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Neutral pion production measurements at SciBooNE
Nuclear Physics B (Proc. Suppl.) 217 (2011) 214–216 www.elsevier.com/locate/npbps
Neutral pion production measurements at SciBooNE J. Catala-Pereza a
IFIC (CSIC/U. Valencia)
Neutrino-induced neutral pion production is an important measurement for next generation neutrino oscillation experiments. Neutral current (N C) neutral pion production is a direct background for electron neutrino appearance experiments, while charged current (CC) neutral pion production affects experiments looking for muon neutrino disappearance. Located in the Booster Neutrino Beam at Fermilab, SciBooNE is a neutrino scattering experiment designed to accurately measure muon neutrino and anti-neutrino cross sections on carbon near 1 GeV neutrino energy. In this talk I will present recent SciBooNE results on neutral pion production, including the total cross section measurement for both channels relative to the CC inclusive cross section, the separation of the coherent and incoherent contributions to the N C channel, and details on neutral pion production kinematics.
1. SciBooNE MOTIVATIONS AND CAPABILITIES SciBooNE is intended to measure the cross section of the neutrino-nucleus interaction at neutrino energies near 1 GeV . 1.1. Detectors SciBooNE is composed by three sub-detectors: SciBar is a fine grained, fully active detector with dE/dx and timing capabilities. It can reconstruct complicated event topologies and gives good angular resolution. SciBar can also detect vertex activity. The Electron Catcher (EC) is used for the recovery of energy leaks and gamma conversions in EC. The Muon Range Detector (M RD) is used only for muon tagging in these analyses. 2. NEUTRAL CURRENT 2.1. Motivations νμ induced N Cπ 0 is one of the largest background for the νμ → νe search. N Cπ 0 backgrounds can mimic a νe quasi-elastic event if one of the gammas produced by the π 0 is undetected. For T2K, the goal is to obtain a N Cπ 0 /CCinc cross section ratio with error below 10%. 0920-5632/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2011.04.105
Theoretical models differ in the prediction of pion kinematics distribution after Final State Interactions (FSI). There is an interest in measuring π 0 kinematics as well as total cross-section. K2K [1] and SciBooNE [2] found CC coherent π production much lower the Rein & Sehgal model prediction [3,4]. On the other hand, MiniBooNE observed N C coherent π 0 production. Therefore it is of interest to check MiniBooNE results with SciBooNE. 2.2. Signal definition and event selection The signal definition used in the NC analysis [5] is at least one π 0 emitted from the interaction nucleus after FSI. An N Cπ 0 event is observed as two isolated tracks due to the conversions of the two gamma resulting of the decay of the π 0 . The background events come from events other than N Cπ 0 in SciBar, neutrino interactions in the materials outside the detector and cosmic rays. In order to reduce the background events, several event selections are performed. Cosmic rays and CC events are rejected by identifying the muon. Tracks identified as muons are those escaping from the side of Scibar, those stopping in SciBar leaving decay electrons, and those that penetrate into the EC. The external background is rejected by using
the upstream layers of SciBar as a veto and requiring the neutral pion reconstructed decay position to be inside the SciBar detector and the reconstructed invariant mass of the two gamma ray candidates to be close to the π 0 mass. 2.3. N Cπ 0 /CCinc cross-section ratio The cross section is measured as a ratio to reduce the neutrino flux systematic error. To calculate the ratio, we subtract the number of background events estimated by the MC to the total number of events in the N Cπ 0 selected sample and then we make an efficiency correction. The ratio of the neutral current π 0 production to the total charged current cross section is measured to be (7.7 ± 0.5stat. ± 0.5sys. ) × 10−2 evaluated at < Eν >= 1.14 GeV . The main source of systematic error comes from the detector response and the ν interaction nuclear model. The overall error, taking in account the statistical and the systematic source is 0.7 × 10−2 and thus the goal of 9% error is achieved. 2.4. π 0 kinematics To test the nuclear model of the NEUT neutrino interaction generator with SciBooNE data, we study π 0 kinematics after FSI after unfolding detector effects. After subtracting the estimated backgrounds the unfolding is performed using the MC simulation to define the unfolding matrix. Finally we perform the efficiency correction to obtain the true π 0 kinematics. In Figure 1 the NEUT expectation is compared with the measured true angle distribution. In order to compare the shapes of the distributions, the total number of entries in the distributions are normalized to unity. 2.5. N C coherent π 0 production A new method to extract the N C coherent π 0 production fraction [6] has been used taking advantage of the lack of recoil nucleon together with small angle with respect to the neutrino beam of the outgoing neutral pions in a coherent event. The recoil nucleon is detected in SciBar as energy deposition around the interaction vertex. In order to perform the extraction, we perform
Fraction of Events
J. Catala-Perez / Nuclear Physics B (Proc. Suppl.) 217 (2011) 214–216
215
0.4
statistic error
systematic error
0.2 MC expectation
-1 -0.5 0 0.5 1 0 Corrected π direction (Cosine)
—
Figure 1. Reconstructed pion angular distributions, normalized to unit area, with statistical and systematic uncertainties. Both data (black data points) and MC (blue dotted histogram) are shown.
a simultaneus fit over two Eπ0 (1 − cosθπ0 ) distributions, with and without vertex activity. The N Ccoh.π0 /CCinc. cross section ratio, evaluated at < Eν >= 0.8 GeV , is measured to be: σ(N Ccohπ 0 ) = (1.16 ± 0.24) × 10−2 . σ(CC)
(1)
3. CHARGED CURRENT 3.1. Motivation and challenges π 0 production is an important fraction of the CC inelastic interactions and its knowledge is important for νμ disappearance measurements. The SciBar track finding efficiency drops with the number of tracks in the event, mostly due to track overlapping. Given the typical topology of a CCπ 0 event, with at least one muon and two electromagnetic showers coming from the conversion of the neutral pion decay gammas, the statistics for this analysis are very low.
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J. Catala-Perez / Nuclear Physics B (Proc. Suppl.) 217 (2011) 214–216
3.2. Event Selection The CCπ 0 signal definition is one muon and one π 0 emmited from the interaction nucleus after FSI. Resonant and DIS channels contribute to the signal. The CCπ 0 event selection [7] is performed by using track properties and event topology information. Once a muon is identified we search for photon-like tracks with no leakages through the sides or the upstream layer of the SciBar detector. We use dE/dx information to reject protons and the disconnection of the track from the muon vertex to reject charged pions. With at least 2 gamma candidates in the event, we discard those with a small opening angle with respect each other (< 0.3 rad). Usually they are one single converted photon whose track is splitted by the reconstruction algorithm. Only events with a single π 0 candidate are accepted in the final sample. The main source of background comes from events that contain a π 0 . Either N Cπ 0 , or CC with secondary π 0 , or multiπ 0 events contribute to this background. 3.3. Event kinematics At this stage of the analysis we are able to reconstruct the muon, photons and π 0 in the event with their kinematic properties (angle and momentum). We find also a peak in the plot of the mass (Figure 2), calreconstructed π 0 invariant culated as Mγγ = Eγ1 Eγ2 (1 − cosθ), close to the expected π 0 mass. 4. CONCLUSIONS SciBooNE recently published [5,6] new results for N Cπ 0 production. Both inclusive π 0 production (dominated by resonance production) and coherent π 0 production agree well with the respective Rein and Sehgal model used to form SciBooNE expectations. The inclusive production cross section relative to the CC inclusive cross section has been measured with an uncertainty of 9%. In addition, the reconstructed π 0 kinematics have been used to test the NEUT nuclear model, finding agreement at the same level of uncertainty. In the CC analysis and despite the low statis-
Figure 2. Reconstructed π 0 invariant mass. MC (filled areas) is normalized to the total CC events.
tics, we are able to reconstruct π 0 kinematics and we aim to obtain a measurement on the CCπ 0 over CC inclusive cross section ratio as well as a value for the CCπ 0 absolute cross section. 5. ACKNOWLEDGMENTS The SciBooNE collaboration gratefully acknowledges support from various grants and contracts from the Department of Energy (U.S.), the National Science Foundation (U.S.), the MEXT (Japan), the INFN (Italy) and the Spanish Ministry of Science and Education. REFERENCES 1. M. Hasegawa et al. [K2K Collaboration], Phys. Rev. Lett. 95 (2005) 252301. 2. K. Hiraide et al. [SciBooNE Collaboration], Phys. Rev. D 78 (2008) 252301. 3. D. Rein and L. M. Sehgal, Nucl. Phys. B 223 (1983) 29. 4. D. Rein and L. M. Sehgal, Phys. Lett. B 657 (2007) 207. 5. Y. Kurimoto et al. [SciBooNE Collaboration], Phys. Rev. D 81 (2010) 033004. 6. Y. Kurimoto et al. [SciBooNE Collaboration], Phys. Rev. D 81 (2010) 111102(R). 7. J. Catala-Perez, AIP Conf. Proc. 1189 (2009) 347.
Neutral pion production measurements at SciBooNE J. Catala-Pereza a
IFIC (CSIC/U. Valencia)
Neutrino-induced neutral pion production is an important measurement for next generation neutrino oscillation experiments. Neutral current (N C) neutral pion production is a direct background for electron neutrino appearance experiments, while charged current (CC) neutral pion production affects experiments looking for muon neutrino disappearance. Located in the Booster Neutrino Beam at Fermilab, SciBooNE is a neutrino scattering experiment designed to accurately measure muon neutrino and anti-neutrino cross sections on carbon near 1 GeV neutrino energy. In this talk I will present recent SciBooNE results on neutral pion production, including the total cross section measurement for both channels relative to the CC inclusive cross section, the separation of the coherent and incoherent contributions to the N C channel, and details on neutral pion production kinematics.
1. SciBooNE MOTIVATIONS AND CAPABILITIES SciBooNE is intended to measure the cross section of the neutrino-nucleus interaction at neutrino energies near 1 GeV . 1.1. Detectors SciBooNE is composed by three sub-detectors: SciBar is a fine grained, fully active detector with dE/dx and timing capabilities. It can reconstruct complicated event topologies and gives good angular resolution. SciBar can also detect vertex activity. The Electron Catcher (EC) is used for the recovery of energy leaks and gamma conversions in EC. The Muon Range Detector (M RD) is used only for muon tagging in these analyses. 2. NEUTRAL CURRENT 2.1. Motivations νμ induced N Cπ 0 is one of the largest background for the νμ → νe search. N Cπ 0 backgrounds can mimic a νe quasi-elastic event if one of the gammas produced by the π 0 is undetected. For T2K, the goal is to obtain a N Cπ 0 /CCinc cross section ratio with error below 10%. 0920-5632/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2011.04.105
Theoretical models differ in the prediction of pion kinematics distribution after Final State Interactions (FSI). There is an interest in measuring π 0 kinematics as well as total cross-section. K2K [1] and SciBooNE [2] found CC coherent π production much lower the Rein & Sehgal model prediction [3,4]. On the other hand, MiniBooNE observed N C coherent π 0 production. Therefore it is of interest to check MiniBooNE results with SciBooNE. 2.2. Signal definition and event selection The signal definition used in the NC analysis [5] is at least one π 0 emitted from the interaction nucleus after FSI. An N Cπ 0 event is observed as two isolated tracks due to the conversions of the two gamma resulting of the decay of the π 0 . The background events come from events other than N Cπ 0 in SciBar, neutrino interactions in the materials outside the detector and cosmic rays. In order to reduce the background events, several event selections are performed. Cosmic rays and CC events are rejected by identifying the muon. Tracks identified as muons are those escaping from the side of Scibar, those stopping in SciBar leaving decay electrons, and those that penetrate into the EC. The external background is rejected by using
the upstream layers of SciBar as a veto and requiring the neutral pion reconstructed decay position to be inside the SciBar detector and the reconstructed invariant mass of the two gamma ray candidates to be close to the π 0 mass. 2.3. N Cπ 0 /CCinc cross-section ratio The cross section is measured as a ratio to reduce the neutrino flux systematic error. To calculate the ratio, we subtract the number of background events estimated by the MC to the total number of events in the N Cπ 0 selected sample and then we make an efficiency correction. The ratio of the neutral current π 0 production to the total charged current cross section is measured to be (7.7 ± 0.5stat. ± 0.5sys. ) × 10−2 evaluated at < Eν >= 1.14 GeV . The main source of systematic error comes from the detector response and the ν interaction nuclear model. The overall error, taking in account the statistical and the systematic source is 0.7 × 10−2 and thus the goal of 9% error is achieved. 2.4. π 0 kinematics To test the nuclear model of the NEUT neutrino interaction generator with SciBooNE data, we study π 0 kinematics after FSI after unfolding detector effects. After subtracting the estimated backgrounds the unfolding is performed using the MC simulation to define the unfolding matrix. Finally we perform the efficiency correction to obtain the true π 0 kinematics. In Figure 1 the NEUT expectation is compared with the measured true angle distribution. In order to compare the shapes of the distributions, the total number of entries in the distributions are normalized to unity. 2.5. N C coherent π 0 production A new method to extract the N C coherent π 0 production fraction [6] has been used taking advantage of the lack of recoil nucleon together with small angle with respect to the neutrino beam of the outgoing neutral pions in a coherent event. The recoil nucleon is detected in SciBar as energy deposition around the interaction vertex. In order to perform the extraction, we perform
Fraction of Events
J. Catala-Perez / Nuclear Physics B (Proc. Suppl.) 217 (2011) 214–216
215
0.4
statistic error
systematic error
0.2 MC expectation
-1 -0.5 0 0.5 1 0 Corrected π direction (Cosine)
—
Figure 1. Reconstructed pion angular distributions, normalized to unit area, with statistical and systematic uncertainties. Both data (black data points) and MC (blue dotted histogram) are shown.
a simultaneus fit over two Eπ0 (1 − cosθπ0 ) distributions, with and without vertex activity. The N Ccoh.π0 /CCinc. cross section ratio, evaluated at < Eν >= 0.8 GeV , is measured to be: σ(N Ccohπ 0 ) = (1.16 ± 0.24) × 10−2 . σ(CC)
(1)
3. CHARGED CURRENT 3.1. Motivation and challenges π 0 production is an important fraction of the CC inelastic interactions and its knowledge is important for νμ disappearance measurements. The SciBar track finding efficiency drops with the number of tracks in the event, mostly due to track overlapping. Given the typical topology of a CCπ 0 event, with at least one muon and two electromagnetic showers coming from the conversion of the neutral pion decay gammas, the statistics for this analysis are very low.
216
J. Catala-Perez / Nuclear Physics B (Proc. Suppl.) 217 (2011) 214–216
3.2. Event Selection The CCπ 0 signal definition is one muon and one π 0 emmited from the interaction nucleus after FSI. Resonant and DIS channels contribute to the signal. The CCπ 0 event selection [7] is performed by using track properties and event topology information. Once a muon is identified we search for photon-like tracks with no leakages through the sides or the upstream layer of the SciBar detector. We use dE/dx information to reject protons and the disconnection of the track from the muon vertex to reject charged pions. With at least 2 gamma candidates in the event, we discard those with a small opening angle with respect each other (< 0.3 rad). Usually they are one single converted photon whose track is splitted by the reconstruction algorithm. Only events with a single π 0 candidate are accepted in the final sample. The main source of background comes from events that contain a π 0 . Either N Cπ 0 , or CC with secondary π 0 , or multiπ 0 events contribute to this background. 3.3. Event kinematics At this stage of the analysis we are able to reconstruct the muon, photons and π 0 in the event with their kinematic properties (angle and momentum). We find also a peak in the plot of the mass (Figure 2), calreconstructed π 0 invariant culated as Mγγ = Eγ1 Eγ2 (1 − cosθ), close to the expected π 0 mass. 4. CONCLUSIONS SciBooNE recently published [5,6] new results for N Cπ 0 production. Both inclusive π 0 production (dominated by resonance production) and coherent π 0 production agree well with the respective Rein and Sehgal model used to form SciBooNE expectations. The inclusive production cross section relative to the CC inclusive cross section has been measured with an uncertainty of 9%. In addition, the reconstructed π 0 kinematics have been used to test the NEUT nuclear model, finding agreement at the same level of uncertainty. In the CC analysis and despite the low statis-
Figure 2. Reconstructed π 0 invariant mass. MC (filled areas) is normalized to the total CC events.
tics, we are able to reconstruct π 0 kinematics and we aim to obtain a measurement on the CCπ 0 over CC inclusive cross section ratio as well as a value for the CCπ 0 absolute cross section. 5. ACKNOWLEDGMENTS The SciBooNE collaboration gratefully acknowledges support from various grants and contracts from the Department of Energy (U.S.), the National Science Foundation (U.S.), the MEXT (Japan), the INFN (Italy) and the Spanish Ministry of Science and Education. REFERENCES 1. M. Hasegawa et al. [K2K Collaboration], Phys. Rev. Lett. 95 (2005) 252301. 2. K. Hiraide et al. [SciBooNE Collaboration], Phys. Rev. D 78 (2008) 252301. 3. D. Rein and L. M. Sehgal, Nucl. Phys. B 223 (1983) 29. 4. D. Rein and L. M. Sehgal, Phys. Lett. B 657 (2007) 207. 5. Y. Kurimoto et al. [SciBooNE Collaboration], Phys. Rev. D 81 (2010) 033004. 6. Y. Kurimoto et al. [SciBooNE Collaboration], Phys. Rev. D 81 (2010) 111102(R). 7. J. Catala-Perez, AIP Conf. Proc. 1189 (2009) 347.