T2K at J-PARC

Nuclear Physics B (Proc. Suppl.) 143 (2005) 269–276 www.elsevierphysics.com

T2K at J-PARC Y.Hayato for the T2K collaborationa a

Institute of Particle and Nuclear Studies, High Energy Accelerator Research Organization(KEK), Oho, Tsukuba, Ibaraki, 305-0801, JAPAN The Tokai to Kamioka long baseline neutrino oscillation experiment (T2K), which utilizes the 50GeV PS at J-PARC, was approved in 2004. This experiment is expected to be started in 2009. The main objectives of the first stage of the T2K experiments are a sensitive search of νµ → νe oscillation and a precision measurement of θ23 and ∆m23 . Compared to the current experimental results, one order of magnitude improvements in all three measurements are expected.

1. Introduction

stand the masses and mixings of lepton sector.

The existence of neutrino oscillation and nonzero neutrino mass has been established by several independent experiments. In 1998, the evidence of neutrino oscillation in the atmospheric neutrino was reported by the SuperKamiokande(SK) collaboration [1]. This phenomena has been confirmed by the first accelerator based long baseline experiment, K2K[2]. Also, a new analysis of atmospheric neutrino data by the SK collaboration shows the distinctive pattern of neutrino oscillation[3]. Recently, solar neutrino experiments and reactor based neutrino oscillation experiments, such as SNO[4], SK[5] and KamLAND[6], have shown that the origin of the solar neutrino deficit was neutrino oscillation. The 3 flavor neutrino oscillation is described by 6 independent parameters: two mass squared differences (∆m212 and ∆m223 ), three mixing angles (θ12 ,θ23 , and θ13 ) and one complex phase (δ). Among of these 6 oscillation parameters, two mass differences and two mixing angles (θ12 and θ23 ) have been measured. However, the other mixing angle (θ13 ) was found to be small and only the upper limit was obtained. Also, there is no information obtained about the phase δ, which is related to the CP violating process. Therefore, one of the main purposes of the next generation neutrino oscillation experiments is to measure these unknown parameters. At the same time, it is also important for those experiments to measure the other parameters much more precisely to under-

2. The T2K experiment

0920-5632/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2005.01.117

The next generation accelerator based neutrino oscillation experiments require very powerful accelerators to perform precise measurements. In Japan, the Tokai to Kamioka long baseline neutrino oscillation experiment(T2K), which utilizes the 50GeV proton synchrotron (PS) in Japan proton accelerator research complex (J-PARC), has been proposed[7] and it was approved in 2004. J-PARC is a new high intensity proton accelerator facility which is now being built in Japan Atomic Energy Research Institute (JAERI) in Tokai-village. As shown in Figure 1, this facility consists of 3 accelerators; the 400MeV proton linear accelerator, 25Hz 3GeV rapid cycle proton synchrotron and 50GeV PS. The construction of these accelerators was started in 2001 and will be completed in 2007. This 50GeV PS is expected to provide us about 50 times intense proton beam compared to the current 12GeV PS in KEK, which has been used for the K2K experiment. The neutrino beamline is expected to be available in 2009. 3. T2K neutrino beamline In constructing the neutrino beamline for T2K at J-PARC, we adopt the off axis beam (OAB) configuration[8]. OAB is one of the methods to produce a narrow band neutrino beam. In the OAB configuration, axis of the beam optics is in-

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Figure 1. Accelerator complex of J-PARC

tentionally shifted by a few degrees from the far detector direction. With a finite decay angle, the neutrino energy becomes almost independent of the parent pion energy due to characteristics of the two body decay. As a result, the width of the neutrino beam becomes narrower. The peak energy of neutrino beam depends on the off-axis angle. Therefore, it is possible to tune the neutrino beam energy to maximize the sensitivity of the oscillation parameters by changing the off axis angle. In the T2K beamline, the off axis angle can be changed from 2.0 to 3.0 degree. This corresponds that the mean energy of neutrino can be adjusted from 0.5 to 0.9GeV. The layout of the J-PARC neutrino beamline is shown in Fig. 2. The main PS ring is designed to accelerate protons up to 50GeV. However, maximum energy of proton is limited to 40GeV in the beginning. The design intensity of proton is 3.3 × 1014 protons/pulse at a repetition rate of ∼0.31(0.28)Hz. Therefore, the maximum power of the beam is 0.75MW. The fast extracted beam width is 5.6µsec and there are 8 (or 15) bunches in a spill. The width of each bunch is 58ns. The accelerated protons are extracted toward the inside of the PS ring. Then, they are bent by almost 90◦ to SK direction by the transport line with a radius of about 110m. This transport beamline can be divided into three parts: the preparation, the arc, and the final focusing sec-

Figure 2. Schematic view of the neutrino beamline for the T2K experiment

tions. The preparation section consists of series of normal conducting magnets and collimators to adjust the extracted beam for the transportation. The arc section bends the beam about 80◦ . In this arc section, 28 combined function superconducting magnets will be placed. This is the first application of this type of super-conducting magnets. These combined function magnets enable us to reduce the number of magnets and to have larger acceptance of the primary beam. The final focusing section makes the beam parallel and adjusts the direction of the beam to hit the production target properly. The production target is made of graphite. Because the intensity of the beam is so high and the beam width is short, instantaneous temperature rise due to the energy deposit in the target becomes extremely high. Therefore, one shot of

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the direction of SK (off-axis detectors). The onaxis detector is mainly used to monitor the neutrino beam direction (profile) and the off-axis detector is used to measure the energy spectrum of neutrinos, estimate the contamination of electron neutrino, and study neutrino interactions. As a far neutrino detector, SK detector will be used and the distance from J-PARC to SK is 295km. 4. T2K neutrino beam properties As described, we adopt the off axis beam configuration to get the narrow band neutrino beam with optimized peak energy. In order to obtain the neutrino energy spectrum and yield, MonteCarlo simulations using GEANT have been performed. The hadronic interaction part was simulated by GCALOR and GFLUKA model. Fig. 3 shows the expected energy spectra of neutrinos with 40GeV proton beam. About 2200 νµ inter-

a.u

the beam will destroy the target if we use high-Z material. Among of the candidate materials, we select graphite because the melting point is high and it has good thermal stress resistance. Also, graphite is very stable and easy to handle. The dimensions of the target are 3 cm in diameter and 90 cm in length, which corresponds to about 2 interaction length. In this target, about 80% of protons interact and generate pions and kaons, which decay into neutrinos. The heat load from the beam is about 60kJ/spill. In order to focus the charged pions generated in the target to the forward direction, three electromagnetic horns will be used. The target will be put inside of the inner conductor of the first horn to collect and focus the pions as much as possible. These horns are driven by the pulsed current of 320kA synchronized with the beam. This entire target and horn system will be put in the Helium vessel to reduce the production of radio active materials and NOx. After the horn, there is a decay volume, whose length is about 110m. While traveling this decay tunnel, pions and kaons will decay into neutrinos. The shape of the decay tunnel is rectangular and the height of the tunnel is gradually increased toward the end to accept both 2 degree and 3 degree off axis beams. This decay volume is also filled with Helium to reduce the absorption of pions. The wall of this decay volume is made by iron plate equipped with water cooling circuit to remove the heat load by the secondary particles. The entire decay volume is surrounded by 6m thick of concrete radiation shield. At the end of the decay volume, there is a beam dump to stop the hadrons and mesons. This beam dump is also cooled by air and water to remove the heat load from the particles which hit the dump. After the beam dump, there is a muon monitor to detect muons. The profile of energetic muons is known to have a good relation to the profile of neutrinos. Therefore, this muon detector makes it possible to monitor the neutrino beam direction spill by spill basis. A neutrino detector hall will be placed 280m from the production target. There will be two independent detector systems. One is on the proton beam axis (on-axis detectors) and the other is on

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Figure 3. Energy spectrum of neutrinos with various off-axis angle. The solid line shows the onaxis, the dotted, dashed, and dash-dotted lines show the OAB 2.0◦ , 2.5◦ and 3.0◦ , respectively.

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actions per year are expected for 2.5◦ off-axis configuration. The νe contamination was estimated to be ∼0.4% around the peak energy of νµ . The sources of these νe are π → µ → e decay chain and kaon decay. The fraction of these two decays were 54% for µ decay and 46% for kaon decay, respectively. In the following sections, we use 40GeV proton beam and define a typical one year operation as 1021 protons on target(POT), which corresponds to about 130 days of operation. 5. Measurement of θ13 The mixing parameter θ13 has been extensively studied by using the disappearance of ν¯e from reactors. However, no signature of finite value of θ13 has been observed and only the upper limit was obtained. Therefore, one of the most important purpose of the next generation neutrino oscillation experiment is to measure this oscillation parameter. Currently, the best upper limit of θ13 was given by the CHOOZ experiment[9]. In the T2K experiment, θ13 can be measured by searching for the appearance of νe , because the νµ to νe oscillation is a sub-leading oscillation of νµ involving ∆m231 . The νµ to νe oscillation probability can be approximately expressed as follows: P (νµ → νe ) =

sin2 2θ13 · sin2 θ23 · sin2 Φ31 , (1)

where Φ31

= 1.27∆m231 [eV2 ]L[km]/Eν [GeV],

∆m231 is the mass squared difference between the third and the first generation, L is the flight distance (295km for T2K experiment), and Eν is the neutrino energy, respectively. Here, ∆m212 is known to be small compared to ∆m223 and thus, |∆m231 | is almost same as |∆m223 |. Therefore, the νµ to νe oscillation signal is expected to be observed around the oscillation maximum of νµ to ντ oscillation. In order to search for the νe appearance signal in SK, charged current quasi-elastic (CCQE) interaction is used. Based on the current knowledge of ∆m2 , oscillation maximum comes around 600MeV in the T2K experiment. In this energy region, dominant interaction mode of neutrino is charged current quasi-elastic scattering(CCQE).

CCQE is two body interaction and by assuming the target nucleon to be at rest, it is possible to reconstruct the energy of neutrino by using the observed lepton momentum and direction as follows: mN El − m2l /2 , (2) Eνrec = mN − El + pl cos θl where mN and ml are the masses of nucleon and charged lepton, El and pl are the energy and momentum of lepton and θl is the angle of the lepton relative to the neutrino beam, respectively. The νe CCQE events are observed as single showering (e-like) ring event in SK. Therefore, the primary event selection criteria are as follows: select one ring event in the fiducial volume, no activity in the outer detector, the ring should be e-like, and total energy of the event should be larger than 100MeV. Next, we reject events with decay electrons. These decay electrons are generated by invisible muons or pions, whose momenta are small and the rings can not be identified. With this cut, it is possible to eliminate large amount of charged current νµ events and pion production events. Then, the events, whose reconstructed neutrino energy is larger than 350MeV and smaller than 850MeV, are selected. With this cut, the events around the oscillation signal energy region are selected. After applying these cuts, the background from misidentification of µ was estimated to be negligible because of the excellent µ to e separation of SK. As a result, the major background events remaining after these cuts described above are π 0 events. When one of the γs from π 0 decay was missed or two γ rings have been overlapped, number of rings could have been identified as one. Here, the energy of T2K neutrino beam is typically below 1GeV. Therefore, the energy of generated π 0 is fairly low and the minimum opening angle of π 0 is large enough to be identified as two ring events. On the other hand, when the decay is asymmetric, the scattered photons of energetic γ hides the other ring generated by the lower energy γ. Therefore, additional π 0 rejections are applied as follows: • reject very forward peaking events: Some fraction of π 0 background events have

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steep forward peak. These events are likely to be generated by coherent π 0 production. Therefore, the events with cos θνe > 0.9 are rejected, where θνe is the angle between the direction of neutrino and that of observed electron ring. • Invariant mass cut and number of rings likelihood cut: We search for the second ring candidate in an event. Then, calculate two likelihoods. One assumes there is only one ring and the other assumes there are two rings. Then, compare these two likelihoods. At the same time, we calculate the invariant mass with these two rings. If the obtained invariant mass is larger than 100MeV/c2 , or the existing of the second rings is more likely (∆L > 80), event is rejected. After applying the cuts mentioned above, signal efficiency is estimated to be 42% whereas 99% of νµ neutral current events are rejected as shown in Tab.1. Fig. 4 shows the reconstructed neutrino energy in five years of running, where the ∆m231 = 2.5×10−3eV2 and sin2 2θ13 = 0.1 are assumed. A clear peak is observed at the oscillation maximum. The sensitivity of sin2 2θ13 = 0.008 at 90% C.L. can be archived in five years operation as shown in Fig. 5. 6. Precise measurement of ∆m223 and θ23 with νµ disappearance In the T2K experiment, precise measurement of two oscillation parameters ∆m223 and θ23 becomes also important. The existing experimental results favor sin2 2θ23 to be 1 but still this value has ∼10% of uncertainty. This uncertainty is fairly large if we try to determine mixing angle θ23 itself. In order to measure these two parameters (∆m223 and θ23 ) in T2K, νµ disappearance channel is used. The survival probability of νµ is approximately expressed as follows: P (νµ → νµ ) = 1 − sin2 2θ23 × cos4 θ13 × sin2 (1.27∆m223 [eV2 ]L[km]/Eν [GeV]) −P (νµ → νe ).

(3)

Figure 4. Reconstructed neutrino energy distributions. The points with error bars show the expected signal+BG, the solid histogram shows the total BG and dashed histogram shows the BG from νµ interactions for 5 years of exposure of off-axis 2.5◦ .

Therefore, if one measure the energy spectrum of neutrino at the far detector, a clear dip is expected to be observed. The position of this dip in the neutrino energy spectrum corresponds to ∆m223 and the depth of the dip corresponds to θ23 . Based on the current knowledge of ∆m223 , oscillation maximum comes around 600MeV in the T2K experiment. As described, the dominant interaction mode of neutrino is CCQE in this energy region. For the CCQE selection to reconstruct the neutrino energy, the same selection criteria as those used in the atmospheric neutrino analysis by SK collaboration are used: no activity is found in the outer detector, the number of ring is one, the particle type is identified as µ-like, reconstructed electron equivalent energy should be larger than 30MeV, and reconstructed vertex is in the fiducial volume of 22.5kt. In order to extract the neutrino energy spectrum, it is necessary to subtract the contribu-

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OAB 2.5◦ FC, FV Evis >100MeV 1ring e-like no decay electron 350< Eνrec <850MeV π 0 rejections

νµ C.C. 2215

νµ N.C. 847

beam νe 184

signal 243

1043 39 12 1.8 0.7

220 175 156 47 9

88 85 71 21 13

204 202 187 146 103

Table 1 Expected number of events after each cut of νe appearance search for 5 year operation (5 ×1021 p.o.t.) with OAB 2.5◦ configuration. For the calculation of oscillated νe , ∆m2 = 2.5 × 10−3 eV2 and sin2 2θµe = 0.1 are assumed. In the table, FC and FV are fully contained events and vertex in the fiducial volume, respectively. Detail of the π 0 rejection cuts are described in the text.

Figure 5. The 90% C.L. sensitivity contour for 5 years exposure of Off-Axis 2.5◦ (solid line) together with the 90% C.L. excluded region of CHOOZ experiment(shaded region).

tion of non-CCQE background from the reconstructed energy spectrum of neutrino. Then we take the ratio between the “measured” spectrum and expected one without oscillation and fit the function (eqn.3). Because the non-CCQE background shape is also distorted by this oscillation, the background spectrum is also updated by the

fit results at each iteration of the fitting. In order to examine the expected precision of the oscillation parameter determination, full SK Monte-Carlo simulation and analysis have been performed. Here, the exposure is expected to be five years and θ13 is approximated to be zero and thus, cos4 θ13 is fixed to 1. The reconstructed energy of neutrino at SK without and with oscillation are shown in Fig.6(a) and (b). Fig. 6-(c) shows the ratio of observed (oscillated) energy spectrum to the non oscillated spectrum, which corresponds to the survival probability of νµ (P (νµ → νµ )) together with the fit result of the oscillation analysis. As shown in this figure, the oscillation pattern is clearly seen. We have studied several configurations in the range of ∆m223 from 1 × 10−3 to 1 × 10−2 eV2 and the obtained sensitivities are shown in Fig. 7. In the case of sin2 2θ23 = 0.9, which is the lower bound suggested by the atmospheric neutrino measurement at SK, the precision gets slightly worse due to the non-oscillated neutrino events at oscillation maximum. At the same time, the effect of various systematic uncertainties are studied. Basically, the disappearance signal dip can be enhanced by selecting the bin at the oscillation maximum and thus, the contribution from the systematic uncertainties are largely suppressed. In order to reduce the effects of systematic uncertainties as same level as the effect from the statistical error, it is necessary to reduce each of

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Figure 6. Reconstructed energy of neutrino at SK. (a) Solid histogram shows the energy spectrum without oscillation. (b) The data points with errors show the energy spectrum of 1 ring µ-like events with oscillation (sin2 2θ, ∆m2 )=(1.0, 2.7 × 10−3eV2 ). The error bars show the statistical error. In figures (a) and (b), hatched histogram show the non-CCQE events. (c) The ratio of the measured spectrum with neutrino oscillation to the expected one without neutrino oscillation. The solid histogram show the fit result of the oscillation.

7. summary The next generation long baseline neutrino oscillation experiment, the T2K experiment, has been proposed and it was approved in 2004. This experiment utilizes J-PARC 50GeV PS. The expected proton intensity is 50 times larger compared to the current K2K experiment. The main objectives are the search of νµ → νe oscillation and the precision measurement of θ23 and ∆m223 in the first stage of T2K experiments and the expected sensitivity will be order of magnitude better than the current experiments. The T2K

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the systematic errors as follows: 5% in the flux normalization, 4% in the energy scale, and 10% in the non-QE fraction. These systematic errors are expected to be reduced to this level by the neutrino flux measurements using CCQE events, detailed non-CCQE background measurements at the near detectors, and pion production experiments. Also, the systematic uncertainties in the far/near ratio will be negligible with the possible intermediate detector located at a few km from the production target. As a result, the overall precisions for sin2 θ23 and ∆m223 are expected to be 0.01 and better than 1×10−4eV2 , respectively.

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Figure 7. The expected 90% C.L. sensitivity of the neutrino oscillation parameters: ∆m223 (left) and θ23 (right). The sin2 2θ23 is set to 1. The offaxis angle is set to 2.5◦ . To obtain these figures, only the statistical error is considered.

experiment is expected to be started in 2009. REFERENCES 1. Y.Fukuda et al., Phys. Rev. Lett. 81(1998) 1562 2. M.H.Ahn et al., Phys. Rev. Lett. 90(2003)

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040801 3. Y.Ashie et al., Phys. Rev. Lett. 93(2004) 101801 4. S.N.Ahmed et al., Phys. Rev. Lett. 92(2004) 181301 5. M.Smy et al., Phys. Rev.D 69(2004) 011104 6. K.Eguchi et al., Phys. Rev. Lett. 92 (2004) 071301, T.Araki et al., Accepted by Phys. Rev. Lett. 7. Y.Itow et al., hep-ex/0106019 8. D.Beavis et al., Proposal of BNL AGS E-889 (1995) 9. M.Apollonio et al. Eur. Phys. J. C27(2003) 331