J-PARC MLFにおけるステライルニュートリノ探索実験

J-PARC MLFにおけるステライル
ニュートリノ探索実験
平岩聡彦
阪大RCNP
on behalf of the J-PARC P56 collaboration
(We requested the 1st stage approval at the 19th PAC on Dec 2014.)
Contents
• Introduction
- LSND
- Current status of sterile neutrino searches.
• Sterile neutrino search at MLF (J-PARC P56)
- Experimental principle.
- Experimental features.
• Background measurement at candidate
sites
- Results
- Sensitivity
• Summary and outlook 21st ICEPP Symposium
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Introduction
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LSND anomaly
• LSND:
signal: nm  ne (appearance)
PRD 64 (2001) 112007
• Using m+ decay at rest (m+ DAR):
𝜋 + ⟶ 𝜇+ 𝜈𝜇
𝑒 + 𝜐𝑒 𝜐𝜇
Oscillations ?
𝜐𝑒
• Detected by Liq. Scinti.:
nep  e+n (IBD),
followed by neutron capture g
(2.2 MeV)
• Excess events:
87.9 ± 22.4 ± 6.0 events.
 3.8 s evidence for oscillation  Sterile neutrino(?)
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Status of sterile neutrino search (Dm2 ~ 1 eV2)
• Positive results:
Experiments
Neutrino
source
signal
type
Significance
σ
LSND
μ Decay-At-Rest
νμ→νe
appearance
3.8
MiniBooNE
π Decay-In-Flight
νμ→νe
appearance
3.4
νμ→νe
appearance
2.8
combined
3.8
Gallium
e capture
νe→νX
disappearance
2.7
Reactors
Beta decay
νe→νX
disappearance
3.0
• There are several negative results:
- MiniBooNE (disappearance).
- KARMEN etc.
• A definite search is awaited. (high statistics and low background)
Sterile neutrino search at J-PARC MLF
(J-PARC P56 experiment)
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Material and Life science Facility (MLF)
400 MeV Linac
Candidate site (3F)
L = 24 m 3 GeV
Synchrotron
Rapid Cycle Synchrotron
Energy:3GeV
Repetition:25Hz
Design Power:1MW
40ms
600ns
100ns x2
J-PARC P56 experiment
• Search for the LSND anomaly using m+
decay at rest (m+DAR) :
- nm  ne (appearance).
• Detector:
- Gd-loaded liquid scintillator.
(25 tons x 2 ~ total 50 tons)
• Measurement principle:
- “Delayed Coincidence”:
- ne + p  e+ + n (IBD)
prompt signal
- 8 MeV g from n-capture
by Gd  delayed signal
(capture time
~ several tens msec)
- En = Ee(visible) + 0.8 MeV
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Experimental
features
beam bunch
• Pulsed beam and muon long life
time enable the separation of
mDAR. (top fig.)
• Due to nuclear absorption,
neutrinos from p- and m- decay
(main BG) are highly suppressed
to the same level of the signals.
 Signature of oscillation by
spectrum shape. (bottom fig.)
• Well-defined energy spectrum
shape of n from mDAR.
• Well-known cross section for IBD
(ne + p  e+ + n).
mDAR
Dm2 = 5.5 eV2
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Neutrino energy
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Background measurements at
candidate sites
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Background measurement
• BKG measurements were performed at the
candidate sites (MLF 3F). (Apr-Jun 2014)
• Detectors:
- 500 kg plastic scintillation counter
(yellow): main detector
veto eff:
- Inner veto counter (red).
- Outer veto counter (green).
> 99.9 %
• 2 different data sets:
- beam-on
- beam-off (to subtract the beam-unrelated
BKGs.)
• 3 different points: point-1, 2, 3.
• The results for “point 2”(L ~ 20 m) are
presented here.
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Point1: L ~ 17 m
Point2: L ~ 20 m
Point3: L ~ 40 m
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BKG(1): Michel-e from beam fast n
On bunch
νe + p→
e+ + e- →2γ
e+ ,
IBD
No
activities
n+C→X+π
→m
Michel-e
by beam n
Clipping
muons
(Cosmic)
delayed
Prompt
+
Selection criteria
n
+Gd →γ
time
〜30μs
〜2.2μs
μ→e
• Prompt signal:
1 < Tp [ms] < 10
20 < Ep [MeV] < 60
• Delayed signal:
Tp < Td [ms] < 100
7 < Ed [MeV] < 12
Thermalized n
captured by Gd
time
time
Huge, but rejected by
charged veto (eff > 99.9 %)
• Beam-associated fast neutrons (T > 200 MeV) can produce pions, followed by
Michel electrons. (p  m  e)
• Michel electrons from beam fast neutrons:
- Michel-e from beam fast neutron mimics the IBD signals.
• The “Michel-e” signals have activities on bunch timing, whereas the “IBD”
signals have no activities on bunch timing.
Before “on-bunch activity cut”
Energy vs Hit Time
Beam bunch
projection
after veto
Before charged cosmic
veto
After “on-bunch activity cut”
(require Eonbunch>4MeV)
“beam on”
/spill/300kW
“beam off”
/spill/300kW
Before cosmic
veto
(1.68±0.03)×10
-4
(1.64±0.03)×104
(4.0±4.2)×10-6
After veto
(1.58±0.09)×10
(1.52±0.09)×10-
(0.6±1.3)×10-6
(4.91±0.28)×10-
(-0.3±1.6)×10-7
(90%C.L. UL；=
<13 /5y/50t/MW）
20<E<60MeV
1.75<t(ms)<4.65
-5
p bunch
After on-bunch
cut
(4.60±1.53)×10
-7
5
7
(expectation)
subtraction
No Michel-e from beam fast n !!
= beam off data (veto) x
accidental on-bunch
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BKG(2): Accidental backgrounds
• Accidental background rate:
Racc = Rprompt x Rdelay x Dvtx x Nspill
- Rprompt: BKG rate for prompt signal.
- Rdelay: BKG rate for delayed signal.
- Dvtx: Rejection factor of spatial correlation cut
(= 1/50)
- Nspill: # of spills (= 1.5 x 109 /5 years)
• Rprompt and Rdelay were measured:
- Prompt: cosmic gammas and neutrons.
- Delayed: - Beam associated gammas.
- Beam neutrons
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Cosmic g and n (Prompt):
• Measurements using small NE213(< 1 kg)
and NaI (2’’f x 2’’) @ Tohoku. (identify g
and n)
• Scaled to 500 kg scinti. at MLF 3F.(right fig)
 Consistent within 6%.
• Gammas and neutrons are dominant.
(neutrons can be removed by PID of the
P56 detector.)
Beam-associated g (Delayed):
• Beam neutrons are thermalized and
captured by the concrete floor, and
g’s are emitted.
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12.5 cm thickness lead under
the detector  1/1000 g’s
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BKG summary and Sensitivity
Source
Contents
Number of Event/50t/5y
Comments
BG
νe from μ-
237
Main BKG. L = 24.
12C(ν ,e )12N
e g.s
16
Michel-e from beam fast n
<13 (90%Cl UL)
Fast neutron (cosmic)
37
Accidental
32
Based on measurement.
480
Δm2=3.0 sin2θ=0.003
342
Δm2=1.2 sin2θ=0.003
Signal
Sensitivity (MW x 5 years, L = 24 m)
Based on measurement.
5s sensitivity as a function of MW x years
Δm2>2.0eV2
LSND 90%CL
Allowed region
(lower edge)
(high Dm2 region)
We can discuss the all LDND allowed region (90% C.L.) with 3s (MW x 5 years).
Especially for Dm2 > 2.0 eV2, we can conclude with 5s (MW x 4 years).
Summary and outlook
• We plan to perform a definite search for sterile
neutrinos at J-PARC MLF.
• Background measurements at the candidate sites
were performed, and the experimental feasibility
was examined.
• We can conclude all the LSND allowed regions
(90 % C.L.) with 3s. (1MW x 5 years)
• We can start the experiment within 1-2 years after
getting the budgets.
New challengers, especially young scientists,
are very welcome. Please join us !!
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J-PARC P56 collaboration
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Backup slide
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Success in RCS 1-MW trial
NOTE: This is a very short term test.
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BKG(2): Accidental BKG for “Prompt”
• Acccidental BKG:
Racc = Rprompt x Rdelay x Dvtx x Nspill
- Rprmpt, Rdelay: BG rates for prompt and delay.
- Dvtx: spatial correlation (rejection power: 1/50)
- Nspill: # of spills: 3x108/year
• Measurement @ Tohoku (Left figure):
- Using NaI and NE213 (w/ PID capability), surrounded by cosmic veto counters.
- ratio: g : n = 3 : 1 (20 < E [MeV] < 60)
• Measurement @ MLF 3F (right figure):
- Consistent with the rate predicted by the Tohoku results within 6 %.
• g’s and neutrons are dominant. (neutrons can be rejected by PID)
x 30 larger than
that in proposal !!
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BKG(3): Accidental BKG for “Delayed“ (beam g)
• Event rate @ “point 2”:
> 1 kHz (E > 1 MeV)
• 10 times larger than that @ “point
3”
• Assumption:
beam associated neutrons are
thermalized and captured by the
concrete floor and g’s are emitted.
Energy spectra for “delayed”
•  It can well reproduce the
measured spectrum.
• Beam g’s can be reduced by putting
12.5 cm thickness lead under the
detector down to 1/10.
(Checked by small plastic scintillator
counter.)
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BKG(4): Accidental BKG for “Delayed“ (beam n)
Beam bunch
• Beam associated neutron (Tn > 10 MeV) can
reach the fiducial volume and are
thermalized and captured by Gd.
 Delayed BKG: 0.016/spill/MW/25t
On-bunch n
• Strong spatial correlation between “onbunch neutron” and “delayed captured g”
• DVTXOB-delayed cut:
 Delayed n rate: 4x10-4 /spill/MW/25t
• Signal inefficiency due to accidental onbunch hit:
< 2.0 %
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