Direct photon cross section with conversions at CDF

We present a measurement of the isolated direct photon cross section in p p collisions at s p 1:8 TeV and jj < 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 0 ! and ! 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