Precision direct photon spectra at high energy and comparison to the 8 TeV ATLAS data

TL;DR

The paper tackles precision predictions for the isolated direct photon spectrum at high transverse energy and its comparison to ATLAS data at $8$ TeV. It combines $N^3LL$ threshold resummation (via PeTeR) with fixed-order fragmentation (via JetPhox), including isolation and electroweak Sudakov corrections, and matches resummed results to NLO with fragmentation. Key findings show that adding threshold resummation and EW logarithms progressively improves agreement with data, with EW effects notably reducing the high-$E_T$ cross section and PDF uncertainties remaining dominant at large $E_T$. This work demonstrates the necessity of both QCD threshold logs and electroweak corrections for precision photon spectra and highlights the potential of high-$x$ PDF constraints from direct photon measurements.

Abstract

The direct photon spectrum is computed to the highest currently available precision and compared to ATLAS data from 8 TeV collisions at the LHC. The prediction includes threshold resummation at next-to-next-to-next-to-leading logarithmic order through the program PeTeR, matched to next-to-leading fixed order with fragmentation effects using JetPhox and includes the resummation of leading-logarithmic electroweak Sudakov effects. Remarkably, improved agreement with data can be seen when each component of the calculation is added successively. This comparison demonstrates the importance of both threshold logs and electroweak Sudakov effects. Numerical values for the predictions are included.

Figures (5)

• Figure 1: Theoretical uncertainties. The hard, jet and scale uncertainties come from varying the scales by a factor of 2 around their default values. The electroweak uncertainty is taken from Becher:2015yea. The PDF uncertainty is taken from ATLAS Aad:2016xcr, who computed it using JetPhox. Note from the bottom left panel that by matching to the resummed distribution, the factorization scale uncertainty of JetPhox is severely reduced.
• Figure 2: Comparison between the prediction from JetPhox and ATLAS data (black). Darker bands are scale uncertainties, lighter bands also include PDF uncertainty. These plots use JetPhox default $\alpha_e = \frac{1}{137}$.
• Figure 3: Comparison between the prediction from JetPhox with $\alpha_e=\frac{1}{129}$ instead of JetPhox's default value of $\alpha_e = \frac{1}{137}$ and ATLAS data. Darker bands are scale uncertainties, lighter bands also include PDF uncertainty.
• Figure 4: Comparison between the prediction from PeTeR matched to JetPhox and ATLAS data. Darker bands are scale uncertainties, lighter bands also include PDF uncertainty.
• Figure 5: Comparison between the prediction from PeTeR including electroweak corrections and ATLAS data. Darker bands are scale uncertainties, lighter bands also include PDF uncertainty.