Comparison of SANC with KORALZ and PHOTOS

TL;DR

Addresses the need for precise one-loop electroweak predictions including real photon emissions in B → f fbar γ decays. The authors compare SANC with KORALZ for Z decays and with PHOTOS for W and Higgs decays, using identical initializations and a common set of observables. They report sub-percent agreement for Z decays, and leading-log consistency with PHOTOS for W and H decays, with Higgs showing near-perfect agreement and W showing larger non-leading discrepancies, enabling a quantified uncertainty assessment of PHOTOS. The results validate SANC as a reliable one-loop SM calculator and offer practical guidance for LEP2/LHC phenomenology and for improving bremsstrahlung treatments in PHOTOS.

Abstract

Using the SANC system we study the one-loop electroweak standard model prediction, including virtual and real photon emissions, for the decays of on-shell vector and scalar bosons B --> f anti-f (gamma), where B is a vector boson, Z or W, or a Standard Model Higgs. The complete one-loop corrections and exact photon emission matrix element are taken into account. For the phase-space integration, the Monte Carlo technique is used. For Z decay the QED part of the calculation is first cross-checked with the exact one-loop QED prediction of KORALZ. For Higgs boson and W decays, a comparison is made with the approximate QED calculation of PHOTOS Monte Carlo. This provides a useful element for the evaluation of the theoretical uncertainty of PHOTOS, very interesting for its application in ongoing LEP2 and future LC and LHC phenomenology.

Figures (6)

• Figure 1: Comparisons (ratios) of the SANC and KORALZ predictions for the $Z$ decay. Observables A and B: ratios of the photon energy (left-hand side) and muon energy (right-hand side) distributions from the two programs. The dominant contribution is of leading-log (collinear) nature.
• Figure 2: Comparisons (ratios) of the SANC and KORALZ predictions for the $Z$ decay. Observables C and D: ratios of the photon angle with respect to $\mu^-$ (left-hand side) and $\mu^-\mu^+$ acollinearity (right-hand side) distributions from the two programs. The dominant contribution is of infrared non-leading-log nature for the left-hand side plot, and non-infrared non-leading-log nature for the right-hand side one.
• Figure 3: Comparisons (ratios) of the SANC and PHOTOS predictions for the $H$ decay. Observables A and B: ratios of the photon energy (left-hand side) and muon energy (right-hand side) distributions from the two programs. The dominant contribution is of leading-log (collinear) nature.
• Figure 4: Comparisons (ratios) of the SANC and PHOTOS predictions for the $H$ decay. Observables C and D: ratios of the photon angle with respect to $\mu^-$ (left-hand side) and $\mu^-\mu^+$ acollinearity (right-hand side) distributions from the two programs. The dominant contribution is of infrared non-leading-log nature for the left-hand side plot, and non-infrared non-leading-log nature for the right-hand side one.
• Figure 5: Comparisons (ratios) of the SANC and PHOTOS predictions for the $W$ decay. Observables A and B: ratios of the photon energy (left-hand side) and muon energy (right-hand side) distributions from the two programs. The dominant contribution is of leading-log (collinear) nature.