for Sterile NeutrinosSearch for Sterile Neutrinos

Outline

1 A i S il i1. Active  vs. Sterile neutrinos2. Cosmological hints3. LSND  and  MiniBooNE4. Reactor  antineutrino anomaly5. Absolute flux estimation at RENO6. Prospect and Conclusion

Active Neutrinos vs Sterile NeutrinosActive Neutrinos vs. Sterile Neutrinos

l k• Only 3 neutrinos are active in weak interaction.• Massive but very light, with mixed flavor:

sin2 12 ~ 0.3, sin2 23 ~ 0.5, sin2 13 ~ 0.023, ,• Mass Hierarchy: Normal or Inverted• Mass‐driven oscillation: sin2 (1.27m2 L/E)

• NOT active in the Standard‐Model interaction.• Oscillation observables:

– Too much disappearance of active neutrinos– Too much appearance of active neutrinosToo much appearance of active neutrinos– Indirect evidence by combined fit of data– If m212  << m312  << m412  : Short baseline anomaly

No of neutrinos in CosmologyNo. of neutrinos in Cosmology

• Cosmological Hints (maybe model‐dependent):Before BICEP2

– Ns = number of thermalized sterile neutrinos– CMB and LSS : Ns = 1.3 +/‐ 0.9, ms < 0.66 eV (95% C.L.)– BBN : Ns < 1,  (95% C.L.)

CMB LSS BBN N 0 85 0 39( 0 56) (95% C L )– CMB + LSS + BBN: Ns = 0.85 +0.39(‐0.56) (95% C.L.)– Standard CDM: 3+1 allowed, 3+2 disfavored

• Unidentified X ray emission from galaxies (Bulbul et al 2014)• Unidentified X‐ray emission from galaxies (Bulbul et al. 2014)– E = (3.55~3.57) +/‐ 0.03 keV– Ms =2E=7.1 keV, and sin2(2) ≈7ⅹ10-11

No of neutrinos in CosmologyNo. of neutrinos in Cosmology

• Light sterile neutrinos ( Archidiacono et al. 2014) 

After BICEP2

– WP: Wilkinson Microwave Anisotropy Probe– High‐l: Atacama Cosmology Telescope + South Pole Telescope (Temp 

fluctuation power, 500<l<3500, 650<l<3000, respectively ), ( )compared to Planck ( l upto 2479)

No of neutrinos in CosmologyNo. of neutrinos in Cosmology

After BICEP2• Light sterile neutrinos ( Archidiacono et al. 2014) 

– LSS: frin WiggleZ Dark Energy Survey– H0: Cepheid distance with Hubble Space Telescope– CFHTLenS: Canada‐France‐Hawaii Telescope Lensing Survey– PSZ : Planck‐Sunayev‐Zel’Dovich catalogue of galuxy clusters 

No of neutrinos in CosmologyNo. of neutrinos in Cosmology

After BICEP2

Including terrestrial neutrino oscillation

• Model dependency:p yThermalized sterile neutrinos conflict with terrestrial neutrino oscillations  .

Sterile neutrinos in our planetSterile neutrinos in our planet

Appearance at LSND and MiniBooNE • LSND: The first evidence in favor of oscillations beyond 3‐flavor 

f kframework.

Sterile neutrinos in our planetSterile neutrinos in our planet

Appearance at LSND and MiniBooNE • MiniBooNE: designed to test LSND, in both neutrino mode and anti‐

neutrino modeneutrino mode.– In neutrino mode: Disfavor most of the parameter space preferred by LSND.– In anti‐neutrino mode: partially consistent with oscillations at m2 ~1eV^2. 

An excess of events at low energy outside the LSND type oscillation energy– An excess of events at low energy, outside the LSND‐type oscillation energy range, was reported.

Sterile neutrinos in our planetSterile neutrinos in our planet

Appearance at LSND and MiniBooNE • MiniBooNE: designed to test LSND, in both neutrino mode and anti‐

neutrino modeneutrino mode.– In neutrino mode: Disfavor most of the parameter space preferred by LSND.– In anti‐neutrino mode: partially consistent with oscillations at m2 ~1eV^2. 

An excess of events at low energy outside the LSND type oscillation energy– An excess of events at low energy, outside the LSND‐type oscillation energy range, was reported.

Sterile neutrinos in our planetSterile neutrinos in our planet

Reactor Antineutrino Anomaly• Observed‐to‐ expected ratio based on old & new spectra. (Mention et al., 

Phys Rev D83 2011)Phys.Rev.D83 2011)• Neutron life time: 885.7s (PDG2012)

Absolute flux estimationAbsolute flux estimation

Far-to-near ratio does depend on

Physical meaning of this gap?

not the normalization of the expected flux, but the slope of the curve.

Multi-detector observations, RENO and Daya Bay,

RENO and Daya Bay

Physical meaning of this gap?Reactor neutrino anomaly

Multi detector observations, RENO and Daya Bay, determined the definite value of angle 13.

Short-baseline oscillation

Measurement of antineutrinos from reactors

NNν̄ =

Np²4πR2 · hσf i · PthhEf i

• Pth:Thermal power

• Np:No of protons in

• <f>:Average Thermal power

• <Ef>:Average energy per 

No. of protons in target = size of the detector

Average cross‐section 

fission• Pth / <Ef>:

Fission rate # of

• :Detection efficiency

• R:

per fission

Fission rate, # of fissions per unit time

• R: baseline, the distance from the reactor core to the detector

Fission rate inside a reactorFission rate inside a reactor

• LWR (Light Water Reactor)> BWR (Boiling WR)> PWR (Pressurized WR)> PWR (Pressurized WR)

• Core thermal power = reactor thermal power – small heatthermal power  small heat from reactor coolant pumps

• In a PWR, ,235U (enrichment) is 3~4% 238U is 95%.

• ‐decay dominant 4 isotopes are 235U, 239Pu, 241Pu and 238U.

Isotope evolution of a typical PWRFission rates at Palo Verde reactor cores.

Fission rate inside a reactorFission rate inside a reactor

E (M V)• Ei (MeV): energy release per fission

• Fission fraction of 4 isotopes,  235U, 239Pu, 241Pu and 238U.

HanbitReactor1

Isotope James (1969)

Kopeikin(2004)

235U 201.7±0.6 201.92±0.46U235(Blue)

Pu239(Red)

238U 205.0±0.9 205.52±0.96239Pu 210.0±0.9 209.99±0.60

• i : averaged fraction quotedPu241(Green)U238(Purple)

241Pu 212.4±1.0 213.60±0.65

(235U :  238U :  239Pu :  241Pu) = (0.574 : 0.081: 0.293 : 0.052)

•• i : the relative fraction of i‐th hEf i =

Pi αiEi

(0.574 : 0.081: 0.293 : 0.052)isotope

Fission rate inside a reactorFission rate inside a reactor

• Fission fraction of 4 isotopes,  235U, 239Pu, 241Pu and 238U.

HanbitReactor1

U235(Blue)

Pu239(Red)

Pu241(Green)U238(Purple)

•• i : the relative fraction of i‐thhEf i =

Pi αiEi

isotope

Measurement of antineutrinos from reactors

NNν̄ =

Np²4πR2 · hσf i · PthhEf i

• Pth:Thermal power

• Np:No of protons in

• <f>:Average Thermal power

• <Ef>:Average energy per 

No. of protons in target = size of the detector

Average cross‐section 

fission• Pth / <Ef>:

Fission rate # of

• :Detection efficiency

• R:

per fission

Fission rate, # of fissions per unit time

• R: baseline, the distance from the reactor core to the detector

Average cross section per fissionAverage cross section per fission

h i P Rφ dEhσf i =

Pi αi

RφiσdE•

• For each isotope,  Y =RφiσdEp

Rφi

is the spectral flux. Neutrinos produced in beta decays decrease depending

is the V-A cross-section of weak interaction , energy dependent IBD rate inon energy.

[reactor]dependent IBD rate in the detector.

Average cross section per fissionAverage cross section per fission

h l• There is no relevant measurement for 238U. Only theoretical method was relied on. 

• Spectrum of neutrinos from  decays of fission fragments:

• In 1980’s, ILL measurements of • “Summation method”

Add up all possible hypothetical beta branches ~10% uncertainty

In 1980 s, ILL measurements of spectrum for 235U, 239Pu and 241Pu.

• Conversion to  spectrum is trivial.E Q E branches.  10% uncertainty 

unavoidable. Eν̄ ≈ Q−Eβ

K. Schreckenbach et al., A. A. Hahn et al. (1985)

Average cross section per fissionAverage cross section per fission

h l• There is no relevant measurement for 238U. Only theoretical method was relied on. 

• Spectrum of neutrinos from  decays of fission fragments:

• In 1980’s, ILL measurements of • “Summation method”

Add up all possible hypothetical beta branches ~10% uncertainty

In 1980 s, ILL measurements of spectrum for 235U, 239Pu and 241Pu.

• Conversion to  spectrum is trivial.E Q E branches.  10% uncertainty 

unavoidable. Eν̄ ≈ Q−Eβ

CON! (Hayes, 2013)

There are 30% forbidden decay channels.The uncertainty should be much larger. The about 6% deficit cannot be confirmed from the current information on reactor fission chains.

K. Schreckenbach et al., A. A. Hahn et al. (1985)

Average cross section per fissionAverage cross section per fission

Average cross section per fissionAverage cross section per fission

• IBD cross section

Average cross section per fissionAverage cross section per fission

• IBD cross section

Average cross section per fissionAverage cross section per fission

• IBD cross section

ProspectProspect

Far-to-near ratio does depend on

Physical meaning of this gap?

not the normalization of the expected flux, but the slope of the curve.

Multi-detector observations, RENO and Daya Bay,

Absolute flux estimation

Physical meaning of this gap?Reactor neutrino anomaly

Multi detector observations, RENO and Daya Bay, determined the definite value of angle 13.

Short-baseline oscillation

ProspectProspect

Absolute flux estimation

G II

P 1

SRP‐IIGoesgen‐II

Goesgen‐IKrasnoyarsk

‐III

P 0 9

Bugey‐3,4

B 3

RENO

ND

P 0.9

ROVNO88,91

SRP‐I

ROVNO88‐2SKrasnoyarsk‐I

Bugey‐3

Goesgen‐III

RENO

FD

100 350 500

P 0.8

ILLKrasnoyarsk‐II

Bugey‐3ToyanalysisbyY.Ko

distance‐probabilitycurve100m 350m 500m

1400m

Concluding remarksConcluding remarks

Issues for sterile neutrinos

1. Three anomalies: LSND(MiniBooNE), Reactor Antineutrino Anomaly, Gallium Anomaly.

2. Although we have several hints for sterile neutrinos, there is no fully consistent picture so far.• Neutrino mode vs anti‐neutrino modeNeutrino mode vs. anti neutrino mode• Disappearance vs. appearance oscillation• Terrestrial experiment vs. cosmology observation

3. The presence of additional neutrino mass states can affect the interpretation of neutrino‐less double beta decay experiments.

Neutrinoless double beta decayNeutrinoless double beta decay

• 3+1 or 1+3 ?