Controlling Systematics in a Future Reactor q13 Experiment

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Controlling Systematics in a Future Reactor q13
Experiment
Jonathan Link Columbia University Workshop on
Future Low-Energy Neutrino Experiments April 30
- May 2, 2003
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A Simple Counting Experiment Study
Look for disappearance in the ratio R, defined as

• Where
• The Ns are the number of observed events
• The Ls are the baselines and
• e is the relative efficiency of the near and far
  detectors.
• Disappearance is measured as a deviation of R
  from 1 and the sensitivity to sin2q13 at 90 CL
  is just

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Counting vs. Shape

• 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.

50 tons, 6 GW, 3 years and 1200 meters
Counting Experiment
Shape Rate
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• Significant Contributions to the Error
• Statistics in the far detector
• Uncertainty in the relative efficiency of the
  near and far detector
• where f is the fraction of run time used for
  cross calibration
• Uncertainty in the background rate in the far
  detector

(with movable detectors)
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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

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Few Words on Methodology

• 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

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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

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Ways of Improving the Statistics at the Far
Detector

• There are three ways
• More target volume at the far detector site
• More reactor power
• More running time
• Twice Volume Twice Power Twice Run Time
• (Statistical errors only)

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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
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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. Ill show later
in this talk that no reactor off running is not
needed.
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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 (3s)
  effect, but we must not be systematics limited.

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Controlling the Relative Efficiency Systematic

• Bugey (the only near/far reactor exp.) had se
  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 se?
• One possibility is movable detectors

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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.

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Sensitivity of Kr2Det
Kr2Det is ultimately limited by the 0.8 error on
the relative efficiency of their two detectors.
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)!
One can do better with a movable far detector
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Sensitivity of Kr2Det with Movable Detectors
10 of the running time is spent doing the cross
calibration.
12 years
With this modification you get to a sensitivity
of 0.01 at Dm2 of 2.510-3 eV2 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.
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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
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Controlling Uncertainty in the Background Rate

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

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Measure Background with Reactor Off Time

• 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 ? sbg 2sfar
• Need 2 months a year to sbg sfar

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
This is not a reliable plan for future
experiments.
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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?

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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.

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The KARMEN Solution (Continued)
For a reactor experiment it might look something
like this
In my studies I assumed a 95 efficient veto.
Then 0.2 bg/ton/day at 300 mwe becomes 0.01
bg/ton/day.
The difference between 150 mwe and 300 mwe
becomes less important. So we might save money
with a shallower site.
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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.
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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.
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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.
n interactions
Proton recoils
Neutron transport simulation Detector resolution
not included
?
From CHOOZ
Can we expect distributions from vetoed events
and events that evade the veto to be the same?
Detailed simulations will tell.
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Conclusions
By controlling the dominant sources of systematic
error and maximizing reactor power a next
generation reactor experiment can be sensitive to
sin2q13 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
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Optimal Baseline
With Dm2 2.510-3 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.