Current status of the Double Chooz experiment

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Current status of the Double Chooz experiment

Julia Haser (MPIK Heidelberg)

on behalf of the Double Chooz collaboration

ICHEP Valencia, 2014/07/04

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Reactor neutrinos and θ

₁₃

P_ν_¯e→ν¯^e ≈1−sin²(2θ13)sin²(1.27^∆^m_E²³¹^L)

◾ reactor neutrinos:

▸ pure ¯νe

▸ low energies

▸ short baselines

▸ no matter effects

▸ high flux

◾ 2 detector principle (cancel systematics)

◾ θ13measurement w/o parameter degeneracies

◾ DC far detector only: MC as a substitute for the near detector

Julia Haser (MPIK Heidelberg) DC-III @ ICHEP 2014 2014/07/04 1 / 16

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Experimental concept

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Double Chooz collaboration

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Neutrino detection

◾ inverse beta decay (IBD) reaction

¯

ν^e+p→e⁺+n

◾ energy threshold of 1.8 MeV

E_vis≈E_ν−0.8 MeV

◾ coincidence signal:

▸ prompt: e⁺annihilation+kinetic energy

▸ delayed: neutron capture

★Gd:E

γ≈8 MeV,τ∼30µs(this talk)

★H:E

γ≈2.2 MeV,τ∼200µs

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Double Chooz Detector

µ

VETOES

◾ Outer Veto: plastic scintillator strips

◾ Inner Veto: liquid scintillator (LAB) with 78 x 8” PMTs

INNER DETECTOR

◾ ν-Target: liquid scintillator PXE+Gd (1 g/l)

◾ γ-Catcher: liquid scintillator PXE (no Gd)

◾ Buffer: transparent mineral oil with 390 x 10” PMTs

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Energy reconstruction

◾ information from lab measurements and detector calibration

1. uniformity correction with a 2D map 2. time stability (data only)

▸ calibrate variation of mean gain

▸ detector response calibration 3. nonlinearity correction (MC only)

▸ readout related

▸ scintillator model related

⇒data to MC agreement over the full energy range

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Backgrounds

Visible Energy (MeV) 2 4 6 8 10 12 14 16 18 20 )/250keV-1Rate (day

10-6 10-5 10-4 10-3 10-2

DC-III (Gd-n) Preliminary

Visible Energy (MeV)

0 5 10 15 20 25 30

Entries / 1.0MeV

1 10 102 103

IBD (standard IBD selection)

IBD above 20 MeV (muon Veto at 30 MeV)

IV Tagged DC-III Gd-n PRELIMINARY

Visible Energy (MeV)

2 4 6 8 10 12

Events/0.50 MeV

0 20 40 60 80 100

120 Double Chooz Preliminary

data MC

natural radioactivity

◾ 0.070±0.003 /day

◾ DC-III/DC-II:0.3 correlation distance cut

DC-II: 2012 DC-III: 2014

fast neutrons, stopping-µ

◾ 0.60±0.05 /day

◾ DC-III/DC-II:0.5 OV and IV veto+

reconstruction likelihood veto

β-n emitter (mainly⁹Li)

◾ 0.97^+0.41_−0.16/day

◾ DC-III/DC-II:0.8 Li+He likelihood veto

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Backgrounds

Visible Energy (MeV) 2 4 6 8 10 12 14 16 18 20 )/250keV-1Rate (day

10-6 10-5 10-4 10-3 10-2

DC-III (Gd-n) Preliminary

0 5 10 15 20 25 30

Entries / 1.0MeV

1 10 102 103

IBD (standard IBD selection)

IBD above 20 MeV (muon Veto at 30 MeV)

IV Tagged DC-III Gd-n PRELIMINARY

2 4 6 8 10 12

Events/0.50 MeV

0 20 40 60 80 100

120 Double Chooz Preliminary

data MC

natural radioactivity fast neutrons, stopping-µ β-n emitter (mainly⁹Li)

◾ reactor OFF-OFF background measurement: 7 events (12.9^+3.1_−1.4 expected)

▸ N_BG(OFF) < ∑N_BG(ON)with compatibility of 9 % (1.7σ)

⇒constraint on possible unaccounted background

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New neutrino selection

PROMPT ENERGY DELAYED ENERGY CORRELATION TIME

DC-II (2012) DC-III

∆T_µ LE: 1 ms, HE: 0.5 s >1 ms prompt energy 0.7 - 12.2 MeV 0.5 - 20 MeV delayed energy 6 - 12 MeV 4 - 10 MeV

∆T 2 - 100µs 0.5 - 150µs

∆R – <1 m

isolation window [−100,+400]µs [−200,+600]µs +improved Light Noise rejection

+improved BG vetoes

◾ benefits from improved active background rejection

▸ wide selection cuts⇒detection systematics reduction

▸ increased S/B (15.6→22)

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Neutrino Candidates

April 2011 – Jan 2013

◾ live-time: 460.7 days

◾ IBD candidates: 17351 (x2 statistics wrt DC-II)

◾ Reactor-off: 7.24 days, 7 candidates

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Normalization uncertainties

source uncertainty (%) improvement wrt DC-II

reactor flux 1.7 –

signal detection

∼

0.6 -40 %

statistics 0.8 -30 %

backgrounds 0.8 -50 %

total 2.1 -20 %

uncertainty relative to signal prediction

◾

detection systematics decreased by almost x2

◾

improvement not only in BG reduction, but uncertainty

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Reactor Rate Modulation results

◾ measureθ13(slope) and BG rate (intercept) at the same time

▸ background model independentθ₁₃analysis possible

▸ unique to DC: additional reactor off data point

◾ result:

▸ sin²2θ13=0.090⁺−^0.0340.035 ^(stat+sys)

▸ B=1.56⁺−^0.180.16 day⁻¹

◾ w/o BG rate constraint result:

▸ sin²2θ13=0.060±0.039(stat+sys)

▸ B=0.93⁺−^0.430.36 day⁻¹

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Rate + Shape results

0.25 MeVEvents

200 400 600 800 1000 1200

DC-III (n-Gd) Preliminary Livetime: 467.90 days

Background-subtracted data No oscillation Systematic uncertainty

= 0.090

¡13 22 Best fit: sin

= 0.00244 eV2 m2 U at

1 2 3 4 5 6 7 8 9 10

0.25 MeVData / Predicted

0.8 0.9 1.0 1.1 1.2

1 2 3 4 5 6 7 8 9 10

0.25 MeVData - Predicted

-100 -50 0 50

◾ other innovations compared to DC-II

▸ range from 0.5-20 MeV (0.25 MeV bins)

▸ measured²³⁸U spectrum in prediction

▸ ∆m²from MINOS 2013(T2K confirmed)

▸ extra bin from 2 reactor off measurement

sin²2θ13=0.090⁺−^0.0320.029 (stat+sys) χ²_min/dof=52.2/40 (p=9.4 %) BG rate after fit: 1.38±0.14 day⁻¹

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Spectrum distortion (1)

◾ spectral distortion above 4 MeV observed

◾ several crosschecks have shown

▸ θ13measurement is not affected

▸ energy scale atE >4 MeV tested (e.g. n-¹²C) and as cause disfavoured

▸ unknown background disfavoured

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Spectrum distortion (2)

◾ RRM fit with free reactor normalization performed for different energy ranges

◾ excess at 4.25−6 MeV consistent with an unaccounted reactor flux

▸ the significance wrt flux prediction is 3σ with BG constraint from our estimation

◾ data-driven study of this energy region:

▸ correlation of excess with reactor power

▸ not only limited to n-Gd sample

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Near detector outlook

◾ detector construction finished

◾ filling this summer

⇒neutrinos in Sept/Oct

◾ 1σwithin [0.015,0.010] after 3 years of ND+FD

▸ BG uncertainty dependent

→statistics dominated!

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Summary

◾ DC-III improvements

▸ x2 more statistics

▸ improved energy reconstruction (non-linearities calibrated)

▸ new selection, active BG rejection→increased efficiency, lower systematics

▸ reduced uncertainty on backgrounds, BG estimations fully based on data

▸ 7.24 days of reactor off-off

◾ θ13 results

▸ R+S: sin²2θ13=0.090^+0.032_−0.029, BG rate: 1.38±0.14 day⁻¹

▸ RRM: sin²2θ13=0.090^+0.034−0.035, BG rate: 1.56±0.17 day⁻¹

▸ RRM (no BG constraint): sin²2θ13=0.060±0.039, BG rate: 0.93±0.40 day⁻¹

◾ deviation from reactor flux prediction observed at high energies

◾ ND ready end of summer 2014

▸ targeted 1σ≥0.01 uncertainty on sin²2θ13(n-Gd only)

new publication! arxiv: 1406.7763

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Appendix

2014/07/04 18

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Energy scale

Visible Energy (MeV) 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

Entries/ 50 keV

0 100 200 300 400 500 600

700 DC-III (Gd-n) Preliminary

data MC

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Accidental background

Correlation Distance (m) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 )/2.5cm-1Rate (day

10-3

10-2

10-1

1

DC-III (Gd-n) Preliminary Data MC Accidentals

2014/07/04 20

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Active background reduction

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R + S results

2014/07/04 22

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Reactor off-off

0 5 10 15 20

)-1MeV-1Background Rate (day

-0.2 -0.1 0 0.1 0.2 0.3 0.4

BG model

2-Off BG

DC-III (n-Gd) Preliminary

◾ 7.24 days of both reactors off→background-only measurement

◾ agreement between reactor off-off and background model

◾ compatibility ofNBG(OFF)and∑NBG(ON)is 9 % (1.7σ)

⇒disfavours unaccounted background

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Carbon-12 n-captures

4.0 4.5 5.0 5.5 6.0

Entries/40 keV

200 250 300 350 400 450 500 550 600

DC-III (Gd-n) Preliminary Data

4.0 4.5 5.0 5.5 6.0

Entries/40 keV

500 1000 1500 2000 2500

DC-III (Gd-n) Preliminary MC

◾ n-C peak in Gamma-Catcher with ∆(data,MC) <0.5 %

2014/07/04 24

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E/L structure

1 2 3 4 5 6 7 8

0.25 MeVData / Predicted

0.8 0.9 1.0 1.1

1.2 Gd-III data (2014) Gd-II data (2012) H-II data (2012) No oscillation Gd-III best fit

Double Chooz Preliminary Background subtracted

◾ same pattern observed in DC-II results with different detection channels (Gd, H) and detector volumes (Target and Gamma-Catcher)

◾ better resolved with DC-III(more statistics, better energy scale and less background)