Direct photon cross section with conversions at CDF

We present a measurement of the isolated direct photon cross section in $p\overline{p}$ collisions at $\sqrt{s}=1.8\mathrm{T}\mathrm{e}\mathrm{V}$ and $|\eta |<0.9$ using data collected between 1994 and 1995 by the Collider Detector at Fermilab (CDF). The measurement is based on events where the photon converts into an electron-positron pair in the material of the inner detector, resulting in a two track event signature. To remove ${\pi }^0\to \gamma \gamma$ and $\eta \to \gamma \gamma$ events from the data we use a new background subtraction technique which takes advantage of the tracking information available in a photon conversion event. We find that the shape of the cross section as a function of photon $p_T$ is poorly described by next-to-leading-order QCD predictions, but agrees with previous CDF measurements.

Figure 1

The two conversion identification requirements. Events between the vertical lines are accepted as conversion candidates, while events in the tails are dominated by fakes. No other selection requirements have been applied to the data in these plots. Top: the spatial separation in the transverse plane between the tracks at the radial location where they are parallel. If the tracks cross each other they are considered to have negative separation. Bottom: the difference in the track $\cot \theta$s.

Figure 2

Vertexed and unvertexed radius of conversion distribution in 8 GeV electron data. The peak at $r=0$ labeled 'Dalitz' is due to ${\pi }^0\to e^{+}{e}^{-}\gamma$ decays and ${\gamma }^{*}\to e^{+}{e}^{-}$ events. The $2\mathrm{c}\mathrm{m}<r<30\mathrm{c}\mathrm{m}$ data selection requirement has been released in this plot.

Figure 3

The diphoton mass spectrum of double conversion events in the data. The data is fit to two Lorentzians plus a third order polynomial. The polynomial is shown as the dotted line. The ${\pi }^0$ and $\eta$ peaks are visible at 0.135 GeV and 0.547 GeV, respectively. The ratio of the areas of the two peaks, along with the Monte Carlo prediction for the ratio of acceptances, gives an $\eta /{\pi }^0$ production ratio of $0.687\pm 0.076$.

Figure 4

An example of the $E/p$ background subtraction fit in two $p_T$ bins. The 16.5 to 18 GeV bin is from the 8 GeV electron (2-tower) data, and the 30 to 34 GeV bin is from the 23 GeV photon (1-tower) data.

Figure 5

The total acceptance times efficiency for both conversion data sets. The decrease in the acceptance at high $p_T$ for the 8 GeV electron data set is due to the 2-tower requirement, which becomes geometrically disfavored. The total conversion probability is not included here.

Figure 6

The isolated direct photon cross section. The result of the conversion technique is compared with CES-CPR and theory. For the conversion measurement the 8 GeV electron data is shown below 30 GeV, and the 23 GeV photon data above. The theory curve is from the authors of Reference [2], and uses the CTEQ5M parton distribution functions with the all scales set to the $p_T$ of the photon. Only the statistical error bars are shown here.

Figure 7

The isolated photon cross section from conversions compared to NLO QCD. The error bars shown here are the combined statistical and $p_T$ dependent systematics, in order to compare the shape of the measured cross section to theory.