PhD-Kopecna-Renata/Chapters/TrackEff/systematics.tex

\subsection{Systematical uncertainties}\label{sec:trackEff-sys}

The sources of potential systematic uncertainties have been investigated for the \runI track reconstruction efficiency measurement~\cite{TrackEffRun1}. The method of measuring the track reconstruction efficiencies remained unchanged, hence the uncertainties are not expected to significantly change in the \runII measurement. 

Changing the \jpsi mass signal model from the sum of two Crysta-ball distributions to the sum of two Gaussian distributions does not change the efficiency significantly compared to the statistical uncertainty. Similarly, changing the background model from an exponential distribution to a linear one leads only to a negligible change in the track reconstruction efficiency. 

Another source of the systematical uncertainty could be the difference between the long method and the combined method. However, the difference is observed to be small relative to the statistical uncertainty and is further reduced in the ratio of the track reconstruction efficiency in data to the efficiency in the simulation. 

The dominant systematical uncertainty in the \runI measurement originates from the choice of the occupancy variable used to improve the agreement of the simulated event sample with the real data. The uncertainty is evaluated by using the number of hits in
the \spd, the number of long tracks in the event and the number of primary interaction vertices as the occupancy variables. The largest deviation observed in \runII  for the correction factors obtained from the combination of all methods in any of the two-dimensional correction tables is 0.8\%. 




%8 Systematic uncertainties
%Small differences in the ratio of efficiencies are seen when reweighting the simulated
%samples in different parameters such as the number of primary vertices, or the number of
%hits or tracks in the different subdetectors. The largest of these differences is taken as
%a systematic uncertainty and amounts to 0.4%. No systematic uncertainty is assigned
%for the agreement of the track reconstruction efficiency determined by the tag-and-probe
%method and the hit-based method (which is on the order of 1%), as the differences cancel
%when forming the efficiency ratio. Accordingly, no systematic uncertainties are assigned
%for the fit model as these cancel when forming the fraction of reconstructed J/ψ decays
%where the probe can be matched to a long track. It has been checked that this is true
%for a range of fit models, the largest variation being 0.2%. Furthermore, no systematic
%uncertainty is assigned to the possible matching of a correctly reconstructed probe track
%to a fake long track, as the requirement for a large overlap in the subdetectors ensure that
%both reconstructed tracks are either real tracks or fake tracks, where the latter would not
%peak at the J/ψ mass. No systematic uncertainty is assigned for the fact that the VELO
%+ T-station method and the long method show slightly different results in Figs. 4–6, as
%both methods probe different momentum spectra and any residual difference will cancel
%15
%when forming the ratio with simulation. No systematic uncertainty is assigned for the
%double-counting of the matching efficiency in the combined method, as this efficiency is
%very close to 100%, and any uncertainty would get further reduced when forming the ratio
%with simulation. No systematic uncertainty is assigned for the large difference for the
%VELO + T efficiency between simulation and data at low momenta in 2011 and 2012, as
%this is automatically taken into account when forming the ratio of efficiencies. Despite this
%difference, the integrated track reconstruction efficiencies between simulation and data are
%in agreement due to compensation of this effect for high momenta, where the efficiency is
%higher in simulation than in data