Conversions

 The method described in section 6.3 was used for the reconstruction of conversions. For locating the primary vertex only conversions with an accurately reconstructed pseudorapidity are of interest. This limits the usable conversions to those with hits from the silicon detectors on the tracks.

Conversions were reconstructed in roads ($\Delta$$\eta$,$\Delta$$\varphi$) = (0.5,0.5) around the identified clusters in the calorimeter and accepted if they had reconstructed transverse momentum above 20 GeV. The cut on p_T effectively rejected all background from conversions arising from photons in $\pi^{0}_{}$ decays of the pile-up. The position of the primary vertex was calculated as

 

z₀ = z_c - r_cT_c. (98)

  

Figure 7.3: The efficiency for reconstructing conversions with hits in the silicon detectors as a function of the conversion radius.

For the conversions in the Higgs events with pile-up there is a reconstruction efficiency of 55% $\pm$ 3% for conversion radii below 40 cm. The resolution as a function of the conversion radius is shown in fig. 7.3. The low efficiency is mainly from conversion radii close to 40 cm; in general conversion searches, where silicon hits on the tracks are not required, these conversions can be found with much larger efficiency as shown in fig. 6.10. Resolution plots for the reconstructed conversions are shown in fig. 7.4. Figures with much better statistics, and for conversions reconstructed in the full Inner Detector, can be found in section 6.3. However, they have been made for 50 GeV p_T single photons and not with the photons from H $\rightarrow$ $\gamma$$\gamma$ events.

  

Figure: The resolution in conversion radius r_c , $\varphi$ , $\eta$ and primary vertex position z₀ for reconstructed conversions with r_c < 40 cm.