Photon Physics at CDF
The study of photon production at a hadron collider is important for
many reasons. As the photon energy is well-measured, compared to
jets, it can be a good tool to further our understanding of
Quantum Chromodynamics (QCD). CDF has performed pure QCD tests with photons
in a number of different ways, the
isolated photon cross section from both
1800 GeV data as well as 630 GeV data, the
photon+1jet angular distribution, the
photon+2jet angular distribution, three
different measurements of
photon+charm production, and
diphoton production.
The second important reason to study photons at a hadron collider
is to measure the gluon distribution inside the proton. In order
to do this both the data and the theory have to be precise, and
the uncertainties of both well under control. At present the only
measurement we believe fits this strict criteria for both data and
theory is the
photon+1jet rapidity distribution, and
this measurement places important constraints on the gluon
distribution. The final important reason to study photons at a
hadron collider, as well as all QCD measurements, is that they are the
backgrounds to new physics. The most famous of these is the Higgs
search at the LHC, where diphoton backgrounds are the most serious
experimental difficulty. In the next section we will illustrate
how all of the photon measurements at CDF mentioned above provide
important insights into diphoton production at the LHC. At the
Tevatron two examples of new physics involving photons are
the 4th generation b quark search, and the bosonic Higgs search.
These searches are underway in CDF.
Now we will tie together all of the photon measurements at CDF
and see how they provide insight into backgrounds for new physics,
specifically the diphoton backgrounds to the Higgs search at the
LHC. Of course diphoton production
at CDF is an important direct test of these backgrounds, and
an important insight from this was demonstrated in our diphoton
publication (PRL70, 2232 (1993)). Here it was shown in the
QCD Monte Carlo PYTHIA that diphotons
at a high energy collider are mostly produced from a
normal single photon+jet event, and the second photon comes from
radiation off the jet. Thus it is very important to study
photon radiation processes off of quark jets, and this is best
done in the high-statistics single photon, photon+1jet, and
photon+2jet samples. The isolated photon
cross section is expected to have a large component of
photon radiation or fragmentation events, and the previous low pt
excess of photons was once attributed to an incomplete treatment
of this in QCD. But we now attribute this to the lack of a
complete parton shower in NLO QCD, which has important implications
of its own. The NEW photon cross section from the 630
GeV data also disagrees with NLO QCD at low pt but is consistent
with NLO QCD + Parton Shower.
The photon+1jet angular distribution
is very sensitive to photon fragmentation effects, and the measurement
is found to be more steep than NLO QCD, perhaps indicating more
fragmentation than is present in that calculation. The
photon+2jet angular distribution has the
unique capability to classify events into "radiation-like" or not, and
at present the "radiation-like" events are also slightly more steep
than QCD, but within present systematics. If the disagreements with
QCD in the angular distributions are due to extra fragmentation events
in the data, it would
indicate the standard QCD tools underestimate photon fragmentation, thus
QCD predictions for diphoton production at the LHC may be underestimated as
well.
The final two photon measurements at CDF provide input into the parton
distributions that contribute to the LHC Higgs search. The gluon
distributions that are constrained by the photon+1jet
rapidity distribution are input to the Higgs production diagram.
The production of diphoton backgrounds has a large component from
charm-anticharm initial states, thus the charm component of the
proton is important, and this is tested with the
photon+charm measurements.
Kuhlmann@Fnald.fnal.gov