1. Controlling Systematics in a Future Reactor  13 Experiment Jonathan Link Columbia University Workshop on Future Low-Energy Neutrino Experiments April 30 − May 2, 2003

2. Look for disappearance in the ratio R, defined as Where: The N’s are the number of observed events The L’s are the baselines and  is the relative efficiency of the near and far detectors. Disappearance is measured as a deviation of R from 1 and the sensitivity to sin 2  13 at 90% CL is just A Simple Counting Experiment Study

3. Huber, Lindner, Schwetz and Winter have shown that a pure shape analysis works well with large statistics. A combined shape and rate analysis improves sensitivity over a pure rate analysis only slightly at the scale of current proposals. Therefore, the counting experiment is sufficient to study/compare these scenarios. Counting vs. Shape 50 tons, 6 GW, 3 years and 1200 meters Counting ExperimentShape & Rate

4. Significant Contributions to the Error 1. Statistics in the far detector 2. Uncertainty in the relative efficiency of the near and far detector where f is the fraction of run time used for cross calibration 3. Uncertainty in the background rate in the far detector (with movable detectors)

5. Kr2Det Proposal This elegant proposal can be simply stated as 2 detectors and one reactor Identical near and far detectors target the dominate source of error in CHOOZ and Palo Verde − flux uncertainty It explicitly address the background error by doubling the depth compared to CHOOZ and has 65 reactor off days a year The reactor power (~2 GW) is low by modern standards The 1000 metes far baseline may not be ideal

6. This analysis starts with the assumptions in the Kr2Det proposal (Mikaelyan et al.): Two identical, 46 ton (fiducial) detectors at 115 and 1000 meters 55 events/day in far detector, 4200 near Reactor is on for 300 days in a year Relative efficiency of near and far detectors know to 0.8% 600 mwe shielding  Background of 0.1 events/ton/day The background rate is measured during reactor off days Few Words on Methodology

7. Spreadsheet Study Allowing the variation of: reactor power run time detector size reactor capacity factor near and far baselines background rate background sensitivity number of far detectors fraction time for cross calibration one or two reactor scenarios

8. Ways of Improving the Statistics at the Far Detector There are three ways… 1. More target volume at the far detector site 2. More reactor power 3. More running time Twice Volume = Twice Power = Twice Run Time (Statistical errors only)

9. More Target Volume at the Far Detector Site Small near detector and bigger far detector: Important errors may not cancel if the detectors are not identical Bigger detectors near and far: Error cancellation intact Possible attenuation problems in large Gd loaded detectors Detectors are impossible to move More same size far detectors: The errors scale like one big detector Could phase in the experiment or improve sensitivity by adding more detectors $$$$

10. Add More Reactor Rower See earlier talk: We can get ~9 GW with French reactor sites ~8 GW in Germany, ~7 GW in the U.S. and Less elsewhere. I’ll show later in this talk that no reactor off running is not needed.

11. More Running Time I think that it is a bad idea to plan on an extra long run (more than 3 years) More time for efficiency to drift (i.e. degradation of Gd loaded scintillator) Hard on young scientists Could get beat by off-axis Extra running time could be useful if we get to the end of our run and we have a marginal (≤3  ) effect, but we must not be systematics limited.

12. Controlling the Relative Efficiency Systematic Bugey (the only near/far reactor exp.) had   = 2% 1.8% if you ignore the solid angle error Kr2Det assumes 0.8% What value should we be using? How will we determine/measure   ? One possibility is movable detectors

13. Movable Detectors This idea originated with Giorgio Gratta and Stan Wojcicki Our idea is to have a far detector(s) that can be moved to sit at the same baseline as the near detector The two detectors record events in the same flux at the same time (head-to-head calibration) Relative efficiency error: Near running fraction of 10 to 15% optimizes the total error A movable detector experiment is best achieved by connecting the two detector sites by a tunnel Such a tunnel might cost $10 to $20 million depending on the site geology, topology and hydrology.

14. Sensitivity of Kr2Det Kr2Det is ultimately limited by the 0.8% error on the relative efficiency of their two detectors. One can do better with a movable far detector… The limit in sensitivity imposed by the 0.8% error. It is possible to overcome this limit with a shape analysis and high statistics (à la Huber, et al.) but only after about 65 years of running (~6000 GW ton yrs)!

15. Sensitivity of Kr2Det with Movable Detectors With this modification you get to a sensitivity of 0.01 at  m 2 of 2.5×10 -3 eV 2 by adding fiducial mass (138 tons) or time (12 years). The effect is even more dramatic when considering reactor sites with higher power, where the systematic limit is reached sooner. 12 years 10% of the running time is spent doing the cross calibration.

16. Moving Detectors at a 6 GW Site Consider 50 ton target detectors at 150 meters and 1200 meters and a 3 year run. The far detector spends 10% of the run time at the near site for cross calibration. Or the relative efficiency is measure to 0.8% with fixed detectors

17. Controlling Uncertainty in the Background Rate 1. Measure background with reactor off time 2. Put detectors very far underground so that the background is insignificant (The KamLAND solution) 3. Create a large effective depth with an external veto/shielding system (The KARMEN solution) 4. Measure the heck out of it Combining 3 and 4 seems to work well

18. This works best at single reactor sites Commercial reactors can have as little as 3 weeks of down time every 18 months. For 3 GW, 300 mwe, 1200 BL   bg ≈ 2×  far Need 2 months a year to  bg ≈  far Measure Background with Reactor Off Time This is not a reliable plan for future experiments. CHOOZ ran the detector before their reactors were commissioned Over time the Gd loading degraded their attenuation length. When they were forced to lower their trigger threshold their background rate changed When extrapolating to zero power at two reactor sites the error scale as so there is no advantage to greater depth. Extrapolation to zero power from CHOOZ

19. The KamLAND Solution KamLAND is so far underground that they estimate only one background event in their entire dataset. Neglecting this event does not significantly affect their result. Finding a site with an acceptable reactor and the ability to get far underground at the optimal baseline would be very hard. Perhaps Dave Reyna has a solution?

20. The KARMEN Solution KARMEN was a surface level neutrino detector that achieved an effective depth of about 3000 mwe by using an active veto shield. Saw background reduction of 97% 3 meter thick steel shield with embedded muon detectors at 2 meters. Spallation neutrons created outside the veto are stopped Muons penetrating the veto are detected.

21. The KARMEN Solution (Continued) For a reactor experiment it might look something like this: The difference between 150 mwe and 300 mwe becomes less important. So we might save money with a shallower site. In my studies I assumed a 95% efficient veto. Then 0.2 bg/ton/day at 300 mwe becomes 0.01 bg/ton/day.

22. Measure the Heck Out of It Even with a 95% efficient veto we still need to estimate the surviving background to within about 25% to make this error significantly smaller than the statistical error. We can achieve this precision by using vetoed events to study distributions of various parameters and use them to extrapolate into the signal region for non-veto events.

23. Measure the Heck Out of It (Continued) Various Distributions from CHOOZ Distributions of Positron energy Neutron capture energy Spatial separation Temporal separation as determined from vetoed events, could be used to estimate correlated backgrounds. These distributions also contain uncorrelated background events.

24. From CHOOZ  interactions ? Neutron transport simulation Detector resolution not included Measure the Heck Out of It (Continued) Matching these vetoed distributions outside the signal range to the data could easily result in a background uncertainty in the signal region of ≥ 25%. Can we expect distributions from vetoed events and events that evade the veto to be the same? Detailed simulations will tell. Proton recoils

25. Conclusions By controlling the dominant sources of systematic error and maximizing reactor power a next generation reactor experiment can be sensitive to sin 2  13 down to 0.01 at 90% CL in 3 years or less. The dominate sources of systematic error Relative efficiency Background Rate can be controlled by designing an experiment with movable detectors and an active external veto shield. Systematics are tied to measurements, they go down as stats go up 9 GW, 50 tons, 1200 m, 3 years 15% cross calib. & 95% eff. veto

26. Optimal Baseline With  m 2 = 2.5×10 -3 the optimal region is quite wide. In a configuration with tunnel connecting the two detector sites, choose a far baseline that gives you the shortest tunnel.