first horn (5) Second horn
2.3 Near Detectors
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In order to study neutrino oscillations between the production point and the far
de-938
tector, it is essential to characterise the unoscillated beam with a precise measurement
939
of the neutrino energy spectrum, the flavour content and the interaction rates before
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oscillating. In the T2K experiment, these measurements are performed by a set of
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detectors located in a facility at 280 m from the target station. There is an on-axis
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detector (INGRID) and an off-axis detector (ND280) as shown in Fig. 2.12. Both are in
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a pit, ND280 is about 24 m below the surface and INGRID is just below. The ND280
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detector is actually composed of many sub-detectors and is described in Chapter 3: the
945
analysis of this thesis is based on data acquired with ND280.
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2.3.1 INGRID, the On-Axis Detector
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Interactive Neutrino GRID (INGRID) is the on-axis near detector, located at 280 m
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from the target. The main purpose of INGRID is to monitor the neutrino beam rate,
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profile, and centre, by detecting neutrino interactions in iron. INGRID is composed
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of 14 identical modules arranged in two identical groups of 7 modules, one along the
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horizontal axis and one along the vertical axis, forming a cross, with 2 extra off-diagonal
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modules, as shown in Fig. 2.13. With this structure, INGRID is designed to sample
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the beam in a transverse section of 10 m × 10 m, sufficiently covering the neutrino
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Figure 2.12: The near detector facility located 280 m from the target station: the off-axis detector ND280 is located in the upper level (about 24 m below the surface); the on-axis detector INGRID is located on the level below (about 33 m deep for the horizontal modules).
beam profile. The centre of the INGRID cross coincides with the centre of the neutrino
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beam, defined as 0◦ with respect to the proton beamline.
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1.5m
~10m
~10m
X Y Beam center
Z
Figure 2.13: Schematic view of the INGRID on-axis detector: the centre module of the vertical row and of the horizontal row corresponds to the neutrino beam centre.
Each module consists of eleven tracking scintillator planes interleaved with nine iron
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target plates. Fig. 2.14 shows a drawing of one INGRID module (left side), where the
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blue planes are the iron layers and the greys are the tracking planes. The ensemble of
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iron and scintillator planes is surrounded by veto scintillator planes to reject charged
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particles that enter the modules from outside (Fig. 2.14, right side). Each iron plate
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is a square of 124×124 cm2 and 6.5 cm thickness, with a total of 7.1 tons of iron
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mass serving as neutrino target per module. Each tracking plane is composed of 24
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scintillator bars in the horizontal axis and 24 in the vertical axis. Scintillation light
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produced by muons from charged-current interactions is collected from each bar and
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transported to a photo-detector with a wavelength shiftingfibre (WLSfibre). The light
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is read out by a Multi-Pixel Photon Counter (MPPC) attached to an end of the WLS
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fibre. Tracking scintillator layers in alternating orientations enable 3D reconstruction
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of the muon paths.
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An extra special module, called Proton Module, is located between the central
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module of the horizontal group and the central module of the vertical group. This
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module is composed of 34 scintillator planes (without iron) which allow 3D tracking
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of low energy particles. Detecting low energy particles, as protons, allows a better
973
understanding of the neutrino interactions in INGRID, and consequently of the neutrino
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beam properties.
975
Figure 2.14: Drawings of an INGRID module (left side): eleven tracking scintillator planes (blue) interleaved with nine iron plates (grey). The ensemble of iron and scintillator planes is surrounded by veto scintillator planes (right side, black).
A typical neutrino event in the INGRID detector is shown in Fig. 2.15: the neutrino
976
enters from the left, interacts in the Proton Module (left module) producing charged
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particles whose tracks are shown as red circles (different sizes depending on the strength
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of the generated signal); the green lines are scintillator planes, the blue lines are veto
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scintillator planes and the grey boxes are the iron plates; the long track exiting the
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Proton Module corresponds to a muon while the short one corresponds to a proton.
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The beam centre is measured counting the number of observed charged-current
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neutrino events in each module (identified by detecting muon tracks). The precision of
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the measurement of the beam centre is better than 10 cm (0.4 mrad precision), defined
984
by the systematic error due to the uncertainty on the neutrino event rate, and stable
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within the statistical error of ∼2 cm. The observed profiles (number of events versus
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position from the INGRID centre) in the x and y directions are fitted with Gaussian
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Figure 2.15: Typical neutrino event in the INGRID detector: the neutrino enters from the left, the interaction happens in the Proton Module (left module), producing charged particles whose energy deposition is shown as red circles.
functions, and the beam centre is defined as the peak of the fit. The two modules off
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the main cross are used to check the axial symmetry of the neutrino beam. Cosmic ray
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data and beam data were used for the calibration of the INGRID detector. INGRID
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measurements shows that the neutrino beam intensity and direction have always been
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stable along the different data taking periods. More details about the performance,
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calibration and simulations for INGRID can be found in [120].
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