Francesco P. Ucci

We compute the next-to-leading order corrections to the radiative return process e + e − → π + π − γ using a dispersive representation of the pion form factor for the calculation of loop integrals. We compare our results with those of the factorized scalar QED approach, as well as with previous predictions obtained by us in the generalized vector meson dominance model. We show the numerical impact of the structure-dependent corrections on various observables of interest for radiative return experiments at flavor factories. Following recent input from the literature, we include in our calculation also corrections due to neutral pseudoscalar poles, and we provide a first estimate of such contributions. We also investigate additional mechanisms contributing to final-state radiation at center-of-mass energies around the ϕ -meson resonance, such as radiative ϕ decays, presenting numerical results that are relevant for the KLOE experiment. These novel features are implemented in the Monte Carlo event generator abaaga@, which can now be used to evaluate the impact of the modeling of pion-photon interaction in radiative return measurements.

A bstract: The radiative return, together with the energy scan, is the method used at flavour factories to measure the pion form factor, which is a crucial input for the data-driven dispersive computation of the leading-order hadronic contribution to the muon anomalous magnetic moment. We consider the radiative hadronic and leptonic channels of main experimental interest, namely the processes e ⁺ e ⁻ → X ⁺ X ⁻ γ , with X = { π , μ }. For such processes, we compute the exact next-to-leading order (NLO) corrections matched to a Parton Shower (PS) to describe exclusive multiple photon emission. All sources of radiative corrections from initial-state and final-state radiation, as well as their interference, are considered according to QED for e ⁺ e ⁻ → μ ⁺ μ ⁻ γ and QED⨁F×sQED (Factorised scalar QED) for e ⁺ e ⁻ → π ⁺ π ⁻ γ . We describe in detail the novel features of our PS approach to compute the fixed-order corrections in association with higher-order contributions to 2 → 3 processes, with a hard photon in the final state. We present validation tests and comparisons with NLO predictions available in the literature to cross-check various ingredients of our formulation. We also show numerical results at NLOPS accuracy according to realistic event selection criteria for precision measurements at flavour factories. Our calculation is implemented in an updated version of the Monte Carlo event generator B aba Y aga @NLO, which can be used for fully exclusive simulations and data analysis in radiative return experiments.

Volume 1 of the FCC Feasibility Report presents an overview of the physics case, experimental programme, and detector concepts for the Future Circular Collider (FCC). This volume outlines how FCC would address some of the most profound open questions in particle physics, from precision studies of the Higgs and EW bosons and of the top quark, to the exploration of physics beyond the Standard Model. The report reviews the experimental opportunities offered by the staged implementation of FCC, beginning with an electron-positron collider (FCC-ee), operating at several centre-of-mass energies, followed by a hadron collider (FCC-hh). Benchmark examples are given of the expected physics performance, in terms of precision and sensitivity to new phenomena, of each collider stage. Detector requirements and conceptual designs for FCC-ee experiments are discussed, as are the specific demands that the physics programme imposes on the accelerator in the domains of the calibration of the collision energy, and the interface region between the accelerator and the detector. The report also highlights advances in detector, software and computing technologies, as well as the theoretical tools/reconstruction techniques that will enable the precision measurements and discovery potential of the FCC experimental programme. The content and structure of this report are guided by the scope and priorities defined in the mandate of the FCC Feasibility Study. It is therefore not intended to serve as an exhaustive review of the full physics potential of FCC. Several topics, already covered in earlier reports such as the FCC CDR, are not reiterated here or are addressed only briefly, in alignment with the study’s focus. This volume reflects the outcome of a global collaborative effort involving hundreds of scientists and institutions, aided by a dedicated community-building coordination, and provides a targeted assessment of the scientific opportunities and experimental foundations of the FCC programme.

Several key observables of the high-precision physics program at future lepton colliders will critically depend on the knowledge of the absolute machine luminosity. The determination of the luminosity relies on the precise knowledge of some reference process, which is, in principle, not affected by unknown physics, so that its cross section can be computed within a well-established theory, like the Standard Model. Quantifying the uncertainties induced by possible new physics effects on such processes is, therefore, crucial. We present an investigation of light and heavy new physics contributions to the small-angle Bhabha process at future e + e − colliders, and we discuss possible strategies to remove the contamination due to heavy degrees of freedom by relying on observables that are independent of the absolute luminosity.

The pion pair production in e ⁺ e ⁻ annihilation at flavour factories plays a crucial role in the determination of the hadronic contribution to the muon anomalous magnetic moment. The recent CMD-3 measurement of the pion form factor via energy scan displays a significant difference with the previous experimental determinations. In order to contribute to an improved theoretical description and simulation of energy scan experiments, we present a calculation of the e ⁺ e ⁻ → π ⁺ π ⁻(γ) hadronic channel at next-to-leading order matched to a Parton Shower algorithm in QED and sQED. According to the recent advances in the literature, particular attention is paid to the treatment of the pion composite structure in loop diagrams beyond the commonly used factorised sQED approach, as well as to the modelling of multiple photon radiation through the Parton Shower algorithm. In particular, we carry out a detailed discussion on the inclusion of the pion form factor in the virtual sQED corrections according to two independent methods, inspired by the generalised vector meson dominance model and the dispersive approach, respectively. We find the two methods to be in remarkable agreement. We show phenomenological results for inclusive and differential observables which are relevant for precision energy scan measurements, focusing on the impact of the radiative corrections and the effect of the various approaches for the treatment of the pion form factor. Our calculation is implemented in an updated version of the Monte Carlo event generator BabaYaga@NLO, that can be used for fully exclusive simulations in data analysis.