Authors
IFAE-Barcelona
Fermilab
Ray Culbertson (rlc@fnal.gov)
Sebastian Grinstein (gris@fnal.gov)
Mario Martinez (mmp@fnal.gov)
Shin Shan Yu (eikoyu@fnal.gov)
Carolina Deluca (cdeluca@fnal.gov)
P.0. Box 500, M.S. 318
Batavia, Illinois 60510
USA
We present preliminary results on the measurement of the inclusive direct photon production cross section
in proton-antiproton collisions at
√s = 1.96 TeV, using data collected with the upgraded Collider Detector at Fermilab in Run II,
and corresponding to an integrated luminosity of 2.5 fb-1. Measurements are performed as a
function of the photon transverse momentum for photons with pT > 30 GeV/c and |η| <
1.0. Photons are required to be isolated in the calorimeter. We use the calorimeter isolation distribution
to estimate the contamination from jets faking isolated photons. The measured cross section is corrected back
to the hadron level and compared to NLO pQCD predictions. The NLO pQCD predictions include non-perturbative corrections.
We find good agreement between data and the theoretical predictions.
Trigger efficiency
The measurement is peformed using a data sample collected via two different trigger paths.
For photons with pT < 90 GeV/c, we use a
trigger path requiring at least one isolated photon with pT > 25 GeV/c.
For photon with transverse momentum above 90 GeV/c, the trigger path requires at least one photon
(no isolation requirement applied) with pT > 70 GeV/c. In all cases, the measurements
are carried out in the kinematic region for which the trigger is fully efficient.
The trigger efficiency for pT < 90 GeV/c is determined using Z->ee data.
For the highest pT trigger, the turn-on efficiency curve is measured using electrons up
to 120 GeV/c, and then using jet data for higher momenta.
Photon Energy Scale
We use Z->ee events to determine small corrections to the absolute photon energy scale in both data and
Monte Carlo simulation that brings the Z mass peak to its nominal PDG value. In the Monte Carlo samples the
correction is of the order of 1.0035, while in the data the corrections are time dependent and vary between
0.998 and 0.988 (see Figure)
Signal and Background Separation
The calorimeter isolation distribution is employed to discriminate photon signal from QCD background (dominated by pi0 decays).
Templates for signal and background are constructed using Monte Carlo simulations and fitted to the data in each bin of pt.
Details for three bins in pT are presented below (linear and log scales). Similar distributions are obtained for the rest
of the bins in pt considered in this measurement.
Photon Fraction
The obtained photon signal fraction varies between 70% and 100% as the photon pT increases.
Unfolding factors
The unfolding factors vary between 0.64 and 0.69 as the photon pT increases. The yellow band includes
the uncertainties on the photon ID acceptance, the photon energy scale and the photon isolation scale.
Systematic Uncertainties
Different sources of systematic uncertianties are considered. The total
systematic uncertainty is dominated by the uncertainties on
the photon signal fraction at low pT and the photon energy scale at high pT.
The systematic uncertainty in the signal fraction is determined by
using different methods to extract the photon signal. The plot
below shows the nominal result, the systematic uncertainty band, and
the result of the different
variations performed. The systematic uncertainty on the photon fraction
is 5% except for the first
bin for which we choose a conservative 13%.
At low pT we developed templates using electrons from Z data samples, and employed the information
from shower-max and pre-radiator detector (CES/CPR in the Figure) to extract the photon signal.
Alternatively, we developed simple 2-bin templates and applied them to obtain
the signal fraction for all the measured pT range.
Finally, we explored the difference between corrected and uncorrected templates.
We quote a 1.5% uncertainty on the photon absolute energy scale that addresses
remaining differences observed between data and Monte Carlo using a Z boson sample.
We quote a 10% uncertainty on the photon isolation energy leading to an uncertainty on the
photon cross secton between 1% and 0.6% as pT increases.
We quote a 3% uncertainty on the acceptance of the
photon ID selection criteria, as extracted from Monte Carlo. Finally, in the pT region below 90 GeV/c,
we included an additional 5% uncertainty on the quoted photon ID selection efficiency.
The total systematic uncertainties are summarized in the plot below. They vary between ~10% and
~15%. An additional 6% uncertainty on the total luminosity is kept separate and is not included
in the Figure below.
The Cross Section Result
The measure pT distribution is unfolded back to the hadron level and is
compared to NLO pQCD predictions by JETPHOX with CTEQ6.1M PDFs and fragmentation functions
given by Bouris et al. (BFG II). The renormalization, factorization and fragmentation scales are all
chosen to be equal and set to the pT of the photon. The theory is
corrected for non-pQCD contributions (from underlying event).
This correction is of the order of 9% and independent of pT .
We find good agreement betweeen data and theory. The difference in shape observed for pT
less than 40 GeV/c has been observed in pervious measurements.
Inclusive isolated photon cross section as a function of the photon pT eps
DATA/THEORY ratio
Ratio to NLO pQCD JETPHOX predictions as a function of the photon pT eps
