M. Shaevitz Columbia University 3 rd Workshop on Future Low-Energy Neutrino Experiments

1. M. Shaevitz Columbia University3rd Workshop on Future Low-Energy Neutrino Experiments March 21, 2004 To appear:Next weekon LANL Preprintserver

2. Outline • Overview of physics • Outline of the method • Details of the method and sensitivity study • Conclusions

3. LEP Z Measurement of the Weak Mixing Angle at a Reactor • Extra physics opportunity in combination with the oscillation measurements • Probes neutrino electroweak physics in new low Q2 regime • Some indications from previous measurements of a discrepancy with expectations for neutrinos • NuTeV measurement • LEP invisible line shape • May give laboratory information on electron neutrino magnetic moment

4. A Reactor Measurement of sin2qW • Probes new physics in the neutrino sector (like NuTeV) • Has low Q2, comparable to APV (Qw) • May be sensitive to neutrino magnetic moment effects at current Lab limits • Uses design similar to near detector proposals • Far Detector Used to Measure Backgrounds • May achieve errors comparable to APV, E158, & NuTeV ... d( sin2qW )= ±0.0019 General conclusion is that a 1% measurement is well-worth pursuing!

5. How to measure sin2qW at a reactor?

6. Overview of Method • Measure the elastic scattering rate in the range 3<Ee<5 MeV using the Near Detector • Measure the neutrino flux using Inverse Beta Decay (IBD) events in the same fiducial volume with same deadtime. • 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.

7. Use “Generic” two detector (near-far) experiment setup to investigate sensitivities and backgrounds 26.5 ton near fiducial volume Try to use conservative and simple estimates for backgrounds and efficiencies Initial Sensitivity Study Similar to Braidwood Setup

8. Veto Reactor WMA Measurement • Use near detector to make the measurement • Needs to be as close as possible to reactor for statistics (~200m) • Needs to be as deep as possible to reduce backgrounds (>300 mwe) • Measure elastic rate between 3 – 5 MeV • Use reduced fiducial volume at center of Gd region and 4p veto system to reduce environmental backgrounds • Use far detector to measure background levels • Overburden near and far should be similar or develop methods to do cross-comparison

9. Questions: 1. Are the statistics high enough? A 3 yr run with 25 ton detector at 200m  ~10,000 events Are the rates from environmental backgrounds (cosmics, radioactivity) low enough? Main background is beta-decay from muon induced long-lived isotopes ~1500 event background for 300 mwe overburden Can the background from inverse beta decay (IBD) be controlled? Neutron veto using fiducial volume inside Gd region ~2000 event background 4. Is the normalization known well enough to make a precise measurement? Yes, using the observed IBD events to measure the ne flux IBD cross section known to 0.3% and 1.8x106 event sample to set normalization Precision Requirements: NuTeV: sin2qW = 0.2274 ± 0.0017 (0.75%) Reactor Exp: Need to measurenee rate to 1.2% between 3 to 5 MeV Key Questions for a Reactor WMA Measurement

10. Question 1: Is the Elastic Scattering event rate high enough? e/free p is 4.3!!! Above 3 MeV, the ES to IBD event rate is about 100:1 Most near detectors are designed to collect ~ 1E6 IBD events so > 10,000 ES events, or a 1% stat error, looks feasible

11. Question 2: Are the Environmental Backgrounds Controllable? Yes if you introduce a cut: 3 < Evis < 5 MeV and use various muon vetos • Cosmic Ray Muons: 3.5 Hz/detector • Most will fire the vetoHigh energy deposited in detectorseparable from other events due to energyWill not fall into visible energy range • Electrons from Muon Decay (Michel electrons) • Electrons from muon decay (Michel electrons) • 12B from muon capture (½life ~20ms) Remove with stopping muon veto • Entering muon with no signal in the lower veto system (0.027 Hz) • Kill all events within 200 ms of stopping muon Part I: Straightforward ones ………

12. Kamland Singles Spectrum (R < 5 m ; 1m from acrylic) EvisWindow Contaminant and Isotope Backgrounds

13. Using a fiducial volume which is far from the acrylic vesselremoves background from contaminants on the balloon. (Position of bis well localized) Cut 40 cm from inner balloon  Giving extremely good efficiency to identify events with an associated neutron Use neutron veto to reduce IBD Backgrounds b-decay oflong-lived isotopes with assoc. neutrons Oil only Scint only Gd dope Fiducial 40 cm Use Inside Fiducial Volume with Gd Buffer

14. Part II: Contaminant Backgrounds It is the b's that are producing the background (no g's are produced w/ energy in the window) a's quench, producing < 1 MeV of visible E Assuming 5x10-17 g/g 232Th  ~100 events/3yr (238U contribution is small) Need to achieve purity levels for U, Th equivalent to KamLAND

15. Part III: High Energy Muon Induced Isotopes • Going deep reduces the rate substantially(Minimum to achieve NuTeV error: 300 mwe) • Lifetimes are too long for a simple cosmic veto • Propose to use a muon veto with a coincident neutron requirement • Muon with “neutron” signal (6-10 MeV within 0.5<Dt<600 ms) opens a 3 s veto window • 0.015 Hz rate  ~5% deadtime • Very important background for measurement: • Use the far detectors to measure thebackground rate in the near detectors... • Far detector measures both the contamination and isotope background if: • Oil purity levels are the same • Overburdens are similar (Can correct using relative cosmic muon rate and calculation of spectrum difference)

16. Summary of Long-lived Isotope Backgrounds • Sources of isotopes that b decay producing potential background in the 3-5 MeV visible energy window. • Isotope decays/day/13 ton detector after applying cuts and vetos • Total rate is 0.81 ± 0.11 (sys) events/day/detector or 1460 ± 200 2% uncertainty  Must be reduced using the far detector  0.5%

17. At 300 mwe, the background is at the 1500 event level giving 2% error Need to use far detector to reduce this uncertainty to 0.5% level Deeper would be better …. 450 mwe reduces the uncertainty to the 1% level Shallow sites (25 – 50 mwe) have cosmic rates worse by x15 and stopping rate increases by even larger factor An elastic scattering measurement requires a close detector at a fairly deep site Gives advantage to a site such as US Braidwood complex where near and far detectors are located with substantial overburden Also, flat overburden x2 better than mountain with same mwe Overburden for the near detector is key for this measurement

18. Identified via the outgoing neutron Require very high efficiency for neutron identification Use both Gd and H capture signature Use 40cm Gd buffer to prevent neutron escape Wide time window (0.5 < Dt <200 ms) Demand Evis > 1.8 MeV Neutron detection inefficiency Time window requirement Early captures: 3×10-5 Late capture: 6×10-4 Neutron capture inefficiency 1×10-3 IBD events in the 3<Evis<5 MeV window = 1,250,000 / 3yrs Background fraction = 0.16%  2000 event background (systematic uncertainty very small since rate is measured and understood very well)  Contributes statistical uncertainty of 45 events (0.4%) Question 3: Is the IBD Background Controllable?

19. Question 4: Is the normalization known well enough? • IBD normalization events can be used to measure the reactor neutrino flux • Cross section as function of energy known to 2% • En = Evis + 1.8 + 2me Can determine the flux vs. true energy • Vetos and fiducial volume cuts applied to IBD normalization events Gives the neutrino flux associated with the fiducial volume with deadtime and geometry correction automatically applied • This flux can then be used to predict the elastic scattering rate as a function of the weak mixing angle, sin2qW, by weighting by the elastic (3-5 MeV) to IBD cross section ratio. Apply xsecratio to IBD

20. k = Normalization: Statistical and Systematic Uncertainties • Statistical Uncertainty: • For a 3 yr run, 2,700,000 IBD events in central fiducial volume Equivalent events after weighting = 1,890,000*0.84  0.08% error • Systematic Uncertainties: • IBD off free protons  Elastic scatters off electrons Need the ratio  From CHOOZ free proton fraction 0.6% error • For normalization sample can use only Gd captures or H and Gd  Measure Gd capture fraction to 0.3% or  Use both then error is related to identifying H captures • IBD and elastic have different energy distributions  Energy resolution smearing can effect the prediction • Simulation shows that thisis a negligible effect

21. > Summary of the Event and Veto Cuts for the Elastic and Normalization Samples

22. Summary of Event Samples Rates for this design

23. Required sensitivity looks possible Measurement is statistics limited Systematics on the backgrounds are fairly large Use far detector to measure these bkgnds Bottom Line- Need close (~200m) and deep (>300 mwe) near detector - Good cross check if near and far at same depth Measurement Sensitivity  d(sin2qW) = 0.0019(compare to NuTeV = 0.0017)

24. Conclusions • A precision measurement of the elastic scattering rate in the near detector could add additional physics opportunities for reactor experiments • Measure the weak mixing angle • Probe for indications of an electron neutrino magnetic moment • Since one is measuring a singles rate in a visible energy region • Premium to have the near detector as close as possible (200 m) with an overburden > 300 mwe. • Measurement can be much more accurate and robust if one uses the far detector to constrain the background • Should be at similar depth to minimize corrections for cosmic rate and spectrum.

25. Detection Assumptions