The Search for θ13 : First Results from Double Chooz. Jaime Dawson, APC

Transcription

1 The Search for θ13 : First Results from Double Chooz Jaime Dawson, APC

2 Contents Brief reminder θ13 current knowledge Reactor experiments Double Chooz Far detector Calibration Neutrinos & Backgrounds Oscillation Analysis Double Chooz and Accelerator Results Conclusion 2

3 Three Neutrino Mixing Pontecorvo Maki Nakagawa Sakata (PMNS) matrix 3 mixing angles 1 CP phase 2 mass splittings Δm2ij We don't know θ13, δcp and sign of Δm231 3

4 Neutrino Oscillations Global fit arxiv: , T. Schwetz et al Before Double Chooz result! θ13 from long baseline accelerator experiments νμ νe appearance - T2K and MINOS 4

5 Reactor Experiments Chooz : R = 1.01 ± 2.8% (stat) ± 2.7% (syst) sin22θ13 < 0.13 (90% C.L) Fogli et al, arxiv: M. Apollonio et. al., Eur.Phys.J. C27 (2003)

6 Reactor Experiments Chooz : R = 1.01 ± 2.8% (stat) ± 2.7% (syst) sin22θ13 < 0.13 (90% C.L) Fogli et al, arxiv: /04/2011 M. Apollonio et. al., Eur.Phys.J. C27 (2003)

7 Modern Reactor θ13 Experiments Disappearance of anti-neutrinos (independent of δcp and sign of Δm31, weak dependence of Δm21) Short distances, ~MeV signals (no matter effects) exaggerated FAR NEAR 7

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9 Concept 2 'identical' detectors Chooz-B 2 x 4.27GW Near 410 m 115 m.w.e ~500 ν/day Far 1050 m 300 m.w.e ~70 ν/day Systematics on reactor power, neutrino spectrum, cross-section and detection are insignificant for a relative measurement 9

10 Near Laboratory Construction 10

11 The Laboratories Outer Muon Veto Plastic scintillator strips with x,y positioning Electronics Glove Boxes and Calibration systems 11

12 The Detectors 12

13 Muon Tracking Outer Veto Tag near miss muons Entry point of any muon Inner Veto Efficient tag of muons and secondaries Track muon Muon Electronics Attenuated output of Inner Detector PMTs Track muon 13

14 Inside the Far Detector 14

15 Electronics & ReadOut 15

16 Waveform Digitisers Single photoelectrons as seen by the waveform digitisers 500 MHz 8-bit flash ADC (developed with Caen V1721X) Dead-time-less (for our event rate) In-house firmware allows choice of event size based on Info from trigger Time between consecutive events 16

17 Event Display Muon in Inner Veto Muon in Inner Detector 17

18 Muons 18

19 Calibration 19

20 Calibration neutron, gamma and positron sources Non-linearity from electronics & charge reconstruction Charge bias in z-direction 20

21 Neutron Calibration 252 Cf source inside target, Deployed along z-axis Calculate neutron capture Gd/(H+Gd) Find 2% correction between data and MC 21

22 Calibration 68 Ge (positron emitter) spectrum well modelled 22

23 Energy Threshold and Trigger 23

24 Detect Neutrinos Detect anti-neutrinos via inverse beta decay p + ν n + e+ In Gd- loaded scintillator Prompt e+ signal 1-8MeV Prompt e+ e- annihilation(2 x 511 kev) Evis = Eν (Mn-Mp)+me Delayed neutron capture on Gd ~30 μs ~ 8 MeV (>80%) H ~200 μs 2.2 MeV 24

25 Neutrinos delayed signal Tag helps define Neutrino event 8 MeV less background Rapid capture time less background Neutron capture on Gd Fiducial volume Gd only in target 25

26 Neutrino Selection time Muon Veto P R O M P T Coincidence D E L A Y E D No other signals Muon veto - No preceding muons (1 ms) No PMT light noise events. Real signals have Homogeneous spread across PMTs Small spread in arrival times Systematics Energy Prompt [0.7, 12] MeV Delayed [6, 12] MeV Coincidence [2, 100] μs No other signals [100 μs before prompt and 400 μs after] 26

27 Accidental Background Singles spectrum Dominant source of accidentals is Radioactivity (from PMTs) Hz (>700keV) Thermal neutrons - 20 n/hr Shift coincidence time window [1,100]ms away from neutrino candidates Accidental spectrum 27

28 Correlated Backgrounds muons Cosmogenics (β-neutron): Li-9, He-8 Main production mechanism spallation on C Search for showering muons (e.g. Edep> 600 MeV) ~257ms 28

29 muons Correlated Background Fast neutrons e - Proton Recoil (positron-like signal) followed by neutron capture Chimney Stopping Muons Muon stopping in chimney followed by Michel e± Sample events of energies > neutrinos [12,30] MeV Tag events with Inner Veto 29

30 Expected number of anti-neutrinos Np ε N (E,t) = exp ν 4 π L2 Pth (t) Ef Pth(t) - Thermal Power monitored by EDF σf Isotopic content of core: k=235u, 238U, 239Pu, 241Pu αk - fractional fission rate Full core simulation MURE EDF inputs: initial fuel loading, geometry etc Np - number of target protons ε - detector efficiency L - distance reactor-detector <Ef> - Mean energy per fission = αk (t) E f Ef k Sk(E) reference anti-neutrino spectra k Use Bugey-4 as reference and account for different core composition. Suppresses uncertainty on Sk(E). σibd(e) Inverse Beta Decay cross-section <σf> - Mean cross-section per fission σf k = de S ( E) σ k IBD Systematics (E) 0 σf = σf Bugey + ( α kdc (t) α kbugey(t) ) σ f k k 30

31 Oscillation Analysis EXAMPLE Two independent measures Normalisation (RATE) Spectral Distortion (SHAPE) Or use both Energy MeV 31

32 Oscillation Fitting Includes uncertainties in anti-neutrino signal, detector response, signal and background statistics, and background spectral shape. Correction for MC/Data differences 32

33 Rate & Shape Oscillation Analysis sin2 2θ13 = ± (stat) ± (syst) Or, < sin2 2θ13 < 0.16 at 90% C.L No oscillation excluded at 94.6% 4121 neutrino candidates (inc. ~ 328 bg events) No oscillation, expect 4344±165 events! No neutrinos above 8 MeV constrain Li-9 and fast neutrons 33

34 Changing Reactor Power 101 days (effective) 2 months with just 1 reactor 1 day both off Another way of measuring θ13 From slope: sin2 2θ13 = ± 0.030(stat)±0.076(sys) Estimate of background rate! / per day 34

35 Both Reactors Off 3 candidate neutrino events - 2 Li-9 candidates and 1 stopping muon in chimney 35

36 Double Chooz, T2K and MINOS 0.003< sin2 2θ13 <0.219 at 3σ 36

37 Conclusions First results from Double Chooz 9th November 2011 LowNu11 presentation by Herve de Kerret and press release 29th December 2011 publication on arxiv hep-ex: Hint for a non-zero value of θ13 sin2 2θ13 = ± (stat) ± (syst) < sin2 2θ13 < 0.16 at 90% C.L. The near detector will be operational by early 2013 Aim for 1σ precision on sin2 2θ13 ~