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.
Highlights
If we denote everything other than the photon in the event as X, by pure kinematical considerations, the mass of X, MX = p2X, must go to zero as ET → ECM/2 and the energy of X must approach ECM/2
As the jet mass translates directly into ET, we can use the domination of the mass by soft and collinear physics, which are well understood in QCD, to see that the photon ET spectrum is dominated by soft and collinear physics
The resummation of large logarithms was done to nextto-leading logarithmic order (NLL) [9,10,11] in the late 1990s
Summary
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. Including the associated large logarithms, resummed to all orders, leads to a a precision calculation beyond the NLO level. The effect of including these logs is to lower the direct photon cross section at high ET by up to around 10%.
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