The Near Detector Purpose and Conceptual Design
A.6 Fixed On-axis Component of the DUNE ND
A.6.1 Motivation and Introduction
A.5.4.2 DUNE-PRISM Linear Combination Analysis
In addition to identifying problems in cross section modeling, DUNE-PRISM measurements provide a mechanism for creating FD predictions directly from the ND data that is largely independent of neutrino interaction modeling. By constructing linear combinations of measurements taken under exposure to different neutrino fluxes, it is possible to determine the distribution of any observable (e.g. reconstructed neutrino energy) for a different neutrino flux of interest. This means, for example, from the ND data alone it is possible to create a distribution of the expected reconstructed neutrino energy distribution at the FD in the event of oscillations with a specific set of parameters. This distribution, created using this data-driven technique, can then be compared to that seen in the FD with a reduced dependence on the flux and neutrino interaction models and their associated uncertainties.
A few example fits of the off-axis ND muon neutrino spectra to an oscillated FD muon neutrino energy spectrum are shown in Figure A.36. Good agreement is seen near the first and second oscillation maxima at 2.5 GeV and 0.7 GeV, respectively. The ability to fit the FD spectrum breaks down outside the central range of energy because the constituent off-axis spectra used in the fit extend only slightly outside this range and cannot duplicate the spectrum in any combination. This does not pose a significant problem for the oscillation analysis because the fit is good for the bulk of the events and in the region that drives the CP sensitivity. This technique can also be applied to match the off-axis muon neutrino spectra to the ND intrinsic electron neutrino spectrum, in order to make a precise measurement of σ(νe)/σ(νµ) with a common flux, or to the FD oscillated electron neutrino energy spectra for the measurement of δCP.
(GeV) Eν
0 1 2 3 4 5 6
per POT)-2 cm-1 (GeVνΦ
0 5 10 15 20 25 30 35 40
−9
×10
eV2 10-3
× = 0.0022 23 m2
∆ ) = 0.5;
θ23 2( sin
Oscillated FD Flux Composite ND Flux Fit region
σ
± Decay pipe radius
σ
± Horn current
σ
± Water layer
[GeV]
Eν
0 1 2 3 4 5 6
FD (unosc.)
ND - FD (osc.)
−0.4
−0.2 0 0.2 0.4
(GeV) Eν
0 1 2 3 4 5 6
per POT)-2 cm-1 (GeVνΦ
0 5 10 15 20 25 30 35 40
−9
×10
eV2 10-3
× = 0.0025 23 m2
∆ ) = 0.65;
θ23 2( sin
Oscillated FD Flux Composite ND Flux Fit region
σ
± Decay pipe radius
σ
± Horn current
σ
± Water layer
[GeV]
Eν
0 1 2 3 4 5 6
FD (unosc.)
ND - FD (osc.)
−0.4
−0.2 0 0.2 0.4
Figure A.36: Linear combinations of off-axis fluxes giving far-detector oscillated spectra for a range of oscillation parameters. The FD oscillated flux is shown in black, the target flux is shown in green, and the linearly combined flux obtained with the nominal beam MC is shown in red. Systematic effects due to 1 σvariations of the decay pipe radius (green), horn current (magenta) and horn cooling water layer thickness (teal) are shown.
broken upstream target fins. This was observed and diagnosed prior to target autopsy by the change in the observed near detector neutrino event spectrum [126]. Figure A.37 shows MINOS near detector data in bins of reconstructed neutrino event energy. Within each bin are points representing successive periods of the data taking run. A time-dependent shift in the three peak bins, i.e., a change in the spectral shape, is obvious in the plot. Another example of a significant and noticeable change in the beam spectra at NuMI was caused by magnetic horn tilt due to degradation of a supporting washer [127]. Yet another significant wiggle in the beam spectral shape has been observed in the MINERvA and NOvA medium energy run [62]. Notably, this wiggle is not as significant in the off-axis data taken by the NOvA near detector as it is for on-axis MINERvA. MINERvA studies indicate the observed spectral shift is best modeled by a shifted horn position or a slight change in the inner horn radius relative to the expected value.
Figure A.37: The low energy run MINOS event rate as a function of neutrino energy broken down in time. From [126].
Given the past experience from NuMI, it is thought to be critically important that the DUNE ND monitor and track the beam spectrum over time. ArgonCube and the MPD will make on-axis neutrino beam spectrum measurements when they are located on-axis. There are two broadly different concepts under discussion for how to monitor the on-axis beam when ArgonCube and the MPD move off-axis as required by the DUNE-PRISM program. One concept is to have a dedicated on-axis beam monitor that is capable of measuring the neutrino beam spectrum as a function of time. Such a capability would require target mass and a magnet or range stack with tracking to measure the momentum of muons arising from CC νµ interactions. The other concept assumes a dedicated on-axis neutrino interaction rate monitor. Both the on-axis rate monitor and the off-axis spectrometer would each track beam stability. In the case of an observed instability of either the rate or the off-axis spectrum, ArgonCube and the MPD would move back on-axis to make a spectrum measurement. It is likely the DUNE-PRISM run plan would include intermittent on-axis measurements as well regardless of observed instabilities. Since the first concept involves a magnet or large range stack, it is likely to be more expensive to implement than the option that makes use of the dedicated rate monitor on-axis. On the other hand, the latter option with intermittent on-axis spectral measurements involves additional movements of the large, DUNE- PRISM detectors, precision comparison of spectral measurements separated in time with detectors that have moved in the interim, and accepting the risk that rate and off-axis spectral monitoring are both less sensitive to some changes in the beam than on-axis spectral monitoring. Figure A.38 provides an illustration, for a small subset of relevant beam parameters, of the differing sensitivities of integrated rate monitoring as compared to spectral monitoring of the beam. Additional studies
are in progress.
Figure A.38: Comparison of rate monitoring and spectral monitoring of the neutrino beam for 1-sigma shifts of the horn positions. On the left is shown the significance of the variation in the observed rate for one week of running with a seven ton fiducial mass detector. It is shown as a function of off-axis angle (including zero, i.e., on-axis). On the right is shown the significance of the shape change as a function of energy for one week of running with an 8.7 ton fiducial mass on-axis spectrometer.
The reference design described below in Section A.6.2 assumes the beam monitoring is done with a dedicated on-axis magnetic spectrometer. The device described is a capable beam monitor in that it has the required mass and muon momentum resolution as will be shown in Section A.6.2.4.
In addition, the on-axis spectrometer described below has capabilities that go beyond beam mon- itoring, which are useful for building confidence in the flux model and providing information that is potentially useful for the evolution of the neutrino interaction model. Other concepts, including one that utilizes an on-axis rate monitor, are under consideration.