νeνe νeνe νeνe νeνe νeνe νeνe Distance (L/E) Probability ν e 1.0 ~1800 meters 3 MeV) Reactor Oscillation Experiment Basics Unoscillated flux observed here Well understood, isotropic source of electron anti-neutrinos Oscillations observed as a deficit of ν e sin 2 2θ 13 νeνe νeνe νeνe νeνe νeνe νeνe Detectors are located underground to shield against cosmic rays. πE ν /2Δm 2 13

The Existing Limit on θ 13 sin 2 2  13 < 0.13 at 90% CL Come from the Chooz and Palo Verde reactor experiments Neither experiments found evidence for e oscillation The null result eliminated  → e as the primary mechanism for the atmospheric deficit Remember the oscillation probability So these experiments are sensitive to sin 2 2θ 13 as a function of Δm 2 13

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

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

CHOOZ and Palo Verde Results sin 2 2  13 < 0.18 at 90% CL (at  m 2 =2.0×10 -3 ) Future experiments should try to improve on these limits by at least an order of magnitude. Down to sin 2 2  In other words, a 1% measurement is needed! < ~ Neither experiments found evidence for e oscillation. This null result eliminated  → e as the primary mechanism for the Super-K atmospheric deficit.

Nuclear Reactors as a Neutrino Source A typical commercial reactor, with 3 GW thermal power, produces 6×10 20 ν e /s The observable e spectrum is the product of the flux and the cross section. Nuclear reactors are a very intense sources of ν e coming from the  -decay of the neutron-rich fission fragments. Arbitrary Flux Cross Section Observable ν Spectrum From Bemporad, Gratta and Vogel

The reaction process is inverse β-decay Two part coincidence signal is crucial for background reduction. Minimum energy for the primary signal of 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 capture e p→ e + n n capture Reactor Neutrino Event Signature n H → D  (2.2 MeV) n m Gd → m+1 Gd  ’s (8 MeV) E ν = E e MeV ( =m n  m p +m e  1.022)

With Gd Without Gd With Gd Without Gd Why 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 μs ~200 μs

Inverse β-decay makes a nice coincidence signal in the detector. First burst of light from the positron. 10’s of μs later… Delayed burst of light from neutron capture. Neutrino Interactions in the Detector νeνe p n e+e+ e-e-

A Simple Sensitivity Model Where N is the number of observed signal events, L is the baseline and ε is the relative efficiency (≈1). Then… Where… < 1– 3σ R means an effect is observed Statistics Relative Normalization Background How Do You Measure a Small Disappearance?

Statistics Ways to optimize statistics… Reactor power Reactor power Daya Bay is one of the most powerful nuclear plants in the world with 6 cores online by 2011 Detector mass Detector mass With 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 Run time Three years run time will be two years more than previous experiments Optimized baseline for known value of Δm 2 Optimized baseline for known value of Δm 2

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

Background 1.Fast neutron ─ fast neutron enters detector, creates prompt signal, thermalizes, and is captured 2.β+n decays of spallation isotopes ─ such as 9 Li and 8 He with β+n decay modes can be created in μ 12 C spallation event The vast majority of backgrounds are directly related to cosmic rays There are three types of background: 1.Random coincidence ─ two unrelated events happen close together in space and time (1%)

Random Coincidence Background Assuming KamLAND concentrations of 40 K, 232 Th and 238 U and 450 mwe Calculated rates for Braidwood. Plot by Hannah Newfield-Plunkett The rate of coincident events can be determined by studying the rates for positron and neutron capture like events in the detector The 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 MeV Neutron events are ~6 to ~10 MeV and include neutron captures from muon induced neutrons which are not shown

Fast Neutron Backgrounds 1.Two 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 μs. 2.Proton recoil off fast neutron ─ dominate effect. 3.Fast neutron excitation of 12 C ─ interesting, but not significantly different than 2. Energy spectrum peaks at particular values (like 4.4 MeV, first 12 C excited state) There are three main processes for the prompt “positron-like” events

Tagging Muons at Daya Bay The 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.  n  p n

Correlated Spallation Isotopes Isotopes like 9 Li and 8 He can be created in μ spallation on 12 C and can decay to β+n KamLAND found that the spallation is almost exclusively 9 Li This production is correlated with μ’s that shower in the detector from the thesis of Kevin McKinny Therefore we can account for these events by looking at the separation in time of candidate events from energetic showers muon showers.

Background Summary Total expected background rates: far site < 0.4 events/det/day Daya Bay site < 6 events/det/day Ling Ao site < 4 events/det/day (1%) (a) (d) (c) (b)

Sensitivity To Shape Deformation 90%CL at Δm 2 = 3×10 -3 eV 2 Assumes negligible background; σ cal relative near/far energy calibration σ norm relative near/far flux normalization σ norm relative near/far flux normalization Huber et al hep-ph/ Statistical error only Fit uses spectral shape only Exposure (ton GW th year) sin 2 2θ 13 Sensitivity

For three years of Daya Bay data and Δm 2 ≈ 2.5×10 -3 eV 2 90% CL limit at sin 2 2θ 13 < σ discovery for sin 2 2θ 13 > Daya Bay Projected Sensitivity Sensitivity to sin 2 2θ 13 Source of Uncertainty% Far Statistical per Det.0.3 Near Statistical per Det.0.1 Reactor Related0.1 Relative Normalization0.38 Background (Near)0.3 Background (Far)0.1 With Swapping

The reactor oscillation experiment is what is known as a disappearance experiment since it is only sensitive to the original neutrino type (ν e ) Electron neutrinos oscillate into ν μ or ν τ which can’t produce a μ or τ at reactor energies Therefore, when oscillations occur a fraction of neutrinos seem to disappear Another class of experiments, known as appearance experiments, are sensitive to the new neutrino types But the expression for the oscillation probability is much more complicated in these experiments Non-Reactor Handles on θ 13

The oscillation probability for ν μ →ν e is given by P(ν μ  ν e ) = sin 2 θ 23 sin 2 2θ 13 sin 2 (1.27 Δm 13 2 L/E) + cos 2 θ 23 sin 2 2θ 12 sin2(1.27 Δm 12 2 L/E) + cos 2 θ 23 sin 2 2θ 12 sin2(1.27 Δm 12 2 L/E) ± J sin δ sin(1.27 Δm 13 2 L/E) (CP Violating Term) ± J sin δ sin(1.27 Δm 13 2 L/E) (CP Violating Term) + J cos δ cos(1.27 Δm 13 2 L/E) + J cos δ cos(1.27 Δm 13 2 L/E) where J = cosθ 23 sin 2θ 12 sin 2θ 23 × sin(1.27 Δm 13 2 L/E) sin(1.27 Δm 12 2 sin 2θ 13 L/E) sin(1.27 Δm 13 2 L/E) sin(1.27 Δm 12 2 sin 2θ 13 L/E) T2K (Japan) Appearance ν μ →ν e (or ν μ →ν e with separate running)Appearance ν μ →ν e (or ν μ →ν e with separate running) Off-axis beam results in a mono-energetic ν μ beamOff-axis beam results in a mono-energetic ν μ beam Long baseline (300 – 900 km)Long baseline (300 – 900 km) Needs a very large detectorNeeds a very large detector Accelerator Based θ 13 Oscillation Experiments NO A MINOS NOνA (Fermilab)

ν e Appearance Probability CP Asymmetry…

The θ 23 Degeneracy Problem Atmospheric neutrino measurements are sensitive to sin 2 2θ 23 But the leading order term in ν μ →ν e oscillations is If the atmospheric oscillation is not exactly maximal (sin 2 2θ 23 ≠1) then sin 2 θ 23 has a twofold degeneracy 45º90º 2θ2θ2θ2θθθ sin 2 sin 2 2θ 23 sin 2 θ 23 No 2!

Daya Bay (3 yrs) + Nova Nova only (3yr + 3yr) Double Chooz + Nova 90% CL Daya Bay +T2K T2K only (5yr, -only) Double Chooz+T2K 90% CL More On Degeneracies There are additional degeneracies due to the unknown CP phase and the unknown sign of the mass hierarchy Combining experimental results can resolve these degeneracies Need the more sensitive reactor experiment to resolve degeneracies McConnel & Shaevitz hep-ex/

Sensitivity to CPV and Mass Hierarchy The accelerator experiments may be sensitive to CP violation and the mass hierarchy, but if Daya Bay sets a limit on sin 2 2θ 13 these questions can not be resolved by Noνa and T2K. ???? McConnel & Shaevitz hep-ex/ Daya Bay