Simulation Studies on LBNE Neutrino Beam Flux in Different Decay Pipe Materials, air/He

UTA-HEP-IF0003 Nov. 11, 2013

Seongtae Park, Jae Yu, Timothy Watson, Amit Bashyal

High Energy Physics Group Department of Physics

The University of Texas at Arlington

Abstract In the beam line neutrino oscillation experiment, providing high intensity neutrino beam with

appropriate energy is important. The main purpose of this study is to facilitate the final decision on the decay pipe filling material in the LBNE neutrino beam line by comparing neutrino fluxes in different decay pipe materials. The studies were done with beam simulation tool g4lbne which is being developed for LBNE neutrino beam simulation. To get neutrino flux and event rate information at the far detector and the near detector, simulations were done with two different decay pipe materials: air and He, using two different physics lists, QGSP_BERT and FTFP_BERT. The simulation results showed that He produced more 𝜈𝜇 flux (about 12% more flux at around 4GeV) and less �̅�𝜇 flux than air in neutrino mode runs. The result implies that He is preferable to air as the decay pipe fill material. 1. Introduction

So far, the Standard Model (SM) of particle physics has excellently explained the elementary particles and their interactions. However, research results from the last decade that the three known types of neutrinos have nonzero mass, mix with one another and oscillate between generations[1], imply physics beyond the Standard Model[2]. There has been remarkable progress in this decade towards understanding the oscillations of neutrinos. Enough information on these oscillations has accumulated for high energy physicists to begin comprehensive experiments of neutrino oscillations.

The Long Baseline Neutrino Experiment (LBNE) is one of the neutrino experiments utilizing Fermilab beam line as the neutrino source[3]. The optimal base line of 1300km was selected such that the oscillation asymmetry between ν and �̅� due to the non-CP-violating matter effect can be clearly separated from the effect generated from true CP violation[4]. A high power, broadband, high purity, sign-selected neutrino beam with a spectrum that can cover full oscillation patterns at the optimal baseline is required for a successful experiment[5]. In order to experimentally prove the neutrino oscillation behavior as they travel, we need information of precise neutrino fluxes at both the source and detector, which are separated by the baseline distance. The near detector will provide the correct flux information on the source side. However, good estimations of the flux information from simulations will be very useful in beamline design and data analysis.

The primary goal of this report is to describe neutrino beam simulation results. The beam fluxes at the near detector and the far detector are investigated using different physics models, QGSP_BERT and FTFP_BERT, and different decay pipe materials, Air and He.

2. LBNE beam line

The LBNE beamline at Fermilab will be designed to provide a neutrino beam of sufficient intensity and energy to meet the requirements of the LBNE experiment with respect to long-baseline neutrino-oscillation physics. The design is a conventional, horn-focused neutrino beam line[Fig.1]. The components of the beamline will be designed to extract a proton beam from the Fermilab Main Injector (MI) and transport it to a target area where the collisions generate a beam of charged particles. The secondary beam, aimed toward the far detector, is then led to a decay-pipe where the particles of the secondary beam decay to generate the neutrino beam. At the end of the decay pipe, an absorber pile removes the residual hadrons.

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The proton energy can be tuned from a minimum of 60GeV to a maximum of 120GeV. Approximately 4.9x1013 protons are extracted every 1.33 seconds at 120 GeV, resulting in a beam power of 708 kW and about 6.5x1020 protons on target per year. The beam size (sigma) at target will be 1.3mm.

The proton beam hits a 96 cm long graphite target and produces secondary particles, mainly pions with small portion of kaons. Eighty-five percent of protons interact with the target and remaining protons fly to the absorber and during the travel they produce additional hadrons in the decay pipe. The secondary particles are focused by the magnetic focusing horns. The charge sign of the focused secondary particles are chosen by setting the horn current direction. In the current setting, positive current(+200kA) collects positive particles resulting in neutrino mode.

Downstream of the target hall is a decay pipe where the secondary particles decay to produce neutrinos. The dimensions of the decay pipe (D=4m, L=204m) are chosen to facilitate pions and kaons decay into neutrinos in the energy range useful for the experiment (0.5-5 GeV). At the end of the decay pipe an absorber stops both the secondary particles that did not decay and the protons that did not interact in the target, leaving only neutrinos to pass through.

Fig. 1 LBNE beamline showing major components of neutrino beam. Figure taken from LBNE CDR

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3. G4LBNE as a simulation tool 3.1 g4lbne and how to use it

The ‘g4lbne’ is a Geant4 based beam simulation tool dedicated to LBNE. Anyone who wants to work on the g4lbne beam simulation must have a valid authority and will need to setup the proper environments. The following provides some steps as well as information about working on g4lbne.

1. Getting FNAL ID for Services account and Kerberos account. Useful information can be found

here. https://fermi.service-now.com/kb_view.do?sysparm_article=KB0010542 2. Once you get the ID, login to here https://fermi.service-now.com/ and request LBNE account and

FNALU account. 3. Setup local computer to access Fermilab computers remotely. Refer to this page for more

information. https://wiki.bnl.gov/dusel/index.php/Computing#User_level

Once you have access to Fermilab computers and lbne computers, two work spaces are assigned to you. One is for data storage(/lbne/data/users/[yourdirectory]) and the other is for g4lbne program install and programming(/lbne/app/users/[yourdirectory]). The first thing you have to do is install the g4lbne in your account area. To install the program, follow the instructions found here https://cdcvs.fnal.gov/redmine/ projects/lbne-beamsim/wiki.

Once the program is installed, go to the installed g4lbne directory(<userID>/lbne-beamsim/g4lbne/) and find the ‘README.txt’ file. The file contains step by step instructions on LBNE beam simulation. You can find the file here as well. https://cdcvs.fnal.gov/redmine/projects/lbne-beamsim/repository/entry/ g4lbne/README.txt

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3.2 Working on the Fermilab Grid

Once you have the program working, you can submit jobs for data with high statistics. Before submitting actual jobs on the grid, you can test the job submission using simple script called ‘hello world’. More detailed help on the test run can be found here. https://cdcvs.fnal.gov/redmine/projects/ifront/wiki/ Getting_Started_on_GPCF#Interactive-Example-Running-Hello-World-as-an-LBNE-User.

For the actual job submission on the grid, first you need to login to ‘gpsn01.fnal.gov’ using secure connection(ssh) and then again login to <userID>@lbnegpvm01. This will bring you to the work area, ‘/afs/fnal.gov/files/home/roomN/UserID’. The room number(roomN) varies. For the job submission, one needs three inputs, input card file, macro file and physics list. The job submission is implemented using a script file written in Python language (submit_flux.py). The file ‘submit_flux.py’ must be located in the directory ‘/afs/fnal.gov/files/home/roomN/UserID’ and the job submission can be done by typing ‘python submit_flux.py --physics_list=QGSP -i CD1-CDR_Geo -n 100000 -f 1 -l 250’ on your command line. In this example, the parameter QGSP is the physics list, CD1-CDR_Geo is the input card file name containing the geometry parameters of the beam line, 100000 is the number of protons on target and 1 and 250 are the first and last indexes for the jobs being submitted which will eventually be the index of the output Ntuple files. The default macro file name is ‘nubeam-G4PBeam-stdnubeam.mac’ and it is defined in ‘submit_flux.py’. 4. Simulation output and analysis program

The output of the simulation is Ntuples[6]. The output files are written under the directory of ‘/lbne/data/users/<userID>/fluxfiles/g4lbne/’. The rest of the directory names are automatically created from the parameters used in the command line during the job submission. For example, if you used ‘QGSP_BERT’ as a physics list, ‘CD1-CDR_Geo’ as the geometry file and ‘nubeam-G4PBeam-stdnubeam.mac’ as the macro file, then the output files will be written in the directory ‘/lbne/data/users/<userID>/fluxfiles/g4lbne/CD1-CDR_Geo/QGSP_BERT/nubeam-G4PBeam-stdnubeam/flux’. But this is solely dependent on the script used for job submission, and the directory structure and names can be changed through the modification of the script file.

The output Ntuples are used to analyze the beam characteristics. The current version of Ntuple contains 63 branch variables and the meaning of each of the variables are described in ref[7]. The files ‘eventRates.C’ and ‘eventRates.h’ are used to create neutrino flux histograms. The program outputs many histograms such as neutrino (antineutrino) fluxes, charged current event rates and neutral current event rates for both neutrinos and antineutrinos and fast MC results for each histogram. Before executing the program, the user must provide correct information about the target Ntuples. This can be done by setting the correct values in the ‘eventRates.h’ file. The values which need to be set are the input card file name, the detector location with the detector name and the name of the physics model. 5. Results and discussion

5.1 Decay processes and neutrino production Figure 2 shows two oscillation maxima as a function of neutrino energy. The LBNE beamline design

is optimized to cover both oscillation maxima. The higher energy neutrinos, 1.5~5GeV, correspond to focused pions of approximately 3.5 to 12 GeV, which are relatively straightforward to focus with toroidal, or horn magnetic focusing elements. However, the low energy pions and kaons, which produce low energy neutrinos, are more scattered and emerge at large angles making tight focusing difficult. The LBNE decay pipe, with a length of 204 m and a diameter of 4 m, is designed to cover both energy ranges. The focused secondary particles decay into other particles as they fly, and relevant various decay modes are listed in Table 1.

In this simulation study, we input 1,250x100,000 protons on target (POT) for each simulation run and the resulting events were written in Ntuple files. From the Ntuples, we calculated neutrino flux at the far detector and the near detector. The far detector is located 1,300 km away from the neutrino source and the near detector will sit onsite at Fermilab 479 m away from the source. Detector weight and importance weight are applied to each event.

𝜈𝜇 and �̅�𝜇 production decay modes are shown in Fig. 3. The weighted flux was calculated at the far

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detector using two different decay pipe materials, air and He. As can be seen in the plots, most of the 𝜈𝜇 comes from 𝜋+ decay, on the other hand the main source of the �̅�𝜇 is 𝜋−. Some of 𝜈𝜇 come from kaon or muon decays, however the portion is small (Table 2). For example, in the total neutrino flux produced with air filled decay pipe, 90.82% is 𝜈𝜇 and almost all of them (90.31%) come from the decay process 𝜋+ → 𝜈𝜇 + 𝜇+ which is represented by the decay code 13 in g4lbne. On the other hand, 8.54% of �̅�𝜇 comes from decay process 14(𝜋− → �̅�𝜇 + 𝜇−). However, the absolute flux of 𝜈𝜇 is 10 times higher than �̅�𝜇 in air, and 12 times in He. In this calculation, only the neutrinos that have energy of less than 5GeV were considered and tau neutrinos were omitted.

Figure 4 shows neutrino (anti-neutrino) production locations observed in the energy range of 0.3~3GeV. The simulation was done with QGSP_BERT physics list and the signals were observed at the far detector. Both 𝜈𝜇 and �̅�𝜇 production rates are high around the target area and gradually decrease downstream. The red solid line represents air as the decay pipe fill material and green line is for He. As shown in the Fig. 4(a), more 𝜈𝜇is produced in He than in air. This is expected because more 𝜋+ will be absorbed in the air than in He during the flight in the decay pipe. On the other hand, more �̅�𝜇 are created in air than in He(Fig. 4(b)). This inversion of higher �̅�𝜇 flux in air is due to higher interaction probability of the residual protons (those that did not interact in the target) with air than He in decay pipe. Since the results came from the neutrino mode runs, the contribution of the �̅�𝜇 production in the decay pipe due to the residual protons is significant and the effect on the �̅�𝜇 flux is dominant. This excess of �̅�𝜇 flux in air over in He in low energy region (0.3~3GeV) is also shown in Fig. 5.

Fig. 2 The energy range of the first and second

oscillation peaks Table 1 Neutrino production process.

Ndecay is the variable used to store decay modes in g4lbne

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Decay code

air(%) He(%) 𝜈𝜇 �̅�𝜇 𝜈𝑒 �̅�𝑒 𝜈𝜇 �̅�𝜇 𝜈𝑒 �̅�𝑒

1 0 0 0.05 0 0 0 0.05 0 2 0 0 0 0.06 0 0 0 0.05 3 0.04 0 0 0 0.04 0 0 0 4 0 0.04 0 0 0 0.04 0 0 5 0.37 0 0 0 0.32 0 0 0 6 0 0 0.1 0 0 0 0.09 0 7 0.08 0 0 0 0.07 0 0 0 8 0 0.08 0 0 0 0.06 0 0 9 0 0 0 0.02 0 0 0 0.02 10 0 0.01 0 0 0 0.01 0 0 11 0 0.37 0.38 0 0 0.38 0.39 0 12 0.02 0 0 0.02 0.02 0 0 0.02 13 90.31 0 0 0 91.59 0 0 0 14 0 8.04 0 0 0 6.85 0 0

Total 90.82 8.54 0.53 0.1 92.04 7.34 0.53 0.09 Table 2 Portions of decay processes contributing to 𝜈𝜇, �̅�𝜇 , 𝜈𝑒 , �̅�𝑒 flux in different decay pipe materials.

Neutrinos in the energy range of 0~5GeV were considered, Neutrino mode run.

Fig. 3 (a)𝜈𝜇 production decay modes (b)�̅�𝜇 production decay modes

Fig. 4 Neutrino production location (a)𝜈𝜇 (b)�̅�𝜇. Used physics list QGSP_BERT, decay pipe fill material

air and He, Neutrino mode run

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Fig. 5 𝜋+ production location

Decay pipe material air He

pion production region(cm) Weight sum Portion(%) Weight sum Portion(%)

π+

Target area (z<0)

(Target volume, 0.74x1.8x95.4) 5.90405e-11

(1.02887e-10) 49.14

(85.64) 6.50208e-11

(1.13087e-10) 50.10

(87.14) First horn area(0<z<300)* (Excluding target, 60.4cm)

5.6562e-11 (7.5661e-12)

47.08 (6.30)

6.19914e-11 (8.27020e-12)

47.77 (6.37)

Second horn area(660<z<960) 6.75626e-13 0.56 6.82012e-13 0.53 Decay pipe area(1730<z<22130) (Material only inside DK pipe)

3.20378e-12 (2.73063e-12)

2.67 (2.27)

1.37967e-12 (8.65771e-13)

1.08 (0.67)

Absorber area(22130<z<22800) 4.63242e-14 0.04 5.03084e-14 0.04 Other gap areas 6.14715e-13 0.51 6.67969e-13 0.51 Total 1.20143e-10 1.29781e-10

π-

Target area (z<0)

(Target volume, 0.74x1.8x95.4) 4.27868e-12 (1.1285e-11)

26.46 (69.78)

4.58104e-12 (1.22703e-11)

29.26 (78.38)

First horn area(0<z<300)* (Excluding target, 6.04cm)

8.18521e-12 (9.1642e-13)

50.61 (5.67)

8.93837e-12 (9.90820e-13)

57.09 (6.33)

Second horn area(660<z<960) 5.39451e-13 3.34 5.73469e-13 3.66 Decay pipe area(1730<z<22130) (Material only inside DK pipe)

2.6224e-12 (2.15845e-12)

16.22 (13.35)

9.85138e-13 (5.16151e-13)

6.29 (3.30)

Absorber area(22130<z<22800) 3.84886e-14 0.24 4.6492e-14 0.30 Other gap areas 5.08388e-13 3.14 5.31299e-13 3.39 Total 1.61726e-11 1.56558e-11

Table 3 Weight sum for π+, π- production in the individual beam line element area. *Target overlaps with the first horn by 60.4cm. The simulations were done with neutrino mode

5.2 Neutrino parent(π+, π-) production locations As shown in the table 2, pion is the main source of the muon neutrino. Figure 5 shows pion(π+)

production locations in the beam line. In this analysis, 10M POTs was used for neutrino mode run and 1,106,709 and 1,136,560 π+ are produced with air and He as decay pipe materials, respectively. The neutrinos with energy greater than 0.3GeV were considered. The color chart represents weighted flux, and only the flux less than 3x10-16 are plotted for more clear visualization of weighted flux distribution.

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Table 3 shows summarized weighted flux for π+ and π- production locations in the two different decay pipe materials. As can be seen in the table, more than 85% of π+ are generated in the target, and small portion is produced in the decay pipe. Notice that beam line component materials like focusing horns and decay pipe wall are also the sources of the pions. In case of π+, the total π+ produced in decay pipe area is 2.67% and the air inside the decay pipe contributes 2.27%. Similarly, the first horn produces 6.30% of π+ (excluding target), but this number includes the horn material(Al) and others like air inside the horn, and it is difficult to separate the horn material from the others because of the complex geometrical shape of the horn. As can be seen in the table, the π- production portion in decay pipe is significantly lower when in He than when in air. Since this results were from neutrino mode run, the lower wrong-sign neutrino source (�̅�𝜇) in He indicates that He is preferable to air as decay pipe material.

5.3 Beam flux comparison in air and He

The main purpose of this study is to investigate effects of decay pipe fill materials on the neutrino beam flux. We started with air and He was chosen as a comparison in this study. The Geant4 material codes for air and He are 15 and 27 respectively. The material selection in the simulation can be done by setting corresponding material code in the geometry input card file which is found in ‘/g4lbne/inputs’ directory. More detailed definitions on various materials can be found in ‘LBNEMaterials.cc’ file.

Figure 6 and 7 show neutrino flux comparison with different decay pipe materials, air and He. Both simulations were done with neutrino mode and used physics list was QGSP_BERT. Weighted flux was calculated at the far detector. Tables 4 and 5 are summary of integrated flux and charged current event rate ratios, respectively. As can be seen from the plots and tables, He produces more right-sign neutrinos than air in the neutrino mode run, about 12% higher flux at around 4GeV. On the other hand, He produces less flux for the wrong-sign neutrinos(�̅�𝜇) at low energy range(<3GeV). Figure 8 is charged current (CC) event rate plots. The cross section data for Ar was used for event rate calculation.

Fig. 6 𝜈𝜇, �̅�𝜇, 𝜈𝑒, �̅�𝑒 flux measured at LBNE far detector. Used physics list QGSP_BERT

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Fig. 7 𝜈𝜇, �̅�𝜇, 𝜈𝑒, �̅�𝑒 flux ratio of He to air calculated from Fig 5.

Fig. 8 𝜈𝜇, �̅�𝜇, 𝜈𝑒, �̅�𝑒 CC event rates.

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Energy

cut range 0~0.5GeV 0.5~1.5GeV 1.5~5GeV 5~10GeV

𝜈𝜇 0.9401±0.0010 0.8807±0.0012 1.0480±0.0010 0.8511±0.0019

�̅�𝜇 0.8667±0.0013 0.9460±0.0010 0.8463±0.0020 1.0503±0.0010

𝜈𝑒 0.9920±0.0166 0.9078±0.0158 1.0400±0.0123 0.9037±0.0148

�̅�𝑒 0.8844±0.0192 1.0258±0.0179 0.9174±0.0209 1.0649±0.0137 Table 4. Integrated flux ratio of He to air. Flux was measured at LBNE far detector, used physics list

was QGSP_BERT

Energy cut range 0~0.5GeV 0.5~1.5GeV 1.5~5GeV 5~10GeV

𝜈𝜇 0.9502±0.0015 0.8528±0.0023 1.0550±0.0010 0.8575±0.0021

�̅�𝜇 0.8383±0.0022 0.9529±0.0015 0.8558±0.0024 1.0594±0.0012

𝜈𝑒 0.9999±0.0209 0.9134±0.0210 1.0452±0.0133 0.9029±0.0158

�̅�𝑒 0.8897±0.0270 1.0352±0.0234 0.9322±0.0248 1.0701±0.0156 Table 5. Integrated CC event rate ratio of He to air. Flux was measured at LBNE far detector, used

physics list was QGSP_BERT

Figure 9 shows flux ratio of near detector to far detector. Apparently, the near detector sees more flux than far detector by order of 6. Since the far detector is located 1,300km away from the beam source, it sees the source volume almost like a point. On the other hand, the near detector is close to the source volume, thus it sees the source volume with large angle variation. The large angle variation reflects the energy variation, resulting in the variation of flux ratio with peak structures at low energy region.

In order to compare the flux ratio of He to air at far detector and near detector, we calculated the ratio of ratio defined as;

R=𝐹𝑄𝐺𝑆𝑃_𝐵𝐸𝑅𝑇, 𝐻𝑒,𝑁𝐷/𝐹𝑄𝐺𝑆𝑃_𝐵𝐸𝑅𝑇, 𝐴𝑖𝑟,𝑁𝐷

𝐹𝑄𝐺𝑆𝑃_𝐵𝐸𝑅𝑇, 𝐻𝑒,𝐹𝐷/𝐹𝑄𝐺𝑆𝑃_𝐵𝐸𝑅𝑇, 𝐴𝑖𝑟,𝐹𝐷.

Where FQGSP_BERT, He(Air),ND(FD) stands for the flux simulated with QGSP_BERT physics list, decay pipe

material of He(Air) and calculated at near(far) detector. And the resulting plots are shown in Fig. 10. By comparing these two flux ratios at different detector locations, we can figure out how each detector sees the flux as a function of energy. As can be seen in the R plot for 𝜈𝜇, the near detector sees more flux than far detector at low energy region. The variation of the near/far ratio going from He to air is about 4%.

As mentioned earlier, in the simulation studies, two different physics lists(QGSP_BERT and FTFP_BERT) were used for comparison and Fig. 10-13 show the results. The two physics models showed significantly different results in the flux plotted as a function of energy. For example, QGSP_BERT produces about 12% more 𝜈𝜇 in the total flux than FTFP_BERT and 31% more flux for �̅�𝜇. However, the two physics models show almost the same results in the flux changes with different decay pipe materials as shown in Fig. 13.

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Fig. 9 𝜈𝜇, �̅�𝜇, 𝜈𝑒, �̅�𝑒 flux ratio of near detector to far detector, Physics list QGSP_BERT, decay pipe

material He.

Fig. 10 𝜈𝜇, �̅�𝜇, 𝜈𝑒, �̅�𝑒 flux ratio comparison at near detector and far detector about different decay

pipe materials. He, air.

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Fig. 11 𝜈𝜇, �̅�𝜇, 𝜈𝑒, �̅�𝑒 flux measured at LBNE far detector. Used physics list QGSP_BERT and

FTFP_BERT, decay pipe material air

Fig. 12 𝜈𝜇, �̅�𝜇, 𝜈𝑒, �̅�𝑒 flux ratio of FTFP_BERT to QGSP_BERT calculated from Fig. 8.

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Fig. 13 Flux ratio comparison of FTFP_BERT to QGSP_BERT about different decay pipe

materials(He, air) at FD.

6. Conclusions and future work

In this study, we compared neutrino flux changes with different decay pipe materials, air and He, to determine the preferred decay pipe material for the LBNE beam line. The simulations were done with two different physics lists, QGSP_BERT and FTFP_BERET, and neutrino fluxes were calculated at near detector and far detector. From the simulation results, we found that He produced more 𝜈𝜇 flux (about 12% more flux at around 4GeV) and less �̅�𝜇 flux than air in neutrino mode run. This result implies that the He is preferable to air for the decay pipe material. We also studied neutrino parent (π+, π-) production locations to see the spatial distribution of the main source of neutrinos in the beam line. From the results, we found that the target produced more than 85% of π+, meaning the target is the major source of the π+ production. Meanwhile, the portion of π+ production in decay pipe material was only 2.51% in the case where we use air instead.

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translated in Sov. Phys. JETP 6: 429. 1957. 2. R. N. Mohapatra et al., "Theory of Neutrinos: A White Paper" 2005. arXiv:hepph/0510213v2. 3. See the LBNE collaboration web site, http://lbne.fnal.gov/ 4. H. Nunokawa, S. Parke, J.W.F. Valle, CP violation and neutrino oscillations, Progress in Particle and

Nuclear Physics 60 (2008) 338–402. 5. Long-Baseline Neutrino Experiment (LBNE) Project CDR vol.1, 2012. 6. http://root.cern.ch/drupal/; http://root.cern.ch/download/doc/ROOTUsersGuideChapters/Trees.pdf

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7. Alexander I. Himmel, The NuMI Beam Simulation with Flugg, 2010, P9., MINOS Document 6316-v10

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