- Email: [email protected]
Backgrounds for νe appearance at T2K using a water Cherenkov detector 2km away from the source
Nuclear Physics B (Proc. Suppl.) 155 (2006) 209–210 www.elsevierphysics.com
Backgrounds for νe appearance at T2K using a water Cherenkov detector 2km away from the source Maximilien Fechnera∗ a
CEA Saclay DAPNIA, SPP, 91191 Gif/Yvette CEDEX, France
The 2KM detector complex for T2K is presented. The 2KM Water Cherenkov detector has been fully simulated. A complete Monte-Carlo νe appearance analysis was performed independently at Super-K and this detector. A simple scaling method is enough to predict the background at Super-Kamiokande from this detector to better than the 10% goal.
1. 2KM detector for T2K The T2K experiment has been described in [1]. A 2.5◦ off-axis νµ beam will be produced at Tokai (Japan) and detected at Super-Kamiokande 300 km away. In such an experiment near detectors are necessary to measure the beam properties before the oscillation takes place. A set of detectors located 280 m from the neutrino source will be built. However as can be seen in fig. 1 the νµ spectrum at this location is significantly different from the spectrum measured at SuperKamiokande, mostly because the ν source (120 m long decay tunnel) is not point-like. This makes the extrapolation of a measurement at 280 m to Super-Kamiokande very dependant on beam Monte-Carlo predictions, thereby introducing systematics because of the difficulty of calculating hadron production at the target level. Besides, Super-Kamiokande is a water Cherenkov detector, and having a similar detector at a near position would help reduce systematics. A detector complex located 2 km away from the source is distant enough so that the neutrino energy spectra at 2 km and Superk-Kamiokande are similar (fig. 1) : the ratio of the near and far fluxes is flat to about ∼ 4%. At this distance the event rate is low enough to place a water Cherenkov device. The proposed 2KM complex is composed of a liquid argon TPC, a water Cherenkov detector and a muon ranger. It will be placed at the same ∗ email
address:[email protected]
0920-5632/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2006.02.051
0.1
0.08
0.06
0.04
0.02
0 0
1
2
3
4
5 Eν (GeV)
Figure 1. νµ fluxes at Super-Kamiokande (black), 2 km (red) and 280 m (blue), for a 2.5◦ off axis beam.
off-axis angle as Super-Kamiokande. The goal of the water Cherenkov detector is to minimize systematics in the prediction by using the same algorithms and techniques as Super-Kamiokande to measure neutrino interactions on water. Since there is a momentum threshold for the production of Cherenkov photons such a device cannot detect the proton produced in ν charged-current quasi-elastic (CCQE) interactions. The liquid Argon TPC is extremely fine-grained and will provide excellent separation between CCQE and non-QE interactions, as well as exclusive crosssection measurements (see [2]). The muon ranger will catch the high energy end of the spectrum, allowing studies of the ν coming from kaon decays.
M. Fechner / Nuclear Physics B (Proc. Suppl.) 155 (2006) 209–210
A GEANT4 simulation has been developped for the whole 2KM complex. Several different geometries can be chosen, including the K2K-1kton geometry. Water scattering lengths and other relevant parameters were tuned using beam and cosmic ray data from the K2K-1kton detector, making the simulator therefore suitable for simulating the 2KM geometry : the cylindrical volume of water is 13.1 m long with a 9.1 m diameter, with a 100t fiducial volume. The K2K-1kton reconstruction software was adapted to this geometry. The philosophy in the design of this device is to match the response of Super-Kamiokande even if it means using a slightly different hardware configuration. Studies show that 5660 8inch PMTs (rather than fewer 20 inch PMTs) match SK performance for ring-counting, particle ID, fiducial volume determination and electron/π 0 separation. 3. νe appearance analysis and extrapolation at Super-Kamiokande For νe appearance there are three main sources of background : NC π 0 production (most serious background), intrinsic beam νe contamination, and finally mis-identified charged-current νµ events. The analysis cuts aim at selecting CCQE νe events. Additional π 0 rejection cuts are used to remove π 0 which appear to be one ring using a dedicated fitter. T2K beam events are simulated at SK using the SK spectra and software, and independently at 2KM using the 2KM tools. The same analysis is performed, and the 2KM results are extrapolated to SK, simply scaling every bin with the ratio of fiducial masses and spherical ν flux attenuation : 2 MSK i i SK = N2km × LL2km ×M . NSK 2km This formula assumes identical efficiencies at both locations, and no beam Monte-Carlo corrections2 . A preliminary, simple and conservative estimate of the systematics shows that the total error is ∼ 7%. This is within the 10% goal 2 for
mis-ID νµ background the oscillation “survival” probablility is applied
for T2K phase I. For 5 years of T2K operation, the SK simulation predicts 24.4 events, and the extrapolation from 2KM gives 25.6 ± 1.8(7.0%), including systematics (see fig. 2), demonstrating the power of such a detector at this location.
Predicted BG for νe app. at SK
Reconstructed Eν, after the π0 cuts 9
beam νe
8
νe+NC
7 6
total : νe+NC+νµ CC
5 4
evts/22.5kt/5yrs/100MeV
2. 2KM Water Cherenkov detector : simulation
events/22.5kt/5yrs/100MeV
210
9
νe+NC
7 6
total : νe+NC+νµ CC
5 4
3
3
2
2 1
1 0 0
beam νe
8
1000
2000
3000
4000 5000 rec. ν energy (MeV)
0 0
1000
2000
3000
4000 5000 reconstructed Eeν (MeV)
Figure 2. Background at SK from SK simulation (left) and extrapolated 2KM background (right). The agreement is very good within the signal window.
4. Conclusion A 2KM detector complex, including a liquid argon TPC, a water Cherenkov detector and a muon ranger is highly desirable for T2K phase I. Studies show that a water Cherenkov detector optimized to minimize systematics errors will predict the νe background at Super-Kamiokande to less than the needed 10%, even relying only on simple scaling of mass and distance. Acknowledgements: The author would like to thank J. Bouchez & F. Pierre (Saclay), C. Walter & K. Scholberg (Duke U.), E. Kearns (Boston U.), T. Kajita (ICRR) for their useful comments. REFERENCES 1. Y. Yamada, these proceedings. 2. A. Meregaglia, these proceedings.
Backgrounds for νe appearance at T2K using a water Cherenkov detector 2km away from the source Maximilien Fechnera∗ a
CEA Saclay DAPNIA, SPP, 91191 Gif/Yvette CEDEX, France
The 2KM detector complex for T2K is presented. The 2KM Water Cherenkov detector has been fully simulated. A complete Monte-Carlo νe appearance analysis was performed independently at Super-K and this detector. A simple scaling method is enough to predict the background at Super-Kamiokande from this detector to better than the 10% goal.
1. 2KM detector for T2K The T2K experiment has been described in [1]. A 2.5◦ off-axis νµ beam will be produced at Tokai (Japan) and detected at Super-Kamiokande 300 km away. In such an experiment near detectors are necessary to measure the beam properties before the oscillation takes place. A set of detectors located 280 m from the neutrino source will be built. However as can be seen in fig. 1 the νµ spectrum at this location is significantly different from the spectrum measured at SuperKamiokande, mostly because the ν source (120 m long decay tunnel) is not point-like. This makes the extrapolation of a measurement at 280 m to Super-Kamiokande very dependant on beam Monte-Carlo predictions, thereby introducing systematics because of the difficulty of calculating hadron production at the target level. Besides, Super-Kamiokande is a water Cherenkov detector, and having a similar detector at a near position would help reduce systematics. A detector complex located 2 km away from the source is distant enough so that the neutrino energy spectra at 2 km and Superk-Kamiokande are similar (fig. 1) : the ratio of the near and far fluxes is flat to about ∼ 4%. At this distance the event rate is low enough to place a water Cherenkov device. The proposed 2KM complex is composed of a liquid argon TPC, a water Cherenkov detector and a muon ranger. It will be placed at the same ∗ email
address:[email protected]
0920-5632/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2006.02.051
0.1
0.08
0.06
0.04
0.02
0 0
1
2
3
4
5 Eν (GeV)
Figure 1. νµ fluxes at Super-Kamiokande (black), 2 km (red) and 280 m (blue), for a 2.5◦ off axis beam.
off-axis angle as Super-Kamiokande. The goal of the water Cherenkov detector is to minimize systematics in the prediction by using the same algorithms and techniques as Super-Kamiokande to measure neutrino interactions on water. Since there is a momentum threshold for the production of Cherenkov photons such a device cannot detect the proton produced in ν charged-current quasi-elastic (CCQE) interactions. The liquid Argon TPC is extremely fine-grained and will provide excellent separation between CCQE and non-QE interactions, as well as exclusive crosssection measurements (see [2]). The muon ranger will catch the high energy end of the spectrum, allowing studies of the ν coming from kaon decays.
M. Fechner / Nuclear Physics B (Proc. Suppl.) 155 (2006) 209–210
A GEANT4 simulation has been developped for the whole 2KM complex. Several different geometries can be chosen, including the K2K-1kton geometry. Water scattering lengths and other relevant parameters were tuned using beam and cosmic ray data from the K2K-1kton detector, making the simulator therefore suitable for simulating the 2KM geometry : the cylindrical volume of water is 13.1 m long with a 9.1 m diameter, with a 100t fiducial volume. The K2K-1kton reconstruction software was adapted to this geometry. The philosophy in the design of this device is to match the response of Super-Kamiokande even if it means using a slightly different hardware configuration. Studies show that 5660 8inch PMTs (rather than fewer 20 inch PMTs) match SK performance for ring-counting, particle ID, fiducial volume determination and electron/π 0 separation. 3. νe appearance analysis and extrapolation at Super-Kamiokande For νe appearance there are three main sources of background : NC π 0 production (most serious background), intrinsic beam νe contamination, and finally mis-identified charged-current νµ events. The analysis cuts aim at selecting CCQE νe events. Additional π 0 rejection cuts are used to remove π 0 which appear to be one ring using a dedicated fitter. T2K beam events are simulated at SK using the SK spectra and software, and independently at 2KM using the 2KM tools. The same analysis is performed, and the 2KM results are extrapolated to SK, simply scaling every bin with the ratio of fiducial masses and spherical ν flux attenuation : 2 MSK i i SK = N2km × LL2km ×M . NSK 2km This formula assumes identical efficiencies at both locations, and no beam Monte-Carlo corrections2 . A preliminary, simple and conservative estimate of the systematics shows that the total error is ∼ 7%. This is within the 10% goal 2 for
mis-ID νµ background the oscillation “survival” probablility is applied
for T2K phase I. For 5 years of T2K operation, the SK simulation predicts 24.4 events, and the extrapolation from 2KM gives 25.6 ± 1.8(7.0%), including systematics (see fig. 2), demonstrating the power of such a detector at this location.
Predicted BG for νe app. at SK
Reconstructed Eν, after the π0 cuts 9
beam νe
8
νe+NC
7 6
total : νe+NC+νµ CC
5 4
evts/22.5kt/5yrs/100MeV
2. 2KM Water Cherenkov detector : simulation
events/22.5kt/5yrs/100MeV
210
9
νe+NC
7 6
total : νe+NC+νµ CC
5 4
3
3
2
2 1
1 0 0
beam νe
8
1000
2000
3000
4000 5000 rec. ν energy (MeV)
0 0
1000
2000
3000
4000 5000 reconstructed Eeν (MeV)
Figure 2. Background at SK from SK simulation (left) and extrapolated 2KM background (right). The agreement is very good within the signal window.
4. Conclusion A 2KM detector complex, including a liquid argon TPC, a water Cherenkov detector and a muon ranger is highly desirable for T2K phase I. Studies show that a water Cherenkov detector optimized to minimize systematics errors will predict the νe background at Super-Kamiokande to less than the needed 10%, even relying only on simple scaling of mass and distance. Acknowledgements: The author would like to thank J. Bouchez & F. Pierre (Saclay), C. Walter & K. Scholberg (Duke U.), E. Kearns (Boston U.), T. Kajita (ICRR) for their useful comments. REFERENCES 1. Y. Yamada, these proceedings. 2. A. Meregaglia, these proceedings.