An investigation into the effects of smearing on the measurement of forward-backward charge asymmetry at the Z pole in the dimuon channel at D0

Abstract

Dimuon events in a 32 GeV bin of mass centred in the Z pole are divided up into statistically independent subsamples based on several discriminating variables. The agreement in data and MC is assessed in the invariant mass, $M_{\mu\mu}$, distribution for the subsamples. Several subsamples show an excess of events shifted from the peak in $M_{\mu\mu}$. The hypothesis is excluded that an excess of final state bremsstrahlung in the MC is the dominant cause. The forward-backward charge asymmetry as a function of mass, $A_{fb}(M_{\mu\mu})$, is then extracted. $A_{fb}(M_{\mu\mu})$ is found to be poorly described regardless of the quality of the $M_{\mu\mu}$ distribution. This is also found to be the case in all statistically independent subsamples. The hypothesis is excluded that the standard MC correction of $p_{T}$-oversmearing exacerbates the problem. The hypothesis is excluded that reweighting the binned MC $\cos(\theta^{*})$ distributions (or any other subsample investigated) to achieve perfect agreement in subsample $M_{\mu\mu}$ distributions will improve the agreement in $A_{fb}(M_{\mu\mu})$ at the subsample and total sample level. A custom $M_{\mu\mu}$ smearing which moves events around the $M_{\mu\mu}$ distribution without reweighting them is then implemented. This is shown to have the capacity for altering aspects of the $A_{fb}(M_{\mu\mu})$ distribution. The MC $A_{fb}(M_{\mu\mu})$ is brought into good agreement with the data in the region $\pm10$ GeV either side of the $M_{Z}$ peak, but at the cost of ruining the agreement in the $M_{\mu\mu}$ distribution. Suggestions for further work are then made.

Date of Award	31 Dec 2012
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Terence Wyatt (Supervisor)