## RSS Few-Body Systems

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1. ### Abstract

These proceedings report the current plans to upgrade the ATLAS detector at CERN for the High Luminosity LHC (HL-LHC). The HL-LHC is expected to start operations in the middle of 2026, aiming to reach an ultimate peak instantaneous luminosity of $$7.5\times 10^{34}~\hbox {cm}^{-2}~\hbox {s}^{-1}$$ , corresponding to approximately 200 inelastic proton–proton collisions per bunch crossing, and to deliver over a period of twelve years more than ten times the integrated luminosity of the large hadron collider (LHC) Runs 1–3 combined (up to $$4000\,\text{ fb }^{-1}$$ ). This is a huge challenge to all sub-systems of the detector which will need extensive upgrades to allow the experiment to pursue a rich and interesting physics programme in the future.

2. ### Abstract

The Efimov effect for three bosons in three dimensions requires two infinitely large s-wave scattering lengths. We assume two identical particles with very large scattering lengths interacting with a third particle. We use a novel mathematical technique where the centrifugal barrier contains an effective dimension parameter, which allows efficient calculations precisely as in ordinary three spatial dimensions. We investigate properties and occurrence conditions of Efimov states for such systems as functions of the third scattering length, the non-integer dimension parameter, mass ratio between unequal particles, and total angular momentum. We focus on the practical interest of the existence, number of Efimov states and their scaling properties. Decreasing the dimension parameter from 3 towards 2 the Efimov effect and states disappear for critical values of mass ratio, angular momentum and scattering length parameter. We investigate the relations between the four variables and extract details of where and how the states disappear. Finally, we supply a qualitative relation between the dimension parameter and an external field used to squeeze a genuine three dimensional system.

3. ### Abstract

BGO–OD is a new experiment at the ELSA accelerator at the University of Bonn. It combines a large-aperture forward magnetic spectrometer with a central BGO calorimeter, thus providing almost 4 $$\pi$$ charged particle tracking and high energy-resolution photon detection. This makes the setup ideally suited to investigate the photoproduction of forward going mesons with residual ground or excited state baryons remaining with little momentum transfer. These kinematics are especially interesting for associated strangeness photoproduction, and first results are presented.

4. ### Abstract

The exclusive electroproduction process $$ep \rightarrow e'p'\pi ^{0}$$ was measured in the range of the photon virtualities $$Q^{2} = 0.4$$ –1.0 GeV $$^{2}$$ , and the invariant mass range of the $$p\pi ^{0}$$ system $$W = 1.1$$ –1.8 GeV. For the first time these kinematics are covered in exclusive $$\pi ^{0}$$ electroproduction off the proton with the nearly complete angular coverage in the $$p\pi ^{0}$$ center of mass system with high statistics. The cross section was measured and the structure functions $$\sigma _T+\epsilon \sigma _L$$ , $$\sigma _{TT}$$ and $$\sigma _{LT}$$ were extracted via fitting the $$\phi ^{*}_{\pi ^{0}}$$ dependance. Analysis of these results has revealed the data sensitivity to the contribution from the nucleon resonances $$N(1650)1/2^-$$ , $$N(1685)5/2^{+}$$ , and $$\varDelta (1700)3/2^{-}$$ . Combined studies of $$\pi ^{+}n$$ and $$\pi ^{0}p$$ electroproduction off proton data from CLAS at $$W>1.6$$  GeV will provide the first results on the high lying $$N^{*}$$ and $$\varDelta$$ electrocouplings at $$Q^{2}$$ < 1.0 GeV $$^{2}$$ for all excited nucleons with substantial decays to the $$N\pi$$ final states. These new experimental data will extend the insight into the complex interplay between the inner quark core and outer meson–baryon cloud in the structure of nucleon resonances with masses above 1.6 GeV.

5. ### Abstract

For the flavor-singlet heavy quark system of bottomonia, we compute the masses of the ground state mesons in four different channels, namely, pseudo-scalar ( $$\eta _{b}(1S)$$ ), vector ( $$\varUpsilon (1S)$$ ), scalar ( $$\chi _{b_0}(1P)$$ ) and axial vector ( $$\chi _{b_{1}}(1P)$$ ). We also calculate the weak decay constants of the $$\eta _{b}(1S)$$ and $$\varUpsilon (1S)$$ as well as the charge radius of $$\eta _{b}(1S)$$ . It complements our previous study of the corresponding charmonia systems: $$\eta _c(1S)$$ , $$J/\varPsi (1S)$$ , $$\chi _{c_0}(1P)$$ ) and ( $$\chi _{c_{1}}(1P)$$ ). The unified formalism for this analysis is provided by a symmetry-preserving Schwinger–Dyson equations treatment of a vector  $$\times$$  vector contact interaction. Whenever a comparison is possible, our results are in fairly good agreement with experimental data, model calculations based upon Schwinger–Dyson and Bethe–Salpeter equations involving sophisticated interaction kernels as well as Lattice QCD. Within the same framework, we also report the elastic and transition form factors to two photons for the pseudo-scalar channels $$\eta _{c}(1S)$$ and $$\eta _{b}(1S)$$ in addition to the elastic form factors for the vector mesons $$J/\varPsi$$ and $$\varUpsilon$$ for a wide range of photon momentum transfer squared ( $$Q^2$$ ). For $$\eta _{c}(1S)$$ and $$\eta _{b}(1S)$$ , we also provide predictions of an algebraic model which correlates remarkably well between the known infrared and ultraviolet limits of these form factors.