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Overview of Reactor Neutrino Measurements

Overview of Reactor Neutrino Measurements. Michael Shaevitz Columbia University NOW2004 With the recent confirmation by Kamland and isolation of the D m solar 2 in the LMA region, the field is turning to: Measuring the last mixing angle q 13

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Overview of Reactor Neutrino Measurements

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  1. Overview of Reactor Neutrino Measurements Michael Shaevitz Columbia University NOW2004 • With the recent confirmation by Kamland and isolation of the Dmsolar2 in the LMA region, the field is turning to: • Measuring the last mixing angle q13 • Obtaining better precision on q23,q12,Dmsolar2 and Dmatm2(along with checking LSND)  Road to measuring n-mass hierarchy and n-CP violation • Reactor oscillation measurements unique for: • Measuring q13 • Constraining q23

  2. Outline • Reactor neutrino measurements • Physics capabilities • Experimental method: • How to measure a small disappearance signal • Possible experimental sites • Sensitivity studies • Comparisons and combinations with offaxis experiments

  3. A reactor-based weak mixing angle (sin2W) measurement Probe new physics in neutrino sector at low Q2 With errors comparable to APV, Moller Scattering, and NuTeV Also, sensitivity to  magnetic moment (~x3 better than current). Moller (e-e-)E158 – Prelim Brief Aside one Elastic Scattering • Use Osc. Experiment near detector for measurement • Use far detector to measure background

  4. Measure the elastic scattering rate using the Near Detector e e–  e e– Measure the neutrino flux using Inverse Beta Decay (IBD) events in the same fiducial volume with same deadtime. e p e–n Reduction of backgrounds Only use events in the 3 < Evisible < 5 MeV window Veto events with delayed coincident neutrons Veto events with various types of associated cosmic muons Use the Far Detector to monitor the non-elastic background rate. Experiment needs a close detector (< 200m) with > 300 mwe shielding Detector requirements very similar to oscillation experiment  Low cost addition to osc. Program  Need to obtain reduced backgnds Weak Mixing Angle Measurement  d(sin2qW) = 0.0019(compare to NuTeV = 0.0017) (See J.Conrad, J.Link, and M.S., hep-ex/0403048)

  5. Nuclear reactors are a very intense sources ofνe with a well understood spectrum 3 GW ≈ 2×1021 MeV/s → 6×1020ne/s Reactor spectrum peaks at ~3.7 MeV Oscillation Max. for Dm2=2.510-3 eV2at L = 1.8 km Osc prob (sin2q13=1.0) = 0.81 for 1.8 km = 0.55 for 1.05 km From Bemporad, Gratta and Vogel Arbitrary Observable n Spectrum Cross Section Flux Reactor Measurements of 13 • Disappearance Measurement:Look for small rate deviation from 1/r2 measured at a near and far baselines • Counting Experiment • Compare events in near and far detector • Energy Shape Experiment • Compare energy spectrum in near and far detector

  6. Reactor Neutrino Event Signature • The reaction process is inverse β-decay followed by neutron capture • Two part coincidence signal is crucial for background reduction. • Positron energy spectrum implies the neutrino spectrum • In undoped scintillator the neutron will capture on hydrogen • More likely the scintillator will be doped with gadolinium to enhance capture Eν = Evis + 1.8 MeV – 2me nH → Dg (2.2 MeV) nmGd → m+1Gdg’s (8 MeV)

  7. Physics of θ13 at Reactors • The reactor experiment can make an unambiguous measurement of the mixing parameter sin22θ13. • Small dependence on: • mass hierarchy • CP violation phase, CP • sin223 and 23 degeneracy • sin212 and Dmsolar2

  8. Recent global fits including solar + KamLAND + CHOOZ Kamland spectral distortion fixes Dm2Solar  Compare to solar gives q13 Gives reduced 90% CL upper limits especially at smaller Dm132 sin2213 <0.09 @ Dm132=2.510-3 eV2 sin2213 <0.12 @ Dm132=1.310-3 eV2 CHOOZ and Palo Verde Experiments Neither experiments found evidence forne oscillation sin22q13< 0.14 at 90% CL (at Dm2=2.5×10-3 eV2) Chooz Systematic Uncertainties Current Status of 13 Knowledge CHOOZ Only Maltoni et al.hep-ph/0405172

  9. Physics of θ13 at Reactors • The reactor experiment can make an unambiguous measurement of the mixing parameter sin22θ13. • Sensitivity should reach sin22θ13 ≤ 0.01 to 0.03 at 90% CL in three years of running. Better sensitivity is also possible. • In conjunction with the off-axis (beam experiments) the reactor experiment breaks the degeneracy of 23 <45 or >45.

  10. The θ23 Degeneracy Problem Disappearance neutrino measurements are sensitive to sin22θ23 But the leading order term in offaxis νμ→νe oscillations is Super-K / Minos / T2K Measures Offaxis q13 Measures Example: Measurement of sin2223 = 0.95  23 = 38 or 52 Prediction for appearance rate  sin223  sin223 = 0.38 or 0.62 (x1.6 uncertainty)

  11. Physics of θ13 at Reactors • The reactor experiment can make an unambiguous measurement of the mixing parameter sin22θ13. • Sensitivity should reach sin22θ13≤ 0.01 to 0.03 at 90% CL in three years of running. Better sensitivity is possible. • In conjunction with the off-axis (beam experiments) the reactor experiment breaks the degeneracy of 23 <45 or >45 • Direct knowledge of the mixing angles is important in its own right! Could be crucial to constructing a theory of flavor. • The reactor measurement determines the feasibility of CP violation and mass hierarchy studies in off-axis.

  12. How Do You Measure a Small Disappearance? • Use identical near and far detectors to cancel many sources of systematics.

  13. Sin22θ13 Reactor Experiment Basics νe νe νe νe νe νe sin22θ13 Well understood, isotropic source of electron anti-neutrinos Oscillations observed as a deficit of νe Eν≤ 8 MeV 1.0 Unoscillated flux observed here Probability νe Survival Probability Distance 1200 to 1800 meters

  14. How Do You Measure a Small Disappearance? • Use identical near and far detectors to cancel many sources of systematics. • Design detectors to eliminate the need for analysis cuts that may introduce systematic error.

  15. Detector Design Basics • Homogenous Volume • Viewed by PMT’s • Coverage of 20% or better • Gadolinium Loaded, Liquid Scintillator Target • Enhances neutron capture • Unloaded Scintillator Region • To capture energy from gamma rays. Eliminates need for fiducial volume cut. • Pure Mineral Oil Buffer • To shield the scintillator from radioactivity in the PMT glass. Allows you to set an energy cut well below the 1 MeV e+e- annihilation energy.

  16. How Do You Measure a Small Disappearance? • Use identical near and far detectors to cancel many sources of systematics. • Design detectors to eliminate the need for analysis cuts that may introduce systematic error. • Detector cross calibration may be used to further reduce the near/far normalization systematic error. •  Use common sources to cross calibrate •  Move far detectors to near site for cross calibration

  17. How Do You Measure a Small Disappearance? • Use identical near and far detectors to cancel many sources of systematics. • Design detectors to eliminate the need for analysis cuts that may introduce systematic error. • Detector cross calibration may be used to further reduce the near/far normalization systematic error. • Reduce background rate and uncertainty

  18. Backgrounds • There are two types of background… • Uncorrelated − Two random events that occur close together in space and time and mimic the parts of the coincidence. • This BG rate can be estimated by measuring the singles rates, or by switching the order of the coincidence events. • Correlated − One event that mimics both parts of the coincidence signal. • These may be caused by fast neutrons (from cosmic m’s) that strike a proton in the scintillator. The recoiling proton mimics the e+ and the neutron captures. • Or they may be cause by muon produced isotopes like 9Li and 8He which sometimes decay to β+n. • Estimating the correlated rate is much more difficult!

  19. How Do You Measure a Small Disappearance? • Use identical near and far detectors to cancel many sources of systematics. • Design detectors to eliminate the need for analysis cuts that may introduce systematic error. • Detector cross calibration may be used to further reduce the near/far normalization systematic error. • Reduce background rate and uncertainty • Go as deep as you can • Veto

  20. Veto Detectors p n n m m Veto Background Events Fast neutrons Veto m’s and shield neutrons 9Li and 8He E ≤ 10.6 MeV τ½ = 0.18 to 0.12 s 0.075 produced/ton/day (450 mwe) 50% to 16% correlated β+n Shielding KamLAND Data A ½ second veto after every muon that deposits more that 2 GeV in the detector should eliminate 70 to 80% of all correlated decays. 6 meters

  21. How Do You Measure a Small Disappearance? • Use identical near and far detectors to cancel many sources of systematics. • Design detectors to eliminate the need for analysis cuts that may introduce systematic error. • Detector cross calibration may be used to further reduce the near/far normalization systematic error. • Reduce background rate and uncertainty • Go as deep as you can • Veto • Use vetoed events to measure the background • Redundant measurements to give convincing signal • Multiple detectors at each site • See osc. signal in both rate and spectral distortion

  22. Scales of Experiments and Sensitivities small: sin22q13~0.03 • Goal: fast experiment to explore region x3-4 below the Chooz limit. • Sensitivity through rate mainly • Example: “Double-Chooz” experiment (300 GW-ton-yrs) medium: sin22q13~0.01 • Make a discovery of q13 in region of interest for the next 10-20 year program • Sensitivity enough to augment the physics of offaxis measurements • Sensitivity both to rate and energy shape • Example: Braidwood, Daya Bay (3000 GW-ton-yrs) large: sin22q13~0.002-0.004?? • Measurement capability comparable to second generation offaxis experiments • Sensitivity mainly through energy shape distortions • MiniBooNE/Kamland sized detector (20,000 GW-ton-yrs)

  23. Counting (Rate) vs Energy Shape Measurement Small detectors give a rate comparison near to farLarge detectors can show a energy shape distortion near to far medium sin22θ13 Sensitivity Fit uses spectral shape only Statistical error only From Huber et al.hep-ph/0303232 large small Exposure (GW·ton·years) The location of the transition from rate to shape depends on the level of systematic error snorm = relative normalization of the two detectors

  24. Proposed Reactor Oscillation Experiments

  25. Proposed Sites Around the World Stop On Hold Go Many Sites have been investigated as potential hosts to a reactor neutrino experiment. This is appropriate since getting the cooperation of the reactor company and host country is a main challenge. See Talks this afternoon by G. Mention and M. Goodman

  26. Proposed Sites Around the World far near near Status

  27. Kashiwazaki-Kariwa, Japan ( ) 6m Diameter Shaft – 200 ~ 300m depth Minakata, Sugiyama, Yasuda, Inoue, and Suekane hep-ph/0211111 • 7 Reactors, 24 GWth • Three ~8.5 ton detectors • Two near detectors at baselines of ~400 m • One far detector at 1.3 ~ 1.8 km • 21 different baselines but not a problem • Sensitivity of sin22θ13≤0.025 in 2 years • Fast! Reaches systematics limit quickly • Currently working its way through the • Japanese system • (2 yr R&D budget approved) far near near

  28. Proposed Sites Around the World Status

  29. Double CHOOZ , France • Use old far detector hall at ~1050 meters • Near detector at 125-250 meters (~50 mwe) • 11 ton Gd loaded detectors. • Sensitivity of sin22θ13≤0.03 in 3 years • Fast and Inexpensive • Has scientific approval • Released an LOI (hep-ex/0405032)

  30. Experimental Goals: Statistical error = 0.4% Background error = 1% Relative detector error = 0.6% Advantages: Infrastructure exists for far site Reactor company would build near site if approved Disadvantages: Shallow near detector site (50-100 mwe) Deadtime = 500 ms / muon 30 - 60% Deadtime Far baseline only 1km which limits sensitivity especially for low Dm2 Cost estimate: Detectors: ~ $7M Civil Construction: ~ $10M Schedule: Now : Securing approvals from agencies and company Feb. 05: If approved, complete design and put out bids May 07: Complete far detector Early 08: Complete near detector 1.5×10-3 eV2 sin22q13limit 2.0×10-3 eV2 3.0×10-3 eV2 1 2 3 4 5 6 7 8 9 10Exposure in Years Double – CHOOZ Experiment

  31. Proposed Sites Around the World Status

  32. Braidwood, Illinois • Four 65 ton detectors • Near detectors at 200 meters & 450 mwe • Two far detectors located at 1500 meters, 450 mwe • Sensitivity of sin22θ13≤0.01 in 3 years • High level of cooperation with utility Braidwood

  33. Experimental Goals: Statistical error = 0.15% Background error = 0.4% Relative detector error = 0.6% Advantages: Deep near site allows other reactor physics measurements Favorable geology and low bkgnd Redundancy and cross checks Both rate and spectral distortion analysis Surface transport of detectors for cross calibration Multiple near and far detectors Disadvantages: Infrastructure costs high due to new undeveloped site Cost estimate: Civil: ~ $35M Detectors: $15M Schedule: 2005: R&D proposal submission. 2006: Full proposal submission 2007: Project approval; start const. 2009: Start data collection Braidwood Experiment Braidwood3 yrs

  34. Proposed Sites Around the World Status

  35. Daya Bay, China • 4 Reactors, 11.5 GWth • Several ~8 ton detectors • Near detectors at baseline of 300 and 400 meters, 200 to 250 mwe • Far detectors at baselines of 1800 and 2400 meters, 1100 mwe • Sensitivity of sin22θ13≤0.01 in 3 years • Utility/government approval is likely • China would support civil construction, but foreign support is needed for detectors

  36. Experimental Goals: Statistical error = 0.2% Background error = 0.3% Relative detector error = 0.4% Advantages: Horizontal access tunnel approach Large overburden reduces bkgnd Flexibility to change baseline Easy to service detector Cross calibration by moving detectors to near site Disadvantages: Uncertainties associated with the Chinese approval system Cost estimate: ~ $25M Schedule: 2005: R&D, engineering design, and secure funding 2006-2007: Construction 2008: Start data collection Daya Bay Experiment

  37. Offaxis experimental inputs: JPARC to SuperK (T2K) exp. Rates for n andn from LOI with / without upgrade x5 rate Offaxis NuMI (Nova) exp. Rates for n andn new LOI appendix with / without proton driver upgrade (x5 rate) Reactor Sensitivity Studies(Comparing and Combining with Offaxis Measurements) (K. McConnel and M. Shaevitz – hep-ex/0409028) • Try to do estimates including all effects: • m232 = 2.510-3 eV2 and allowed to vary with  = 0.110-3 eV2 • Include both mass hierarchies m232 > 0 and m232 < 0 • sin2223 = 0.95 and allowed to vary with  = 0.01 • Include ambiguity of 23 < 45 or > 45 • m122 and 12 fixed at current values • dCP allowed to vary between 0 to 360 • Reactor experimental inputs: • Small scale: d(sin22q13) = 0.03 • Medium scale: d(sin22q13) = 0.01 • Large scale: d(sin22q13) = 0.005(90% CL upper limit sensitivities at m2 = 2.510-3 eV2)

  38. Comparisons of Reactor and Off-axis Sensitivities large medium small reactor ×5 beamrate 90% CL upper limits for underlying null sin22θ13 (= 0) A medium scale reactor experiment, gives a more stringent limit on sin22θ13 in several regions than off axis, even with proton driver like statistics (×5 beam rate). After a medium scale reactor limit, only a small window of opportunity exists for an observation of νμ to νe with an off-axis. Green: Offaxis exp. (5yr  only)Blue: Combined Medium Reactor plus OffaxisWhite: Offaxis Only (x5 rate) combinewith med.reactor combinewith med.reactor 5 yrs -only

  39. Comparison of Reactor to Off-axis Signal Meas. Chooz-like, small scale Braidwood-like medium scale 90% CL regions for sin22θ13 = 0.05, δCP=0 and Δm2 = 2.5×10-3 eV2 A medium scale reactor experiment makes a significantly better measurement of sin213 than the offaxis experiments. Green: Offaxis exp. OnlyBlue: Combined Medium Reactor plus OffaxisRed: Combined Small Reactor plus Offaxis 5 yrs -only

  40. Resolving the θ23 Degeneracy Example: sin2223 = 0.95  23 = 38 or 23 = 90-38  = 52 (23TRUE= 38) • For sin213= 0.05, combining with off-axis ν andν running • Medium scale reactor breaks the θ23 degeneracy. • Small reactor is not sufficient to break the degeneracy. (3 yrs  + 3 yrs )

  41. Regions where 23 Degeneracy Resolved at 2(Is 23 < 450 or > 450 ?) 3 yrs  + 3 yrs 3 yrs  + 3 yrs

  42. T2K  - 5 years Constraining the CP Violation Parameter, CPand Determining the Mass Hierarchy • Use Medium Reactor (d(sin22q13) = 0.01) can predict the neutrino prob. • Oscillation probability vs dCP for offaxis exp(m2 = 2.5x10-3 eV2 , sin2213 = 0.05) P(e) sin2213=0.1 Neutrino, normal hierarchy Neutrino, inverted hierarchy CP

  43. For large sin213, can restrict CP with reactor and -only offaxis data. For smaller sin213, need offaxis data to restrict CP. In all cases, precision on determining sin213 comes from reactor data. Reactor Contribution to CP Violation (3 yrs) 3 yrs  + 3 yrs 5 yrs -only

  44. large medium small reactor large medium small reactor CP Violation 3 Discovery Regions Reactor measurement does not contribute much to measuring CP violation… But a null reactor measurement, even at the Double Chooz sensitivity, can mostly rule out accessible regions for T2K / Nova. with proton driver with x5 upgrade To the right of the curve, CP=0 or p is excluded by at least three standard deviations m2 = 2.5×10-3 eV2 sin2223 = 0.95  0.01 with x5 upgrades 3 yrs  + 3 yrs

  45. large medium small reactor large medium small reactor Determining Reach in Mass Hierarchy To the right of the curve, mass hierarchy is resolved by at least two sigma Reactor measurement does not contribute much to resolving the mass hierarchy… Again, a null reactor measurement, even at the Double Chooz sensitivity, can mostly rule out accessible regions for T2K and Nova. m2 = 2.5×10-3 eV2 sin2223 = 0.95  0.01 with proton driver with x5 upgrades 3 yrs  + 3 yrs

  46. Hyper-K alone makes poor determination of CP violation phase, CP 23 degeneracy limits accuracy Mass hierarchy introduces wide range  Medium scale reactor data can lift 23 degeneracy (black dashed curves)  Combination with Nova can determine hierarchy Bottom Line: Combination of reactor plus Hyper-K plus Nova can make precise determination(black dashed curves) Far Future  Reactor plus Hyper-K + Nova(x10)

  47. Conclusions • A reactor experiment is “the” prime and only unambiguous measurement of q13 • q13 is a important physics parameter • Needed to constrain the models of lepton mixing matrix • If very small, probably indicates a new symmetry • q13 is key for planning future long-baseline experiments to measure CP violation and the mass hierarchy • If sin22q13 is > ~0.02, T2K and Nova make a nice program • If sin22q13 is < ~0.01, need other techniques to access the physics (1st,2nd max. measurements; Superbeam exps, Neutrino Factory….) • Reactor measurements are important for sorting out the q23 ambiguity (q23 vs 900- q23) • Again this is an important, fundamental physics parameter (like q13) • May be important for CP violation and mass hierarchy measurements • Reactor experiments are being pursued at many sites and it is likely that several will be approved and go forward.

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