The three-nucleon system as a laboratory for nuclear physics: the need for 3N forces

Recent experimental results in three-body systems have unambiguously shown that calculations based on nucleon-nucleon forces fail to accurately describe many experimental observables and one needs to include effects which are beyond the realm of the two-body potentials. This conclusion owes its significance to the fact that experiments and calculations can both be performed with a high accuracy. In this short review, a sample of recent experimental results along with the results of the state-of-the-art calculations will be presented and discussed.Comment: Commissioned article for Nuclear Physics News, 8 pages, 6 figure

WASA-FRS EXPERIMENTS IN FAIR PHASE-0 AT GSI

We have developed a new and unique experimental setup integrating the central part of the Wide Angle Shower Apparatus (WASA) into the Fragment Separator (FRS) at GSI. This combination opens up possibilities of new experiments with high-resolution spectroscopy at forward 0◦ and measurements of light decay particles with nearly full solid-angle acceptance in coincidence. The first series of the WASA-FRS experiments have been successfully carried out in 2022. The developed experimental setup and two physics experiments performed in 2022 including the status of the preliminary data analysis are introduced.</p

Observation of a Vector Charmoniumlike State at 4.7 GeV/c2 and Search for Zcs in e+e- →k+K-J/ψ

Using data samples with an integrated luminosity of 5.85 fb-1 collected at center-of-mass energies from 4.61 to 4.95 GeV with the BESIII detector operating at the BEPCII storage ring, we measure the cross section for the process e+e-→K+K-J/ψ. A new resonance with a mass of M=4708-15+17±21 MeV/c2 and a width of Γ=126-23+27±30 MeV is observed in the energy-dependent line shape of the e+e-→K+K-J/ψ cross section with a significance over 5σ. The K+J/ψ system is also investigated to search for charged charmoniumlike states, but no significant Zcs+ states are observed. Upper limits on the Born cross sections for e+e-→K-Zcs(3985)+/K-Zcs(4000)++c.c. with Zcs(3985)±/Zcs(4000)±→K±J/ψ are reported at 90% confidence levels. The ratio of branching fractions {[B(Zcs(3985)+→K+J/ψ)]/B[Zcs(3985)+→(D¯0Ds∗++D¯∗0Ds+)]} is measured to be less than 0.03 at 90% confidence level. © 2023 authors. Published by the American Physical Society

Matter radius of the doubly-magic ⁵⁶ Ni measured in a storage ring

The matter radius of the doubly magic 56Ni was extracted from a measurement of the differential cross section by employing, for the first time, elastic proton scattering in inverse kinematics with a radioactive beam at Ekin= 390.2 MeV/nucleon circulating in a storage ring and passing an internal hydrogen gas-jet target with a revolution frequency of around 2 MHz. The novel experimental scheme is based on UHV-compatible Si detectors operated as active vacuum windows, which were implemented in the ESR storage ring at GSI. A matter radius &lt;rm2&gt;1/2=3.74-0.06+0.03 fm was extracted for the doubly-magic self-conjugate nucleus 56Ni.</p

Observation of a Vector Charmoniumlike State at 4.7 GeV/c2 and Search for Zcs in e+e- →k+K-J/ψ

Using data samples with an integrated luminosity of 5.85 fb-1 collected at center-of-mass energies from 4.61 to 4.95 GeV with the BESIII detector operating at the BEPCII storage ring, we measure the cross section for the process e+e-→K+K-J/ψ. A new resonance with a mass of M=4708-15+17±21 MeV/c2 and a width of Γ=126-23+27±30 MeV is observed in the energy-dependent line shape of the e+e-→K+K-J/ψ cross section with a significance over 5σ. The K+J/ψ system is also investigated to search for charged charmoniumlike states, but no significant Zcs+ states are observed. Upper limits on the Born cross sections for e+e-→K-Zcs(3985)+/K-Zcs(4000)++c.c. with Zcs(3985)±/Zcs(4000)±→K±J/ψ are reported at 90% confidence levels. The ratio of branching fractions {[B(Zcs(3985)+→K+J/ψ)]/B[Zcs(3985)+→(D¯0Ds∗++D¯∗0Ds+)]} is measured to be less than 0.03 at 90% confidence level. © 2023 authors. Published by the American Physical Society

Measurement of the cross section of e + e − → ηπ + π − at center-of-mass energies from 3.872 GeV to 4.700 GeV

Using data samples with an integrated luminosity of 19 fb−1 at twenty-eight center-of-mass energies from 3.872 GeV to 4.700 GeV collected with the BESIII detector at the BEPCII electron-positron collider, the process e+e− → ηπ+π− and the intermediate process e+e− → ηρ0 are studied for the first time. The Born cross sections are measured. No significant resonance structure is observed in the cross section lineshape. [Figure not available: see fulltext.]

Experimental Studies of Few-nucleon Systems at Intermediate Energies

Systems composed of 3 nucleons are a subject of precise experimental studies for many years. At the first stage the investigations were mainly focused on elastic nucleon-deuteron scattering, slowly extending to systematic measurements of the deuteron breakup reaction. Intermediate energies, below the threshold for pion production, deserve special attention: it is the region where comparison with exact theoretical calculations is possible, while the sensitivity to various aspects of interaction, like subtle effects of the dynamics beyond the pairwise nucleon-nucleon force, is significant. Moreover, the Coulomb interaction and relativistic effects show their influence in the observables of the breakup reaction. All these effects vary with energy and appear with different strength in certain observables and phase space regions, what calls for systematic investigations of a possibly rich set of observables determined in a wide range of energies. The next step in complication of the system are studies of reactions involving 4 nucleons—more sensitive, as expected, to subtle dynamics beyond the pairwise interaction. A brief survey of recent and planned experiments in the 3- and 4-nucleon systems is given