QCD anatomy of photon isolation

Energetic photons are a short-distance probe to search for new physics and test the standard model at colliders. In order to differentiate photons from different origins at hadron colliders, it is necessary to impose isolation requirements. These requirements restrict the hadronic radiation inside a cone around the photon. In this thesis, we present a factorization theorem for the QCD effects associated with photon isolation. We show that for small cone radius R, photon isolation effects can be captured by a fragmentation function describing the decay of a parton into a photon accompanied by hadronic radiation. We compute this fragmentation function for different isolation criteria. For small isolation energy, the cone fragmentation function factorizes further into jet and soft functions. Using renormalization-group methods, we resum the leading logarithms of R and the non-global logarithms of the ratio εγ of the isolation energy and the photon energy, so that we control all logarithmically enhanced terms in the cross section. Finally, in the limit of small R
and small εγ, we provide a simple formula to convert NNLO cross-section results from smooth-cone isolation to fixed-cone isolation.

2023

dissertation

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