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  • Distance (L/E)Probability e1.0~1800 meters(@ 3 MeV)Reactor Oscillation Experiment BasicsWell understood, isotropic source of electron anti-neutrinosDetectors are located underground to shield against cosmic rays.

  • The Existing Limit on 13sin22q13< 0.13 at 90% CL Come from the Chooz and Palo Verde reactor experimentsNeither experiments found evidence for ne oscillationThe null result eliminated nmne as the primary mechanism for the atmospheric deficitRemember the oscillation probability

    So these experiments are sensitive to sin2213 as a function of m213

  • CHOOZ Homogeneous detector 5 ton, Gd loaded, scintillating target 300 meters water equiv. shielding 2 reactors: 8.5 GWthermal Baselines 1115 m and 998 m Used new reactors reactor off data for background measurementChooz Nuclear Reactors, France

  • Palo Verde 32 mwe shielding (Shallow!) Segmented detector: Better at handling the cosmic rate of a shallow site 12 ton, Gd loaded, scintillating target 3 reactors: 11.6 GWthermal Baselines 890 m and 750 m No full reactor off runningPalo Verde Generating Station, AZ

  • CHOOZ and Palo Verde Results sin22q13< 0.18 at 90% CL (at Dm2=2.010-3) Future experiments should try to improve on these limits by at least an order of magnitude.Down to sin22q13 0.01In other words, a 1% measurement is needed!

  • Nuclear Reactors as a Neutrino SourceA typical commercial reactor, with 3 GW thermal power, produces 61020 e/s

    The observable ne spectrum is the product of the flux and the cross section.

    Nuclear reactors are a very intense sources of e coming from the b-decay of the neutron-rich fission fragments.

  • The reaction process is inverse -decay

    Two part coincidence signal is crucial for background reduction.Minimum energy for the primary signal of 1.022 MeV from e+e annihilation at process threshold. Positron energy spectrum implies the anti-neutrino spectrum

    In pure scintillator the neutron would capture on hydrogen

    Scintillator will be doped with gadolinium which enhances capturene p e+n n captureReactor Neutrino Event Signaturen H D g (2.2 MeV)n mGd m+1Gd gs (8 MeV)E = Ee + 0.8 MeV ( =mn-mp+me-1.022)

  • With GdWithout GdWith GdWithout GdWhy Use Gadolinium?Gd has a huge neutron capture cross section. So you get faster capture times and smaller spatial separation. (Helps to reduce random coincidence backgrounds)Also the 8 MeV capture energy (compared to 2.2 MeV on H) is distinct from primary interaction energy.~30 s~200 s

  • Inverse -decay makes a nice coincidence signal in the detector.First burst of light from the positron.10s of s later Delayed burst of light from neutron capture.Neutrino Interactions in the Detectore+e-

  • A Simple Sensitivity ModelWhere N is the number of observed signal events, L is the baseline and is the relative efficiency (1). ThenWhere< 1 3R means an effect is observedStatisticsRelative NormalizationBackgroundHow Do You Measure a Small Disappearance?

  • Statistics Ways to optimize statistics Reactor power Daya Bay is one of the most powerful nuclear plants in the world with 6 cores online by 2011 Detector massWith a total of 80 tons at the far site and no fiducial mass cut Daya Bay will be an order of magnitude larger than any previous short baseline reactor neutrino experiment Run time Three years run time will be two years more than previous experiments Optimized baseline for known value of m2

  • Relative NormalizationThe use of a near detector eliminates the normalization uncertainty due to Inverse -decay reaction cross section neutrino production in the reactor core reactor powerTruly identical detectors would eliminate the remaining sources of normalization uncertainty which are detector efficiency gadolinium fraction (neutron detection efficiency) free proton count (neutrino target size and density) geometric acceptance

  • BackgroundFast neutron fast neutron enters detector, creates prompt signal, thermalizes, and is captured+n decays of spallation isotopes such as 9Li and 8He with +n decay modes can be created in 12C spallation eventThe vast majority of backgrounds are directly related to cosmic raysThere are three types of background:

    Random coincidence two unrelated events happen close together in space and time(1%)

  • Random Coincidence BackgroundAssuming KamLAND concentrations of 40K, 232Th and 238U and 450 mweCalculated rates for Braidwood.Plot by Hannah Newfield-PlunkettThe rate of coincident events can be determined by studying the rates for positron and neutron capture like events in the detectorThe singles rates from long lived spallation isotopes and the U, Th and K decay chains is shown below. Hannah was a bright high school student who worked with me for a couple of summers and is now a Cornell undergraduate student.Positron-like events are between ~2 and 8 MeVNeutron events are ~6 to ~10 MeV and include neutron captures from muon induced neutrons which are not shown

  • Fast Neutron BackgroundsTwo neutron captures from the same cosmic This should be tagged the vast majority of the time, but it sets the tag window for tagged muons at 100 s.Proton recoil off fast neutron dominate effect.Fast neutron excitation of 12C interesting, but not significantly different than 2. Energy spectrum peaks at particular values (like 4.4 MeV, first 12C excited state)There are three main processes for the prompt positron-like events

  • Tagging Muons at Daya BayThe basic idea is to tag muons that pass near the detector so that we can reject the fast neutron background. Neutrons from farther away should be mostly ranged out.

  • Correlated Spallation IsotopesIsotopes like 9Li and 8He can be created in spallation on 12C and can decay to +n KamLAND found that the spallation is almost exclusively 9LiThis production is correlated with s that shower in the detectorfrom the thesis of Kevin McKinnyTherefore we can account for these events by looking at the separation in time of candidate events from energetic showers muon showers.

  • Background SummaryTotal expected background rates: far site < 0.4 events/det/dayDaya Bay site < 6 events/det/dayLing Ao site < 4 events/det/day(1%)(a)(d)(c)(b)

  • Sensitivity To Shape Deformation 90%CL at m2 = 310-3 eV2Assumes negligible background; cal relative near/far energy calibration norm relative near/far flux normalizationHuber et al hep-ph/0303232Statistical error onlyFit uses spectral shape onlyExposure (ton GWth year)sin2213 Sensitivity4008000

  • For three years of Daya Bay data and m2 2.510-3 eV290% CL limit at sin2213 < 0.008 3 discovery for sin2213 > 0.015Daya Bay Projected SensitivitySensitivity to sin2213

    Source of Uncertainty%Far Statistical per Det.0.3Near Statistical per Det.0.1Reactor Related0.1Relative Normalization0.38Background (Near)0.3Background (Far)0.1

  • Non-Reactor Handles on 13

  • The oscillation probability for e is given by P(e) = sin223 sin2213 sin2(1.27 m132 L/E) + cos223 sin2212 sin2(1.27 m122 L/E) J sin sin(1.27 m132 L/E) (CP Violating Term) + J cos cos(1.27 m132 L/E)where J = cos23 sin 212 sin 223 sin(1.27 m132 L/E) sin(1.27 m122 sin 213 L/E)Appearance e (or e with separate running)Off-axis beam results in a mono-energetic beamLong baseline (300 900 km)Needs a very large detectorAccelerator Based 13 Oscillation Experiments

  • e Appearance ProbabilityCP Asymmetry

  • The 23 Degeneracy ProblemAtmospheric neutrino measurements are sensitive to sin2223 But the leading order term in e oscillations is

    If the atmospheric oscillation is not exactly maximal (sin22231) then sin223 has a twofold degeneracy459022sin2sin2223sin223No 2!

  • Daya Bay +T2K T2K only (5yr,n-only) Double Chooz+T2K90% CLMore On DegeneraciesThere are additional degeneracies due to the unknown CP phase and the unknown sign of the mass hierarchyCombining experimental results can resolve these degeneraciesNeed the more sensitive reactor experiment to resolve degeneraciesMcConnel & Shaevitzhep-ex/0409028

  • Sensitivity to CPV and Mass HierarchyThe accelerator experiments may be sensitive to CP violation and the mass hierarchy, but if Daya Bay sets a limit on sin2213 these questions can not be resolved by Noa and T2K. ?McConnel & Shaevitzhep-ex/0409028 Daya BayDaya Bay