- Email: [email protected]
T2K Phase-I
Nuclear Physics B (Proc. Suppl.) 149 (2005) 154–156 www.elsevierphysics.com
T2K Phase-I T. Ishidaa , on behalf of the T2K collaboration∗ a
Institute of Particle and Nuclear Studies, KEK 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan The T2K project, the first superbeam long-baseline neutrino oscillation experiment connecting 295 km between the J-PARC 50 GeV Proton Synchrotron at Tokai and Super-Kamiokande, was approved and beam-line construction at Tokai has been started towards the commissioning scheduled in early 2009. A tunable off-axis neutrino beam will enable us to observe the νe appearance signal, and thus to explore a finite θ13 value, the last unknown mixing angle in the MNS matrix, in five years of its phase-I operation.
1. MOTIVATION After the first announcement by the SuperKamiokande (SK) collaboration that the neutrino has mass, remarkable progress has been made to understand the mass and mixing of neutrinos through neutrino oscillation. The oscillation probabilities from one neutrino flavor to another are expressed in terms of a 3×3 MNS mixing matrix and two independent squared differences of the mass eigenvalues. More than one order of magnitude difference in eV2 has been measured between the solar+reactor and the atmospheric neutrino experiments, ∆m2sol =(7.7∼8.8)×10−5 eV2 [1] and ∆m2atm =(1.9∼3.0)×10−3 eV2 [2], respectively. Since the atmospheric neutrino anomaly is known to be due to a large mixing between the second and third generations, we can safely assume ∆m212 ≡ ∆m2sol << ∆m223 ∼ ∆m213 ≡ ∆m2atm . Meanwhile, K2K, the first longbaseline neutrino oscillation experiment between KEK and Kamioka, has observed about one hundred neutrino events so far. Since the E/L∼ 1.3 GeV/250 km is similar to ∆m2atm , the oscillation probabilities for relevant oscillation modes can be approximated by two-flavor oscillation: Pµ→τ
∼ cos4 θ13 · sin2 2θ23 · sin2 φatm ,
Pµ→e
∼ sin2 θ23 · sin2 2θ13 · sin2 φatm , (φatm ≡ ∆m2atm · L/4Eν ),
∗ The
T2K collaboration was formed in May 2003, with ∼150 physicists from 53 institutions of 12 countries.
0920-5632/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2005.05.037
Figure 1. Cross section of the T2K layout.
by dropping the relatively small ∆m212 contributions. The νµ disappearance mode analysis has given (1.9 ∼ 3.6)×10−3 eV2 at sin2 2θ23 =1[3]. It is almost of similar precision to the constraint obtained by atmospheric neutrinos so far, although it is limited by the statistics. The νe appearance mode analysis on half of the data gives the constraint on sin2 2θ13 to be less than 0.3∼0.4 at ∆m2atm [4]. It is also close to the upper limit, 0.12, obtained by the CHOOZ reactor experiment[5]. These results show a potential that a similar long-baseline oscillation experiment with an ∼100-times stronger beam will work out as a very sensitive probe for precision measurements on these parameters. Accordingly, the T2K is proposed[7] as the first super-beam long base-line neutrino oscillation experiment. The goals for its first five years of phase-I operation are to explore the νµ disappearance mode with unprecedented precision, and to discover the signal of the νe appearance. If the latter is observed, it is quite possible that CP violation could be observable, by comparing the νµ -to-νe and ν µ -to-ν e modes.
T. Ishida / Nuclear Physics B (Proc. Suppl.) 149 (2005) 154–156
Figure 2. Expected number of neutrino interactions at SK as a function of the true neutrino energy, for off-axis angles of 2◦ , 2.5◦ , and 3◦ , overlayed with that for WBB.
This is the main purpose in the phase-II, which would occur after apparatus upgrades[6]. 2. EXPERIMENTAL SETUP T2K will employ the J-PARC 50 GeV proton synchrotron(PS), which has been under construction at Tokai since April, 2001, to produce neutrino super-beam, and Super-Kamiokande, a 50kt water Cherenkov neutrino detector with a fiducial mass of 22.5 kt at Kamioka, as a far neutrino detector. The baseline length is 295 km and spill-by-spill synchronisation by GPS can reduce the background such as atmospheric neutrino events. The 50 GeV PS is designed[8] to deliver 3.3 × 1014 protons every 3.64 seconds with a primary beam power of 750 kW. A beam spill will have 8 bunches in 5 µs duration. With 130 days of operation per year, it will accumulate 1021 protons-on-target, which is an order of magnitude bigger than that of the K2K design value for its 5 years of operation, 1020 p.o.t.. The scheme to produce neutrinos is basically the same as in K2K. A horn-focused wide band beam (WBB) has been conventionally used in neutrino-beam experiments so far. In this configuration, the far neutrino detector is placed on the axis of the neutrino beam-line optics so as to collect as many neutrino events as possible. Meanwhile, since the momentum and angular acceptance of the horn system is large, the produced neutrino spectrum becomes wide. In the high-energy re-
155
gion (Eν >1GeV), inelastic interactions of neutrinos can be critical backgrounds for our precision measurements on the oscillation parameters, especially for the νe appearance search. To avoid this, we decided to adopt the Off-Axis Beam (OAB) configuration[9], which is an attractive option to produce a narrower and lower neutrino energy spectrum than that of WBB. The axis of the beam optics is displaced by a few degrees from the far detector direction. With a finite decay angle, the neutrino energy becomes almost independent of the parent pion momentum because of the Lorentz boost; we can thus obtain a quasi-monochromatic neutrino spectrum. The peak of the energy spectrum can be adjusted by choosing the off-axis angle. We have decided to construct the beamline apparatus, decay volume and beam dump, towards underground, with the off-axis angle being adjustable over the range of 2◦ ∼ 3◦ . Fig. 2 shows the expected νµ spectrum at the far detector with typical OAB angles in the range, overlayed with that for the WBB. The average Eν value can range from 0.78 to 0.52 GeV, which is around the expected oscillation maximum. νe contamination, which intrinsically exists in the beam, is expected to be only ∼0.2%. It is about one order lower than that of K2K, since the shorter decay volume length reduces the number of electron neutrinos due to muon decay, and the OAB places the high energy electron neutrinos from kaon decays well above the narrow peak energy of muon neutrinos from pion decays. With the 2◦ OAB configuration, about 4,500 νµ interactions (3,000 out of them are from charged current interactions) are expected at SK for 22.5 kt · yr exposure, in the case of no oscillation. A near detector will be located 280 m downstream of the target, which is a kind of detector with fine-granularity with sufficiently fast event handling. In addition to the detector at Tokai, we also have a plan to construct an intermediate neutrino detector at ∼2 km from the target, where the neutrino spectrum is almost the same as that of SK and the water-Cherenkov technique can work with an adequate event rate. A budget request for the beam-line construction was approved in last December, and construction is now being started at Tokai towards completion in 2008. It
156
T. Ishida / Nuclear Physics B (Proc. Suppl.) 149 (2005) 154–156
Figure 3. Physics outputs expected from T2K Phase-I, 5×1021 p.o.t.. (a) The 90% confidence level sensitivities to sin2 2θ23 and ∆m223 as a function of the true ∆m223 . (b) 90% confidence level sensitivity on (∆m213 , sin2 2θ13 ).
also covers the cost for the 280 m detector. Meanwhile, the budget for the 2 km detector has yet to be approved. The details of the beamline apparatus and R&D were presented elsewhere in this workshop [10]. 3. EXPECTED SENSITIVITIES The νµ disappearance signal is observed as a distortion of the neutrino energy spectrum at the far detector. Here, the neutrino energy is reconstructed from the measured momentum and angle of the outgoing muon by assuming a two-body CC quasi-elastic (CCQE) interaction, νµ + N → µ + P . The precision of the measurements for the oscillation parameters around the relevant mass
region is estimated to be δ(sin2 2θ23 ) ∼0.01, and δ(∆m223 ) ≤ 1 × 10−4 eV2 , as shown in Fig. 3(a) as a function of the true ∆m223 value. Here, systematic errors, such as far-near ratio, energy scale etc are assumed to be at a similar level to those for K2K. On the other hand, νe appearance events can be identified as e-like single ring events at SK by using µ/e identification based on the Cherenkov ring image pattern, being more than 99% accurate. The backgrounds are expected to come from physical processes, i.e. from intrinsic νe contamination in the beam, and also from a miss-identification of the 2-gamma decay of π 0 ’s, which are produced mainly via neutral current (NC) single-pion production. The event-selection criteria are (1) the vertex is in the fiducial region, (2) a single e-like ring, (3) no µ-e decay electron observed, (4) visible energy greater than 100 MeV to reject NC elastic events, (5) a tight e/π 0 separation scheme with various kinematic constraints on the observed EM shower(s) which is under development, and (6) reconstructed Eν between 0.4 and 1.2 GeV. In the case of sin2 2θ13 =0.1, i.e., just under the CHOOZ 90% C.L. limit, the expected number of νe appearance events for 5 years of operation with the OAB 2◦ option is 123.2. On the other hand, the number of expected background events is only 22.2 in total, where the intrinsic νe contamination and π 0 s make equal contributions. Fig. 3(b) shows the 90% sensitivity contour on (∆m213 ,sin2 2θ13 ) by assuming a background subtraction error of 10%. In the relevant ∆m2 region, we have a sensitivity of sin2 2θ13 in the range greater than 0.006, which is a factor of 20 improvement past the CHOOZ upper limit. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
T. Mitsui, these proceedings. T. Kajita, these proceedings. K. Kaneyuki, these proceedings. M. Yoshida et al. PRL. 93(2004) 051801. M. Apollonio et al., PL. B466 (1999) 415. M. Shiozawa, these proceedings. Y. Itow et al., hep-ex/0106019. M. Furusaka et al., KEK Report 99-4. D. Beavis et al., BNL AGS E-889(1995). T. Nakadaira, these proceedings.
T2K Phase-I T. Ishidaa , on behalf of the T2K collaboration∗ a
Institute of Particle and Nuclear Studies, KEK 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan The T2K project, the first superbeam long-baseline neutrino oscillation experiment connecting 295 km between the J-PARC 50 GeV Proton Synchrotron at Tokai and Super-Kamiokande, was approved and beam-line construction at Tokai has been started towards the commissioning scheduled in early 2009. A tunable off-axis neutrino beam will enable us to observe the νe appearance signal, and thus to explore a finite θ13 value, the last unknown mixing angle in the MNS matrix, in five years of its phase-I operation.
1. MOTIVATION After the first announcement by the SuperKamiokande (SK) collaboration that the neutrino has mass, remarkable progress has been made to understand the mass and mixing of neutrinos through neutrino oscillation. The oscillation probabilities from one neutrino flavor to another are expressed in terms of a 3×3 MNS mixing matrix and two independent squared differences of the mass eigenvalues. More than one order of magnitude difference in eV2 has been measured between the solar+reactor and the atmospheric neutrino experiments, ∆m2sol =(7.7∼8.8)×10−5 eV2 [1] and ∆m2atm =(1.9∼3.0)×10−3 eV2 [2], respectively. Since the atmospheric neutrino anomaly is known to be due to a large mixing between the second and third generations, we can safely assume ∆m212 ≡ ∆m2sol << ∆m223 ∼ ∆m213 ≡ ∆m2atm . Meanwhile, K2K, the first longbaseline neutrino oscillation experiment between KEK and Kamioka, has observed about one hundred neutrino events so far. Since the E/L∼ 1.3 GeV/250 km is similar to ∆m2atm , the oscillation probabilities for relevant oscillation modes can be approximated by two-flavor oscillation: Pµ→τ
∼ cos4 θ13 · sin2 2θ23 · sin2 φatm ,
Pµ→e
∼ sin2 θ23 · sin2 2θ13 · sin2 φatm , (φatm ≡ ∆m2atm · L/4Eν ),
∗ The
T2K collaboration was formed in May 2003, with ∼150 physicists from 53 institutions of 12 countries.
0920-5632/$ – see front matter © 2005 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2005.05.037
Figure 1. Cross section of the T2K layout.
by dropping the relatively small ∆m212 contributions. The νµ disappearance mode analysis has given (1.9 ∼ 3.6)×10−3 eV2 at sin2 2θ23 =1[3]. It is almost of similar precision to the constraint obtained by atmospheric neutrinos so far, although it is limited by the statistics. The νe appearance mode analysis on half of the data gives the constraint on sin2 2θ13 to be less than 0.3∼0.4 at ∆m2atm [4]. It is also close to the upper limit, 0.12, obtained by the CHOOZ reactor experiment[5]. These results show a potential that a similar long-baseline oscillation experiment with an ∼100-times stronger beam will work out as a very sensitive probe for precision measurements on these parameters. Accordingly, the T2K is proposed[7] as the first super-beam long base-line neutrino oscillation experiment. The goals for its first five years of phase-I operation are to explore the νµ disappearance mode with unprecedented precision, and to discover the signal of the νe appearance. If the latter is observed, it is quite possible that CP violation could be observable, by comparing the νµ -to-νe and ν µ -to-ν e modes.
T. Ishida / Nuclear Physics B (Proc. Suppl.) 149 (2005) 154–156
Figure 2. Expected number of neutrino interactions at SK as a function of the true neutrino energy, for off-axis angles of 2◦ , 2.5◦ , and 3◦ , overlayed with that for WBB.
This is the main purpose in the phase-II, which would occur after apparatus upgrades[6]. 2. EXPERIMENTAL SETUP T2K will employ the J-PARC 50 GeV proton synchrotron(PS), which has been under construction at Tokai since April, 2001, to produce neutrino super-beam, and Super-Kamiokande, a 50kt water Cherenkov neutrino detector with a fiducial mass of 22.5 kt at Kamioka, as a far neutrino detector. The baseline length is 295 km and spill-by-spill synchronisation by GPS can reduce the background such as atmospheric neutrino events. The 50 GeV PS is designed[8] to deliver 3.3 × 1014 protons every 3.64 seconds with a primary beam power of 750 kW. A beam spill will have 8 bunches in 5 µs duration. With 130 days of operation per year, it will accumulate 1021 protons-on-target, which is an order of magnitude bigger than that of the K2K design value for its 5 years of operation, 1020 p.o.t.. The scheme to produce neutrinos is basically the same as in K2K. A horn-focused wide band beam (WBB) has been conventionally used in neutrino-beam experiments so far. In this configuration, the far neutrino detector is placed on the axis of the neutrino beam-line optics so as to collect as many neutrino events as possible. Meanwhile, since the momentum and angular acceptance of the horn system is large, the produced neutrino spectrum becomes wide. In the high-energy re-
155
gion (Eν >1GeV), inelastic interactions of neutrinos can be critical backgrounds for our precision measurements on the oscillation parameters, especially for the νe appearance search. To avoid this, we decided to adopt the Off-Axis Beam (OAB) configuration[9], which is an attractive option to produce a narrower and lower neutrino energy spectrum than that of WBB. The axis of the beam optics is displaced by a few degrees from the far detector direction. With a finite decay angle, the neutrino energy becomes almost independent of the parent pion momentum because of the Lorentz boost; we can thus obtain a quasi-monochromatic neutrino spectrum. The peak of the energy spectrum can be adjusted by choosing the off-axis angle. We have decided to construct the beamline apparatus, decay volume and beam dump, towards underground, with the off-axis angle being adjustable over the range of 2◦ ∼ 3◦ . Fig. 2 shows the expected νµ spectrum at the far detector with typical OAB angles in the range, overlayed with that for the WBB. The average Eν value can range from 0.78 to 0.52 GeV, which is around the expected oscillation maximum. νe contamination, which intrinsically exists in the beam, is expected to be only ∼0.2%. It is about one order lower than that of K2K, since the shorter decay volume length reduces the number of electron neutrinos due to muon decay, and the OAB places the high energy electron neutrinos from kaon decays well above the narrow peak energy of muon neutrinos from pion decays. With the 2◦ OAB configuration, about 4,500 νµ interactions (3,000 out of them are from charged current interactions) are expected at SK for 22.5 kt · yr exposure, in the case of no oscillation. A near detector will be located 280 m downstream of the target, which is a kind of detector with fine-granularity with sufficiently fast event handling. In addition to the detector at Tokai, we also have a plan to construct an intermediate neutrino detector at ∼2 km from the target, where the neutrino spectrum is almost the same as that of SK and the water-Cherenkov technique can work with an adequate event rate. A budget request for the beam-line construction was approved in last December, and construction is now being started at Tokai towards completion in 2008. It
156
T. Ishida / Nuclear Physics B (Proc. Suppl.) 149 (2005) 154–156
Figure 3. Physics outputs expected from T2K Phase-I, 5×1021 p.o.t.. (a) The 90% confidence level sensitivities to sin2 2θ23 and ∆m223 as a function of the true ∆m223 . (b) 90% confidence level sensitivity on (∆m213 , sin2 2θ13 ).
also covers the cost for the 280 m detector. Meanwhile, the budget for the 2 km detector has yet to be approved. The details of the beamline apparatus and R&D were presented elsewhere in this workshop [10]. 3. EXPECTED SENSITIVITIES The νµ disappearance signal is observed as a distortion of the neutrino energy spectrum at the far detector. Here, the neutrino energy is reconstructed from the measured momentum and angle of the outgoing muon by assuming a two-body CC quasi-elastic (CCQE) interaction, νµ + N → µ + P . The precision of the measurements for the oscillation parameters around the relevant mass
region is estimated to be δ(sin2 2θ23 ) ∼0.01, and δ(∆m223 ) ≤ 1 × 10−4 eV2 , as shown in Fig. 3(a) as a function of the true ∆m223 value. Here, systematic errors, such as far-near ratio, energy scale etc are assumed to be at a similar level to those for K2K. On the other hand, νe appearance events can be identified as e-like single ring events at SK by using µ/e identification based on the Cherenkov ring image pattern, being more than 99% accurate. The backgrounds are expected to come from physical processes, i.e. from intrinsic νe contamination in the beam, and also from a miss-identification of the 2-gamma decay of π 0 ’s, which are produced mainly via neutral current (NC) single-pion production. The event-selection criteria are (1) the vertex is in the fiducial region, (2) a single e-like ring, (3) no µ-e decay electron observed, (4) visible energy greater than 100 MeV to reject NC elastic events, (5) a tight e/π 0 separation scheme with various kinematic constraints on the observed EM shower(s) which is under development, and (6) reconstructed Eν between 0.4 and 1.2 GeV. In the case of sin2 2θ13 =0.1, i.e., just under the CHOOZ 90% C.L. limit, the expected number of νe appearance events for 5 years of operation with the OAB 2◦ option is 123.2. On the other hand, the number of expected background events is only 22.2 in total, where the intrinsic νe contamination and π 0 s make equal contributions. Fig. 3(b) shows the 90% sensitivity contour on (∆m213 ,sin2 2θ13 ) by assuming a background subtraction error of 10%. In the relevant ∆m2 region, we have a sensitivity of sin2 2θ13 in the range greater than 0.006, which is a factor of 20 improvement past the CHOOZ upper limit. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
T. Mitsui, these proceedings. T. Kajita, these proceedings. K. Kaneyuki, these proceedings. M. Yoshida et al. PRL. 93(2004) 051801. M. Apollonio et al., PL. B466 (1999) 415. M. Shiozawa, these proceedings. Y. Itow et al., hep-ex/0106019. M. Furusaka et al., KEK Report 99-4. D. Beavis et al., BNL AGS E-889(1995). T. Nakadaira, these proceedings.