Experimental activities in few-body physics

Understanding the few-nucleon system remains one of the challenges in modern nuclear and hadron physics. Observables in few-nucleon scattering processes are sensitive probes to study the two and many-body interactions between nucleons in nuclei. In the past decades, several facilities provided a large data base to study in detail the three-nucleon interactions below the pion-production threshold by exploiting polarized proton and deuteron beams and large-acceptance detectors. Only since recently, the four-nucleon scattering process at intermediate energies has been explored. In addition, there is a focus to collect data in the hyperon-nucleon sector, thereby providing access to understand the more general baryon-baryon interaction. In this contribution, some recent results in the few-nucleon sector are discussed together with some of the preliminary results from a pioneering and exclusive study of the four-nucleon scattering process. Furthermore, this paper discusses the experimental activities in the hyperon sector, in particular, the perspectives of the hyperon program of PANDA.Comment: 9 pages, 4 figures, proceedings of the 21st European Conference on Few Body Problems in Physics, Salamanca, Spain, 30 August - 3 September 201

Studying the ω\omega-mass in-medium in γ+A→π0γ+X\gamma + A\to \pi^0\gamma +X reactions

Simulations based on a coupled-channel transport model have been performed to analyze the feasibility to study the in-medium $\omega$ mass exploiting the process $\gamma+A\to\pi^0\gamma+X$ for C, Ca and Nb nuclei. The distortions due to final-state interactions of the $\pi^0$ and background contributions from the reaction $\gamma+A\to\pi^0\pi^0+X$ are found to be small in the mass range of interest ($0.6<M_{\pi^0\gamma}<0.8$ GeV). Furthermore, the effect of the detector resolution on the $\pi^0\gamma$-mass determination is discussed

Proton-deuteron radiative capture cross sections at intermediate energies

Differential cross sections of the reaction $p(d,^3{\rm He})\gamma$ have been measured at deuteron laboratory energies of 110, 133 and 180 MeV. The data were obtained with a coincidence setup measuring both the outgoing $^3$He and the photon. The data are compared with modern calculations including all possible meson-exchange currents and two- and three- nucleon forces in the potential. The data clearly show a preference for one of the models, although the shape of the angular distribution cannot be reproduced by any of the presented models.Comment: 6 pages, 6 figures, accepted for publication in EPJ

Measurement of Proton Electromagnetic Form Factors in e(+) e(-) -&gt; p(p)over-bar in the Energy Region 2.00-3.08 GeV

The process of e+e-→pp̄ is studied at 22 center-of-mass energy points (s) from 2.00 to 3.08 GeV, exploiting 688.5 pb-1 of data collected with the BESIII detector operating at the BEPCII collider. The Born cross section (σpp̄) of e+e-→pp̄ is measured with the energy-scan technique and it is found to be consistent with previously published data, but with much improved accuracy. In addition, the electromagnetic form-factor ratio (|GE/GM|) and the value of the effective (|Geff|), electric (|GE|), and magnetic (|GM|) form factors are measured by studying the helicity angle of the proton at 16 center-of-mass energy points. |GE/GM| and |GM| are determined with high accuracy, providing uncertainties comparable to data in the spacelike region, and |GE| is measured for the first time. We reach unprecedented accuracy, and precision results in the timelike region provide information to improve our understanding of the proton inner structure and to test theoretical models which depend on nonperturbative quantum chromodynamics

Evidence for a Neutral Near-Threshold Structure in the K S0 Recoil-Mass Spectra in e+e- → K S0 Ds+ D∗- and e+e- → K S0 Ds∗+ D-

We study the processes e+e-→KS0Ds+D∗- and e+e-→KS0Ds∗+D-, as well as their charge conjugated processes, at five center-of-mass energies between 4.628 and 4.699 GeV, using data samples corresponding to an integrated luminosity of 3.8 fb-1 collected by the BESIII detector at the BEPCII storage ring. Based on a partial reconstruction technique, we find evidence of a structure near the thresholds for Ds+D∗- and Ds∗+D- production in the KS0 recoil-mass spectrum, which we refer to as the Zcs(3985)0. Fitting with a Breit-Wigner line shape, we find the mass of the structure to be (3992.2±1.7±1.6) MeV/c2 and the width to be (7.7-3.8+4.1±4.3) MeV, where the first uncertainties are statistical and the second are systematic. The significance of the Zcs(3985)0 signal is found to be 4.6σ including both the statistical and systematic uncertainty. We report the Born cross section multiplied by the branching fraction at different energy points. The mass of the Zcs(3985)0 is close to that of the Zcs(3985)+. Assuming SU(3) symmetry, the cross section of the neutral channel is consistent with that of the charged one. Hence, we conclude that the Zcs(3985)0 is the isospin partner of the Zcs(3985)+

Measurement of the e+e- →π+π-J /ψ cross section in the vicinity of 3.872 GeV

We report a measurement of the cross section for the process e+e-→π+π-J/ψ around the X(3872) mass in search for the direct formation of e+e-→X(3872) through the two-photon fusion process. No enhancement of the cross section is observed at the X(3872) peak and an upper limit on the product of electronic width and branching fraction of X(3872)→π+π-J/ψ is determined to be Γee×B(X(3872)→π+π-J/ψ)&lt;7.5×10-3 eV at 90% confidence level under an assumption of total width of 1.19±0.21 MeV. This is an improvement of a factor of about 17 compared to the previous limit. Furthermore, using the latest result of B(X(3872)→π+π-J/ψ), an upper limit on the electronic width Γee of X(3872) is obtained to be &lt;0.32 eV at the 90% confidence level

Erratum to: Measurement of e + e − → ωπ + π − cross section at s = 2.000 to 3.080 GeV (Journal of High Energy Physics, (2023), 2023, 1, (111), 10.1007/JHEP01(2023)111)

The original article was published in Jan. 2023 (JHEP 01 (2023) 111), which the value of (Formula presented.) μ = (2.213 ± 0.106 ± 0.149) × 10−10 is not correct. It should be a!(Formula presented.) μ = (0.135 ± 0.007 ± 0.009)×10−10. The other parts of the paper are correct other than the a!(Formula presented.) μ value to be modified. The corresponding preprint is arXiv:2208.04507v3

Observation of the y (4230) and evidence for a new vector charmoniumlike state y (4710) in e+e- → KS0 KS0 J/ψ

Cross sections for the process e+e-→KS0KS0J/ψ at center-of-mass energies from 4.128 to 4.950 GeV are measured using data samples with a total integrated luminosity of 21.2 fb-1 collected by the BESIII detector operating at the BEPCII storage ring. The Y(4230) state is observed in the energy dependence of the e+e-→KS0KS0J/ψ cross section for the first time with a statistical significance of 26.0σ. In addition, an enhancement around 4.710 GeV, labeled as the Y(4710), is seen with a statistical significance of 4.2σ. There is no clear structure around 4.484 GeV. Using a fit with a coherent sum of three Breit-Wigner functions, we determine the mass and width of the Y(4230) state to be 4226.9±6.6±22.0 MeV/c2 and 71.7±16.2±32.8 MeV, respectively, and the mass and width of the Y(4710) state to be 4704.0±52.3±69.5 MeV/c2 and 183.2±114.0±96.1 MeV, respectively, where the first uncertainties are statistical and the second are systematic. In addition, the average Born cross section ratio σBorn(e+e-→KS0KS0J/ψ)σBorn(e+e-→K+K-J/ψ) is measured to be 0.388-0.028+0.035±0.016, or 0.426-0.031+0.038±0.018 if three-body phase space is considered