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MDC post-mortem redux. D. A. Petyt 11 th May 2005. Status as of last CC meeting:

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Mdc post mortem redux
MDC post-mortem redux

D. A. Petyt 11th May 2005

  • Status as of last CC meeting:

    • True values of cross-section and oscillation parameters were used to reweight the ND and FD MC samples to perform a match-up with MDC datasets. No obvious improvement as agreement was OK even with nominal systematic parameters

    • BMPT parameters unknown. Used -7% normalisation as a guesstimate of the effect of BMPT reweighting. This value did not appear to be supported by fits to the MDC data.

  • This meeting

    • Obtained BMPT reweighting histograms that were generated by Mark for the MDC

    • Used BMPTHistoWeightCalculator method in MCReweight to calculate a weighting factor for each MC event, based on neutrino parent PID, px, py, pz

    • This does not tell us what the tweaked BMPT parameters were, but at least it allows us to determine their effect on the ND and FD CC-like event samples

Robert’s slide on BMPT weighting from Argonne

log10 pt


log10 pz

Weights are stored in 2D histograms of p_z vs p_t for each parent particle type. These weights are extracted by the BMPTHistoWeightCalculator method in MCReweight

Neutrino parent near
Neutrino parent – Near

  • True energy distribution of selected CC-like events in the Near Detector by neutrino parent particle type

BMPT weight vs neutrino parent - 2


  • BMPT weighting affects overall normalisation of CC-like events by –2.8%

  • Some structure is evident due to the different weights given to pi+, pi-, K-, p











X sec and total weight
X-sec and total weight


  • True X-sec parameters were

    • Ma_qel=1.0123 (nominal 1.032)

    • Ma_res=1.0605 (nominal 1.032)

    • Disfact=1.0251 (nominal 1.0)

  • Overall effect of x-sec reweighting: 1.2% change in normalisation

  • BMPT reweighting changes normalisation by –2.8%

  • Overall effect is –1.6%. This is why we saw no obvious excursions in any of the MDC/MC matchups with ND data.



Energy dependence
Energy dependence

BMPT weight Cross-section weight BMPT * x-sec weight

Visible Energy True Energy

3% change at the peak of the LE spectrum

Mdc mc match up
MDC/MC match-up

This plot: ratio of MDC data to MC with tweaked x-sec parameters and nominal BMPT weights (i.e. 1.0). The chisq fit is to the straight line at 1.0.

This plot: ratio of MDC data to MC with both tweaked x-sec and BMPT parameters. The chisq fit is also to the straight line at 1.0. The fit is acceptable, although slightly worse than before

We clearly do not have enough MDC data to observe the effect of beam reweighting at this level

Divining the parameters
Divining the parameters…

  • We don’t know which BMPT parameters were changed, and by how much, but can we guess?

  • Having gained some familiarity with the effect of BMPT weights during the MDC, I spotted the similarity between the true MDC weights returned by MCReweight (plot at left) and the effect of changing a_pi and alpha_pi in the BMPT model.

  • Can we reproduce the left-hand plot by some combination of these two parameters?

a_pi 6%

alpha_pi 5%

=  ?

Fit to a pi and alpha pi
Fit to a_pi and alpha_pi

68,90% C.L.

  • Used the BMPTWeightCalculator method of MCReweight to reweight the MC events as the parameters a_pi and alpha_pi are changed.

  • Perform a fit in the variable Evis to the ‘true’ BMPT weighted spectrum to obtain best-fit values of these parameters

  • The fit does a reasonable job of describing the true distribution below 20 GeV. The discrepancy above that is presumably due to the effect of K-

  • This means that a reduced parameter fit similar to that we performed for the MDC (using a_pi, alpha_pi and A_pi) would be sufficient to account for the BMPT model excursions that existed in the MDC. Unfortunately, they were so small as to be undetectable given the statistics we had.

Neutrino parent – Far

  • Now perform the same analysis on FD data and see how applying the correct BMPT weights affects the oscillation fits.

BMPT weight vs neutrino parent - far


  • Overall shape very similar to ND sample, although proton peak is suppressed.










Comparison of nd fd weights
Comparison of ND/FD weights

BMPT weight Cross-section weight BMPT * x-sec weight

  • ND/FD differences are rather small…



Far near ratios
Far/Near ratios

  • Here I am plotting the product of the beam and x-sec weights…

  • Effect of reweighting cancels in F/N ratio



Fd fit with reweighted mc
FD fit with reweighted MC

  • The effect of re-weighting is to reduce the size of the peak in the MC sample. The oscillation fit therefore favours a slightly smaller value of sin22q which moves the best-fit slightly closer to the true values.

  • The quality of the fit does not actually improve (c2=38.3/37 versus 37.6/37), although both are perfectly acceptable

Nominal, 68,90% C.L.

Nominal, no osc.

Weighted, 68,90% C.L.

Nominal, best fit


Nominal, best fit

Weighted, no osc.


Weighted, best fit

Weighted, best fit

NC contamination

MDC ‘data’


True parameters


  • We are now able to perform a full post-mortem of the MDC with the ability to reweight events according to true beam, x-sec and oscillation parameters

  • The effect of beam reweighting is fairly small (overall –2.8% effect on the normalisation and ~-4% at the peak). This is partially compensated by the effect of x-sec reweighting – the overall effect is –1.6%.

  • There is insufficient MDC data available to detect excursions that are as small as this.

  • It appears that a reduced set of BMPT parameters is sufficient to describe the effect of beam reweighting in the MDC. Perhaps these shape and normalisation parameters are sufficiently general that they can be used outside the scope of the BMPT model?

  • The FD fit is only marginally affected by applying the true beam and x-sec weights, the shift in parameter values is much smaller than the size of the 68% C.L contour. Not surprising given the small size of the reweighting factors applied.