Final state photon production at LEP

Abstract

We present a detailed study of photon production in hadronic events in electronpositron annihilation at LEP energies. We show that estimates of the inclusive photon spectrum using the quark-to-photon fragmentation function determined using the ALEPH 'photon' + 1 jet data agree well with the observations of the OPAL collaboration. This agreement shows that the photon fragmentation function determined in this way can be used for inclusive observables. We also compare next-to-leading order and beyond leading logarithm predictions obtained using the numerically resummed solutions of the fragmentation function evolution equation of Bourhis, Fontannaz and Guillet and Glück, Reya and Vogt with the data. Moreover, in order to check the general behaviour of the fragmentation function, we consider an analytic series expansion in the strong coupling. We find that the parameterizations are inaccurate at large x values. While the OPAL data is in broad agreement with estimates based on any of these approaches, the ALEPH data prefers the resummed BFG parameterization. Finally, there is some ambiguity as to whether the fragmentation function is treated as O(α) or O(α/α s ). We show that at present this ambiguity affects mainly the prediction for the 'photon' + 1 jet rate at large z.

NLO QCD+EW predictions for V + jets including off-shell vector-boson decays and multijet merging

“…For the definition of jets we employ the anti-k T algorithm [82] with R = 0.4. More precisely, in order to guarantee infrared safeness in presence of NLO QCD and EW corrections, we adopt a democratic clustering approach [83][84][85], where QCD partons and photons are recombined. In order to ensure the cancellation of collinear singularities that arise from collinear photon emissions off charged leptons and quarks, collinear pairs of photons and charged fermions with ∆R γf < 0.1 are recombined via four-momentum addition, and all observables are defined in terms of such dressed fermions.…”

Diphoton production at the LHC: a QCD study up to NNLO

“…2-4. The QCD calculation that includes NLO direct+LO fragmentation components is carried out by using the MCFM program [31], and we use either the NLO fragmentation functions of the BFG set II [92] (as in our customary NLO calculations) or the LO fragmentation functions of the GdRG LO set [96]. At the central value of the scales and using BFG fragmentation functions, we obtain the following values of total cross sections: σ M CF M standard = 29.66 pb with E T max = 2 GeV, and σ M CF M standard = 28.46 pb with E T max = 10 GeV (the GdRG LO set of fragmentation functions leads to results that are larger by approximately 1 pb).…”

NLO electroweak automation and precise predictions for W + multijet production at the LHC

“…In presence of NLO EW corrections, it is clear that the requirement of IR safeness needs to be extended to the singularities associated with q → qγ, q →qγ and γ → qq QED splittings. In principle, this can be easily achieved through the so-called democratic jet clustering approach [71][72][73], where photons and QCD partons are handled on the same footing at each clustering step. Jets resulting from democratic clustering contain photons, quarks and gluons, and their four-momenta are determined by the sum of all jet constituents, including photons.…”