KamLAND. Introduction Data Analysis First Results Implications Future

Transcription

1 KamLAND Introduction Data Analysis First Results Implications Future Bruce Berger 1 Tohoku University, Sendai, Japan University of Alabama University of California at Berkeley/LBNL California Institute of Technology Drexel University University of Hawaii at Manoa Louisiana State University University of New Mexico Stanford University University of Tennessee Triangle Universities Nuclear Laboratory Institute of High Energy Physics, Beijing, China

2 Introduction to KamLAND Kamioka Liquid-Scintillator Antineutrino Detector KamLAND is a neutrino oscillation experiment that uses a terrestrial source of neutrinos to look at the solar neutrino problem KamLAND reactor exclusion region (3 years) KamLAND is a long-baseline experiment to study the disappearance of electron antineutrinos (ν e ) Bruce Berger 2

3 Detecting Reactor Neutrinos Liquid Scintillator allows us to detect energy depositions down to sub-mev levels Detected E ν spectrum (no oscillations) Coincidence signal: we detect prompt e + annihilation (E = E ν 0.8 MeV) delayed n capture (~190 µs) (E = 2.2 MeV) No directional information Reactor ν e spectrum Bruce Berger 3 Cross section for ν e + p e + + n

4 Why Japan? Kashiwazaki Takahama KamLAND uses the entire Japanese nuclear power industry as a long-baseline source KamLAND Ohi 80% of flux from baselines km Bruce Berger 4

5 Effects of Oscillations Neutrino oscillations change both the rate and the energy spectrum of detected events Multiple reactors at different baselines complicate the signal Example spectra (L.A.Winslow) Top: m 2 = , tan 2 θ =0.41 ( LMA II ) Bottom: m 2 = , tan 2 θ =0.41 ( LMA I ) *top 4 reactors at full thermal power only Bruce Berger 5

6 KamLAND Detector Rock 1 kton liquid scintillator 80% dodecane 20% pseudocumene 1.5 g/l PPO Calibration Systems Electronics (E-Hut) Paraffin outside the 120-µm nylon balloon radon barrier PMT s 18m Steel Sphere 1879 PMT's " - fast " - efficient 13m Nylon Balloon 225 Veto PMT's Water erenkov Bruce Berger Outer Detector 6

7 Rapid Construction September 1999 Detector Construction Started September 28, 2000 PMT Installation Completed March 2001 Balloon Installed and Tested September 24, 2001 Oil Filling Completed January 2002 Infrastructure Completed January 22, 2002 Data Collection Started First results released December 9, 2002 Bruce Berger 7

8 KamLAND Data antineutrino candidate two events (color is time) Prompt (e + ) event E = 3.20 MeV t = 111 µs R = 34 cm Bruce Berger 8 Delayed (neutron) event E = 2.22 MeV

9 Data Analysis ADC counts (~120 µv) KamLAND raw data are waveforms collected with custom low-deadtime front-end electronics We analyze waveforms to extract pulse arrival times (at the 1 ns level) and total pulse charges Vertexing: we determine the position of the energy deposition from the timing information ~25 cm resolution Energy: we determine the event energy from the sum of pulse charges essentially counting PE ~7.5% [E(MeV)]-½ resolution 12-channel KAMFEE board Blue: raw data Red: pedestal Green: pedestal subtracted Samples (~1.5 ns) Bruce Berger 9

10 Antineutrino Candidates Candidate selection cuts - total efficiency 78.3 ± 1.6% fiducial volume: R < 5 m (both prompt and delayed) time correlation: 0.5 µs < t < 660 µs vertex correlation: R < 1.6 m delayed energy: 1.8 MeV < E delay < 2.6 MeV z-axis: R delay,xy > 1.2 m Muon spallation vetoes 2 ms for full volume (neutrons) 2 sec within 3m of track ( 8 He, 9 Li, etc.) Showering (high E): 2 sec for full volume 11.4% deadtime Bruce Berger 10

11 Backgrounds Accidentals ± Li, 9 He 0.94 ± 0.85 delayed neutron emitters: prompt signal is β from β-decay delayed signal is neutron capture lifetimes: 8 Li 119 ms; 9 He 178 ms Fast neutrons < 0.5 spallation in rock by undetected muons prompt signal is neutron shower delayed signal is neutron capture Geoneutrinos eliminate with cut E prompt > 2.6 MeV Total 1 ± 1 Bruce Berger 11

12 Systematics Systematic Uncertainties: Total LS mass 2.1 Fiducial mass ratio 4.1 Energy threshold 2.1 Efficiency of cuts 2.1 Live time 0.07 Reactor power 2.0 Fuel composition 1.0 Time lag 0.28 ν e spectra 2.5 Cross section 0.2 Total 6.4% Fiducial volume estimated from uniformity of spallation neutrons Energy calibration with sources on z-axis plus backgrounds throughout detector Bruce Berger 12

13 Antineutrino Rate Analysis Observed 54 (145.1 days livetime) No-oscillation expectation 86.8 ± 5.6 (syst) Background 1 ± 1 (N obs N BG )/N no-osc = ± (stat) ± (syst) (statistics above on 54 events) Probability that 86.8 events would fluctuate down to 54 is < 0.05% Standard ν e propagation is ruled out at the 99.95% confidence level curve, shaded region: global-fit solar LMA Bruce Berger 13

14 Rate + Shape Analysis Fit prompt (positron) energy spectrum above 2.6 MeV with full reactor information (power, fuel, flux), 2-flavor mixing Energy spectrum shape provides additional constraints on oscillation parameters Bruce Berger 14

15 Observation of Oscillations? 2-ν oscillation: best fit χ 2 /8 d.o.f. = 0.31 data and best oscillation fit are consistent at 93% C.L. no oscillation; flux suppression data and best oscillation fit are consistent at 54% C.L. (Monte Carlo studies) We have not directly observed neutrino oscillation Constraints on mixing parameters are due to the absence of large distortions Bruce Berger 15

16 Reactor plus Solar Solar picture before KamLAND KamLAND rate analysis: Everything but LMA excluded (two-flavor, CPT conserved ) KamLAND rate + shape: LMA constrained New global fits 2 LMA subregions (Bahcall et al., hep-ph/ ) LMA II LMA I Bruce Berger 16

17 KamLAND future reactor sensitivity? Monte Carlo study: 1000 sets of 500 events for each of: LMA II : m 2 = , tan 2 θ =0.41 LMA I : m 2 = , tan 2 θ =0.41 Top 16 reactors, full thermal power, energy resolution smearing Fit for mixing parameters with shape-only analysis above 2.6 MeV No systematics included Clear separation of LMA I and LMA II L.A.Winslow Better fractional resolution on m 2 for LMA I (4%) than LMA II (5%) (95% CL) tan 2 θ 12 to ± 0.2 level (95% CL) (without rate!) Bruce Berger 17

18 KamLAND Solar Phase Idea: directly detect solar 7 Be neutrinos Measures tan 2 θ 12 Singles measurement Low energy threshold Low backgrounds required! Radiopurity vs. design goals: 238 U < g/g < (3.5 ± 0.5) g/g 232 Th < g/g < (5.2 ± 0.8) g/g 40 K < g/g < g/g But other (unanticipated) backgrounds must also be reduced substantially: 85 Kr, 210 Pb, 210 Bi Have to make substantial upgrades to the purification system KamLAND proposal plots not actual backgrounds! Bruce Berger 18

19 Future Reactor neutrino sensitivity Continue to take reactor data 3-4 years total for ~500 events Reactors on (Kashiwazaki is now offline!) Increase the fiducial volume: 5 m 5.5 m adds 33% to fiducial volume 4π calibration system Continue to work to understand the detector better to reduce systematics (Fluctuations?) Solar phase Lots of work to meet purification requirements Other measurements Solar antineutrinos, geoneutrinos, spallation measurements, etc. Bruce Berger 19