- Email: [email protected]
The K2K SciBar Detector
Nuclear Physics B (Proc. Suppl.) 139 (2005) 289–294 www.elsevierphysics.com
The K2K SciBar Detector H. Maesakaa for The K2K SciBar Group a
Department of Physics, Kyoto University Oiwake-cho, Kita-shirakawa Sakyo, Kyoto, 606-8502, Japan. SciBar is a fully-active scintillator tracking detector, which was newly installed at the near detector site of K2K long baseline neutrino oscillation experiment. The purposes of SciBar are to improve the measurement of the neutrino flux and to study neutrino interactions. Its construction was completed in August 2003. The neutrino data have been taken stably since October 2003. We describe its design, operation and basic performance. Preliminary results from an analysis of charged current events are also presented.
1. Introduction The KEK-to-Kamioka long baseline neutrino oscillation experiment (K2K) [1] has been running since 1999, to study the muon neutrino oscillation observed in the atmospheric neutrino results from Super-Kamiokande [2]. By using 12 GeV proton synchrotron at KEK, we produce almost pure muon neutrino beam, whose energy spectrum is widely spread around the mean energy 1.3 GeV. The beam is detected by both a near detector system (ND) at KEK and SuperKamiokande (SK) which is about 250 km far from KEK. By comparing both the number of events and the neutrino energy spectrum between the SK observation and the expectation from ND measurements, we determine the parameters of neutrino oscillations. The neutrino energy spectrum is determined by using charged current quasi-elastic (CC-QE) scattering (νµ + n → µ− + p), which is a dominant process around 1 GeV. The latest results from K2K [3] show indications for the disappearance of muon neutrinos, and are consistent with the atmospheric neutrino results. From those results, the neutrino energy of the oscillation maximum is expected to be around 0.6 GeV. Therefore, a measurement of neutrino flux below 1GeV is the most important. To improve the determination of the neutrino energy spectrum and to understand neutrino interactions with nuclei, we designed and constructed a fully-active scintillator detector named
0920-5632/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2004.11.225
SciBar (scintillator bars) as an upgrade of ND. The design, operation and basic performance of SciBar are presented in the following sections. We also show preliminary results from an analysis of charged current (CC) events. 2. SciBar Detector SciBar consists of extruded scintillator strips, which are readout by wavelength shifting (WLS) fibers and Multi-anode PMTs. A schematic drawing of SciBar is shown in Figure 1. The extruded scintillator is developed and produced by Fermilab. The scintillator is made of polystyrene infused with fluors. The ingredient of the scintillator is the same as that used for MINOS detector [4,5] at Fermilab. The dimension of the scintillator strip is 2.5 × 1.3 × 300 cm3 with a hole (1.8 mm in diameter) in the middle. Thin (0.25 mm thick) reflective coating, composed of TiO2 infused in polystyrene, surrounds the entire scintillator bar. Both the hole and coating are extruded together with scintillator. In total, 14848 strips are arranged vertically and horizontally one after another. The total volume of SciBar is 3 × 3 × 1.7 m3 and the total mass is about 15 tons. A schematic view of the readout system is shown in Figure 2. A wavelength shifting fiber is inserted into the hole of each strip, and is connected to a multi-anode PMT which has 64 pixels. To monitor the PMT gain, a clear fiber is
290
H. Maesaka / Nuclear Physics B (Proc. Suppl.) 139 (2005) 289–294
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GE
VT
QP
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Figure 1. A schematic drawing of SciBar. Extruded scintillator strips are arranged vertically and horizontally. The size of scintillator part is 3 × 3 × 1.7 m3 and the weight is about 15 tons. Each strip has a hole in the middle, in which a WLS fiber is inserted. At the end of the fiber, a multi-anode PMT is attached. In addition, an electro-magnetic calorimeter is placed behind the scintillator part. Light injection module
C lear fiber
F rontend board
64ch MAP MT
T o V ME B oard
V A/T A C hips WLS fiber (x64) E xtruded s cintillator
Holder cookie Detector s tructure
Figure 2. A schematic view of the readout system. A WLS fiber absorbs and transports scintillation photons to a multi-anode PMT. The PMT is readout by a VA/TA system. In addition, a clear optical fiber is drawn from a blue LED to a light injection module attached to a fiber bundle, to monitor the PMT gain.
connected from a blue LED to a light injection module attached to a fiber bundle. We have chosen to use 1.5 mm diameter fibers, in order that they fit the pixel size of the PMT (2 × 2 mm2 ). The attenuation length is 3.5 m from a laboratory measurement. The 64-pixel multi-anode PMT has a bialkali photo-cathode and 12 % quantum efficiency for a green photon, while 20 % for a blue photon. The typical gain is 6 × 105 . With this gain, the response linearity is preserved up to 200 photoelectrons (p.e.). Since the light yield is about 10 p.e./MeV (see Section 4.1), the linearity is sufficient to measure up to 20 MeV/cell. Both timing and charge information from the PMT are digitized by readout electronics, which are assembled from VA/TA ASIC chip-sets. The VA/TA chip-set is composed of VA and TA chips, and it has 32 inputs. Therefore, two sets are used to read one PMT. The VA integrates the signals from a PMT and, if triggered by an external signal, holds the voltage proportional to the charge. Then, it sends the voltage signal to a VME ADC board sequentially. The TA discriminates the pulse height by a given threshold, and provides an OR’ed logical signal over 32 channels. The TA signal is used to make a hold signal for the VA and is recorded by a TDC. An electro-magnetic calorimeter (EC) was installed behind of the main part to measure νe contamination in the beam and π 0 yield from neutrino interactions. EC is comprised of 2 planes of 30 horizontal and 32 vertical modules re-used from the CHORUS experiment [6]. The module is made of lead sheets and scintillating fibers. It has a dimension of 4×8×262 cm3 , and consists of two of 4 × 4 cm2 cells. EC has 11 radiation length along the beam axis and covers 2.6 × 2.6 m2 . The energy resolution is 14/ E (GeV) %. The construction of SciBar finished in August 2003 and neutrino data has been taken since October 2003. Since the neutrino beam is shot every 2.2 seconds as 1.1µsec duration pulse, the beam data taking is triggered by the accelerator. An event display of a CC-QE candidate is shown in Figure 3. So far, 2.0 × 1019 protons have been delivered to the production target since the installation of SciBar. In addition to beam data,
H. Maesaka / Nuclear Physics B (Proc. Suppl.) 139 (2005) 289–294
K2K Fine-Grained Detector (Side View) Run 4999 Spill 19016 TRGID 1 SBEv 23252 2003 10 7 14 44 40 0 Nvtx 0
10 cm
Figure 3. An event display of a CC-QE candidate. Each hit is shown as a closed circle, whose area is proportional to ADC counts. Two tracks are generated at the vertex. One extends to MRD, so that it is a muon. The other stops within SciBar and gives larger ADC outputs than the muon track. Therefore, it is a proton.
pedestal, LED and cosmic ray data are taken together during an interval between beams. LED and cosmic ray data are used, respectively, for PMT gain monitoring and for absolute energy calibration. 3. Physics Capability Since the segmentation is 2.5 × 1.3 cm2 , SciBar can detect a track as short as 10 cm. It corresponds to 0.4 GeV/c for a proton, and 0.1 GeV/c for a muon and a charged pion. Therefore, SciBar has high efficiency to detect CC-QE as a 2track event. Furthermore, since SciBar measures dE/dx along a track, protons can be discriminated from muons and pions. Thus, we can distinguish CC-QE events from other CC non-quasielastic events (non-QE) by using the particle identification together with a kinematic requirement. Consequently, SciBar helps us to determine the neutrino energy spectrum more precisely. On the other hand, there is a somewhat ad-
291
verse point that the neutrino target is scintillator (carbon and hydrogen nuclei), while that of SK is water (oxygen and hydrogen nuclei). This difference may affect the expectation of both the number of events and the neutrino energy spectrum at SK. Since the Fermi momentum is different by about 5 % between carbon and oxygen, the Pauli suppression for protons and neutrons can be slightly changed. In the K2K case, it corresponds to about 1 % difference in the total neutrino interaction cross section. Final state interactions, such as the re-scattering of protons and pions or absorption of pions in the target nucleus, can also differ. We can study these effects with SciBar data. Study of various neutrino interactions is also necessary for neutrino oscillation analyses. As for the neutrino interactions around 1GeV, there are still many quantities which are not measured precisely enough, such as cross sections and form factors. These study are also important for future neutrino oscillation experiments. SciBar can determine following quantities as a function of neutrino energy or squared 4-momentum transfer: CC-QE cross section, CC resonance production cross sections without a neutron in the final state (νµ + p → µ− + p + π + and νµ + p → µ− + p + π 0), neutral current (NC) elastic scattering cross section, NC / CC cross section ratio, and νe contamination in the beam. Neutrino energy and 4momentum transfer can be determined by using all final state products. As a result, form factors for each interaction mode can also be evaluated. NC elastic scattering can tell us the strange quark contribution to the spin component of nucleons [7]. Furthermore, SciBar can measure following quantities: the cross sections for CC coherent pion production (νµ + 12 C → µ− + π + + 12 C), CC multi-pion production (νµ + N → µ− + N + π + π + . . .), NC resonance production cross section. 4. Basic Performance In the following sections, we describe some basic performance of SciBar. Using cosmic ray data, we evaluate the light yield and timing resolution for each strip. Then, we show the stability of
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H. Maesaka / Nuclear Physics B (Proc. Suppl.) 139 (2005) 289–294
PMT gain which is monitored by an LED system. 4.1. Light Yield After finding a track from a cosmic ray event, we require that the track angle with respect to the beam direction is less than 45 degrees. If the track makes one and only one hit in a given layer, the hit is used by this analysis. If there is no attenuation effect, mean values of light yield histograms distribute around the central value of 9.1 p.e./MeV with the standard deviation of 2.6 p.e./MeV. Even if the attenuation effect is the largest (3.9 p.e./MeV), the statistical fluctuation of energy response is better than 1.6/ E (GeV) %. For example, typical 1 GeV/c protons and muons give, respectively, 3.0 and 2.1 MeV/cm to scintillator. Therefore, the energy deposit for a 10cm-muon is measured to be 21 ± 2.3 MeV, while that for a 10cm-proton is 30 ± 2.8 MeV. They can be clearly separated. Thus, the light yield is sufficient for energy measurement and particle identification. 4.2. Time Resolution The time resolution is measured by comparing hit timing between neighboring channels along a cosmic ray track. From the time difference distribution, timing resolution is found to be 1.3 nsec. It is enough for identifying a bunch of the beam spill (125 nsec spacing. See figure 5). Furthermore, it is adequate to determine the direction of a particle by using time-of-flight, if the track length is as long as 1 m. 4.3. Gain Monitoring The gain drift of each PMT is monitored by an LED system. A pulse of blue LED light is transmitted to fiber bundles through clear fibers, as shown in Figure 2. WLS fibers are uniformly illuminated and bring photons to a PMT surface. In addition, the intensity of the LED is monitored by both a photo-diode and a 2-inch PMT calibrated by an Am-NaI stable light source. Figure 4 shows a stability of a channel of multi-anode PMT. The PMT gain is corrected within 1 % level. The deviation of corrected gain is smaller than the statistical fluctuation of energy measurement.
Figure 4. A gain stability of a PMT channel. Each figure shows the time variation of ADC counts from cosmic rays. The upper figure is before the correction of PMT gain. There is a large deviation from November to December because of the temperature change due to the disorder of an air conditioner. The lower one shows after the correction. The PMT gain is followed within 1%.
5. Charged Current Analysis To study neutrino oscillations, the analysis of charged current (CC) events is necessary, because we need to determine the neutrino energy spectrum at the near site by using CC-QE interaction and test our MC prediction of CC interactions. We describe the results from an inclusive charged current analysis. To select a CC event, at least one of reconstructed tracks is required to extend to Muon Range Detector (MRD) [8] located downstream of SciBar. We select two event types: 3D matching (3D) and first-layer-hit matching (1L) events. If the extension of a SciBar track is joined to an MRD track, it is selected as a 3D event. If a SciBar track is not a 3D event but matches with hits in the first layer of MRD, it is identified as a 1L event. Then, the selected track is required to start from 9.38-ton fiducial volume. Both 3D and 1L are used for the inclusive CC analysis. Figure 5 shows a timing distribution of 3D and 1L events. The bunch structure of the beam is clearly seen. Since there are no events before or after the beam, the amount of background is negligible.
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Data NC CC multi π CC coherent π CC single π CC−QE
1000 1200 1400 1600 1800 2000 2200 2400 nsec
Figure 5. A timing distribution of a inclusive muon sample. The micro-bunch structure of the beam is clearly seen. In addition, there are no background before or after the beam.
Then, the muon energy is evaluated from the range through SciBar, EC and MRD. The muon angle is determined by the least square fit to SciBar hits. A muon momentum and a cosine of a muon angle with respect to the neutrino beam are shown in figure 6 and 7, respectively. Figure 8 shows a reconstructed q 2 distribution, where the reconstructed q 2 is the square of 4-momentum transfer assuming CC-QE. As for the muon momentum, the data agree with MC well. On the other hand, the muon angle distribution shows a deficit in the forward direction. A discrepancy is also seen in low q 2 region of the q 2 distribution. The reason for the deficit is under investigation of detector systematics and neutrino interactions. Except for the forward going muons, each histogram shows a good agreement between data and MC. Thus, SciBar detector itself and analysis methods are working well and ready for a neutrino oscillation analysis. To investigate particle identification capability, we select muon enhanced tracks and proton enhanced tracks by kinematic criteria. First of all, the primary track, which is matched with a MRD track, is recognized as a muon. To select proton enhanced track, we extract 2-track events from the inclusive CC sample. Here, the definition of a 2-track event is that the number of tracks reconstructed from the start point of the primary
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 GeV/c
Figure 6. An inclusive muon momentum distribution. Open circles with error bars are data. The error bars show statistical errors only. A solid line shows the MC normalized by the number of events. In addition, the MC is shown by each interaction mode.
Data NC CC multi π CC coherent π CC single π CC−QE
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Figure 7. An inclusive muon angle distribution with respect to the neutrino beam. The horizontal axis shows the cosine of the muon angle.
Data NC CC multi π CC coherent π CC single π CC−QE
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 22 GeV
Figure 8. Reconstructed q 2 distribution of an inclusive CC sample.
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rent analysis, inclusive muon distributions agree well with MC expectation. Thus, SciBar has been working well and is ready for neutrino oscillation analysis. 7. Acknowledgment We thank KEK technical staffs and volunteer students for the grateful efforts on the construction of SciBar. We also appreciate Fermilab staffs for the development and the production of extruded scintillators. REFERENCES
Figure 9. Muon likelihood distributions for a muon enhanced sample (top) and a proton enhanced sample (bottom). The likelihood of a minimum ionizing track is close to 1, while that of another is close to 0. Muons and protons are clearly separated.
track is 2, including the primary track. Then, we can evaluate expected proton direction from muon momentum and angle assuming CC-QE by using 2-body kinematics. If the angle between this expectation and the observed second track is less than 25 degrees, the observed track is classified to a proton enhanced sample. The proton purity of the proton enhanced sample is 95 % in MC data. By using the muon sample, we construct muon likelihood from dE/dx information. In the present analysis, only 4 planes are used for the likelihood. Figure 9 shows muon likelihood distributions for both muon and proton samples. Protons and muons are clearly separated. The separation power will be improved in a future analysis. 6. Conclusions SciBar was constructed to improve the determination of the neutrino energy spectrum especially below 1GeV. The basic performance is obtained to be as well as expected. From the charged cur-
1. S. H. Ahn et al. Phys. Lett. B 511, 178 (2001) 2. Y. Fukuda et al. Phys. Rev. Lett. 81, 1562 (1998) 3. M. H. Ahn et al. Phys. Rev. Lett. 90, 041801 (2003) 4. The MINOS Collaboration, “Technical Design Report” NuMI-L-337 (1998). 5. A. Pla-Dalmau, FERMILAB-Conf-00/343 (2001). 6. S. Buontempo et al., Nucl. Instrum. Meth. A 349, 70 (1994). 7. R. Tayloe, Nucl. Phys. Proc. Suppl. 105 (2002) 62. 8. T. Ishii et al. Nucl. Instrum. Meth. A 482, 244 (2002) [Erratum-ibid. A 488, 673 (2002)]
The K2K SciBar Detector H. Maesakaa for The K2K SciBar Group a
Department of Physics, Kyoto University Oiwake-cho, Kita-shirakawa Sakyo, Kyoto, 606-8502, Japan. SciBar is a fully-active scintillator tracking detector, which was newly installed at the near detector site of K2K long baseline neutrino oscillation experiment. The purposes of SciBar are to improve the measurement of the neutrino flux and to study neutrino interactions. Its construction was completed in August 2003. The neutrino data have been taken stably since October 2003. We describe its design, operation and basic performance. Preliminary results from an analysis of charged current events are also presented.
1. Introduction The KEK-to-Kamioka long baseline neutrino oscillation experiment (K2K) [1] has been running since 1999, to study the muon neutrino oscillation observed in the atmospheric neutrino results from Super-Kamiokande [2]. By using 12 GeV proton synchrotron at KEK, we produce almost pure muon neutrino beam, whose energy spectrum is widely spread around the mean energy 1.3 GeV. The beam is detected by both a near detector system (ND) at KEK and SuperKamiokande (SK) which is about 250 km far from KEK. By comparing both the number of events and the neutrino energy spectrum between the SK observation and the expectation from ND measurements, we determine the parameters of neutrino oscillations. The neutrino energy spectrum is determined by using charged current quasi-elastic (CC-QE) scattering (νµ + n → µ− + p), which is a dominant process around 1 GeV. The latest results from K2K [3] show indications for the disappearance of muon neutrinos, and are consistent with the atmospheric neutrino results. From those results, the neutrino energy of the oscillation maximum is expected to be around 0.6 GeV. Therefore, a measurement of neutrino flux below 1GeV is the most important. To improve the determination of the neutrino energy spectrum and to understand neutrino interactions with nuclei, we designed and constructed a fully-active scintillator detector named
0920-5632/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2004.11.225
SciBar (scintillator bars) as an upgrade of ND. The design, operation and basic performance of SciBar are presented in the following sections. We also show preliminary results from an analysis of charged current (CC) events. 2. SciBar Detector SciBar consists of extruded scintillator strips, which are readout by wavelength shifting (WLS) fibers and Multi-anode PMTs. A schematic drawing of SciBar is shown in Figure 1. The extruded scintillator is developed and produced by Fermilab. The scintillator is made of polystyrene infused with fluors. The ingredient of the scintillator is the same as that used for MINOS detector [4,5] at Fermilab. The dimension of the scintillator strip is 2.5 × 1.3 × 300 cm3 with a hole (1.8 mm in diameter) in the middle. Thin (0.25 mm thick) reflective coating, composed of TiO2 infused in polystyrene, surrounds the entire scintillator bar. Both the hole and coating are extruded together with scintillator. In total, 14848 strips are arranged vertically and horizontally one after another. The total volume of SciBar is 3 × 3 × 1.7 m3 and the total mass is about 15 tons. A schematic view of the readout system is shown in Figure 2. A wavelength shifting fiber is inserted into the hole of each strip, and is connected to a multi-anode PMT which has 64 pixels. To monitor the PMT gain, a clear fiber is
290
H. Maesaka / Nuclear Physics B (Proc. Suppl.) 139 (2005) 289–294
'N
GE
VT
QP
%
CN
QT
KO
GV
O
GT
$GCO
O O
Figure 1. A schematic drawing of SciBar. Extruded scintillator strips are arranged vertically and horizontally. The size of scintillator part is 3 × 3 × 1.7 m3 and the weight is about 15 tons. Each strip has a hole in the middle, in which a WLS fiber is inserted. At the end of the fiber, a multi-anode PMT is attached. In addition, an electro-magnetic calorimeter is placed behind the scintillator part. Light injection module
C lear fiber
F rontend board
64ch MAP MT
T o V ME B oard
V A/T A C hips WLS fiber (x64) E xtruded s cintillator
Holder cookie Detector s tructure
Figure 2. A schematic view of the readout system. A WLS fiber absorbs and transports scintillation photons to a multi-anode PMT. The PMT is readout by a VA/TA system. In addition, a clear optical fiber is drawn from a blue LED to a light injection module attached to a fiber bundle, to monitor the PMT gain.
connected from a blue LED to a light injection module attached to a fiber bundle. We have chosen to use 1.5 mm diameter fibers, in order that they fit the pixel size of the PMT (2 × 2 mm2 ). The attenuation length is 3.5 m from a laboratory measurement. The 64-pixel multi-anode PMT has a bialkali photo-cathode and 12 % quantum efficiency for a green photon, while 20 % for a blue photon. The typical gain is 6 × 105 . With this gain, the response linearity is preserved up to 200 photoelectrons (p.e.). Since the light yield is about 10 p.e./MeV (see Section 4.1), the linearity is sufficient to measure up to 20 MeV/cell. Both timing and charge information from the PMT are digitized by readout electronics, which are assembled from VA/TA ASIC chip-sets. The VA/TA chip-set is composed of VA and TA chips, and it has 32 inputs. Therefore, two sets are used to read one PMT. The VA integrates the signals from a PMT and, if triggered by an external signal, holds the voltage proportional to the charge. Then, it sends the voltage signal to a VME ADC board sequentially. The TA discriminates the pulse height by a given threshold, and provides an OR’ed logical signal over 32 channels. The TA signal is used to make a hold signal for the VA and is recorded by a TDC. An electro-magnetic calorimeter (EC) was installed behind of the main part to measure νe contamination in the beam and π 0 yield from neutrino interactions. EC is comprised of 2 planes of 30 horizontal and 32 vertical modules re-used from the CHORUS experiment [6]. The module is made of lead sheets and scintillating fibers. It has a dimension of 4×8×262 cm3 , and consists of two of 4 × 4 cm2 cells. EC has 11 radiation length along the beam axis and covers 2.6 × 2.6 m2 . The energy resolution is 14/ E (GeV) %. The construction of SciBar finished in August 2003 and neutrino data has been taken since October 2003. Since the neutrino beam is shot every 2.2 seconds as 1.1µsec duration pulse, the beam data taking is triggered by the accelerator. An event display of a CC-QE candidate is shown in Figure 3. So far, 2.0 × 1019 protons have been delivered to the production target since the installation of SciBar. In addition to beam data,
H. Maesaka / Nuclear Physics B (Proc. Suppl.) 139 (2005) 289–294
K2K Fine-Grained Detector (Side View) Run 4999 Spill 19016 TRGID 1 SBEv 23252 2003 10 7 14 44 40 0 Nvtx 0
10 cm
Figure 3. An event display of a CC-QE candidate. Each hit is shown as a closed circle, whose area is proportional to ADC counts. Two tracks are generated at the vertex. One extends to MRD, so that it is a muon. The other stops within SciBar and gives larger ADC outputs than the muon track. Therefore, it is a proton.
pedestal, LED and cosmic ray data are taken together during an interval between beams. LED and cosmic ray data are used, respectively, for PMT gain monitoring and for absolute energy calibration. 3. Physics Capability Since the segmentation is 2.5 × 1.3 cm2 , SciBar can detect a track as short as 10 cm. It corresponds to 0.4 GeV/c for a proton, and 0.1 GeV/c for a muon and a charged pion. Therefore, SciBar has high efficiency to detect CC-QE as a 2track event. Furthermore, since SciBar measures dE/dx along a track, protons can be discriminated from muons and pions. Thus, we can distinguish CC-QE events from other CC non-quasielastic events (non-QE) by using the particle identification together with a kinematic requirement. Consequently, SciBar helps us to determine the neutrino energy spectrum more precisely. On the other hand, there is a somewhat ad-
291
verse point that the neutrino target is scintillator (carbon and hydrogen nuclei), while that of SK is water (oxygen and hydrogen nuclei). This difference may affect the expectation of both the number of events and the neutrino energy spectrum at SK. Since the Fermi momentum is different by about 5 % between carbon and oxygen, the Pauli suppression for protons and neutrons can be slightly changed. In the K2K case, it corresponds to about 1 % difference in the total neutrino interaction cross section. Final state interactions, such as the re-scattering of protons and pions or absorption of pions in the target nucleus, can also differ. We can study these effects with SciBar data. Study of various neutrino interactions is also necessary for neutrino oscillation analyses. As for the neutrino interactions around 1GeV, there are still many quantities which are not measured precisely enough, such as cross sections and form factors. These study are also important for future neutrino oscillation experiments. SciBar can determine following quantities as a function of neutrino energy or squared 4-momentum transfer: CC-QE cross section, CC resonance production cross sections without a neutron in the final state (νµ + p → µ− + p + π + and νµ + p → µ− + p + π 0), neutral current (NC) elastic scattering cross section, NC / CC cross section ratio, and νe contamination in the beam. Neutrino energy and 4momentum transfer can be determined by using all final state products. As a result, form factors for each interaction mode can also be evaluated. NC elastic scattering can tell us the strange quark contribution to the spin component of nucleons [7]. Furthermore, SciBar can measure following quantities: the cross sections for CC coherent pion production (νµ + 12 C → µ− + π + + 12 C), CC multi-pion production (νµ + N → µ− + N + π + π + . . .), NC resonance production cross section. 4. Basic Performance In the following sections, we describe some basic performance of SciBar. Using cosmic ray data, we evaluate the light yield and timing resolution for each strip. Then, we show the stability of
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H. Maesaka / Nuclear Physics B (Proc. Suppl.) 139 (2005) 289–294
PMT gain which is monitored by an LED system. 4.1. Light Yield After finding a track from a cosmic ray event, we require that the track angle with respect to the beam direction is less than 45 degrees. If the track makes one and only one hit in a given layer, the hit is used by this analysis. If there is no attenuation effect, mean values of light yield histograms distribute around the central value of 9.1 p.e./MeV with the standard deviation of 2.6 p.e./MeV. Even if the attenuation effect is the largest (3.9 p.e./MeV), the statistical fluctuation of energy response is better than 1.6/ E (GeV) %. For example, typical 1 GeV/c protons and muons give, respectively, 3.0 and 2.1 MeV/cm to scintillator. Therefore, the energy deposit for a 10cm-muon is measured to be 21 ± 2.3 MeV, while that for a 10cm-proton is 30 ± 2.8 MeV. They can be clearly separated. Thus, the light yield is sufficient for energy measurement and particle identification. 4.2. Time Resolution The time resolution is measured by comparing hit timing between neighboring channels along a cosmic ray track. From the time difference distribution, timing resolution is found to be 1.3 nsec. It is enough for identifying a bunch of the beam spill (125 nsec spacing. See figure 5). Furthermore, it is adequate to determine the direction of a particle by using time-of-flight, if the track length is as long as 1 m. 4.3. Gain Monitoring The gain drift of each PMT is monitored by an LED system. A pulse of blue LED light is transmitted to fiber bundles through clear fibers, as shown in Figure 2. WLS fibers are uniformly illuminated and bring photons to a PMT surface. In addition, the intensity of the LED is monitored by both a photo-diode and a 2-inch PMT calibrated by an Am-NaI stable light source. Figure 4 shows a stability of a channel of multi-anode PMT. The PMT gain is corrected within 1 % level. The deviation of corrected gain is smaller than the statistical fluctuation of energy measurement.
Figure 4. A gain stability of a PMT channel. Each figure shows the time variation of ADC counts from cosmic rays. The upper figure is before the correction of PMT gain. There is a large deviation from November to December because of the temperature change due to the disorder of an air conditioner. The lower one shows after the correction. The PMT gain is followed within 1%.
5. Charged Current Analysis To study neutrino oscillations, the analysis of charged current (CC) events is necessary, because we need to determine the neutrino energy spectrum at the near site by using CC-QE interaction and test our MC prediction of CC interactions. We describe the results from an inclusive charged current analysis. To select a CC event, at least one of reconstructed tracks is required to extend to Muon Range Detector (MRD) [8] located downstream of SciBar. We select two event types: 3D matching (3D) and first-layer-hit matching (1L) events. If the extension of a SciBar track is joined to an MRD track, it is selected as a 3D event. If a SciBar track is not a 3D event but matches with hits in the first layer of MRD, it is identified as a 1L event. Then, the selected track is required to start from 9.38-ton fiducial volume. Both 3D and 1L are used for the inclusive CC analysis. Figure 5 shows a timing distribution of 3D and 1L events. The bunch structure of the beam is clearly seen. Since there are no events before or after the beam, the amount of background is negligible.
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Data NC CC multi π CC coherent π CC single π CC−QE
1000 1200 1400 1600 1800 2000 2200 2400 nsec
Figure 5. A timing distribution of a inclusive muon sample. The micro-bunch structure of the beam is clearly seen. In addition, there are no background before or after the beam.
Then, the muon energy is evaluated from the range through SciBar, EC and MRD. The muon angle is determined by the least square fit to SciBar hits. A muon momentum and a cosine of a muon angle with respect to the neutrino beam are shown in figure 6 and 7, respectively. Figure 8 shows a reconstructed q 2 distribution, where the reconstructed q 2 is the square of 4-momentum transfer assuming CC-QE. As for the muon momentum, the data agree with MC well. On the other hand, the muon angle distribution shows a deficit in the forward direction. A discrepancy is also seen in low q 2 region of the q 2 distribution. The reason for the deficit is under investigation of detector systematics and neutrino interactions. Except for the forward going muons, each histogram shows a good agreement between data and MC. Thus, SciBar detector itself and analysis methods are working well and ready for a neutrino oscillation analysis. To investigate particle identification capability, we select muon enhanced tracks and proton enhanced tracks by kinematic criteria. First of all, the primary track, which is matched with a MRD track, is recognized as a muon. To select proton enhanced track, we extract 2-track events from the inclusive CC sample. Here, the definition of a 2-track event is that the number of tracks reconstructed from the start point of the primary
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 GeV/c
Figure 6. An inclusive muon momentum distribution. Open circles with error bars are data. The error bars show statistical errors only. A solid line shows the MC normalized by the number of events. In addition, the MC is shown by each interaction mode.
Data NC CC multi π CC coherent π CC single π CC−QE
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Figure 7. An inclusive muon angle distribution with respect to the neutrino beam. The horizontal axis shows the cosine of the muon angle.
Data NC CC multi π CC coherent π CC single π CC−QE
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 22 GeV
Figure 8. Reconstructed q 2 distribution of an inclusive CC sample.
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rent analysis, inclusive muon distributions agree well with MC expectation. Thus, SciBar has been working well and is ready for neutrino oscillation analysis. 7. Acknowledgment We thank KEK technical staffs and volunteer students for the grateful efforts on the construction of SciBar. We also appreciate Fermilab staffs for the development and the production of extruded scintillators. REFERENCES
Figure 9. Muon likelihood distributions for a muon enhanced sample (top) and a proton enhanced sample (bottom). The likelihood of a minimum ionizing track is close to 1, while that of another is close to 0. Muons and protons are clearly separated.
track is 2, including the primary track. Then, we can evaluate expected proton direction from muon momentum and angle assuming CC-QE by using 2-body kinematics. If the angle between this expectation and the observed second track is less than 25 degrees, the observed track is classified to a proton enhanced sample. The proton purity of the proton enhanced sample is 95 % in MC data. By using the muon sample, we construct muon likelihood from dE/dx information. In the present analysis, only 4 planes are used for the likelihood. Figure 9 shows muon likelihood distributions for both muon and proton samples. Protons and muons are clearly separated. The separation power will be improved in a future analysis. 6. Conclusions SciBar was constructed to improve the determination of the neutrino energy spectrum especially below 1GeV. The basic performance is obtained to be as well as expected. From the charged cur-
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