K2K Events with Nuance

Simulating K2K Events

I have modified the Nuance event generator to use the K2K beam rather than atmospheric neutrinos to produce event vectors in Super-Kamiokande. The beam was taken from one of the standard HBOOK files produced by the beam channel group.  I believe the total flux in this HBOOK file corresponds to 1020 protons on target, with horn current at the design value of 250 kA.  The fluxes are scaled to a distance of 250 km. Only muon neutrino interactions are simulated in this run.

Contained Interactions

Events originating in water are produced with the standard Nuance physics model, incorporating all types of charged- and neutral-current reactions (quasi-elastic, resonances, deep-inelastic, coherent, and electron scattering). Nuclear effects (Pauli blocking and final-state interactions of hadrons) are included. Events are generated in the full water volume of SuperK, including the outer detector and the insensitive region.

Entering Events

Events originating in the nearby rock are produced with the same algorithm used to simulate upward-going muons in SuperK. Only charged-current interactions are simulated, and hadrons produced in the interaction are ignored (these should be absorbed quickly in the rock). Nuclear effects, including Pauli blocking, are neglected; this probably over-estimates the event rate slightly, depending on what energy threshold is appropriate. Muon propagation is handled with routines provided by Bartol, which have been tuned on cosmic rays. For the low energies of the K2K beam, energy loss mainly involves continuous ionization processes.

Event Samples

The samples generated correspond to 10 times the neutrino flux in the beam file, or 1021 protons on target.

Contained Events

In the full water volume of SuperK (50 ktons), 9517 events are produced. The breakdown of these events by physics process is:

  Charged Current Neutral Current
Quasi-elastic 2263 1599
Resonance 2880 963
Deep-inelastic 1265 455
Coherent 184 103
Electron scattering - 5

Neutral current quasi-elastic (NCQE) events are invisible in nearly all cases, so they will be ignored in the following discussion. Taking into account the ratio of fiducial to total volume, and the assumed proton luminosity, the number of non-NCQE events in the fiducial volume per 1020 protons on target is then 356.3, and the number of numu-charged current events is 296.6.

Figure 1, at right, shows the parent numu neutrino energy of contained events (in MeV). The blue histogram is for all non-NCQE events, and the red is for charged-current interactions only. Hence the difference between the histograms represents visible neutral current events. As expected, the neutral current events tend to be higher in energy (since quasi-elastics are excluded).
Figure 1: Neutrino energy for visible contained events (blue) and charged-current (red)
The second figure shows the distribution of muon momentum (in MeV/c) for charged-current interactions in the water volume.

The black curve is for no oscillations.

The blue curve is for =2.5×10-3.

The red curve is for 3.5×10-3.

The green curve is for 5×10-3 (eV)2.
Figure 2: Muon momentum for charged-current interactions at various values of .
Figure 3 shows the angle (in degrees) of the muon with respect to the neutrino vs. the muon momentum (in MeV/c). The angle shown does not include possible reconstruction uncertainties, which should be considerably smaller.
Figure 3: Muon-K2K angle vs. muon momentum for charged-current interactions

Entering Events

For the same integrated proton luminosity as the contained event sample, 1285 events produced in the surrounding rock enter the SuperK outer detector. Muons of energy down to 200 MeV are retained (although clearly these will not be visible in the SK inner detector). The breakdown of these events is given in the table below:

Quasi-elastic 621
Resonance 511
Deep-inelastic 153
The figure at right shows the parent neutrino energy for entering events from the K2K beam. Comparison with Figure 1 shows the neutrino spectrum for these events is somewhat harder than for contained events. This is as expected, since the target volume for lower energy neutrinos is smaller, and conversely higher energy neutrinos have a larger effective target volume from which a muon can reach the water of SuperK.
Figure 4: Parent neutrino energy for entering K2K events
Figure 5 shows the momentum spectrum of muons produced in rock by the K2K beam and entering the SuperK outer detector at various values of . The color-coding of is the same as in Figure 2. Since the entering events have a somewhat harder spectrum, the oscillation effects are less than for contained events. This is particularly true for muons with sufficient momentum to reach the SK inner detector. This threshold momentum will be quite sensitive to the selection cuts used, and also to the specific event geometry (pathlength to reach the SK inner detector), but it is likely to be at least 500-700 MeV/c.
Figure 5: Muon momentum spectrum of entering events, for various values of .

Data Files

A total of four data files are available, two each for contained and entering events. The data files are stored in a downloadable zip file. For contained and entering events, a .kin file is provided which can be used as input to APDETSIM, and a .hbook file, with Ntuples (one entry per event) and histograms (beam spectrum, cross-section) is also included.

Kate Scholberg has already run the contained event sample through APDETSIM. The resulting ZBS data file is on the suketto cluster: /net/sukop2g/mnt2/tmp/k2kmc/k2k.dat. (Due to a bug in the first generation of the entering events, the corresponding detector simulation file will have to be re-run.)

Future Plans

The next obvious step is to simulate events in the K2K near detectors. Because the neutrino flux is position-dependent at the near detectors, this will require a bit more effort, however please let me know if there is an urgent need for such samples (e.g. when the near detector MC is ready for production). Feel free to contact me if you have any problems or questions concerning the data files.

Dave Casper