We have studied prompt diphoton production in proton-antiproton
collisions at a center-of-mass energy of 1.8 TeV by the
Collider Detector at Fermilab CDF.
Diphoton production in hadron-hadron collisions is one of the clear probe
for testing the Next-to-Leading Order (NLO) calculation of the
Quantum Chromodynamics (QCD) because of the advantage of the
detection of photons with small systematic uncertainty
and good energy resolution. The measurement
of the diphoton production cross section
can be used
to check the QCD calculation.
By looking at the transverse momentum of the diphoton system,
diphoton production is also a good probe
for measuring the effects of multiple initial soft gluon radiations and
the intrinsic transverse momentum of the initial state partons, which are
beyond the NLO prediction.
were collected by the CDF detector placed at B\O \,area of the
Fermilab Tevatron collider
during 1992\,-\,1995 collider run. The total integrated luminosity was
The trigger system, including both the hardware and the software modules,
required the collision events to have two large and
isolated electromagnetic energy depositions in the Central
At offline level, we selected EM clusters which had
no tracks associated with them.
Using the Central Electromagnetic Strip chamber (CES),
were embedded in the CEM near the shower maximum,
we required that
the EM shower profile should be
consistent with that obtained from testbeam electrons.
Candidate photons with large additional CES clusters
we obtained 652 events as the diphoton candidate events.
The efficiencies of each selection criterion for the photons were
calculated with various CDF physics datasets.
The backgrounds against prompt photons come from neutral mesons\,
(\,$\pi^0$'s, $\eta$'s, and $K_s^0$'s\,)\,decaying into multiple
We cannot separate them
on event-by-event basis,
so we subtracted the backgrounds statistically by using the
information from the CES shower profile
and the conversion rate
in the Central Preradiator detector (CPR) placed
in front of the CEM.
The photon fraction in the diphoton candidates was evaluated
as a function of the transverse momentum ($p_T$) of the photons. It showed
a large photon purity at high $p_T$ region\,(0.6 at $p_T=25$\,GeV/c).
The differential cross section for diphoton production was measured
as a function of the photon $p_T$, the invariant
mass ($M$) of the diphotons, the azimuthal angle between diphotons (
the ratio of the transverse momenta of the two photons ($Z$) and
the $p_T$ of the diphoton system. The results were compared
with the NLO QCD prediction with CTEQ2M parton distribution functions.
The results, as a function of the photon $p_T$,
$M$ and $\Delta\phi$, were consistent
with the NLO prediction, while the results
on the $Z$ and the diphoton system $p_T$ showed slight differences
between data and theory.