Meeting Minutes from March 25

Stan Wojcicki's Talk on Off-Axis Experiment Beam Studies
Stan has set up a stand-alone code that he used for optimization studies with the numi beamline. The beauty of this code is that it can be used to get neutrino fluxes with high statistical accuracy in 30 seconds or so, but the drawback is that it's a simplified model: no secondary interactions of the pions once they are produced, and the multiple scattering is applied when the particle leaves the horn location, not "as the pion is going through the horn material". But he can study what things it pays to worry about, and what things it doesn't pay to worry about. Minute-taker's comment: this is a really interesting way to look at this problem, and I highly recommend looking at this talk!

The main point here is that optimizing the off-axis flux is by no means equivalent to optimizing the on-axis flux. By varying the target and horn z positions, and slightly changing the "off axis" angle, Stan was able to arrive at an optimized beam, which is peaked at 2.5GeV instead of 2GeV. The two figures of merit he considered concerned the nue background, and the nc background. The figures of merit were the signal (which has the sin^2 delta mL/E weighting) divided by the sqrt(nue background) or sqrt(nc background) where he evaluated the nc background assuming a detector with perfect energy resolution, but no particle id. The nc background really comes from events in the peak, not the events in the high energy tail, once you're at this level of a high energy tail. Thus the comment: " we have met the enemy, and she is us".

The "optimal" beam he arrived at by these considerations has about a factor of two higher figure of merit over the LE beam, and about a factor of 30-50% higher figure of merit over the ME beam (page 31 gives the summary). The biggest increase in figure of merit comes from moving the target back 1m from the nominal Low energy position, and moving horn 2 to 24m, or 14m downstream of the nominal low energy position. By changing the target to be thinner and longer he gets about a 10% increase in neutrino flux, and Mark Messier pointed out that the R&D for "medium" or "high energy" targets has already been done so that's not as painful a change as one might imagine...

Stan also looked at collimation or putting a third horn into the beamline, neither of these things increased the flux (or the two figures of merit, which are related to the signal/sqrt(background) ) by more than a few per cent.

In these slides delta p/p refers to the width of the neutrino beam (p is the neutrino momentum). One interesting question is: what's the best width of the neutrino beam, or conversely, what's the goal for your energy resolution in an off-axis detector?

These studies should be followed up with a more realistic simulation (i.e. geant, which would have reinteractions and better multiple scattering), although this study shows what the interesting variables are to tune.

Another thing Stan looked at was the use of a near off-axis detector. Since the off-axis flux is not very dependent on the momenta of the pions, and the momenta of the pions is well detected by the on-axis neutrino detector, that helps. Also, since the off-axis detector nue background is primarily from muon decays not kaon decays, again this is constrained by the on-axis near detector measurements (or the muon monitors). Finally, he suggests that one could determine the off-axis nc background by using the far detector data itself--since the NC process has a well-defined y (visible hadronic energy divided by total neutrino energy) distribution, one could arguably extrapolate from low visible hadronic energy events, and look at the "pi0 energy/visible energy" distributions and extrapolate under the peak. (this has certainly been done in high energy neutrino beams). Kevin commented that it's not clear that one would have the statistics to do this with the far detector, and it's slightly worrisome with a near on-axis detector since you certainly wouldn't have the same neutrino energy distribution, and you could never get the underlying neutrino energy distribution for the nc events you do see in the near detector. Finally, one other concern Kevin pointed out about measuring the nc contamination is that at 2GeV there are lots of nuclear/final state effects going on, so the functional form you use to "extrapolate under the peak" may not be justified at the 5 to 10% level, and could very well be the dominant systematic error in the analysis.
Debbie Harris' Talk on Matter Effects Off Axis
These slides show the result of a back of the envelope study, looking at what baselines are interesting for matter effects, and what matters and what doesn't for studying matter effects. What is plotted on many axes is the chi2 difference between making the right and wrong choice for delta m^2, but there was no uncertainty assumed for the value of theta_13 itself, which of course must also be incorporated. The plots might be what you would consider doing given a precise measurement of theta_13, say from JHF. You can see this chi2 difference versus several different things, i.e. the background fraction, the uncertainty on the background fraction, the beam energy, delta m^2, etc., all as a function of baseline. This study shows that if one is willing to take the hit in precision on theta_13 by itself, it's really favorable to go to higher baselines--for a 2GeV beam and a baseline of 900km, the matter effects are as big as 80%!

The plots all assume (unless stated otherwise) that the error from antineutrino running is as small as that from neutrino running, which we saw earlier corresponds to a run time of about 2.5 to 3 times longer. One thing to check is what the optimal use of time would be...what fraction would you want to run in nubar vs nu? Also, the plots all asume the same efficiency and systematic error for nu and nubar, which is certainly not going to be the csse. The systematic errors in nubar running are assumed to be uncorrelated with those in nu running. But from the plots vs systematic errors you can see that there's not much loss in going from 0 syst. error to 10% systematic error.
Deborah Harris
Last modified: Mon Apr 1 11:30:41 CST 2002