Experimental search for the cosmic
background neutrino decay in the cosmic
far-infrared background
TGSW 2014
Sep. 29th, 2014 / University of Tsukuba, Japan

Yuji Takeuchi (Univ. of Tsukuba)
for Neutrino Decay Collaboration
S.H.Kim , K. Takemasa, K.Kiuchi , K.Nagata , K.Kasahara , T.Okudaira, T.Ichimura,M.Kanamaru, K.Moriuchi, R.Senzaki(Univ. of Tsukuba), H.Ikeda, S.Matsuura,T.Wada(JAXA/ISAS), H.Ishino, A.Kibayashi (Okayama Univ), S.Mima (RIKEN),
T.Yoshida, S.Komura, K.Orikasa, R.Hirose(Univ. of Fukui), Y.Kato(Kindai Univ.),
Y.Arai, M.Hazumi(KEK), E.Ramberg, J.H.Yoo, M.Kozlovsky, P.Rubinov,D.Sergatskov(Fermilab) S.B.Kim(Seoul National Univ.)
1

Contents• Introduction to neutrino decay search– Proposed rocket experiment• Candidates for far-infrared single photon
detector/spectrometer– Nb/Al-STJ with diffraction grating– Hf-STJ• Summary2

Neutrino• Neutrino has 3 mass generations (1, 2, 3)• Neutrino flavor states (e, , ) are not mass eigenstates
𝜈𝑒𝜈1𝑈𝑒1 𝑈𝑒2 𝑈𝑒3
𝜈𝜇 = 𝑈𝜇1 𝑈𝜇2 𝑈𝜇3 𝜈2𝜈𝜏𝜈3𝑈𝜏1 𝑈𝜏2 𝑈𝜏3 Neutrino can decay through the loop diagram– 𝜈3 → 𝜈1,2 + 𝛾γ

𝜈1,2e, 𝜇, 𝜏𝑊

𝜈3• Neutrino lifetime is very long
Cosmic neutrino background (CB) is the best neutrino
source for neutrino decay search3Cosmic neutrino background (C𝜈B)

CMB

𝑛𝛾 = 411/cm3
𝑇𝛾 = 2.73 K

CB (~1s after big bang)
3 𝑇𝜈
𝑛𝜈 + 𝑛𝜈 =4 𝑇𝛾

3Not yetobserved
experimentally𝑛𝛾 = 110 cm3134𝑇𝜈 =𝑇𝛾 = 1.95K
11𝑝𝜈 = 0.5meV/c4

Motivation of -decay search in CB• Search for 𝜈3 → 𝜈1,2 + 𝛾 in cosmic neutrino background (C𝜈B)– Direct detection of C𝜈B– Direct detection of transition magnetic dipole moment of neutrino
2– Direct measurement of neutrino mass: 𝑚3 = 𝑚32 − 𝑚1,22𝐸𝛾• Aiming at sensitivity of detecting 𝛾 from 𝜈 decay for 𝜏 𝜈3 = Ο 1017 yr𝑠– SM expectation 𝜏 = Ο(1043 yr𝑠)
– Current experimental lower limit 𝜏 > Ο(1012 yr𝑠)
– L-R symmetric model (for Dirac neutrino) predicts down to 𝜏 = Ο(1017 yr𝑠)for 𝑊𝐿 -𝑊𝑅 mixing angle 𝜁 < 0.02
Neutrino magneticmoment term

𝜈𝑗𝐿 𝜎𝜇𝜈 𝜈𝑖𝑅γ𝜈𝑗𝐿𝜈𝑖𝑅

SM: SU(2)Lｘ U(1)Y𝜈1,2𝐿γ
𝑊𝐿𝑒𝐿 , 𝜇𝐿 , 𝜏𝐿
𝜈3𝐿
𝜈3𝑅𝑚𝜈3

𝚪~ 𝟏𝟎𝟒𝟑 𝒚𝒓−𝟏

Suppressed by 𝑚𝜈 , GIM

LRS: SU(2)LｘSU(2)RｘU(1)B-Lγ
𝑊1

𝜈1,2𝐿

𝜏𝐿𝑚𝜏
𝜏𝑅

≃ 𝑊𝐿 − 𝜁𝑊𝑅𝚪~ 𝟏𝟎𝟏𝟕 𝒚𝒓

𝜈3𝑅−𝟏

PRL 38,(1977)1252, PRD 17(1978)1395

𝑊1cos𝜁
=𝑊2sin𝜁−sin𝜁cos𝜁

𝑊𝐿𝑊𝑅1026
enhancement toSM

Only suppressed by 𝜁~0.025Photon Energy in Neutrino Decay•𝐸𝛾2– Δ𝑚23= |𝑚32 − 𝑚22 | = 2.4 × 10−3 𝑒𝑉 22– Δ𝑚12= 7.65 × 10−5 𝑒𝑉 2𝜈3𝜈2•Two body decay

ν3 → 𝜈1,2 + 𝛾2
𝑚32 − 𝑚1,2𝐸𝛾 =2𝑚3

m3=50meVE =24.8meV(50𝜇𝑚)
m2=8.7meV

m1=1meV

From neutrino oscillationE =24meV(52𝜇𝑚)

E =4.4meV(282𝜇𝑚)From Planck+WP+highL+BAO– ∑𝑚𝑖 < 0.23 eV

 50meV<𝑚3 <87meV
𝑬𝜸 =14~24meV (𝝀𝜸 =51~89m)

𝐸𝛾 distribution in ν3 → 𝜈2 + 𝛾
dN/dE(a.u.)𝛾

𝑚3 = 50 meVRed Shift effectSharp Edge with
1.9K smearing

𝐸𝛾 [meV]
25meV(50𝜇𝑚)
6Surface brightness I [MJy/sr]

Backgrounds to C𝜈B decay
CMB

ZE

AKARI COBE

CIB
𝜆𝐼𝜆 ~0.1-0.5MJy/sr

ZL

ISD
DGL

Zodiacal Emission
𝐼𝜈 ~8MJy/srCB decay

SLwavelength [m]E [meV]

Expected 𝑬𝜸 spectrum

𝑚3 = 50meVCB decay𝜏 = 5 × 1012 yr𝑠𝐼𝜈 ~0.5MJy/sr
𝜏 = 1 × 1014 yrs
𝐼𝜈 ~25kJy/sr
at λ＝50μm
7Detector requirements• Need continuous spectrum of photon energy around 𝜆=50𝜇m(far infrared photon) with highly precise accuracy– Photon-by-photon energy measurement with better than 2% resolution
for 𝐸𝛾 = 25meV (𝜆=50𝜇m ) to achieve better S/N as well as to identify
the sharp edge in the spectrum– A ground based experiment is impossible, so rocket and/or satellite
experiment with this detector is required• Superconducting Tunneling Junction (STJ) detectors in development– Array of 50 Nb/Al-STJ pixels with diffraction grating covering 𝜆 = 40 − 80𝜇m• For rocket experiment aiming at improvement of current lower limitfor 𝝉(𝝂𝟑 ) by 2 order : O(1014yr) in a 200-sec measurement– STJ using Hafnium: Hf-STJ for satellite experiment• Δ = 20𝜇eV : Superconducting gap energy for Hafnium• 𝑁q.p. = 25meV 1.7Δ = 735 for 25meV photon: Δ𝐸 𝐸 < 2% if Fano-factor isless than 0.38

STJ(Superconducting Tunnel Junction）Detector• Superconducting / Insulator /Superconducting
Josephson junction device

2Δ

Δ: Superconducting gap energy

A bias voltage (|V|<2Δ) is applied across the junction.
A photon absorbed in the superconductor breaks Cooper pairs and createstunneling current of quasi-particles proportional to the photon energy.
95mm

STJ examples
5.9KeV X-ray

100m x 100m5mm

H. Sato (RIKEN)• STJs are already in practical use as a single photon
spectrometer for a photon ranging from near-infrared to
X-ray, and shows excellent performances comparing toconventional semiconductor detectors
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STJ energy resolution
Statistical fluctuation in number of quasi-particles determines STJ energyresolution
Smaller superconducting gap energy Δ yields better energy resolution

𝜎𝐸 =Δ: Superconducting gap energy
F: fano factor
E: Photon energy

1.7Δ 𝐹𝐸SiNbAl

Hf9.23

1.20

0.165

Δ[meV] 1100 1.550

0.172

0.020

Tc[K]Nb

Tc :SC critical temperature
Need ~1/10Tc for practicaloperation

Well-established as Nb/Al-STJ

Hf

Nq.p.=25meV/1.7Δ=9.5

Nq.p.=25meV/1.7Δ=735(back-tunneling gain from Al-layer)

Hf-STJ as a photon detector is not established

Poor energy resolution, but
photon counting is possible inprinciple

2% energy resolution is achievable if Fano factor <0.311

Proposal of a rocket experimentExpect 200s measurement at altitude of 200~300km


Telescope with diameter of 15cm and focal length of 1m
All optics (mirrors, filters, shutters and grating) will be cooled below 4K

Diffraction grating covering =40-80m (16-31meV) and array of Nb/AlSTJ pixels: 50()x8() Use each Nb/Al-STJ pixel as a single-photon counting detector for FIR photonof =40−80m (Δ𝜆 = 0.8𝜇𝑚)
 sensitive area of 100mx100m for each pixel (100rad x 100rad)Nb/Al-STJarrayΔ𝜃Δ𝜆𝜆 = 40 − 80𝜇m
𝐸𝛾 = 16~31meV
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Surface brightness I [MJy/sr]

Expected accuracy in the spectrum measurement
Zodiacal Emission

CMB

Telescope parameters• Main mirror– D=15cm, F=1m

• detector

Zodiacal LightISDDGLCB decay

SL

Integrated flux from galaxy counts

wavelength [m]

• Zodiacal emission ⇒ 343Hz / pixel– sensitive area
100mx100m/ pixel– 50x8 arrayΔ𝜆 =40𝜇𝑚= 0.8𝜇𝑚50

𝜏 = 1 × 1014 yr– 200sec measurement: 0.55M events / 8 pixels (at 𝜆 = 50𝜇𝑚)– 0.13% accuracy measurement for each wavelength: 𝜹 𝑰𝝂 =11kJy/sr• Neutrino decay (𝑚3 = 50 meV, 𝜏𝜈 = 1 × 1014 yr): 𝑰𝝂 =25kJy/sr– 2.3σ away from statistical fluctuation in ZE measurement

 decay with 𝝉𝝂 = 𝟏𝟎𝟏𝟒 yrs is possible to detect, or set lower limit!

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Summary• We propose an experiment to search for neutrino radiative
decay in cosmic neutrino background.
• Requirements for the detector is an ability of photon-byphoton energy measurement with better than 2% energy
resolution for 𝐸𝛾 = 25 meV (𝜆 = 50𝜇𝑚)• Nb/Al-STJ array with grating and Hf-STJ are considered forthe experiments and under development.

• It is possible to improve the neutrino lifetime lower limit up toO(1014yrs) for 200-sec measurement in a rocket experimentwith the detector.
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Backup15

Energy/Wavelength/Frequency

𝐸𝛾 = 25 meV

𝜈 = 6 THz
𝜆 = 50𝜇𝑚
16STJ I-V curve• Sketch of a current-voltage (I-V) curve for STJ
 The Cooper pair tunneling current (DC Josephson current) is seen at V =0, and the quasi-particle tunneling current is seen for |V|>2

2ΔB field
Leak current

Josephson current is suppressed by magnetic field
17

STJ back tunneling effect• Quasi-particles near the barrier can mediate Cooper pairs, resulting intrue signal gain

– Bi-layer fabricated with superconductors of different gaps Nb>Al to enhancequasi-particle density near the barrier– Nb/Al-STJ Nb(200nm)/Al(10nm)/AlOx/Al(10nm)/Nb(100nm)• Gain: 2～200

PhotonNbAlAlNb
18

Feasibility of VIS/NIR single photon detection• Assume typical time constant from STJ response to pulsed
light is ~1μs
• Assume leakage is 160nA160𝑛𝐴 = 𝑒 × 1012 𝑠 = 𝑒 × 106 𝜇𝑠
Fluctuation from electron statistics in 1μs is𝑒 × 106 𝜇𝑠 = 103 𝑒 𝜇𝑠
While expected signal for 1eV are(Assume back tunneling gain x10)
1𝑒𝑉

1𝑒𝑉
1.7Δ × 10𝑒 =× 10 = 4 × 103 𝑒
1.7×1.5𝑚𝑒𝑉More than 3sigma away from leakage fluctuation

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Feasibility of FIR single photon detection• Assume typical time constant from STJ response to pulsed light is ~1μs
• Assume leak current is 0.1nA
0.1𝑛𝐴 = 6.25 × 108 𝑒 𝑠 = 6.25 × 102 𝑒 𝜇𝑠
Fluctuation due to electron statistics in 1μs is
6.25 × 102 𝑒 𝜇𝑠 = 25 𝑒 𝜇𝑠
While expected signal charge for 25meV are25meV 1.7Δ × 10𝑒 =•25meV× 10𝑒 = 98𝑒
1.7×1.5meV(Assume back tunneling gain x10)More than 3sigma away from leakage fluctuation
Requirement for amplifier• Noise<16e• Gain: 1V/fC  V=16mV

20

Temperature dependence of Nb/Al-STJ leak current
21This Nb/Al-STJ is
produced by S. Mima(Riken)

Need Ileak<0.1nA for single photon
counting with S/N>10340um2 Nb/Al-STJ I-V curve

Temperature dependence of leak current
Junction size: 100x100um2

Junction size: 40um2

 10nA at T=0.9K
 2nA

T=0.4K
Magnetic field on
Rref=1𝑀Ω

2nA @0.5mV

𝑰

𝑽5nA
500μV

Leak current can be reduced by using
small junction size. We are testing STJof 4m2 junction size

Need T<0.9K for detector operation
 Use 3He sorption or ADR for theoperation in rocket experiment21

T. Okudaira

100x100m2 Nb/Al-STJ response to NIR multi-photons

Response to NIR laser pulse（λ=1.31μm)
10 laser pulses in 200ns

1kNIR laser
through opticalfiber

50μV/DIVI

0.8μs/DIV Observe voltage drop by 250μV

STJV

Signal pulse height distribution

T=1.8K(Depressuring LHe)

We observed a response to NIR photons• Response time ~1μs
• Corresponding to 40 photon detection(estimated by statistical fluctuations innumber of detected photons)

Pulse height dispersion isconsistent with ~40 photons

SignalPedestal-Pulse height (a.u.)22

T. Okudaira4m2 Nb/Al-STJ response to VIS light at single photon level
Assuming a Poisson distribution convoluted with Gaussian which hassame sigma as pedestal noise:
𝑓 𝑥; 𝜇 = 𝑁tot

event

𝑛T=1.8K(Depressuring LHe)

𝑀
𝑥−𝑛𝑛 −𝜇𝜇 𝑒
1𝜇⋅exp −𝑛!
2𝜎𝑝2
2𝜋𝜎𝑝2Best fit f(x;)x2

ndf=85

1photon0photon2photon3photon𝜒 2 minimum at μ=0.93μ𝑁𝛾 = 0.93+0.19−0.14
pulse hight(AU)Currently, readout noise is dominated, but we are detecting VIS light atsingle photon level

23

Development of SOI-STJ

K. Kasahara• SOI: Silicon-on-insulator– CMOS in SOI is reported to work at 4.2K by T. Wada (JAXA), et al.
Phys. 167, 602 (2012)• A development of SOI-STJ for our application with Y. Arai (KEK)– STJ layer sputtered directly on SOI pre-amplifier• Started test with Nb/Al-STJ on SOI with p-MOS and n-MOS FETSTJ Nb
SOI

metal padSTJ

STJ lower layer has electricalcontact with SOI circuit24

Hf-STJ development• We succeeded in observation of Josephsoncurrent by Hf-HfOx-Hf barrier layer in 2010
(S.H.Kim et. al, TIPP2011)

Hf(250nm)
Hf(350nm)
Si wafer

A sample in 2012
B=0 Gauss

K. Nagata

B=10 Gauss

HfOx：20Torr,1hour
anodic oxidation：
45nm

200×200μm2
T=80~177mK

Ic=60μA
Ileak=50A@Vbias=10V
Rd=0.2Ω

However, to use this as a detector, much improvement in leakcurrent is required. (𝐼leak is required to be at pA level or less)25

K. Nagata

Hf-STJ Response to DC-like VIS light
Laser ON

Laser pulse: 465nm, 100kHz

~10μA

I

Laser OFFV
50μA/DIV

10uA/100kHz=6.2×108e/pulse

20μV/DIVWe observed Hf-STJ response to visible light26

4m2 Nb/Al-STJ response to DC-like VIS light
VIS laser pulse (465nm) 20MHz illuminated on 4m2 Nb/Al-STJ

Laser OFFLaser ON

10nA/20MHz=0.5×10-15 C =3.1×103 e~10nA

I

0.45 photons/laser pulse is given inprevious slide

V

Gain from Back-tunneling effect
Gal=6.720nA/div
0.5mV/div

27

Development of SOI-STJ
1 mA /DIV

1 mA /DIV

2mV /DIV

2mV /DIV

50uA /DIV
2mV /DIV

B=150 gauss• We formed Nb/AlSTJ on SOI• Josephson current
observed• Leak current is 6nA@Vbias=0.5mV,
T=700mK
10 nA /DIV
500uV /DIVIDS[A]

B=0n-MOSp-MOS

0.7V

by K. Kasahara-0.7V

VGS[V]• n-MOS and p-MOS
in SOI on which
STJ is formed• Both n-MOS and pMOS works at
T=750~960mK28

STJ on SOI response to laser pulse
SOI上のNb/Al-STJの光応答信号
Iluminate 20 laser pulses (465nm, 50MHz) on 50x50um2 STJ which is formed on SOI
Estimated number of photons from output signal pulse
height distribution assuming photon statistics

𝑀2Nγ = 2 2 ~ 206±112
σ −σ𝑝

M : Meanσ : signal RMS
σp : pedestal RMS

NoisePedestal

SignalSignal
500uV /DIV.
1uS /DIV.

Confirmed STJ formed on SOI responds to VIS laser pulses
29