Measurement of the ratio of cross sections for inclusive isolated-photon production in $pp$ collisions at $\sqrt{s}=13$ and $8$ TeV with the ATLAS detector
2019
The ratio of the cross sections for inclusive isolated-photon production in pp
collisions at centre-of-mass energies of 13 and 8 TeV is measured using the ATLAS detector
at the LHC. The integrated luminosities of the 13 TeV and 8 TeV datasets are 3.2 fb−1 and
20.2 fb−1
, respectively. The ratio is measured as a function of the photon transverse energy
in different regions of the photon pseudorapidity. The predictions from next-to-leading-order
perturbative QCD calculations are compared with the measured ratio. The experimental
systematic uncertainties as well as the uncertainties affecting the predictions are evaluated
taking into account the correlations between the two centre-of-mass energies, resulting
in a reduction of up to a factor of 2.5 (5) in the experimental (theoretical) systematic
uncertainties. The predictions based on several parameterisations of the proton parton
distribution functions agree with the data within the reduced experimental and theoretical
uncertainties. In addition, this ratio to that of the fiducial cross sections for Z boson
production at 13 and 8 TeV using the decay channels Z → e
+e
− and Z → µ
+µ
− is made
and compared with the theoretical predictions. In this double ratio, a further reduction
of the experimental uncertainty is obtained because the uncertainties arising from the
luminosity measurement cancel out. The predictions describe the measurements of the
double ratio within the theoretical and experimental uncertainties.
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