PoS(NEUTEL2015)015 http://pos.sissa.it/ 6 MeV a spectral distortion com- − . 13 Θ is extracted from a background model independent 13 Θ . In the energy region around 4 . This smallest of the three known mixing angles is determined from 029 032 . . 13 0 0 Θ − + 090 . 0 on behalf of the Double Chooz Collaboration ∗ ) = 13 Θ 2 ( 2 [email protected] Speaker. pared to the reactor neutrino fluxhas no prediction significant is impact observed on and the discussed. determination of This distortion however the disappearance of electron antineutrinos at ain distance Chooz, of France. about Neutrino 1 interactions km with fromlator the two produce nuclear protons a reactors of coincidence H signal atoms ofbackground in a suppression. an prompt organic positron liquid The and scintil- value a of delayed neutron allowing efficient The main goal of the Double Chooz reactorneutrino neutrino mixing experiment is angle a precision measurement of the rate only analysis as well asof from sin a fit to the observed energy spectrum. The latter gives a result ∗ Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. c

XVI International Workshop on Neutrino Telescopes, 2-6 March 2015 Palazzo Franchetti - Istituto Veneto, Venice, Italy Max-Planck-Institut für Kernphysik Heidelberg, 69117 Heidelberg,E-mail: Germany Christian Buck The Double Chooz experiment PoS(NEUTEL2015)015 ] mea- 4 ], could just 6 Christian Buck ] and Palo Verde [ 5 ] lead to a deficit in the observed 3 , . 2 13 Θ ] in the 50ies of the last century. 1 ], with improved statistical and systematical uncertainties 9 . For the energy of reactor neutrinos the first oscillation min- ]. 12 10 Θ ] and RENO [ . The key feature of these new experiments was the concept of having near 8 13 Θ ], Daya Bay [ 7 The far laboratory of the Double Chooz experiment is located at an average distance of 1.05 km The design of the inner detectors is identical in near and far position. The neutrinos are de- Together with solar neutrino experiments the KamLAND reactor neutrino experiment [ Since then many experiments followed, most of them placed at distances around 100 m and This concept is illustrated for the case of Double Chooz in Figure 1. The near detector at a Nuclear reactors are a strong, pure and free source of electron antineutrinos which are emitted Whereas the first generation of these experiments, Chooz [ can be achieved without relying on flux predictions, which typically dominate the uncertainty 2 km, which is sensitive to the neutrino mixing angle 13 − from the two Chooz reactors. Ita is rock the shielding same of detector position 300 ofat mw.e. the an The former average distance near Chooz 0.41 experiment detector with km. laboratory The was rock specially shielding built of for the Doubletected near Chooz laboratory in is an 115 mw.e. inversetarget beta scintillator decay (Target). reaction The interaction on signaturethreshold protons of of of neutrinos 1.8 with MeV the energies is hydrogen aboveFrom a the atoms the coincidence reaction measured signal in positron of an energy a organic deposition prompt in liquid positron the and Target a the delayed neutrino neutron energy capture. can be extracted. 2. The Double Chooz detector neutrino rate, which can not be explained in the standard framework ofsured 3 the neutrino neutrino families. mixing angle was able to measure below. For a longreactor time flux predictions these and short set stringent baseline1 limits eV. experiments for In seemed neutrino 2011, mixing to a at be reevaluation mass of splittings in the of good more reactor agreement than flux with prediction [ and far detectors. By aΘ comparison of the flux atand the sensitivity near in such and a far measurement. detector a pure measurement of distance of about 400 m measuresyet the relevant. neutrino rate The at far aand position detector detects where at the the disappearance about oscillation of effect 1simple the is km electron not site distance antineutrinos. configuration sits There with close areFirst advantages just to the of the two such flux first a reactors related rather oscillationbackgrounds. and uncertainty minimum Moreover, two is the negligible detectors experiment compared profits almostin from to periods at the with other iso-flux opportunity both uncertainties to reactors positions. measure off as [ the efficiency backgrounds and imum for the "solarbaseline parameters" experiments is and observed the KamLAND at distance1 baselines there is of a about class 50 of experiments km. with baselines In of between the short isotropically. Therefore the vicinity ofto reactors study is a neutrino suitable properties. location Thefirst for measurement history neutrino of experiments of a and reactor neutrino neutrino by experiments Reines already and Cowan started [ with the Double Chooz 1. Neutrino mixing at nuclear reactors set upper limits on theChooz [ smallest of the three mixing angles, a new generation, including Double PoS(NEUTEL2015)015 Christian Buck ) and to increase the energy of the gammas emitted s µ The survival probability of an electron antineutrino is plotted versus the distance from the reactor The Target with a mass of 8.3 tons is contained in an 8 mm thick cylindrical acrylic vessel and Two systems are available for muon detection. The so-called "Inner Veto" (IV) consists of Several systems are available for detector calibration. There are weekly calibrations with a surrounded by about 18 tons ofof Gd-free the liquid positron scintillator annihilation called in "Gamma the Catcher" Targettarget as (GC). well volume, Gammas as but after can the still neutron be captureby on detected Gd 80 in might the escape tons GC. the non-scintillating Thecontained liquids buffer in of a oil the steel outside Inner vessel Detector that thethe optically are inner separates acrylic completed walls the of vessel Inner this Detector of steel from vesselof the a each 10 Muon detector GC. inch. Veto is system. equipped This At with mineral 390 photomultiplier oil tubes (PMTs) is 70 tons of linear aklylby benzene 78 (LAB) based PMTs liquid of scintillator. 8detector, The inch. the scintillation "Outer light In Veto" is addition (OV). observed detector The there shielding are is strategy flat surrounded is layers by different ofradioactivity in from 15 plastic the the scintillator cm rock two strips by of detectors. 1 on steel m top The of whereas of far water. the the nearmulti detector wavelengths is LED light shielded injectionRadioactive against system gamma natural to sources, monitor a the californiuminside PMT (Cf) the behaviour Target neutron along as source the gain and verticalbe variations. a z-axis positioned laser in at the ball various detector can locationsfixed center. be to inside The the deployed the acrylic radioactive vessels. sources GC can routing also them along a stainless steel guide tube in the neutron capture (8 MeV), the target scintillator is doped with 1 g/l of gadolinium (Gd). Figure 1: To keep the coincidence time short (about 30 source position for an energy of 3by MeV. The the positions of vertical the bars. Double Chooz near and far detector are indicated Double Chooz PoS(NEUTEL2015)015 13 Θ Christian Buck The Double Chooz far detector. Figure 2: In the uniformity calibration a correction is applied as a function of the radial and vertical A bias of the baseline estimation can result in gain non-linearities especially at low charges of Other non-linearities are introduced either by the the modeling of the readout system and The visible energy of the data is corrected for time variations of the mean gain and detector A precise knowledge of the energy scale is of crucial importance in particular for the positions to convert thethe number detector. of photoelectrons The calibration forhydrogen. is each This done correction position factor using to ranges the upfor the to single the one 5 data 2.2 % as at MeV inside well the the gamma as Target. in center after The the correction neutron of Monte map capture Carlo is simulation. on applied few photoelectrons. Gain and gaincalibration system non-linearity with of different light all intensities channels andlinearity light is injection can measured positions. change using The after gain the and power LED-fiber upon its cycles each non- of power-cycle period. the readout electronics, therefore the gain ischarge measured integration algorithm orenergy by deposited the in the scintillator detector liquids. due toor two positron Scintillation effects. energies, the The light second first Cerenkov is is light ionization production. notlight quenching production The at linear contribution increases low of to with electron the increasing latter the to energies. the total analysis using the spectral information. Thenumber visible energy of in photoelectrons the with detector is severalformity reconstructed corrections. of from the the These detector correction response, factors non-linearities, take time variations care and of the nonuni- absolute energy scale. 3. Energy reconstruction Double Chooz PoS(NEUTEL2015)015 Christian Buck s. No extra triggers µ ]. 11 150 − 5 . 20 MeV. Also the delayed energy − Cf calibration data and IBD candidate 5 . Cf source at the detector center is used. 252 252 t cut from 0 ∆ Po-decay (8.8 MeV) which show up around 212 ] there is the coincidence signal of the prompt positron 11 s after the prompt candidate. µ 10 MeV and the − s before and 600 E selection efficiency based on the results of our data. µ ∆ t, Cf calibration data at the detector center. In the spontaneous fission of the Cf, ∆ 252 ] the prompt energy window was enlarged to 0 12 , 7 Several new cuts were introduced to reduce backgrounds. The prompt and delayed event must The main source of inefficiency in the neutrino detection of Double Chooz is related to the For the reduction of cosmogenic backgrounds, events within a 1 ms time window after a muon The neutrons produced in the IBD reaction are basically captured either on Gd or on H. There In the Gd analysis of Double Chooz [ To determine the absolute energy scale data of the were allowed 200 be reconstructed within less than 1as m well distance. as Moreover, from information vertex obtained reconstructionall by are these the used IV selection to and cuts tag OV and 17351With veto the neutrino correlated modified candidates background and events. could optimized With beincreased to selection observed more cuts in than the 460.7 20. signal days to of background live ratio time. in Double5. Chooz Detection efficiency neutron capture. The trigger efficiency reachesthe 100% detection at efficiency of 500 the keV prompt with event iswith negligible close the uncertainty. to detection Also 100%. of The the systematic delayed uncertainty signal associated is estimated with and prompt events with ayses coincident [ signal in the OV are rejected. Compared to previous anal- event and the highenergies energy of (8 natural MeV) radioactivity. delayed Foruniform "physics event in events" space after we and neutron first simultaneous capture in requireinstrumental time that on light at the production Gd, the from light PMTs. the well signal This PMT arrives above is bases. to the avoid background events due to window and the coincidence timeniques, could which be in widened consequence increased duedelayed the to detection energy improved efficiency window background and is rejection the from tech- signal 4 to noise ratio. The events. The Double Chooz simulation is checkedon and corrected Gd if and needed the for the fraction of captures gammas (prompt signal) as well as neutrons (delayed signal) are emitted. The background in is just a small contributionsured from using captures on C. The fraction of neutron captures on Gd is mea- Double Chooz response. The observed variationsmagnitude. are on The the time per variations cent(2.2 are level extracted MeV). with and The some corrected energy systematic using dependenceneutron uncertainty the captures of and neutron on the capture Gd energy peak (8 dependence on MeV) is and H then alphas from studied using data from 4. Neutrino selection 1 MeV visible energy due to quenching effects. From the position offound the to 2.2 be MeV 186 peak photoelectronssystematic with of uncertainty good neutrons on agreement the captured between energy on scale data was H and calculated Monte the to Carlo. be absolute 0.74 In energy % total scale [ the is PoS(NEUTEL2015)015 Christian Buck events/day. This rate is found to be ) ,n) reactions, were found to be negligible syst α ( 0026 . 0 ± ) stat ( ]. was applied. ) 11 0003 . 0 syst ( ± analysis [ 0041 . 0701 0 . 13 ± Θ ) stat ( 0011 . 08%. To match the result of the simulation to the one of the data a correction factor of . 0 0 ± ± As accidental background we name random coincidences of events that meet the conditions of Correlated signals of fast neutrons (FN) or stopping muons (SM) are another source of back- The neutron selection efficiency over the full detector volume was evaluated using two in- Finally, there is a class of events in which the neutron is detected in a different volume than The backgrounds in Double Chooz can be classified in three categories. One is accidental 30 . 9750 -n-decays. Other contributions, as background due to ( . the energy cuts and fall within theeffectively reduced coincidence with time the window. distance The cut rate and ofcoincidence it this time can type window be of by measured background more is with thanrate high 1 precision s is by from measured shifting the to the prompt candidate. be In 0 this way the background 6.1 Accidental background ground in DC. Fast neutronsand can producing be spallation produced neutrons. by muonsentering The missing the neutrino the detector signal from veto outside, systems can creating of then recoil the be protons detector mimicked from by neutron scattering these (prompt neutrons event). stable over the data taking period. 6.2 Fast neutrons and stopping muons Double Chooz this calibration data canplicity be condition. effectively reduced In by mostconcentration a cases used prompt in there cut Double is Chooz at more85 the 4 than fraction MeV of one neutron or neutron captures by emitted0 on using per Gd was a fission. determined multi- to At be the Gd- dependent methods. One withcandidates. calibration Since data the from data the wasmethods in neutron no very source correction good and was agreement applied. the with the The other results associated using uncertainty of IBD was the determined simulations to forthe be both positron. 0.19% only. For example theredepositing the can energy be there, an whereas IBD theGd. reaction neutron When in might neutrons the travel enter to Gamma the the Catcherthe Target target with GC we and we the call can call positron it be it "spill-in" capturedeffective "spill-out" events, on increase for events. of neutrons Since the leaving fiducial there the volume are2.08%. Target in to more The the related spill-in detector systematic than uncertainty beyond spill-out is themodels events determined Target. there by and The the is parameter net comparison an of spill-in variations different0.27 flux simulation in is %. the Combining MC. theon As three the contributions a target and result proton adding number, the othermajor we improvement uncertainty uncertainties end compared as was up to e.g. with previous determined DC the a to analyses. knowledge total be uncertainty for the efficiency6. part of Backgrounds 0.6%, a background which has the smallestvery high contribution precision. of Then there all are backgroundsfast two neutrons categories and of and can correlated stopping backgrounds. be muons The determinedβ and first the with of second those are one long-lived cosmogenic isotopes undergoing and do not impact our PoS(NEUTEL2015)015 events/day. Christian Buck 41 16 . . 0 0 + − 97 . ). This reactor off-off σ 7 . events is predicted in the off-off 1 4 . . 3 1 + − 9 . 051 events/day estimated by extrapolation . 0 ± 604 He. Since the half life of these isotopes is above Li increases with the muon energy. Therefore the . 8 9 Li or 9 47 events are expected from residual reactor neutrinos. From the sum of residual neutrinos -n emitters like the dominant . 0 β ± One of the unique features in the Double Chooz experiment is the possibility of measuring A lower limit for this background was computed separately by selecting a Li-enriched sample Background from SM can be largely suppressed using information from the vertex reconstruc- The highest background in DC with largest uncertainty is coming from cosmogenically pro- 57 . period. The compatibility to theresult is observed used number to of constrain events the is total 9.0 background rate % in (1 the neutrino oscillation analyses. and background expectations a slightly higher rate of 12 the background in reactor offthe periods. far detector So data far taking.live-time. there were In After two total applying of 571 all these neutrino reactor the candidates off vetoes were periods 7 measured during events during remained. 7.24 days From of dedicated simulation studies The spectral shape was determinedthe from IBD captured Li on candidate H events to improvesimulation including statistics. studies. those The with spectrum is neutrons consistent from with the one obtained6.4 from Reactor off-off measurements taking into account muon energy information, lateraladditional distances neutrons to in the the muon muon track interactions. andsignificantly production In reduced of this and an way assymetric the uncertainty uncertainty isby to obtained. the a After lower subtracting dedicated side Li veto, could events rejected be the final background rate from cosmogenic isotopes is 0 most precise fit result islower obtained energies using an muons additional depositing cut more onto than the reduce 600 distance accidental MeV of pairs. the in muon the tracks detector. to At the prompt vertex is applied tion, which assumes point like eventsand in charge the distribution central from part of muonsfore the or the detector. background high Those from energy differ FN in electronchecked is PMT events using significantly time in events, higher. the which The chimney are spectralin area. tagged shape the of by IV There- the the scintillator FN IV, beforeenergy since background entering is the fit the neutrons from inner often the detector. depositwhere upper correlated some From end background energy these dominates, of IV a the tagged shapeevidence consistent events neutrino for with and any spectrum a a energy flat (around high dependence spectrumby 12 observed. was the found MeV) The OV. and The to flat no correlated shape 30 background is rate MeV, is also a 0 confirmed region by events vetoed duced Double Chooz The neutron itself (or another one)create can the then delayed be signal. captured intor The the without SM fiducial being volume events of arise tagged the fromchimney. by detector Here muons the and the stopping muon veto track and systems. mimics decaying the prompt in Typically event the these and the detec- muons decay Michel enter electron the the delayed. detector via the from coincidences between 20 and 30 MeV. 6.3 Cosmogenic isotopes 100 ms, it is impossibleof to this veto background them is without evaluated introducing byprevious significant fitting muons. dead-time. the time The The correlation contribution probability between the to IBD produce candidates and the PoS(NEUTEL2015)015 029 032 . . 0 0 were − + 13 -analysis 090 Θ in perfect . 13 0 Θ 034 035 Christian Buck . . 0 0 ) = + − 13 Θ 090 2 . ( 2 is found to be 0 ) ] are presented. The first includes energy 13 value is found at sin , the number of residual neutrinos in the 2 Θ 11 2 2 χ ( m 2 , minimizing it with respect to eight fit parame- ∆ ) 13 Θ 2 ( analyses [ 2 13 039 and as additional fit output a lower rate on the total Θ . 0 ± is obtained including the constraint on the total background rate 060 . 0 13 events/day. Θ is also determined in Double Chooz using a special rate only analysis. = 36 43 . . 0 0 13 13 − + Θ Θ 2 93 . ( 2 40). Several cross-checks were carried out, e.g. removing the constraint to fit pa- / 2 . events/day. The best-fit value for sin 52 41 17 . . = 0 0 + − is carried out over a wide range of sin 64 Highest sensitivity on The mixing angle This oscillation analysis is based on a fit to the antineutrino rate and spectral shape. Data are The analysis considers the uncertainties on In the following the results of two 2 . /d.o.f. χ 2 χ background rate of 0 agreement to the rate +the shape analysis is fit done result. just usingdays the If of background the information both constraint from reactors the off, on measurement a theanalysis with slightly background about lower we seven value rate get of is the sin mixing removed angle and is obtained. In this cross check of 1 In this case, observedpower and conditions. expected There neutrino are candidateoff mainly rates or three are different both compared configurations: reactorsobserved for both off. candidate different rate reactors is reactor on, Furthermore, plotted one versus theconditions. the reactor expected reactors rate From in are a seven not bins linearThis for always fit analysis different can reactor running the be power mixing at performed angle either fulloff-off independent as measurement power. of or well any constraining background as The the model theBackgrounds using background background section. just using the rate the reactor can estimated be rates as estimated. described in the rameters for the correlated background rates or a rate only fit. Consistent values of ( obtained indicating the robustness of this fit. 7.2 Reactor rate modulation analysis ters for each value of the mixing angle. The minimum reactor-off periods, the backgrounds and theof energy scale represented by three parameters. A scan compared to the prediction ofbins an between antineutrino 0.5 MC and sample. 20and The MeV. background The data is prediction are constructed for separated for each the intoto bin. number 40 the Systematic of energy and fit events statistical by including uncertainties the neutrino areenergy bins. use propagated signal From of the a fit information covariance itdue matrix is to in possible the to order data further to points constrain the above properly background, 8 account in MeV, where particular for the correlations number between of predicted neutrino events is very small. in Double Chooz. Higher precisionfirst. is obtained from the rate + shape analysis, which is discussed 7.1 Rate + shape analysis information from the shape ofanalysis. the positron Both spectrum, methods whereas give the consistent second is results a demonstrating dedicated the rate robustness only of the Double Chooz 7. Neutrino oscillation analysis PoS(NEUTEL2015)015 level. σ Christian Buck ]. There is an excess of the 11 experiments as RENO and Daya Bay. 13 Θ Visible Energy (MeV) 2 = 0.090 analysis of Double Chooz was studied and found to be Livetime: 467.90 days B energy spectrum of data and MC simulation. Those 13 DC-III (n-Gd) 2 12 2 13 Θ = 0.00244 eV 2 m at Data No oscillation Reactor flux uncertainty Total systematic uncertainty Best fit: sin 12345678 with the reactor flux uncertainty (green band) and the total systematic uncertainty 13

Θ 0.8 1.2 1.1 1.0 0.9

0.25 MeV 0.25

Data / Predicted / Data Black points show the ratio of the data to the prediction without oscillations. The red line shows The energy scale in this region is confirmed by spallation neutrons captured on carbon (energy The excess in the 5 MeV region is also seen in other Double Chooz has finalized its near detector (ND) construction by the end of 2014 and first A spectral distortion compared to the predicted reactor flux besides the oscillatory signature peak at 5 MeV), for whichis data observed and MC in simulation the agree within comparison 0.5%. of Furthermore, no the distortion preliminary data demonstrated the feasibility oftors. the reactor So far, neutrino with measurement one with detector both only,prediction. the detec- dominant This uncertainty in will Double change Chooz is completely on with the reactor the flux ND, since the flux uncertainty is then strongly observed rate around 5 MeV. TheThe significance impact of this of excess the wassmall. distortion evaluated to For on the be the oscillation on analysis the inrelevant, 3 since Double the Chooz oscillation the effect energy is region expected below mainly 4 at MeV low is energies. much more observations disfavor the energy scaleto as be cause strongly of correlated the to distortion.far the unknown The background reactor rate source power, of which inbackground the the is contribution 5 excess not in is MeV this expected region region. found in isis case Therefore, disfavored caused as it the by well. is hypothesis limitations There of caused of are the an by investigations, reactor if additional a flux the so predictions, excess but those studiesThis are is not an yet additional conclusive. hintsystematics that of the the experiments cause is is rather probably different. not background or detector related, since9. the Prospects with near detector is found above 4 MeV of the prompt energy as shown in Figure 3 [ 8. Spectral distortion Figure 3: the best-fit including (orange band). Double Chooz PoS(NEUTEL2015)015 , , Christian Buck , Phys.Rev. D 64, , Phys.Rev. C 83, 054615 , Phys.Rev. D 86, 052008 with the Double Chooz 13 Θ 015 after few years and could reach . 0 , Phys.Rev. 92, 930-931 (1953). )) = 13 Θ is found in an analysis using rate and shape in- 2 , Phys.Rev. D 83, 073006 (2011). ( 2 032 029 . . 0 0 sin ( + − σ Disappearance in the Double Chooz Experiment 090 . e ¯ ν of 0 ) 13 Detection of the Free Neutrino Θ 2 disappearance in the Double Chooz experiment ( e 2 ¯ ν Improved predictions of reactor antineutrino spectra Search for neutrino oscillations on a long base-line at the CHOOZ nuclear Reactor antineutrino anomaly Final results from the Palo Verde neutrino oscillation experiment Observation of Reactor Electron Antineutrinos Disappearance in the RENO Measurement of Neutrino Oscillation with KamLAND: Evidence of Spectral Spectral Measurement of Electron Antineutrino Oscillation Amplitude and Frequency at , Eur.Phys.J. C 27, 331-374 (2003). Reactor Improved measurements of the neutrino mixing angle Direct measurement of backgrounds using reactor-off data in Double Chooz Indication of Reactor , Phys.Rev.Lett. 108, 191802 (2012). with the far detector only. The accuracy of the energy reconstruction could be im- , Phys.Rev.Lett. 94, 081801 (2005). , Phys.Rev.Lett. 112, 061801 (2014). , JHEP 10 (2014) 086 [Erratum ibid. 02 (2015) 074]. 13 Θ detector Phys.Rev. D 87, 011102(R) (2013). (2012). Experiment Daya Bay Phys.Rev.Lett. 108, 131801 (2012). Distortion power station (2011). 112001 (2001). The Double Chooz reactor neutrino experiment has improved its analysis on the neutrino mix- [8] F.P. An et al., [9] J.K. Ahn et al., [1] F. Reines and C.L. Cowan, [5] M. Appolonio et al., [6] F. Boehm et al., [2] Th.A. Mueller et al., [3] G. Mention et al., [4] T. Araki et al., [7] Y. Abe et al., [12] Y. Abe et al., [11] Y. Abe et al., [10] Y. Abe et al., formation. The result was confirmed incomparing a the spectrum observed and and background expected independenttortion rate neutrino of only rate the analysis for energy different spectrum compared reactorData to power predictions taking conditions. beyond with the A the oscillation dis- near effectChooz detector was sensitivity observed. has are started expected. and further significant improvements of the Double References proved significantly, the neutrino selection wastagging optimized and to rejection techniques reduce were systematics, implemented newAs and background a data result with both a reactors value off for were sin included. ing angle 0.010 with further analysis improvements. Conclusion Double Chooz suppressed due to the simpletors and experimental the set detectors up positioned ofprojected almost sensitivity the on with Double the the Chooz iso-flux ND line. site reaches with Based only on the two current reac- knowledge the