Recent results of KamLAND-‐Zen 11-‐July-‐2012
Hiroshi Nakano
1 Paper reading seminar
[Paper] Measurement of the double-‐β decay half-‐life of 136Xe with the KamLAND-‐Zen experiment (published 19 April 2012) hPp://prc.aps.org/pdf/PRC/v85/i4/e045504 Limits on Majoron-‐Emi[ng Double-‐Beta Decays of 136Xe in KamLAND-‐Zen (Dated May 30, 2012) hPp://arxiv.org/pdf/1205.6372.pdf
reference
• hPp://www.awa.tohoku.ac.jp/rcns/wp-‐content/uploads/2012/06/Results-‐from-‐KamLAND-‐Zen.pdf
• hPp://www.icepp.s.u-‐tokyo.ac.jp/info/sympo/18/torape/Hakuba2012-‐Matsuda.pdf • hPp://www.jahep.org/hepnews/2011/11CKamLAND-‐Zen-‐04final.pdf
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Contents 3
□ Physics mo+va+on □ Detector and Selec+on □ Result
Physics mo`va`on 4 Detector & Selec`on Physics Result
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e–
e–
νe νe
2νββ decay 0νββ decay
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e–
e–
0νββ + Majoron decay
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χ0
0νββ search is the only way for check whether neutrino is Majorana or Dirac. 0νββ rate has rela`on to neutrino mass. There are other mechanisms. (0νββ + Majoron)
lepton # viola`on ! neutrino = an` neutrino !
Detector [1/4]
• Inner Balloon (IB) – radius : 1.54 [m] – film thickness : 25 [μm] – Liquid Scin`llator (LS) : 13 [ton] – 136Xe :
• 300 [kg] • 2.44 ± 0.01 % by weight
– role : • 2 beta decay source • Detector
5 Physics Result Detector & Selec`on
Detector [2/4] 6
• Outer Balloon (OB) – radius : 6.5 [m] – film thickness : 135 [μm] – Liquid Scin`llator (LS) : 1000 [ton] – role :
• Detector • Shield for Inner Balloon
Physics Result Detector & Selec`on
Detector [3/4] 7
• Buffer Oil (BO) – radius : 9 [m] – role :
• shield • Spherical Stainless-‐steel
containment Tank (SST) – radius : 9 [m] – role :
• shield for Liquid Scin`llator – PMT :
• 17 inch : 1325 • 20 inch : 554 • coverage : 34 [%]
Physics Result Detector & Selec`on
Detector [4/4] 8
• Outer Detector (OD) – Water : 3200 [ton]
Physics Result Detector & Selec`on
J Huge LS & low BG à KamLAND is good for 0νββ decay search
DAQ system • Trigger
– 70 or more 17-‐inch PMT hit (~0.4 [MeV]) – 1 [ms] lower energy threshold (~0.25 [MeV]) awer trigger
• Accuracy – vertex resolu`on : 15 [cm] / sqrt(E [MeV]) – energy resolu`on : (6.6 ± 0.3) [%] / sqrt( E [MeV]) – Concerning about
• PMT gain varia`on • solid angle • shadowing • transparency of materials
– Calibra`on • γ from 208Tl decay • β + γ form 214Bi decay • 2.225 [MeV] gamma from neutron capture by proton
9
Some BG occurs sequen`ally. Lower energy threshold cannot be set always.
Physics Result Detector & Selec`on
Candidate eventselec`on [1/2]
• i) Fiducial Volume (FV) : R < 1.2 [m]
10
~ Vertex resolu`on Inner Balloon surface
136Xe 2νββ
136Xe 0νββ 214Bi
134Cs
Ra`o of 134Cs to 137Cs is consistent with contamina`on by fallout. FV cut is performed to mi`gate BG from IB.
FV contains 125 ± 7 [kg] of 136Xe. (2.44 ± 0.01 % by weight)
Physics Result Detector & Selec`on
Candidate eventselec`on [2/2]
• ii) Muon event veto – Muon : 10,000 p.e. event or more than 5 OD hits – Events occurring within 2 [ms] awer muon are also vetoed
11
Muon causes noisy or low efficiency sequen`al events. From physical and electronic side.
• iii) Coincidence cut – 99.97 ± 0.01 % of 214Bi – 212Po BG is removed
• iv) Reactor neutrino veto – KamLAND knows its feature well
• v) VTQ test
Total live `me awer cut : 77.6 [days] for paper in April. 112.3 [days] for paper in May.
Physics Result Detector & Selec`on
Result (paper in April) 12
136Xe 2νββ is dominant in
Low-‐energy region
number of 136Xe 0νββ
is < 15 according to the fit
half-‐life of 136Xe 2νββ is 2.38 ± 0.02 ± 0.14 x 1021 [yr]
lower limit of half-‐life of 136Xe 0νββ is 5.7 x 1024 [yr] (90% C.L.). Corresponding effec`ve mass is <mββ> < (0.3 – 0.6) [eV]
Physics Result Detector & Selec`on
13 Physics Result Detector & Selec`on
• Spectrum for fit to 0νββ region – 100Ag – 88Y – 208Bi – 60Co – 136Xe 0νββ
• All isotopes in the ENSDF database were searched
Fit with 136Xe 0νββ only
Fit with 110Ag + 136Xe 0νββ only
Fit with 208Bi + 136Xe 0νββ only
Result (paper in May) 14
half-‐life of 136Xe 2νββ is 2.30 ± 0.02 ± 0.12 x 1021 [yr]
lower limit of half-‐life of 136Xe 0νββ is 6.2 x 1024 [yr] (90% C.L.). Corresponding effec`ve mass is <mββ> < (0.26 – 0.54) [eV]
Contribu`on from Majoron-‐emi[ng mode to zero
lower limit of half-‐life of 136Xe Majoron-‐emi[ng mode (n=1) is 2.6 x 1024 [yr]
n = 1 ordinary mode
Physics Result Detector & Selec`on
15 Physics Result Detector & Selec`on
• 2νββ spectrum and one of a 0νββ (+Majoron) spectrum are fiPed simultaneously.
16
from slide of Neutrino2012, 6th June
Physics Result Detector & Selec`on
Systema`cs 17
paper in April
FV error 5.2 %
enrichment of 136Xe 0.05 %
Xe concentra`on 2.8 %
detector energy scale 0.3 %
Xe-‐LS edge effect 0.06 %
detec`on efficiency 0.2 %
Total 5.9 % 5.2 %
paper in May
supplemental xenon concentra`on measurement
Physics Result Detector & Selec`on
Future Prospects 18
from slide of Neutrino2012, 6th June
Physics Result Detector & Selec`on