# Direct photon cross section with conversions at CDF

We present a measurement of the isolated direct photon cross
section in $p\bar{p}$ collisions at $\sqrt{s} = 1.8$ TeV 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 $\pi^0 \rightarrow \gamma \gamma$ and $\eta \rightarrow
\gamma \gamma$ events from the data we have developed a novel
background subtraction technique based in electron E/p. The new
measurement improves upon the statistical and systematic errors of
conventional measurements at $p_T < 20$ GeV. We find that the
cross section is larger than next-to-leading order QCD predictions
over the $p_T$ range studied. The conversion cross section is also
30-40\% larger than previous CDF measurements.

hep group
Last modified: Tue Jul 23 15:10:39 EDT 2002