Controlling Systematics in a Future Controlling Systematics in a Future Reactor Reactor 1313 Experiment Experiment

Jonathan Link

Columbia University

Workshop on Future Low-Energy Neutrino Experiments

April 30 − May 2, 2003

εLN

LNR

nearnear

farfar

2

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 sin213 at 90% CL is just

A Simple Counting Experiment StudyA Simple Counting Experiment Study

E

far

R

dEE

LmEE

mS)27.1sin()()(

64.1

213

213

• 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. ShapeCounting vs. Shape

50 tons, 6 GW, 3 years and 1200 meters

Counting Experiment Shape & Rate

Significant Contributions to the ErrorSignificant Contributions to the Error1. 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

far

bgfar

stat N

NN

far

bgratebgbg N

N

fNnear

2 (with movable detectors)

Kr2Det ProposalKr2Det 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

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 MethodologyFew Words on Methodology

den 0.85 g/cm 3̂ L near 150 m coverage 0.2 depth 300 mweflux 2.00E+20 nu/s/GWth L far 1200 m diam_pmt 8 in veto_ineff 0.05power_th 6.1 GWth near flux 4.315E+11 /cm 2̂/s area_pmt 0.032429 m 2̂ eff depth 6000 mweflux0 1.22E+21 nu/s far flux 6.742E+09 /cm 2̂/s near_bg 0.01 /ton/dayyears 3 Hden 7.85E+22 H/cm 3̂ far_bg 0.01 /ton/dayupfrac 0.89 xsec*eff 5.59E-44 cm 2̂ near_bg_err 50 %uptime 3.25E+02 day/year far_bg_err 25 %

tons/unit fid far units active frac near bg sub far bg sub near events near error far tot/unit far err/unit far total eff error R error50 1 1 547.5 547.5 7977144 2837.72 124642 353.82128 124642 0.008 0.008650 1 0.9 492.75 492.75 7179429 2690.84 112178 335.66464 112178 0.001669 0.00362125 2 0.9 246.375 246.375 3589714 1898.72 56089 237.35074 112178 0.00236 0.00364

16.6 3 0.9 163.593 163.593 2383570 1546.1 37243 193.40784 111729 0.002897 0.00366610 5 0.85 93.075 93.075 1356114 1165.49 21189 145.88377 105946 0.003136 0.0036625 10 0.9 49.275 49.275 717942 847.702 11217 106.14271 112178 0.005278 0.00379

far units = The number of identical detectors at the far location (n).active frac = The fraction of time that each far unit spends at the far location (time spent at the near location is 1-active frac).bg sub = The total number of background events subtracted from each unit.near events = The number of signal events (after BG subtraction) seen by the near detector during the active fraction (N_n).far events = The number of signal events seen by each far detector during the active fraction (N_f).eff error = The error on the reletive efficiency (eff) of a far detector wrt the near detector as measured side-by-side.R = far 2̂/near 2̂ * 1/n * sum(N_f*eff)/N_nR error = The error on R.

Spreadsheet StudySpreadsheet StudyAllowing 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

Ways of Improving the Statistics Ways of Improving the Statistics at the Far Detectorat 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)

More Target Volume at the Far Detector SiteMore 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

$$$$

Add More Reactor RowerAdd 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.

More Running TimeMore Running TimeI 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.

Controlling the Relative Efficiency SystematicControlling 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

Movable DetectorsMovable DetectorsThis 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.

fNnear

2

Sensitivity of Kr2DetSensitivity of Kr2DetKr2Det 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)!

Physics Reach of Kr2Det Proposal

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0 5 10 15 20Years

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Kr2Det Proposal

Systematics Limit

Physics Reach of Kr2Det with Movable Detector

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Sensitivity of Kr2Det with Movable DetectorsSensitivity of Kr2Det with Movable Detectors

With this modification you get to a sensitivity of 0.01 at m2 of 2.5×10-3 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.

12 years

10% of the running time is spent doing the cross calibration.

6 GW Reactor, 1200 meter Baseline

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Fixed Detectors

Movable Detectors

Moving Detectors at a 6 GW SiteMoving Detectors at a 6 GW SiteConsider 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

Controlling Uncertainty in the Background RateControlling 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

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 TimeMeasure 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

halffar fN

The KamLAND SolutionThe 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?

The KARMEN SolutionThe 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.

The KARMEN Solution (Continued)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.

Measure the Heck Out of ItMeasure 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.

Measure the Heck Out of It (Continued)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.

From CHOOZ

interactions

?

Neutron transport simulation

Detector resolution not included

Measure the Heck Out of It (Continued)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

ConclusionsConclusionsBy controlling the dominant sources of systematic error and maximizing reactor power a next generation reactor experiment can be sensitive to sin213 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 years15% cross calib.& 95% eff. veto

Optimal BaselineOptimal Baseline

6 GW and 3 Years

0

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500 1000 1500 2000

Baseline (meters)

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dm^2=5.0e-3

dm^2=2.5e-3

dm^2=1.0e-3

With m2 = 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.