Transcript ppt
Reactor Oscillation Experiment Basics νe νe νe νe νe Unoscillatedν flux e observed here Well understood, isotropic source of electron anti-neutrinos Oscillations observed as a deficit of νe Detectors are located underground to shield against cosmic rays. Probability νe 1.0 sin22θ13 πEν /2Δm213 P( ν e ν e ) 1 sin 2 2θ13 sin 2 (m132 L / 4 Eν ) Distance (L/E) ~1800 meters (@ 3 MeV) The Existing Limit on θ13 Come from the Chooz and Palo Verde reactor experiments Neither experiments found evidence for ne oscillation The null result eliminated nm→ne as the primary mechanism for the atmospheric deficit Remember the oscillation probability P( ν e ν x )sin 2 2θ13sin 2 Δm132 L 4 Eν So these experiments are sensitive to sin22θ13 as a function of Δm213 sin22q13< 0.13 at 90% CL 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 measurement Chooz Nuclear Reactors, France Palo Verde Palo Verde Generating Station, AZ • 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 running CHOOZ and Palo Verde Results • Neither experiments found evidence for ne oscillation. • This null result eliminated nm→ne as the primary mechanism for the Super-K atmospheric deficit. • sin22q13< 0.18 at 90% CL (at m2=2.0×10-3) • Future experiments should try to improve on these limits by at least an order of magnitude. Down to sin22q13 <~ 0.01 In other words, a 1% measurement is needed! Nuclear Reactors as a Neutrino Source Nuclear reactors are a very intense sources of νe coming from the b-decay of the neutron-rich fission fragments. A typical commercial reactor, with 3 GW thermal power, produces 6×1020 νe/s The observable ne spectrum is the product of the flux and the cross section. Arbitrary From Bemporad, Gratta and Vogel Observable ν Spectrum Reactor Neutrino Event Signature The reaction process is inverse β-decay ne p→ e+n n capture 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 Eν = Ee + 0.8 MeV ( =mnmp+me1.022) In pure scintillator the neutron would capture on hydrogen n H → D g (2.2 MeV) Scintillator will be doped with gadolinium which enhances capture n mGd → m+1Gd g’s (8 MeV) 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) With Gd Without Gd ~30 μs With Gd Without Gd ~200 μs Also the 8 MeV capture energy (compared to 2.2 MeV on H) is distinct from primary interaction energy. Neutrino Interactions in the Detector Inverse β-decay makes a nice coincidence signal in the detector. νe e+ e np 157 158 64 Gd First burst of light from the positron. 10’s of μs later… Delayed burst of light from neutron capture. How Do You Measure a Small Disappearance? A Simple Sensitivity Model R N far L2far 2 near N near L < 1– 3σR means an effect is observed Where N is the number of observed signal events, L is the baseline and ε is the relative efficiency (≈1). Then… 2 R stat 2 bg2 Where… stat N far N bg N far Statistics pair n 1 Relative Normalization bg rate N bg bg N far Background 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 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 Three years run time will be two years more than previous experiments • Optimized baseline for known value of Δm2 Relative Normalization The use of a near detector eliminates the normalization uncertainty due to • Inverse β-decay reaction cross section • neutrino production in the reactor core • reactor power Truly 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 Background 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. Fast neutron ─ fast neutron enters detector, creates prompt signal, thermalizes, and is captured 2. β+n decays of spallation isotopes ─ such as 9Li and 8He with β+n decay modes can be created in μ 12C spallation event (1%) Random Coincidence Background 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. Assuming KamLAND concentrations of 40K, 232Th and 238U and 450 mwe Calculated rates for Braidwood. Plot by Hannah Newfield-Plunkett 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 Hannah was a bright high school student who worked with me for a couple of summers and is now a Cornell undergraduate student. Fast Neutron Backgrounds There are three main processes for the prompt “positron-like” events 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 12C ─ interesting, but not significantly different than 2. Energy spectrum peaks at particular values (like 4.4 MeV, first 12C excited state) 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. p n m n m Correlated Spallation Isotopes Isotopes like 9Li and 8He can be created in μ spallation on 12C and can decay to β+n KamLAND found that the spallation is almost exclusively 9Li 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 (a) (d) (c) (b) (1%) 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 Sensitivity To Shape Deformation sin22θ13 Sensitivity Assumes negligible background; σcal relative near/far energy calibration σnorm relative near/far flux normalization 90%CL at Δm2 = 3×10-3 eV2 Huber et al hep-ph/0303232 400 8000 Exposure (ton GWth year) Daya Bay Projected Sensitivity For three years of Daya Bay data and Δm2 ≈ 2.5×10-3 eV2 0.006 90% CL limit at sin22θ13 < 0.008 0.013 3 σ discovery for sin22θ13 > 0.015 With Swapping Source of Uncertainty Far Statistical per Det. Near Statistical per Det. Reactor Related Relative Normalization Background (Near) Background (Far) Sensitivity to sin22θ13 % 0.3 0.1 0.1 0.38 0.12 0.3 0.1 Non-Reactor Handles on θ13 The reactor oscillation experiment is what is known as a disappearance experiment since it is only sensitive to the original neutrino type (νe) Electron neutrinos oscillate into νμ or ντ 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 Accelerator Based θ13 Oscillation Experiments • • • • Appearance νμ→νe (or νμ→νe with separate running) Off-axis beam results in a mono-energetic νμ beam Long baseline (300 – 900 km) Needs a very large detector NOnA The oscillation probability for νμ→νe is given by MINOS P(νμνe) = sin2θ23 sin22θ13 sin2(1.27 Δm132 L/E) + cos2θ23 sin22θ12 sin2(1.27 Δm122 L/E) ± J sin δ sin(1.27 Δm132 L/E) (CP Violating Term) + J cos δ cos(1.27 Δm132 L/E) NOνA (Fermilab) where J = cosθ23 sin 2θ12 sin 2θ23 × sin(1.27 Δm132 L/E) sin(1.27 Δm122 sin 2θ13 L/E) T2K (Japan) νe Appearance Probability P (ν μ ν e ) sin 2 θ 23 sin 2 2θ13 sin 2 (1.27m132 L E ) cos2 θ 23 cos2 θ13 sin 2 2θ12 sin 2 (1.27m122 L E ) cosθ 23 sin θ12 sin 2θ13 sin 2θ 23 sin δ sin 2 (1.27m132 L E ) sin(1.27m122 L E ) cosθ 23 sin θ12 sin 2θ13 sin 2θ 23 cosδ sin(1.27m132 L E ) cos(1.27m132 L E ) sin(1.27m122 L E ) CP Asymmetry… P (ν μ ν e ) P (ν μ ν e ) cosθ 23 sin θ12 sin 2θ13 sin 2θ 23 sin δ sin 2 (1.27m132 L E ) sin(1.27m122 L E ) The θ23 Degeneracy Problem Atmospheric neutrino measurements are sensitive to sin22θ23 2 P( ν μ ν x )sin 2 2θ 23sin 2 m23 L 4 Eν But the leading order term in νμ→νe oscillations is P( ν μ ν e )sin 2 θ 23sin 2 2θ13sin 2 m132 L 4Eν No 2! sin2 If the atmospheric oscillation is not exactly maximal (sin22θ23≠1) then sin2θ23 has a twofold degeneracy sin22θ23 sin2θ23 θ 45º θ 2θ 90º 2θ 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 McConnel & Shaevitz hep-ex/0409028 Daya Bay +T2K T2K only (5yr,n-only) Double Chooz+T2K 90% CL Daya Bay (3 yrs) + Nova Nova only (3yr + 3yr) 90% CL Double Chooz + Nova Need the more sensitive reactor experiment to resolve degeneracies Sensitivity to CPV and Mass Hierarchy McConnel & Shaevitz hep-ex/0409028 Daya Bay Daya Bay ? The accelerator experiments may be sensitive to CP violation and the mass hierarchy, but if Daya Bay sets a limit on sin22θ13 these questions can not be resolved by Noνa and T2K.