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New Measurements of Reactor Disappearance with the Double Chooz Far Detector
Available online at www.sciencedirect.com
ScienceDirect Physics Procedia 61 (2015) 331 – 335
New measurements of reactor ν¯ e disappearance with the Double Chooz Far Detector C. Mariani for the Double Chooz Collaboration Center for Neutrino Physics, Virginia Tech, Blacksburg, VA 24061, USA
Abstract We present an update on the results of the Double Chooz experiment. Double Chooz searches for the neutrino mixing angle, θ13 , in the three-neutrino mixing matrix via the disappearance of ν¯ e produced by the dual 4.27 GW/th Chooz B Reactors. Here we discuss updated oscillation fit results using both the rate and the shape of the anti-neutrino energy spectrum. In the most recent oscillation analysis we included data with neutron captures on Gadolinium and Hydrogen along with the reactor off data that we collected. This is an important step in our multi-year program to establish the value of θ13 . © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). c 2011 Published by Elsevier Ltd. Selection and peer review is the responsibility of the Conference lead organizers, Frank Avignone, University of South Carolina, and Wick Haxton, University of California, Berkeley, and Lawrence Berkeley Laboratory
Keywords: Neutrino, neutrino oscillation, reactor neutrino experiment
1. Introduction Neutrino oscillations have been established in the last ten years by various experiments using neutrinos from the sun [1, 2, 3, 4, 5, 6, 7], the Earth’s atmosphere [8, 9], nuclear reactors [10, 11], and accelerators [13, 12]. All these data can be described within a three flavour neutrino oscillation theory, characterized by two mass-squared differences (Δm221 , Δm231 ), three mixing angles (θ12 , θ13 , θ23 ), and one complex phase (δCP ). The sensitivity to sin2 (2θ13 ) of previous reactor experiments was limited by the uncertainty of the rate (and shape) of the detected neutrino spectrum (known to ∼2%) [14]. The oscillation signal will manifest as a deficit on the energy spectrum of the far detector as compared to the normalized near detector spectrum. The location of the deficit depends on the L/E of the experiment in question, where spectrum maximum is ∼4 MeV and L ranges within 1-2 km. Oscillation analyses look for a difference between overall measured fluxes and predicted ones or near detector measured unoscillated flux. The former analyses are considered to be reactor neutrino rate dominated, while the latter aiming for the spectral distortions are considered to be flux shape dominated. 2. The DoubleChooz Experiment The Double Chooz collaboration involves scientists from various countries: France, Germany, Japan, Russia, Spain, UK, US and Brazil. The limited size of the far laboratory (former site of CHOOZ) limits the dimensions of the far detector (8.2 t total, as shown in Fig. 1), but allows using previous background from the CHOOZ experiment [14]. The Double Chooz analysis strategy uses the inverse-β reaction [IBD] (ν¯e + p → e+ + n) to identify electron antineutrinos. The IBD sample provides a good sample of anti-neutrino interactions and excellent background rejection that can easily be achieved with very limited analysis cuts. This strategy allows to keep the systematic uncertainties
1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer review is the responsibility of the Conference lead organizers, Frank Avignone, University of South Carolina, and Wick Haxton, University of California, Berkeley, and Lawrence Berkeley Laboratory doi:10.1016/j.phpro.2014.12.071
332
C. Mariani / Physics Procedia 61 (2015) 331 – 335
very low. Each analysis cut adds one more systematic uncertainty to the measurement of θ13 . An IBD event is identified by the correlated prompt-e+ and delayed-n capture energy depositions by a coincidence energy trigger. The ν energy is accurately measured using the positron energy deposition. The energy threshold due to kinematics is of ∼ 1.8 MeV. The sample of anti-neutrinos detected are those associated with fast decay chains, that allows the antineutrino sample to be distinguished from other anti-neutrinos that can come from the exhausted-fuel stored in the near power plant. The final measured reactor neutrino spectrum is the convolution of the inverse-β cross-section and of the sum over all the β-decay spectra of all fission products. The active volume of the Double Chooz far detector consists of liquid scintillator loaded with 0.1% of Gd. Double Chooz, as other reactor experiments, uses Gd because of its very high cross-section for thermal-neutron capture. The use of the Gadolinium reduces also the time of the coincidence between the positron prompt signal and the neutron delayed signal, granting very good background suppression. The Gd capture also provides a very good energy tag: there are gamma-rays emitted upon the neutron capture amounting to ∼8 MeV. This signal is well separated from the background signal caused by radio-impurities (defined as accidental) in the detector. Therefore, a very good background reduction is achieved by simply applying an energy cut. In the oscillation analysis that will be presented in Sec. 4 we will combine the antineutrino samples identified in the Gd active volume of the detector with the one where the IBD interactions happen in the gamma catcher region, where the neutron capture is on H. In this case the energy deposition is of 2.2 MeV and will happen in the region of the gamma catcher and in the target region. The use of the H-captured samples provide higher statistical samples of interactions with different background contaminations. Figure 1 shows the 4 volume Double Chooz far detector: target (acrylics Gd loaded liquid scintillator), γ-catcher (acrylics unloaded liquid scintillator), buffer (non-scintillating oil used to reduce radioactivity from phototubes and surrounding rocks) and inner-veto (liquid scintillator). There is also an outer inert shield to reject rock radiation and an active tracking outer-veto for cosmogenic background studies. Here is a summary of the capabilities of a typical reactor anti-neutrino detector like the one used by the Double Chooz experiment: i) energy threshold below the expected e+ spectrum (∼ 0.5MeV), ii) low radioactivity rate (rate in target < 10Hz), iii) detector response uniformity (no precise energy trigger), iv) hardware fiducial volume definition within acrylics or distance between the positron and neutron capture signal of 90 cm (H analysis only) v) fast-neutron tagging, vi) information redundancy (useful for calibration and efficiencies uncertainties). The scintillator batches are readout by photomultiplier tubes and the detector optical properties are measured and/or corrected using various calibration systems (LED, laser and radioactive sources). A 3D calibration deployment system is also used for energy calibration and to determine energy nonlinearities with very high accuracy.
Figure 1. The Double Chooz Far Detector [15]
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C. Mariani / Physics Procedia 61 (2015) 331 – 335
3. Backgrounds The most common sources of backgrounds are coming from processes mimicking a time coincidence with nGd/H-capture-like emission. While a radio-pure detector is critical for the purpose of the experiment, all dominating backgrounds are associated with cosmic muons, hence, the overburden of the detector determines its background rate. There are 3 types of backgrounds: i) accidental caused by the random coincidence between natural radioactivity (e+ -like) and the capture of a thermal neutron on Gd or H (n-like) or two natural radioactivity events (e+ -like). ii) correlated caused by an incoming fast-neutron, which first recoils on a proton (e+ -like) and, then, gets captured into Gd or H, once thermalized. iii) unstable spallation isotopes (generated on carbon) cause milliseconds lifetimes β-n decays, which are very difficult to veto. The accidental background is the most dangerous for an oscillation analysis, since its e+ -like spectrum rises dramatically at low energies, where the oscillation deficit is expected. Reactor-off data provides a measurement of the effective integrated background spectrum per detector. This is a unique capability for the Double Chooz detector. This is, however, a very rare event: particularly unlikely in a multi-core power station. 4. Oscillation Analysis The oscillation analysis presented here uses data collected by the Double Chooz far detector between April 13, 2011 and March 15, 2012. The total lifetime is 227.9 days for the Gd-analysis [16] corresponding to 8,249 candidates. For the H-analysis we used 240.1 days corresponding to 36,284 candidate. To extract sin2 θ13 we compare both the rate and shape of the data to the reference E prompt . We used 18 and 31 variable sized bins between 0.7 MeV to 12.2 MeV respectively in the case of the Gd- and H-analysis respectively [16, 17]. The fit procedure is the same between the two analyses and it includes using the prediction of the observed number of signal and background events for each energy bin. The χ2 is defined as in Eq. 1. χ2Rate+S hape =
B
pred T obs Niobs − Nipred Mi−1 + pull terms j Nj − Nj
(1)
i, j
The background events are calculated based on the measured rates and the lifetime of the detector. Systematical and statistical errors are propagated to the fit by the use of a covariance matrix M in order to properly account for correlations between energy bins. There are different sources of uncertainty: signal, detector, statistical, efficiency and background. They are summarized in Tab. 1 for both the Gd and H-analysis. The spectrum shape uncertainties
Source Reactor ν¯ e flux Efficiency 9 Li rate Fast n + stopping μ rate Accidentals rate Total statistical error
Gd selection 1.8% 1.0% 1.5% 0.5% <0.1% 1.12%
H selection 1.8% 1.6% 1.6% 0.6% 0.2% 1.08%
Table 1. Summary of the normalization uncertainties for the Double Chooz Gd and H-analysis [16, 17]
included in our covariance matrices, M, includes the reactor ν¯ e spectrum, energy scale uncertainties, 9 Li spectrum and Fast neutron and stopping muon spectrum. The energy scale uncertainties arise from three sources: time variation, non linearities and non uniformities in the detector response. In both analyses we use the Bugey4 measurement [18] to minimize the systematic uncertainties in the reactor flux prediction. The background rates for the 9 Li and Fast neutron and stopping muons are allowed to vary during the minimization procedure (with the use of a pull term), and they rescale the rate of the corresponding backgrounds during the minimization procedure. A summary of the values for the background rate used as input in the fit and the minimization results are presented in Tab. 4. The elements of the migration matrix are recalculated as a function of the oscillation and other fit parameters (background
334
C. Mariani / Physics Procedia 61 (2015) 331 – 335
Gd
H
Input (rel. unc.)
Fit output (rel. unc.)
9
Li rate FN + SM rate
1.3 ± 0.5 d−1 (40%) 0.7 ± 0.2 d−1 (30%)
1.0 ± 0.3 d−1 (30%) 0.6 ± 0.1 d−1 (20%)
9
2.8 ± 1.2 d−1 (40%) 2.5 ± 0.5 d−1 (20%)
3.9 ± 0.6 d−1 (15%) 2.6 ± 0.4 d−1 (15%)
Li rate FN + SM rate
Table 2. Rate+Shape fit background constraints for both the Gd and H analyses [16, 17]
Background-subtracted signal No oscillation
1200
Best fit: sin2 (2θ13) = 0.109 at Δ m231 = 0.00232 eV 2 (χ2 /d.o.f. = 42.1/35)
1000
Events 0.25 MeV
Events 0.5 MeV
rates) at each step of the minimization. The Δm2 used for the fit is (2.32 ± 0.12) × 10−3 eV2 . The best fit value gives sin2 2θ13 = 0.109±0.039 for the Gd-analysis [16] and sin2 2θ13 = 0.097±0.048 for the H-analysis [17]. Figures 2 show the prompt energy spectrum of neutrino candidates with background subtracted for the Gd [16] and H-analysis [17]. We have also preliminarily combined the Gd and H-analysis measurements. In this case the fit includes the correlation of systematic errors and includes the background constraints as coming out of the reactor-off measurements [19]. Between 2011 and 2012 we have collected a total of 7.5 days of data with both the reactors off [19](called an off-off period). This is a unique capability of Double Chooz. The expected number of events during the reactor off period is 14.8 ± 4.0 in good agreement with the prediction of 8 events. The results for the combined Gd and H fit results together with the off-off data are still preliminary. We measure a sin2 2θ13 of 0.109 ± 0.035 for the rate plus shape results and sin2 2θ13 = 0.107 ± 0.045 for the rate only results. The χ2 /d.o. f. is 61.2/50 for the rate plus shape and 6.1/3 for the rate only result. 1400
Background-subtracted signal No oscillation Best fit: sin2(2θ13) = 0.097
1200
2 = 0.00231 eV2 (χ2 /d.o.f. = 38.9/30) at Δm31
Systematic error
Systematic error
1000
800 800
600 600
400
400 200
1.2
Data - Predicted Data / Predicted 0.25 MeV 0.25 MeV
Data - Predicted Data / Predicted 0.5 MeV 0.5 MeV
200
1.0 0.8 0.6 0 -50 -100 -150 2
4
6
8
10
12
1.2 1.0 0.8 0.6
0 -100 -200 2
4
6
Energy (MeV)
(a)
8
10
12
Energy (MeV)
(b)
Figure 2. Background-subtracted data (black points with statistical error bars) are superimposed on the prompt energy spectra expected in the case of no oscillations (dashed blue line) and for our best fit sin2 2θ13 (solid red line). Solid gold bands indicate systematic errors in each bin. Middle: The ratio of data to the no-oscillation prediction is superimposed on the expected ratio in the case of no oscillations (blue dashed line) and for our best fit sin2 2θ13 (solid red line). Gold bands indicate systematic errors in each bin. Bottom: The difference between data and the no-oscillation prediction is shown in the same style as the ratio. Figures and caption from Ref. [16, 17]
C. Mariani / Physics Procedia 61 (2015) 331 – 335
5. Conclusions The Double Chooz experiment has provided a rich, unique program with the far detector only analysis. The collaboration has used two signal channels Gd and H. The unique capability of Double Chooz in doing reactor offoff measurement contributed substantially to verify and validate the experimental results obtained with the rate + shape oscillation analysis. The far detector only analysis is expected to be finalized by June, 2013 and it will aim to a precision, using just the far detector, of 0.03. The Double Chooz experiment is now the process of building its near detector, which is expected to come online at the end of summer, 2014. The experiment projected precision on sin2 2θ13 using both the near and far detector is of 0.01 [20]. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
B. T. Cleveland et al., Astrophys. J. 496, 505 (1998) GNO, M. Altmann et al., Phys. Lett. B616, 174 (2005), hep-ex/0504037. Super-Kamkiokande, J. Hosaka et al., Phys. Rev. D73, 112001 (2006), hep-ex/0508053. SNO, Q. R. Ahmad et al., Phys. Rev. Lett. 89, 011301 (2002), nucl-ex/0204008. SNO, B. Aharmim et al., Phys. Rev. Lett. 101, 111301 (2008), 0806.0989. SNO, B. Aharmim et al., (2009), 0910.2984. Borexino, C. Arpesella et al., Phys. Rev. Lett. 101, 091302 (2008), 0805.3843. Super-Kamiokande, Y. Fukuda et al., Phys. Rev. Lett. 81, 1562 (1998), hep-ex/9807003. Super-Kamiokande, Y. Ashie et al., Phys. Rev. D71, 112005 (2005), hep-ex/0501064. KamLAND, T. Araki et al., Phys. Rev. Lett. 94, 081801 (2005), hep-ex/0406035. KamLAND, S. Abe et al., Phys. Rev. Lett. 100, 221803 (2008), 0801.4589. K. Abe et al. [T2K Collaboration], Phys. Rev. Lett. 112, 061802 (2014) [arXiv:1311.4750 [hep-ex]]. MINOS, P. Adamson et al., Phys. Rev. Lett. 101, 131802 (2008), 0806.2237. M. Apollonio et al. hep-ex/0301017 F. Ardellier et al. hep-ex/0606025. Y. Abe et al. [Double Chooz Collaboration], Phys. Rev. D 86, 052008 (2012) [arXiv:1207.6632 [hep-ex]]. Y. Abe et al. [Double Chooz Collaboration], Phys. Lett. B 723, 66 (2013) [arXiv:1301.2948 [hep-ex]]. Y. Declais et al. (Bugey), Phys. Lett. B338, 383 (1994). Y. Abe et al. [Double Chooz Collaboration], arXiv:1401.5981 [hep-ex]. C. Mariani, Mod. Phys. Lett. A 27, 1230010 (2012) [arXiv:1201.6665 [hep-ex]].
335
ScienceDirect Physics Procedia 61 (2015) 331 – 335
New measurements of reactor ν¯ e disappearance with the Double Chooz Far Detector C. Mariani for the Double Chooz Collaboration Center for Neutrino Physics, Virginia Tech, Blacksburg, VA 24061, USA
Abstract We present an update on the results of the Double Chooz experiment. Double Chooz searches for the neutrino mixing angle, θ13 , in the three-neutrino mixing matrix via the disappearance of ν¯ e produced by the dual 4.27 GW/th Chooz B Reactors. Here we discuss updated oscillation fit results using both the rate and the shape of the anti-neutrino energy spectrum. In the most recent oscillation analysis we included data with neutron captures on Gadolinium and Hydrogen along with the reactor off data that we collected. This is an important step in our multi-year program to establish the value of θ13 . © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). c 2011 Published by Elsevier Ltd. Selection and peer review is the responsibility of the Conference lead organizers, Frank Avignone, University of South Carolina, and Wick Haxton, University of California, Berkeley, and Lawrence Berkeley Laboratory
Keywords: Neutrino, neutrino oscillation, reactor neutrino experiment
1. Introduction Neutrino oscillations have been established in the last ten years by various experiments using neutrinos from the sun [1, 2, 3, 4, 5, 6, 7], the Earth’s atmosphere [8, 9], nuclear reactors [10, 11], and accelerators [13, 12]. All these data can be described within a three flavour neutrino oscillation theory, characterized by two mass-squared differences (Δm221 , Δm231 ), three mixing angles (θ12 , θ13 , θ23 ), and one complex phase (δCP ). The sensitivity to sin2 (2θ13 ) of previous reactor experiments was limited by the uncertainty of the rate (and shape) of the detected neutrino spectrum (known to ∼2%) [14]. The oscillation signal will manifest as a deficit on the energy spectrum of the far detector as compared to the normalized near detector spectrum. The location of the deficit depends on the L/E of the experiment in question, where spectrum maximum is ∼4 MeV and L ranges within 1-2 km. Oscillation analyses look for a difference between overall measured fluxes and predicted ones or near detector measured unoscillated flux. The former analyses are considered to be reactor neutrino rate dominated, while the latter aiming for the spectral distortions are considered to be flux shape dominated. 2. The DoubleChooz Experiment The Double Chooz collaboration involves scientists from various countries: France, Germany, Japan, Russia, Spain, UK, US and Brazil. The limited size of the far laboratory (former site of CHOOZ) limits the dimensions of the far detector (8.2 t total, as shown in Fig. 1), but allows using previous background from the CHOOZ experiment [14]. The Double Chooz analysis strategy uses the inverse-β reaction [IBD] (ν¯e + p → e+ + n) to identify electron antineutrinos. The IBD sample provides a good sample of anti-neutrino interactions and excellent background rejection that can easily be achieved with very limited analysis cuts. This strategy allows to keep the systematic uncertainties
1875-3892 © 2015 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and peer review is the responsibility of the Conference lead organizers, Frank Avignone, University of South Carolina, and Wick Haxton, University of California, Berkeley, and Lawrence Berkeley Laboratory doi:10.1016/j.phpro.2014.12.071
332
C. Mariani / Physics Procedia 61 (2015) 331 – 335
very low. Each analysis cut adds one more systematic uncertainty to the measurement of θ13 . An IBD event is identified by the correlated prompt-e+ and delayed-n capture energy depositions by a coincidence energy trigger. The ν energy is accurately measured using the positron energy deposition. The energy threshold due to kinematics is of ∼ 1.8 MeV. The sample of anti-neutrinos detected are those associated with fast decay chains, that allows the antineutrino sample to be distinguished from other anti-neutrinos that can come from the exhausted-fuel stored in the near power plant. The final measured reactor neutrino spectrum is the convolution of the inverse-β cross-section and of the sum over all the β-decay spectra of all fission products. The active volume of the Double Chooz far detector consists of liquid scintillator loaded with 0.1% of Gd. Double Chooz, as other reactor experiments, uses Gd because of its very high cross-section for thermal-neutron capture. The use of the Gadolinium reduces also the time of the coincidence between the positron prompt signal and the neutron delayed signal, granting very good background suppression. The Gd capture also provides a very good energy tag: there are gamma-rays emitted upon the neutron capture amounting to ∼8 MeV. This signal is well separated from the background signal caused by radio-impurities (defined as accidental) in the detector. Therefore, a very good background reduction is achieved by simply applying an energy cut. In the oscillation analysis that will be presented in Sec. 4 we will combine the antineutrino samples identified in the Gd active volume of the detector with the one where the IBD interactions happen in the gamma catcher region, where the neutron capture is on H. In this case the energy deposition is of 2.2 MeV and will happen in the region of the gamma catcher and in the target region. The use of the H-captured samples provide higher statistical samples of interactions with different background contaminations. Figure 1 shows the 4 volume Double Chooz far detector: target (acrylics Gd loaded liquid scintillator), γ-catcher (acrylics unloaded liquid scintillator), buffer (non-scintillating oil used to reduce radioactivity from phototubes and surrounding rocks) and inner-veto (liquid scintillator). There is also an outer inert shield to reject rock radiation and an active tracking outer-veto for cosmogenic background studies. Here is a summary of the capabilities of a typical reactor anti-neutrino detector like the one used by the Double Chooz experiment: i) energy threshold below the expected e+ spectrum (∼ 0.5MeV), ii) low radioactivity rate (rate in target < 10Hz), iii) detector response uniformity (no precise energy trigger), iv) hardware fiducial volume definition within acrylics or distance between the positron and neutron capture signal of 90 cm (H analysis only) v) fast-neutron tagging, vi) information redundancy (useful for calibration and efficiencies uncertainties). The scintillator batches are readout by photomultiplier tubes and the detector optical properties are measured and/or corrected using various calibration systems (LED, laser and radioactive sources). A 3D calibration deployment system is also used for energy calibration and to determine energy nonlinearities with very high accuracy.
Figure 1. The Double Chooz Far Detector [15]
333
C. Mariani / Physics Procedia 61 (2015) 331 – 335
3. Backgrounds The most common sources of backgrounds are coming from processes mimicking a time coincidence with nGd/H-capture-like emission. While a radio-pure detector is critical for the purpose of the experiment, all dominating backgrounds are associated with cosmic muons, hence, the overburden of the detector determines its background rate. There are 3 types of backgrounds: i) accidental caused by the random coincidence between natural radioactivity (e+ -like) and the capture of a thermal neutron on Gd or H (n-like) or two natural radioactivity events (e+ -like). ii) correlated caused by an incoming fast-neutron, which first recoils on a proton (e+ -like) and, then, gets captured into Gd or H, once thermalized. iii) unstable spallation isotopes (generated on carbon) cause milliseconds lifetimes β-n decays, which are very difficult to veto. The accidental background is the most dangerous for an oscillation analysis, since its e+ -like spectrum rises dramatically at low energies, where the oscillation deficit is expected. Reactor-off data provides a measurement of the effective integrated background spectrum per detector. This is a unique capability for the Double Chooz detector. This is, however, a very rare event: particularly unlikely in a multi-core power station. 4. Oscillation Analysis The oscillation analysis presented here uses data collected by the Double Chooz far detector between April 13, 2011 and March 15, 2012. The total lifetime is 227.9 days for the Gd-analysis [16] corresponding to 8,249 candidates. For the H-analysis we used 240.1 days corresponding to 36,284 candidate. To extract sin2 θ13 we compare both the rate and shape of the data to the reference E prompt . We used 18 and 31 variable sized bins between 0.7 MeV to 12.2 MeV respectively in the case of the Gd- and H-analysis respectively [16, 17]. The fit procedure is the same between the two analyses and it includes using the prediction of the observed number of signal and background events for each energy bin. The χ2 is defined as in Eq. 1. χ2Rate+S hape =
B
pred T obs Niobs − Nipred Mi−1 + pull terms j Nj − Nj
(1)
i, j
The background events are calculated based on the measured rates and the lifetime of the detector. Systematical and statistical errors are propagated to the fit by the use of a covariance matrix M in order to properly account for correlations between energy bins. There are different sources of uncertainty: signal, detector, statistical, efficiency and background. They are summarized in Tab. 1 for both the Gd and H-analysis. The spectrum shape uncertainties
Source Reactor ν¯ e flux Efficiency 9 Li rate Fast n + stopping μ rate Accidentals rate Total statistical error
Gd selection 1.8% 1.0% 1.5% 0.5% <0.1% 1.12%
H selection 1.8% 1.6% 1.6% 0.6% 0.2% 1.08%
Table 1. Summary of the normalization uncertainties for the Double Chooz Gd and H-analysis [16, 17]
included in our covariance matrices, M, includes the reactor ν¯ e spectrum, energy scale uncertainties, 9 Li spectrum and Fast neutron and stopping muon spectrum. The energy scale uncertainties arise from three sources: time variation, non linearities and non uniformities in the detector response. In both analyses we use the Bugey4 measurement [18] to minimize the systematic uncertainties in the reactor flux prediction. The background rates for the 9 Li and Fast neutron and stopping muons are allowed to vary during the minimization procedure (with the use of a pull term), and they rescale the rate of the corresponding backgrounds during the minimization procedure. A summary of the values for the background rate used as input in the fit and the minimization results are presented in Tab. 4. The elements of the migration matrix are recalculated as a function of the oscillation and other fit parameters (background
334
C. Mariani / Physics Procedia 61 (2015) 331 – 335
Gd
H
Input (rel. unc.)
Fit output (rel. unc.)
9
Li rate FN + SM rate
1.3 ± 0.5 d−1 (40%) 0.7 ± 0.2 d−1 (30%)
1.0 ± 0.3 d−1 (30%) 0.6 ± 0.1 d−1 (20%)
9
2.8 ± 1.2 d−1 (40%) 2.5 ± 0.5 d−1 (20%)
3.9 ± 0.6 d−1 (15%) 2.6 ± 0.4 d−1 (15%)
Li rate FN + SM rate
Table 2. Rate+Shape fit background constraints for both the Gd and H analyses [16, 17]
Background-subtracted signal No oscillation
1200
Best fit: sin2 (2θ13) = 0.109 at Δ m231 = 0.00232 eV 2 (χ2 /d.o.f. = 42.1/35)
1000
Events 0.25 MeV
Events 0.5 MeV
rates) at each step of the minimization. The Δm2 used for the fit is (2.32 ± 0.12) × 10−3 eV2 . The best fit value gives sin2 2θ13 = 0.109±0.039 for the Gd-analysis [16] and sin2 2θ13 = 0.097±0.048 for the H-analysis [17]. Figures 2 show the prompt energy spectrum of neutrino candidates with background subtracted for the Gd [16] and H-analysis [17]. We have also preliminarily combined the Gd and H-analysis measurements. In this case the fit includes the correlation of systematic errors and includes the background constraints as coming out of the reactor-off measurements [19]. Between 2011 and 2012 we have collected a total of 7.5 days of data with both the reactors off [19](called an off-off period). This is a unique capability of Double Chooz. The expected number of events during the reactor off period is 14.8 ± 4.0 in good agreement with the prediction of 8 events. The results for the combined Gd and H fit results together with the off-off data are still preliminary. We measure a sin2 2θ13 of 0.109 ± 0.035 for the rate plus shape results and sin2 2θ13 = 0.107 ± 0.045 for the rate only results. The χ2 /d.o. f. is 61.2/50 for the rate plus shape and 6.1/3 for the rate only result. 1400
Background-subtracted signal No oscillation Best fit: sin2(2θ13) = 0.097
1200
2 = 0.00231 eV2 (χ2 /d.o.f. = 38.9/30) at Δm31
Systematic error
Systematic error
1000
800 800
600 600
400
400 200
1.2
Data - Predicted Data / Predicted 0.25 MeV 0.25 MeV
Data - Predicted Data / Predicted 0.5 MeV 0.5 MeV
200
1.0 0.8 0.6 0 -50 -100 -150 2
4
6
8
10
12
1.2 1.0 0.8 0.6
0 -100 -200 2
4
6
Energy (MeV)
(a)
8
10
12
Energy (MeV)
(b)
Figure 2. Background-subtracted data (black points with statistical error bars) are superimposed on the prompt energy spectra expected in the case of no oscillations (dashed blue line) and for our best fit sin2 2θ13 (solid red line). Solid gold bands indicate systematic errors in each bin. Middle: The ratio of data to the no-oscillation prediction is superimposed on the expected ratio in the case of no oscillations (blue dashed line) and for our best fit sin2 2θ13 (solid red line). Gold bands indicate systematic errors in each bin. Bottom: The difference between data and the no-oscillation prediction is shown in the same style as the ratio. Figures and caption from Ref. [16, 17]
C. Mariani / Physics Procedia 61 (2015) 331 – 335
5. Conclusions The Double Chooz experiment has provided a rich, unique program with the far detector only analysis. The collaboration has used two signal channels Gd and H. The unique capability of Double Chooz in doing reactor offoff measurement contributed substantially to verify and validate the experimental results obtained with the rate + shape oscillation analysis. The far detector only analysis is expected to be finalized by June, 2013 and it will aim to a precision, using just the far detector, of 0.03. The Double Chooz experiment is now the process of building its near detector, which is expected to come online at the end of summer, 2014. The experiment projected precision on sin2 2θ13 using both the near and far detector is of 0.01 [20]. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]
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