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Study of neutrino background to proton decay search using K2K 1kt detector data
Nuclear Physics B (Proc. Suppl.) 138 (2005) 199–202 www.elsevierphysics.com
Study of neutrino background to proton decay search using K2K 1kt detector data Shun’ichi Minea a Department of Physics and Astronomy, University of California, Irvine, Irvine, CA, 92697-4575, USA Backgrounds for proton decay searches at Super-Kamiokande are summarized. A study of the background using the K2K 1kt detector data is described.
1. PROTON DECAY BACKGROUNDS AT SK Most dominant proton decay (PDK) modes predicted by non-SUSY (supersymmetry) and SUSY models are p → e+ π 0 and p → ν¯K + , respectively. The experimental best limits on these modes are obtained by Super-Kamiokande (SK). See [1] for more details. The sensitivity of the partial lifetime lower limit grows linearly with the exposure for a background-free experiment. However, a smaller uncertainty on the background is indispensable in order to achieve better sensitivity for a non-zero background experiment. To check the details of the background for p → e+ π 0 search, a sample of 1000 years (∼22.5 Mtyr) atmospheric neutrino MC is generated in the SK 22.5 kt fiducial volume. The following pre-selections are applied: (1) only νe interactions, (2) no quasi-elastic (QE) and neutral current (NC) modes and (3) one π 0 after the nuclear effect simulation. Only the true MC vector information without detector simulation is studied at this time. Figure 1 shows the total invariant mass and total momentum for the 1000 years MC. There are 46 events (∼2.0 events/Mtyr) in the PDK signal box. The breakdown of the neutrino interaction mode of the background is: 26 via one pion production from resonance, 2 via η production, and 18 via deep inelastic scattering (DIS). The typical interactions of the one pion production are νe n → e− pπ 0 and νe p → e− pπ + , where π + is
0920-5632/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2004.11.047
Figure 1. Total invariant mass and total momentum distribution for 1000 years atmospheric νe MC at SK.
changed to π 0 in the oxygen nucleus (charge exchange). The typical interactions of the DIS are νe n → e− nπ + π 0 , where π + momentum is below Cherenkov threshold, and so on. The dominant energy region (1-3 GeV) of these backgrounds overlaps precisely with the region of the K2K beam. Therefore, the K2K data can be used to check the validity of the neutrino MC used to estimate the background of p → e+ π 0 mode at SK. The NC atmospheric neutrino interaction ν 16O → νΛ0K +15 N γ (when Λ0 → N π is in-
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S. Mine / Nuclear Physics B (Proc. Suppl.) 138 (2005) 199–202
2. K2K 1kt DETECTOR DATA The results of the study using the K2K 1kt detector (1KT) data is briefly shown. See [3] for more details. Although the atmospheric neutrino flux contains all neutrino flavors, the atmospheric neutrino background for p → e+ π 0 mode can be checked by studying νµ N → µπ 0 X interactions (µπ 0 events) produced in the K2K νµ beam, because the dominant uncertainty comes from the uncertainty of the neutrino cross-section in the hadron sector and secondary pion interactions in the oxygen nucleus. The µπ 0 events are selected with essentially the same cuts used in the SK p → e+ π 0 analysis:
1KT preliminary number of events
visible) produces the same signal as the PDK signal 16 O → νK +15 N γ [1]. Again, the dominant energy region of the background matches with the K2K region well according to the kaon MC [2]. The expected background for the prompt γ method is about 1 event/Mtyr.
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Figure 2. Event selection and event rate for 1kt data (full) and MC (blank). The MC event rate is normalized to the data for 50t all neutrino events (the first bin). Enlarged signal box “LSB” (total momentum<450 MeV/c and 600
• 50t fiducial • two or three rings • one muon-like, the other(s) e-like • 85
overall shape of the data is well reproduced by that of the MC. The detection efficiency for such µπ 0 events in the PDK signal box is estimated to be 34% with p → µπ 0 MC at 1kt. The nuclear effect is the dominant contribution to the detection inefficiency. The systematic errors on the number of µπ 0 events at 1kt are roughly estimated in order to check the order of the number of background events in the current SK analysis. The ringcounting error is estimated to be 29% by comparing the manual ring edge finding program to the standard automatic one. The PID error is estimated to be about 21% by comparing the events with or without the PID cuts. The other systematic errors arising from uncertainties in the neutrino energy spectrum, neutrino interaction model, etc. are estimated to be much less than those of the ring-counting and PID. The cancellation of the systematic error between SK and 1kt has not been taken into account yet. Although
S. Mine / Nuclear Physics B (Proc. Suppl.) 138 (2005) 199–202
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Figure 3. Total invariant mass and total momentum of µπ 0 events for 1kt data (top) and MC (bottom).
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background events for the p → e+ π 0 mode at SK can be roughly predicted as Σi Σj [Nevis (j) × P (j, i)×W (i)] ×SK /1KT = 0.12 ±0.05 (stat .)± 0.05 (syst .) events/1489 days(Preliminary), where Nevis (j) is the number of µπ 0 events at 1kt in each jth bin of the visible energy distribution, P (j, i) is the probability for each jth bin of the visible energy as a function of the true neutrino energy bin (i), W (i) is the weight distribution for each ith true energy bin, and SK (1kt ) is the detection efficiency for p → eπ 0 (p → µπ 0 ) events at SK (1kt). We conclude that the background is negligibly small in the current SK analysis. The background estimated by the atmospheric neutrino MC at SK is 0.17 ± 0.10 (stat.) events / 1489 days. The two methods are consistent with each other. According to the K2K νµ MC at 1kt, about 60% of K + from the CC kaon production are visible. K + could be directly identified by measuring both momentum and opening angle of the Cherenkov cone. A few hundreds of such interactions with visible K + are expected for 1020 pot. Another idea is the indirect K + search. Since the NC/CC ratio of the kaon production interaction at 1kt is about 1/5, the CC kaon production νµ n → µ− Λ0 K + , K + → µ+ ν may be the most clear and detectable signal at 1kt by requiring two muons with a few tens of nanoseconds time difference and the second muon momentum ∼236 MeV/c. Several tens of such interactions are expected for 1020 pot. 3. Summary
these systematic error estimations are still very preliminary, they are comparable with the statistical error at this moment. By dividing the atmospheric νe energy distribution at SK by the K2K νµ one at 1kt, the weight distribution used to extrapolate the number of µπ 0 events at 1kt to the number of eπ 0 events at SK is obtained. The observed visible energy is used to calculate the true neutrino energy at 1kt by using a matrix obtained by simulations. Finally, the number of atmospheric neutrino
It is crucial to understand the neutrino background for future PDK searches. Our ability to model the atmospheric neutrino background for p → e+ π 0 at SK is well supported by the K2K 1kt data. Atmospheric neutrino background for current p → e+ π 0 at SK is negligible. The validity of the kaon production model could be checked by the 1kt data. REFERENCES 1. C. Sterner, ‘Experimental Review of Proton Decay’, in this proceedings.
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S. Mine / Nuclear Physics B (Proc. Suppl.) 138 (2005) 199–202
2. Private communication with K. Kobayashi 3. S. Mine, Nucl. Phys. Proc. Suppl. 112 (2002) 154-158. 4. Y. Hayato, Nucl. Phys. Proc. Suppl. 112 (2002) 171-176.
Study of neutrino background to proton decay search using K2K 1kt detector data Shun’ichi Minea a Department of Physics and Astronomy, University of California, Irvine, Irvine, CA, 92697-4575, USA Backgrounds for proton decay searches at Super-Kamiokande are summarized. A study of the background using the K2K 1kt detector data is described.
1. PROTON DECAY BACKGROUNDS AT SK Most dominant proton decay (PDK) modes predicted by non-SUSY (supersymmetry) and SUSY models are p → e+ π 0 and p → ν¯K + , respectively. The experimental best limits on these modes are obtained by Super-Kamiokande (SK). See [1] for more details. The sensitivity of the partial lifetime lower limit grows linearly with the exposure for a background-free experiment. However, a smaller uncertainty on the background is indispensable in order to achieve better sensitivity for a non-zero background experiment. To check the details of the background for p → e+ π 0 search, a sample of 1000 years (∼22.5 Mtyr) atmospheric neutrino MC is generated in the SK 22.5 kt fiducial volume. The following pre-selections are applied: (1) only νe interactions, (2) no quasi-elastic (QE) and neutral current (NC) modes and (3) one π 0 after the nuclear effect simulation. Only the true MC vector information without detector simulation is studied at this time. Figure 1 shows the total invariant mass and total momentum for the 1000 years MC. There are 46 events (∼2.0 events/Mtyr) in the PDK signal box. The breakdown of the neutrino interaction mode of the background is: 26 via one pion production from resonance, 2 via η production, and 18 via deep inelastic scattering (DIS). The typical interactions of the one pion production are νe n → e− pπ 0 and νe p → e− pπ + , where π + is
0920-5632/$ – see front matter © 2004 Published by Elsevier B.V. doi:10.1016/j.nuclphysbps.2004.11.047
Figure 1. Total invariant mass and total momentum distribution for 1000 years atmospheric νe MC at SK.
changed to π 0 in the oxygen nucleus (charge exchange). The typical interactions of the DIS are νe n → e− nπ + π 0 , where π + momentum is below Cherenkov threshold, and so on. The dominant energy region (1-3 GeV) of these backgrounds overlaps precisely with the region of the K2K beam. Therefore, the K2K data can be used to check the validity of the neutrino MC used to estimate the background of p → e+ π 0 mode at SK. The NC atmospheric neutrino interaction ν 16O → νΛ0K +15 N γ (when Λ0 → N π is in-
200
S. Mine / Nuclear Physics B (Proc. Suppl.) 138 (2005) 199–202
2. K2K 1kt DETECTOR DATA The results of the study using the K2K 1kt detector (1KT) data is briefly shown. See [3] for more details. Although the atmospheric neutrino flux contains all neutrino flavors, the atmospheric neutrino background for p → e+ π 0 mode can be checked by studying νµ N → µπ 0 X interactions (µπ 0 events) produced in the K2K νµ beam, because the dominant uncertainty comes from the uncertainty of the neutrino cross-section in the hadron sector and secondary pion interactions in the oxygen nucleus. The µπ 0 events are selected with essentially the same cuts used in the SK p → e+ π 0 analysis:
1KT preliminary number of events
visible) produces the same signal as the PDK signal 16 O → νK +15 N γ [1]. Again, the dominant energy region of the background matches with the K2K region well according to the kaon MC [2]. The expected background for the prompt γ method is about 1 event/Mtyr.
10
5
10
4
10
3
10
2
10
1
A
B
C
D
E selection criteria
Figure 2. Event selection and event rate for 1kt data (full) and MC (blank). The MC event rate is normalized to the data for 50t all neutrino events (the first bin). Enlarged signal box “LSB” (total momentum<450 MeV/c and 600
• 50t fiducial • two or three rings • one muon-like, the other(s) e-like • 85
overall shape of the data is well reproduced by that of the MC. The detection efficiency for such µπ 0 events in the PDK signal box is estimated to be 34% with p → µπ 0 MC at 1kt. The nuclear effect is the dominant contribution to the detection inefficiency. The systematic errors on the number of µπ 0 events at 1kt are roughly estimated in order to check the order of the number of background events in the current SK analysis. The ringcounting error is estimated to be 29% by comparing the manual ring edge finding program to the standard automatic one. The PID error is estimated to be about 21% by comparing the events with or without the PID cuts. The other systematic errors arising from uncertainties in the neutrino energy spectrum, neutrino interaction model, etc. are estimated to be much less than those of the ring-counting and PID. The cancellation of the systematic error between SK and 1kt has not been taken into account yet. Although
S. Mine / Nuclear Physics B (Proc. Suppl.) 138 (2005) 199–202
1KT preliminary total momentum (MeV/c)
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total invariant mass (MeV/c2) total momentum (MeV/c)
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Figure 3. Total invariant mass and total momentum of µπ 0 events for 1kt data (top) and MC (bottom).
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background events for the p → e+ π 0 mode at SK can be roughly predicted as Σi Σj [Nevis (j) × P (j, i)×W (i)] ×SK /1KT = 0.12 ±0.05 (stat .)± 0.05 (syst .) events/1489 days(Preliminary), where Nevis (j) is the number of µπ 0 events at 1kt in each jth bin of the visible energy distribution, P (j, i) is the probability for each jth bin of the visible energy as a function of the true neutrino energy bin (i), W (i) is the weight distribution for each ith true energy bin, and SK (1kt ) is the detection efficiency for p → eπ 0 (p → µπ 0 ) events at SK (1kt). We conclude that the background is negligibly small in the current SK analysis. The background estimated by the atmospheric neutrino MC at SK is 0.17 ± 0.10 (stat.) events / 1489 days. The two methods are consistent with each other. According to the K2K νµ MC at 1kt, about 60% of K + from the CC kaon production are visible. K + could be directly identified by measuring both momentum and opening angle of the Cherenkov cone. A few hundreds of such interactions with visible K + are expected for 1020 pot. Another idea is the indirect K + search. Since the NC/CC ratio of the kaon production interaction at 1kt is about 1/5, the CC kaon production νµ n → µ− Λ0 K + , K + → µ+ ν may be the most clear and detectable signal at 1kt by requiring two muons with a few tens of nanoseconds time difference and the second muon momentum ∼236 MeV/c. Several tens of such interactions are expected for 1020 pot. 3. Summary
these systematic error estimations are still very preliminary, they are comparable with the statistical error at this moment. By dividing the atmospheric νe energy distribution at SK by the K2K νµ one at 1kt, the weight distribution used to extrapolate the number of µπ 0 events at 1kt to the number of eπ 0 events at SK is obtained. The observed visible energy is used to calculate the true neutrino energy at 1kt by using a matrix obtained by simulations. Finally, the number of atmospheric neutrino
It is crucial to understand the neutrino background for future PDK searches. Our ability to model the atmospheric neutrino background for p → e+ π 0 at SK is well supported by the K2K 1kt data. Atmospheric neutrino background for current p → e+ π 0 at SK is negligible. The validity of the kaon production model could be checked by the 1kt data. REFERENCES 1. C. Sterner, ‘Experimental Review of Proton Decay’, in this proceedings.
202
S. Mine / Nuclear Physics B (Proc. Suppl.) 138 (2005) 199–202
2. Private communication with K. Kobayashi 3. S. Mine, Nucl. Phys. Proc. Suppl. 112 (2002) 154-158. 4. Y. Hayato, Nucl. Phys. Proc. Suppl. 112 (2002) 171-176.