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Near detector complex for the T2K long base line experiment
Nuclear Physics B (Proc. Suppl.) 139 (2005) 295–300 www.elsevierphysics.com
Near detector complex for the T2K long base line experiment F.S´anchez, on the behalf of the T2K ND280m working group Universitat Aut`onoma de Barcelona/Institut de F`ısica d’Altes Energies Edifici Cn, E-01983 Bellaterra (Barcelona) Spain The Tokai-2-Kamioka (T2K) project has been recently approved to search for the first signal of νe appearance in a conventional νµ beam, and to measure the θ13 neutrino mixing parameter. T2K sends a beam of neutrinos from the Japan Proton Research Complex(JPARC) to Super-Kamiokande with a baseline of 295 km using the Off-Axis beam technique. The neutrinos are measured in a near site, just after production, for flux normalization and background prediction. The design goals of the near detector for the T2K project are discussed.
1. Introduction The Super-Kamiokande collaboration announced in 1998 the evidence that neutrino has mass[1]. The Super-Kamiokande discovery was based on the measurement of the disappearance (oscillation) of muon neutrinos produced as the product of the collision of high energy cosmic rays with the atmosphere. Since then, the mass and the mixing of the neutrinos has been gradually explored at different experiments. K2K announced in 1999 the detection of neutrinos produced in an accelerator[2] after flying over a distance of 250 km proving that long base line(LBL) experiments are feasible. Recently, K2K[3] and KamLAND[4] have presented evidence of neutrino oscillations with artificial ν sources where initial fluxes can be measured. A new long base-line experiment between the Japan Proton Accelerator Research Complex(JPARC), being built at Tokai, and the Kamioka underground neutrino observatory(T2K)[5] has been recently approved. The T2K experiments is aiming at the measurement of the mixing angle θ13 and the precise measurement of the atmospheric oscillation parameters θ23 and ∆m224 . The angle θ13 is very poorly known, the only measurement is an upper limit( sin2 θ13 < 0.12@95%C.L.) given by the CHOOZ reactor experiment[6]. A non vanishing θ13 is mandatory for the existence of CP-violation in the lepton sector in a similar way to the one discovered in the quark sector in the sixties. 0920-5632/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2004.11.235
CP-violation in the lepton sector could also be discovered at T2K measuring the difference in the oscillation amplitudes of neutrinos and antineutrinos. The new T2K will also provide a normalization of the neutrino flux after production point. Two detectors are being considered for this purpose. One of them located at 280 m and the other at ˇ 2 km based on water Cerenkov technique. The 2 km detector will improve the flux prediction and will allow to measure the neutrino interactions with the same technology as the far detector[5]. In this paper we will discuss the design parameters and goals for the near detector located at 280 m from the target. 2. T2K experiment T2K is a neutrino oscillation experiment based on very intense muon neutrino conventional beam. The neutrino beam will be produced at JPARC colliding protons of 40 GeV with a graphite target. The detection of the neutrinos will take place at the Super-Kamiokande detector located 295 km away from the production site. The spill-by-spill synchronization will be done by GPS following the experience accumulated by the K2K experiment[7]. The 40 GeV proton synchrotron is designed to deliver up to 3.3x1014 protons every 3.4 seconds (primary beam power 750 KW). With 130 days/year of operation, the experiment will accumulate 1021 protons on target which is an order of magnitude bigger that the
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one achieved in the K2K experiment for 5 years of operation. The atmospheric oscillation parameters is measured in the transition νµ → ντ , where ντ is not visible to be below the τ threshold production. The probability of this oscillation is given by: Pνµ →νe ≈ sin2 θ23 sin2 2θ13 sin2 φatm where φatm = φ23 = ∆2 matm L/4Eν (L is the baseline, ∆2 matm is the difference between the square of the masses of the neutrino mass states and Eν the neutrino energy) and θ23 is the mixing angle that dominates atmospheric neutrino oscillations. θ23 is measured to be very close to 45o . The measurement is based on the distortion of the νµ energy spectrum due to the oscillation. The νµ energy is reconstructed by assuming a Charge Current quasi-elastic (CCQE) interaction, νµ N → µp. The main sources of systematic uncertainties for this measurement are the non quasi-elastic contamination in the sample, the flux prediction at the far detector and the detector energy scales. T2K will also measure the mixing angle θ13 via the oscillation channel νmu → νe . The probability for this oscillation is given by: Pνµ →ντ ≈ cos4 θ13 sin2 θ23 sin2 φatm T2K is sensitive to θ13 with a baseline equal to the one atmospheric oscillations. The goal of the T2K experiment is the detection of νe appearance via the electron identification in charge current interactions. The expected signal is very small according to upper limits from CHOOZ experiments[6], and therefore the background is expected to dominate the sensitivity to θ13 . The main sources of background are the intrinsic νe contamination in the beam and the misidentification of the 2-gamma decay of π 0 ’s, which are mainly produced via neutral currents with intermediate ∆ resonances [5]. 2.1. Off-axis neutrino beam A horn-focused wide band beam (WWB) has been widely used in conventional neutrino-beam experiments. In this configuration, the detector
is placed on the neutrino beam axis in order to collect the larger amount of neutrino interactions. The WWB configuration is however not optimal for the νe appearance experiment. The oscillation energy region is very narrow with respect to the full ν spectrum that contributes mainly as background to the measurement. This background appears in two different ways, as π 0 from neutral currents or charge currents non quasi-elastic neutrino interactions and the intrinsic νe beam content. To reduce the contribution of the background a novel technique developed for the proposed experiment E-889 at Brookhaven [8] was adopted for the T2K experiment, the off-axis beam (OAB). This is a very attractive technique since it provides a narrow neutrino spectrum with a neutrino flux at the maximum of the oscillation comparable to the one of the WBB. The OAB exploits the kinematics of the π ± decays placing the detector at a finite angle with respect to the primary pion beam. In this configuration, the ν energy from pion decays become almost independent of the pion energy due to the Lorentz boost, thus providing an almost mono-energetic neutrino beam. The actual off-axis neutrino spectrum for a 2.50 off-axis setting is displayed in Fig.1. This configuration also improves the prediction of the flux since the neutrino energy depends weakly on the primary π(K) energy distributions. The prediction of the neutrino spectrum is one of the most important sources of systematic errors in disappearance neutrino experiments as it has been shown by K2K experiment[3]. The neutrino energy can be also tuned to the maximum of the oscillation changing the off-axis value. According to beam simulations[5], the ν energy peak moves from 0.55 GeV to 0.7 GeV while changing from 30 to 20 off-axis angle. The monitoring of the neutrino beam direction is very important to control the stability of the energy spectrum. The OAB also reduces the high energy neutrino tail and thus the π 0 contamination that might mimic the appearance of νe . On the other hand, the intrinsic νe flux remains flat in the off-axis configuration since the origin of the νe are three body decays (µ decays, and Ke3 ). The OAB improves the relative νe contamination in the region
F. Sánchez / Nuclear Physics B (Proc. Suppl.) 139 (2005) 295–300
of the oscillation, thus improving the sensitivity to the θ13 angle.
Table 1 Total number of νµ interactions per year (107 s) per detector ton and the fraction of the different interactions for an off-axis neutrino beam as predicted by NEUT Monte-Carlo [9]. Interactions 120 000 events/year/ton Reaction Fraction of interactions CC - QE 38.2 % CC - Other 33.0 % NC - 1 π 0 4.4 % NC - Other 24.4 %
3. Detector design goals The near detector will be located at a distance of 280 m from the target position. The detector has to accomplish a monitoring task, provide information on the neutrino energy spectrum, the total neutrino flux and characterize the neutrino interactions in the relevant energy range. The prediction of the NEUT [9] on the different neutrino interactions are shown in Table.1, where it can be seen that more than 50% of the CC interactions are of the quasi-elastic type. 3.1. Muon monitor The direction of the proton beam is monitored by the muon monitor measuring high energy muons escaping the beam dump. The detector monitors µ’s from π’s and Kaons decays in the decay volume. This is, in essence, the same process that provides neutrinos to the beam, so the measurements of the muon yield might also provide information on the neutrino beam. The statistics is large enough to provide stability information in a spill by spill basis. The detector will be build by segmented ionization chamber and/or semi-conductor detector following the experience of the K2K experiment and considering the higher radiation dose they will be exposed to. The muon monitor will be placed
297
down stream the beam dump at the end of the decay volume. The location reduces the heat produced in the beam dump and the contamination from secondary hadrons produced in the interactions of primary protons with the beam dump. The µ monitor location limits the sensitivity of the detector to high energy muons (above 5 GeV). The predicted muon flux is of the order of 108 muons/spill/cm2 for a 5 GeV threshold. At this threshold, the ratio of muon to other charge tracks is around 10. Muons above 5 GeV still carry information on the stability and alignment of the beam according to simulations. It also provides information on possible failures of the horn pulsing, observed as a drastic reduction of the muon flux per pulse. The detector has to measure the muon flux to relatively large angles with respect to the beam axis to improve the sensitivity to misalignment of the beam. The goal is to monitor the proton beam direction with an accuracy of better than 1 mrad for each spill. The monitor coverage will sample the flux with pads distributed as a cross at the end of the beam dump monitoring an area of approximately 3x3 m2 . This area gives enough resolution to resolve displacements of the proton position at the order of 1 cm at the target spill by spill. 3.2. On-axis neutrino monitor The muon monitor is not enough to prove the stability of the beam. The µ monitor looks only at high energy muons from the decay volume without looking at the actual ν beam. Another reason is that the muon monitor integrates muon energies above certain cut value missing some information from the energy dependency of the beam. This is an important observable to identify malfunctioning in the horn focusing system. The onaxis neutrino monitor complements the measurement looking at muons produced in neutrino interactions at the near detector site, 280 m away from the target. This detector should be able to measure the beam profile and intensity in at least two energy bins. To provide a good profile measurements the detector has to cover a very large area, estimated to be ± 4 m in vertical and horizontal direction for 0.1 mrad pointing precision, since the beam profile is rather flat at the beam
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center. To reduce the cost of the detector, several ideas exploiting the concept of a grid detector are under study. The grid concept is based on several detector units distributed as a cross. Each of these modules is able to identify the energy of the muon produced during the neutrino interactions. Two possible alternatives are being considered looking for neutrinos interacting in the detector module or in the soil upstream the beam. 3.3. Off-axis neutrino detector The off-axis neutrino detector is a fine grain detector with capability to study exclusive neutrino interactions. The near detector should provide information on the neutrino energy spectrum, the relative strength of exclusive neutrino interactions with a neutrino spectrum similar to the one of the far detector and predict the background in the far detector from the π 0 production and νe intrinsic beam contamination. The off-axis detector is being design as a multipurpose detector to be located off-axis providing a similar neutrino spectrum to the one observed at the Super-Kamiokande. The detector is being optimized to measure CCQE neutrino interactions, π 0 ’s and electrons from CCQE νe interactions. 3.3.1.
Prediction of energy spectrum at Super-Kamiokande In an ideal case, all the systematic uncertainty would cancel out by using the measured spectra in the near detector. In reality, the near detectors are different from the far detector in terms of material, size, and responses. The closer location to the decay pipe also introduces a large and complicated far-to-near spectrum ratio. This can be seen in 2, where deviations of the order of 20% are predicted between the near and the far beam flux. These deviations would be controlled in a more ˇ systematic way with the water Cerenkov detector located at 2 km from the target [5]. Off-axis 280 m detector also provides useful information about the far to near ν flux ratio. A fully-active fine grained scintillator detector similar to the K2K SciBar[10] detector is being considered. The advantage of a fully-active is the increase of the sensitivity to the low energy
Figure 1. Spectrum of neutrino energies at the near detector for a 2.50 off-axis configuration.
protons from CCQE reactions. The typical momentum of this protons are around 400 MeV/c3 which makes the detection very inefficient. The disadvantage of a scintillator based detector is a different target material (Carbon) to the one of the water based far detector (Oxygen). Two alternatives to the carbon based detectors are being considered. The first one is a sampling detector of water and very highly segmented scintillators. The high segmentation allows to track back the interaction point and identify interactions occurring in the water volume. The second option is to add liquid scintillator to water keeping the advantage of a fully active detector while adding a fraction of neutrino oxygen interactions. A R&D project have been initiated to identify the best scintillator and to study the aging stability of the mixture. The energy scale of the detector is also an important issue that will compromise the resolution on the ∆m223 and the understanding of the spectrum extrapolation from the near to the far detector. It is important that the energy scale is controlled to a good precision. The optimal way
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p
Figure 2. Ratio of the near detector to the SuperKamiokande spectra for 2.50 off-axis configuration. The ratio is fixed to be one.
Figure 3. Proton momentum spectrum at ND280m 2.50 off-axis detector.
of controlling it will be embedding the detector in a magnetic field that allows to measure the energy of the muon with accuracy. A proposal to reuse the UA1/NOMAD magnet coil and iron yoke, adding tracking chambers to the detector is under consideration. The magnet proposal will also help in the reconstruction of neutrino exclusive channels interactions. The magnet opens the possibility of measuring the π ± momentum and identification capabilities of the detector including the lepton charge.
rounded by electromagnetic calorimeters with a very low energy threshold to contain all γ’s from π 0 decays. One open question is the importance of the nuclear target material. Alternative Carbon and Water, as mentioned above, targets are considered as target for the off-axis detector. The impact of the Carbon and Oxygen on the measurement of the total flux still has to be evaluated. The νe flux with respect to the νµ flux is very small (0.4% at the flux peak, 1% integrated, Fig.1). The statistics of νe interactions will be the dominant factor to determine the total fiducial volume of the near detector (≈ 500 CCQE νe interactions/year/ton). The extrapolation of the νe and π 0 backgrounds to the far detector is also delicate. The νe flux comes from 3-body decays (mainly µ decays) and the π0 ’s are produced at neutral current interactions at higher energies where the near to far ratio suffers from large uncertainties. A precise measurement of the near detector flux also influences the prediction of background to νe appearance.
3.3.2. π 0 and intrinsic νe for νe appearance The near detector is being designed to predict the contamination for the νe appearance experiment. The detector should be able to efficiently identify π 0 and electrons covering the phase space of misidentified π 0 at the far detector. The main source of background from π 0 in the far detector is the very asymmetric π 0 decay with overlapping or very feeble rings [5]. The average π 0 energy is 650 MeV as predicted by NEUT[9], see Fig.4. The typical radiation length of scintillator is 40 cm, therefore the detector has to be sur-
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π0
Figure 4. π 0 momentum spectrum at ND280m 2.50 off-axis detector.
detectors aiming at the monitoring of the beam stability, prediction of neutrino flux at the far detector and the study of the basic neutrino interactions with matter at the relevant energies. The detector complex is divided into three apparatus: the muon monitor at the end of the decay volume, the neutrino monitor at the 280m location and the off-axis fine-grained detector. The off-axis detector will be probably magnetized to improve the performance on the momentum measurement and neutrino energy scale. It will consist on active target made of plastic scintillator or scintillator dissolved in water in order to reduce the proton detection threshold. An alternative of passive water followed by a high precision tracker is also considered as an alternative. The detector will also be equipped with gas trackers to profit from the presence of the magnet and low threshold electromagnetic calorimeters to detect γ’s from π 0 decays. REFERENCES
3.3.3. Anti-neutrino run As mention in the introduction, an antineutrino run is mandatory to measure the CPviolation phase in case the θ13 mixing angle is sizable. We should also pay attention to this possibility in the design of the near detector. During anti-neutrino run, a source of background is the intrinsic neutrino contamination of the beam. Anti-neutrinos flux is smaller than the neutrino in conventional beams, and the anti-neutrino crosssection with matter is approximately a factor of 2 smaller [11], giving a final yield of anti-neutrinos with respect to neutrinos that is not very favorable.Magnetizing the detector will help in reducing the neutrino contamination and improving the sensitivity to CP-violation measurement. It is also possible that an anti-neutrino run will take place to resolve the degeneracies in the θ13 parameter induced by the CP phase in order to improve the limit on the θ13 angle. 4. Summary The Near Detector at 280m for the JPARC neutrino experiment (T2K) is a complex set of
1. Y.Fukuda et al. ( Super-Kamiokande collaboration), Phys.Rev.Lett. 81 (1998), 1562. 2. S.H.Ahn et al. (K2K collaboration), Phys.Lett.B511 (2001), 178. 3. S.H.Ahn et al. (K2K collaboration), Phys.Rev.Lett. 90 (2003) 041801. 4. K.Eguchi et al. (KamLAND Collaboration), Phys.Rev.Lett. 90 (2003). 5. Y.Itow et al., “The JHF-Kamioka neutrino project”, KEK Report 2001-4, hepex/0106019. 6. M.Apollonio et al., Phys. Lett. B466 (1999), 415. 7. H.G.Berns and J.R.Wilkes IEEE Trans.Nucl.Sci. 47 (2000) 340. 8. B.Beavis et al., Proposal of BNL AGS E-889 (1995). 9. Y.Hayato, Nucl.Phys.Proc.Suppl.112 (2002) 171. 10. H.Maesaka, “The K2K SciBar detector”, in these proceedings. 11. S. Eidelman et al., Phys.Lett.B 592 (2004) 1.
Near detector complex for the T2K long base line experiment F.S´anchez, on the behalf of the T2K ND280m working group Universitat Aut`onoma de Barcelona/Institut de F`ısica d’Altes Energies Edifici Cn, E-01983 Bellaterra (Barcelona) Spain The Tokai-2-Kamioka (T2K) project has been recently approved to search for the first signal of νe appearance in a conventional νµ beam, and to measure the θ13 neutrino mixing parameter. T2K sends a beam of neutrinos from the Japan Proton Research Complex(JPARC) to Super-Kamiokande with a baseline of 295 km using the Off-Axis beam technique. The neutrinos are measured in a near site, just after production, for flux normalization and background prediction. The design goals of the near detector for the T2K project are discussed.
1. Introduction The Super-Kamiokande collaboration announced in 1998 the evidence that neutrino has mass[1]. The Super-Kamiokande discovery was based on the measurement of the disappearance (oscillation) of muon neutrinos produced as the product of the collision of high energy cosmic rays with the atmosphere. Since then, the mass and the mixing of the neutrinos has been gradually explored at different experiments. K2K announced in 1999 the detection of neutrinos produced in an accelerator[2] after flying over a distance of 250 km proving that long base line(LBL) experiments are feasible. Recently, K2K[3] and KamLAND[4] have presented evidence of neutrino oscillations with artificial ν sources where initial fluxes can be measured. A new long base-line experiment between the Japan Proton Accelerator Research Complex(JPARC), being built at Tokai, and the Kamioka underground neutrino observatory(T2K)[5] has been recently approved. The T2K experiments is aiming at the measurement of the mixing angle θ13 and the precise measurement of the atmospheric oscillation parameters θ23 and ∆m224 . The angle θ13 is very poorly known, the only measurement is an upper limit( sin2 θ13 < 0.12@95%C.L.) given by the CHOOZ reactor experiment[6]. A non vanishing θ13 is mandatory for the existence of CP-violation in the lepton sector in a similar way to the one discovered in the quark sector in the sixties. 0920-5632/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2004.11.235
CP-violation in the lepton sector could also be discovered at T2K measuring the difference in the oscillation amplitudes of neutrinos and antineutrinos. The new T2K will also provide a normalization of the neutrino flux after production point. Two detectors are being considered for this purpose. One of them located at 280 m and the other at ˇ 2 km based on water Cerenkov technique. The 2 km detector will improve the flux prediction and will allow to measure the neutrino interactions with the same technology as the far detector[5]. In this paper we will discuss the design parameters and goals for the near detector located at 280 m from the target. 2. T2K experiment T2K is a neutrino oscillation experiment based on very intense muon neutrino conventional beam. The neutrino beam will be produced at JPARC colliding protons of 40 GeV with a graphite target. The detection of the neutrinos will take place at the Super-Kamiokande detector located 295 km away from the production site. The spill-by-spill synchronization will be done by GPS following the experience accumulated by the K2K experiment[7]. The 40 GeV proton synchrotron is designed to deliver up to 3.3x1014 protons every 3.4 seconds (primary beam power 750 KW). With 130 days/year of operation, the experiment will accumulate 1021 protons on target which is an order of magnitude bigger that the
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one achieved in the K2K experiment for 5 years of operation. The atmospheric oscillation parameters is measured in the transition νµ → ντ , where ντ is not visible to be below the τ threshold production. The probability of this oscillation is given by: Pνµ →νe ≈ sin2 θ23 sin2 2θ13 sin2 φatm where φatm = φ23 = ∆2 matm L/4Eν (L is the baseline, ∆2 matm is the difference between the square of the masses of the neutrino mass states and Eν the neutrino energy) and θ23 is the mixing angle that dominates atmospheric neutrino oscillations. θ23 is measured to be very close to 45o . The measurement is based on the distortion of the νµ energy spectrum due to the oscillation. The νµ energy is reconstructed by assuming a Charge Current quasi-elastic (CCQE) interaction, νµ N → µp. The main sources of systematic uncertainties for this measurement are the non quasi-elastic contamination in the sample, the flux prediction at the far detector and the detector energy scales. T2K will also measure the mixing angle θ13 via the oscillation channel νmu → νe . The probability for this oscillation is given by: Pνµ →ντ ≈ cos4 θ13 sin2 θ23 sin2 φatm T2K is sensitive to θ13 with a baseline equal to the one atmospheric oscillations. The goal of the T2K experiment is the detection of νe appearance via the electron identification in charge current interactions. The expected signal is very small according to upper limits from CHOOZ experiments[6], and therefore the background is expected to dominate the sensitivity to θ13 . The main sources of background are the intrinsic νe contamination in the beam and the misidentification of the 2-gamma decay of π 0 ’s, which are mainly produced via neutral currents with intermediate ∆ resonances [5]. 2.1. Off-axis neutrino beam A horn-focused wide band beam (WWB) has been widely used in conventional neutrino-beam experiments. In this configuration, the detector
is placed on the neutrino beam axis in order to collect the larger amount of neutrino interactions. The WWB configuration is however not optimal for the νe appearance experiment. The oscillation energy region is very narrow with respect to the full ν spectrum that contributes mainly as background to the measurement. This background appears in two different ways, as π 0 from neutral currents or charge currents non quasi-elastic neutrino interactions and the intrinsic νe beam content. To reduce the contribution of the background a novel technique developed for the proposed experiment E-889 at Brookhaven [8] was adopted for the T2K experiment, the off-axis beam (OAB). This is a very attractive technique since it provides a narrow neutrino spectrum with a neutrino flux at the maximum of the oscillation comparable to the one of the WBB. The OAB exploits the kinematics of the π ± decays placing the detector at a finite angle with respect to the primary pion beam. In this configuration, the ν energy from pion decays become almost independent of the pion energy due to the Lorentz boost, thus providing an almost mono-energetic neutrino beam. The actual off-axis neutrino spectrum for a 2.50 off-axis setting is displayed in Fig.1. This configuration also improves the prediction of the flux since the neutrino energy depends weakly on the primary π(K) energy distributions. The prediction of the neutrino spectrum is one of the most important sources of systematic errors in disappearance neutrino experiments as it has been shown by K2K experiment[3]. The neutrino energy can be also tuned to the maximum of the oscillation changing the off-axis value. According to beam simulations[5], the ν energy peak moves from 0.55 GeV to 0.7 GeV while changing from 30 to 20 off-axis angle. The monitoring of the neutrino beam direction is very important to control the stability of the energy spectrum. The OAB also reduces the high energy neutrino tail and thus the π 0 contamination that might mimic the appearance of νe . On the other hand, the intrinsic νe flux remains flat in the off-axis configuration since the origin of the νe are three body decays (µ decays, and Ke3 ). The OAB improves the relative νe contamination in the region
F. Sánchez / Nuclear Physics B (Proc. Suppl.) 139 (2005) 295–300
of the oscillation, thus improving the sensitivity to the θ13 angle.
Table 1 Total number of νµ interactions per year (107 s) per detector ton and the fraction of the different interactions for an off-axis neutrino beam as predicted by NEUT Monte-Carlo [9]. Interactions 120 000 events/year/ton Reaction Fraction of interactions CC - QE 38.2 % CC - Other 33.0 % NC - 1 π 0 4.4 % NC - Other 24.4 %
3. Detector design goals The near detector will be located at a distance of 280 m from the target position. The detector has to accomplish a monitoring task, provide information on the neutrino energy spectrum, the total neutrino flux and characterize the neutrino interactions in the relevant energy range. The prediction of the NEUT [9] on the different neutrino interactions are shown in Table.1, where it can be seen that more than 50% of the CC interactions are of the quasi-elastic type. 3.1. Muon monitor The direction of the proton beam is monitored by the muon monitor measuring high energy muons escaping the beam dump. The detector monitors µ’s from π’s and Kaons decays in the decay volume. This is, in essence, the same process that provides neutrinos to the beam, so the measurements of the muon yield might also provide information on the neutrino beam. The statistics is large enough to provide stability information in a spill by spill basis. The detector will be build by segmented ionization chamber and/or semi-conductor detector following the experience of the K2K experiment and considering the higher radiation dose they will be exposed to. The muon monitor will be placed
297
down stream the beam dump at the end of the decay volume. The location reduces the heat produced in the beam dump and the contamination from secondary hadrons produced in the interactions of primary protons with the beam dump. The µ monitor location limits the sensitivity of the detector to high energy muons (above 5 GeV). The predicted muon flux is of the order of 108 muons/spill/cm2 for a 5 GeV threshold. At this threshold, the ratio of muon to other charge tracks is around 10. Muons above 5 GeV still carry information on the stability and alignment of the beam according to simulations. It also provides information on possible failures of the horn pulsing, observed as a drastic reduction of the muon flux per pulse. The detector has to measure the muon flux to relatively large angles with respect to the beam axis to improve the sensitivity to misalignment of the beam. The goal is to monitor the proton beam direction with an accuracy of better than 1 mrad for each spill. The monitor coverage will sample the flux with pads distributed as a cross at the end of the beam dump monitoring an area of approximately 3x3 m2 . This area gives enough resolution to resolve displacements of the proton position at the order of 1 cm at the target spill by spill. 3.2. On-axis neutrino monitor The muon monitor is not enough to prove the stability of the beam. The µ monitor looks only at high energy muons from the decay volume without looking at the actual ν beam. Another reason is that the muon monitor integrates muon energies above certain cut value missing some information from the energy dependency of the beam. This is an important observable to identify malfunctioning in the horn focusing system. The onaxis neutrino monitor complements the measurement looking at muons produced in neutrino interactions at the near detector site, 280 m away from the target. This detector should be able to measure the beam profile and intensity in at least two energy bins. To provide a good profile measurements the detector has to cover a very large area, estimated to be ± 4 m in vertical and horizontal direction for 0.1 mrad pointing precision, since the beam profile is rather flat at the beam
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center. To reduce the cost of the detector, several ideas exploiting the concept of a grid detector are under study. The grid concept is based on several detector units distributed as a cross. Each of these modules is able to identify the energy of the muon produced during the neutrino interactions. Two possible alternatives are being considered looking for neutrinos interacting in the detector module or in the soil upstream the beam. 3.3. Off-axis neutrino detector The off-axis neutrino detector is a fine grain detector with capability to study exclusive neutrino interactions. The near detector should provide information on the neutrino energy spectrum, the relative strength of exclusive neutrino interactions with a neutrino spectrum similar to the one of the far detector and predict the background in the far detector from the π 0 production and νe intrinsic beam contamination. The off-axis detector is being design as a multipurpose detector to be located off-axis providing a similar neutrino spectrum to the one observed at the Super-Kamiokande. The detector is being optimized to measure CCQE neutrino interactions, π 0 ’s and electrons from CCQE νe interactions. 3.3.1.
Prediction of energy spectrum at Super-Kamiokande In an ideal case, all the systematic uncertainty would cancel out by using the measured spectra in the near detector. In reality, the near detectors are different from the far detector in terms of material, size, and responses. The closer location to the decay pipe also introduces a large and complicated far-to-near spectrum ratio. This can be seen in 2, where deviations of the order of 20% are predicted between the near and the far beam flux. These deviations would be controlled in a more ˇ systematic way with the water Cerenkov detector located at 2 km from the target [5]. Off-axis 280 m detector also provides useful information about the far to near ν flux ratio. A fully-active fine grained scintillator detector similar to the K2K SciBar[10] detector is being considered. The advantage of a fully-active is the increase of the sensitivity to the low energy
Figure 1. Spectrum of neutrino energies at the near detector for a 2.50 off-axis configuration.
protons from CCQE reactions. The typical momentum of this protons are around 400 MeV/c3 which makes the detection very inefficient. The disadvantage of a scintillator based detector is a different target material (Carbon) to the one of the water based far detector (Oxygen). Two alternatives to the carbon based detectors are being considered. The first one is a sampling detector of water and very highly segmented scintillators. The high segmentation allows to track back the interaction point and identify interactions occurring in the water volume. The second option is to add liquid scintillator to water keeping the advantage of a fully active detector while adding a fraction of neutrino oxygen interactions. A R&D project have been initiated to identify the best scintillator and to study the aging stability of the mixture. The energy scale of the detector is also an important issue that will compromise the resolution on the ∆m223 and the understanding of the spectrum extrapolation from the near to the far detector. It is important that the energy scale is controlled to a good precision. The optimal way
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p
Figure 2. Ratio of the near detector to the SuperKamiokande spectra for 2.50 off-axis configuration. The ratio is fixed to be one.
Figure 3. Proton momentum spectrum at ND280m 2.50 off-axis detector.
of controlling it will be embedding the detector in a magnetic field that allows to measure the energy of the muon with accuracy. A proposal to reuse the UA1/NOMAD magnet coil and iron yoke, adding tracking chambers to the detector is under consideration. The magnet proposal will also help in the reconstruction of neutrino exclusive channels interactions. The magnet opens the possibility of measuring the π ± momentum and identification capabilities of the detector including the lepton charge.
rounded by electromagnetic calorimeters with a very low energy threshold to contain all γ’s from π 0 decays. One open question is the importance of the nuclear target material. Alternative Carbon and Water, as mentioned above, targets are considered as target for the off-axis detector. The impact of the Carbon and Oxygen on the measurement of the total flux still has to be evaluated. The νe flux with respect to the νµ flux is very small (0.4% at the flux peak, 1% integrated, Fig.1). The statistics of νe interactions will be the dominant factor to determine the total fiducial volume of the near detector (≈ 500 CCQE νe interactions/year/ton). The extrapolation of the νe and π 0 backgrounds to the far detector is also delicate. The νe flux comes from 3-body decays (mainly µ decays) and the π0 ’s are produced at neutral current interactions at higher energies where the near to far ratio suffers from large uncertainties. A precise measurement of the near detector flux also influences the prediction of background to νe appearance.
3.3.2. π 0 and intrinsic νe for νe appearance The near detector is being designed to predict the contamination for the νe appearance experiment. The detector should be able to efficiently identify π 0 and electrons covering the phase space of misidentified π 0 at the far detector. The main source of background from π 0 in the far detector is the very asymmetric π 0 decay with overlapping or very feeble rings [5]. The average π 0 energy is 650 MeV as predicted by NEUT[9], see Fig.4. The typical radiation length of scintillator is 40 cm, therefore the detector has to be sur-
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Figure 4. π 0 momentum spectrum at ND280m 2.50 off-axis detector.
detectors aiming at the monitoring of the beam stability, prediction of neutrino flux at the far detector and the study of the basic neutrino interactions with matter at the relevant energies. The detector complex is divided into three apparatus: the muon monitor at the end of the decay volume, the neutrino monitor at the 280m location and the off-axis fine-grained detector. The off-axis detector will be probably magnetized to improve the performance on the momentum measurement and neutrino energy scale. It will consist on active target made of plastic scintillator or scintillator dissolved in water in order to reduce the proton detection threshold. An alternative of passive water followed by a high precision tracker is also considered as an alternative. The detector will also be equipped with gas trackers to profit from the presence of the magnet and low threshold electromagnetic calorimeters to detect γ’s from π 0 decays. REFERENCES
3.3.3. Anti-neutrino run As mention in the introduction, an antineutrino run is mandatory to measure the CPviolation phase in case the θ13 mixing angle is sizable. We should also pay attention to this possibility in the design of the near detector. During anti-neutrino run, a source of background is the intrinsic neutrino contamination of the beam. Anti-neutrinos flux is smaller than the neutrino in conventional beams, and the anti-neutrino crosssection with matter is approximately a factor of 2 smaller [11], giving a final yield of anti-neutrinos with respect to neutrinos that is not very favorable.Magnetizing the detector will help in reducing the neutrino contamination and improving the sensitivity to CP-violation measurement. It is also possible that an anti-neutrino run will take place to resolve the degeneracies in the θ13 parameter induced by the CP phase in order to improve the limit on the θ13 angle. 4. Summary The Near Detector at 280m for the JPARC neutrino experiment (T2K) is a complex set of
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