Point-form relativistic quantum mechanics is used to derive an expression for the electromagnetic form factor of a pseudoscalar meson for spacelike momentum transfers. The elastic scattering of an electron by a confined quark-antiquark pair is treated as a relativistic two-channel problem for the qqe and qqeγ states. With the approximation that the total velocity of the qqe system is conserved at (electromagnetic) interaction vertices this simplifies to an eigenvalue problem for a Bakamjian-Thomas type mass operator. After elimination of the qqeγ channel the electromagnetic meson current and form factor can be directly read off from the one-photon-exchange optical potential. By choosing the invariant mass of the electron-meson system large enough, cluster separability violations become negligible. An equivalence with the usual front-form expression, resulting from a spectator current in the q + = 0 reference frame, is established. The generalization of this multichannel approach to electroweak form factors for an arbitrary bound few-body system is quite obvious. By an appropriate extension of the Hilbert space this approach is also able to accommodate exchange-current effects.

In 2017, optics commissioning strategy for low-β Ã operation of the CERN Large Hadron Collider (LHC) underwent a major revision. This was prompted by a need to extend the scope of beam-based commissioning at high energy, beyond the exclusively linear realm considered previously, and into the nonlinear regime. It also stemmed from a recognition that, due to operation with crossing angles in the experimental insertions, the linear and nonlinear optics quality were intrinsically linked through potentially significant feed-down at these locations. Following the usual linear optics commissioning therefore, corrections for (normal and skew) sextupole and (normal and skew) octupole errors in the high-luminosity insertions were implemented. For the first time, the LHC now operates at top energy with beam-based corrections for nonlinear dynamics, and for the effect of the crossing scheme on beta-beating and dispersion. The new commissioning procedure has improved the control of various linear and nonlinear characteristics of the LHC, yielding clear operational benefits.

A new type of fast luminosity separation scans ("Emittance Scans") was introduced at the CERN Large Hadron Collider (LHC) in 2015. The scans were performed systematically in every fill with full-intensity beams in physics production conditions at the Interaction Point (IP) of the Compact Muon Solenoid (CMS) experiment. They provide both transverse emittance and closed orbit measurements at a bunch-by-bunch level. The precise measurement of beam-beam closed orbit differences allowed a direct, quantitative observation of long-range beam-beam PACMAN effects, which agrees well with numerical simulations from an improved version of the TRAIN code. II. BUNCH-BY-BUNCH DIAGNOSTICS AT THE LHC A. Luminosity MeasurementsIn the following, the main observable used for the analysis of Emittance Scans is the bunch-by-bunch luminosity as

Recently we have reconsidered the quantization of relativistic field theories on a Lorentz-invariant surface of the form x µ x µ = τ 2 [1]. With this choice of the quantization surface all components of the 4-momentum operator become interaction dependent, whereas the generators of Lorentz transformations stay free of interactions -a feature characteristic for Dirac's "point form" of relativistic dynamics. Thus we speak of "point-form quantum field theory" (PFQFT). Old papers on PFQFT (see, e.g., [2,3]) dealt mainly with the evolution of quantum fields in the parameter τ and made use of a Fock-space basis which is related to the generators of the Lorentz group. Such a choice for the basis and the "time parameter", however, gave rise to conceptual difficulties. To avoid these problems we have kept the usual momentum basis and considered evolution of the system as generated by the 4-momentum operator [1]. In this way we were able to show for free fields that quantization on the space-time hyperboloid x µ x µ = τ 2 leads to the same Fock-space representation of the Poincaré generators as equaltime quantization. Moreover, we have suggested a generalized interaction picture which leads to a manifestly Lorentz covariant expression for the scattering operator as path-ordered exponential of the interaction part of the 4-momentum operator (along arbitrary timelike paths). We furthermore showed that the perturbative expansion of the scattering operator, defined in such a way, is (order by order) equivalent to usual time-ordered perturbation theory.The nice feature that the operator formalism becomes manifestly Lorentz covariant if fields are quantized on the space-time hyperboloid x µ x µ = τ 2 was not our only motivation to study PFQFT. PFQFT serves also as a natural starting point for the construction of effective interactions, currents, etc., which can be applied to point-form quantum mechanics. The main difficulty of finding a quantum mechanical realization of the Poincaré algebra, which describes a finite number of interacting particles, is caused by the fact that interaction terms in the Poincaré generators have to satisfy non-linear constraints, in general. 1 A procedure that resolves this problem has been proposed by Bakamjian and * E-mail address: wolfgang.schweiger@uni-graz.at 1 These constraints are automatically satisfied if a local interacting field theory is quantized.

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