Mesic Nuclei Research Articles

Probing the existence of η3He mesic nucleus with a few-body approach

Motivated by recent observations in the WASA-at-COSY detector, we investigate the η3He nucleus using the ηNNN few body method. We construct an effective s-wave energy dependent ηN potential that reproduces the ηN subthreshold scattering amplitude as per the 2005 Green-Wycech model, yielding the η separation energy and decay width of 0.19 MeV and 1.71 MeV, respectively. Additionally, we construct various sets of effective s-wave energy independent ηN potentials, where the corresponding complex scattering lengths (aηN) are fallen within the range provided in most theoretical models. Our investigation aims to explore the potential existence of a bound η3He nucleus under these scattering lengths, and to calculate the resulting binding energies and decay widths. Our findings indicate that the η3He nucleus is bound by 1.65 MeV with a decay width of about 5 MeV when aηN is (1.0+0.3i) fm, and by 1.46 MeV with a decay width of about 10 MeV when aηN is (1.0+0.5i) fm.

The WASA-FRS project at GSI and its perspective

A novel technique to study bound states of exotic hadrons in subatomic nuclei, such as hypernuclei and mesic nuclei, has been developed by employing the Fragment Separator FRS and the WASA central detector at GSI. Two experiments, S447 for studying light hypernuclei, especially hypertriton and a Λnn bound state, and S490 for searching for η’ mesic-nuclei, were recently performed. Data analyses are currently in progress, and light charged particles such as protons and π± are clearly observed and identified in the both experiments. For S447, light nuclear fragments that can also be residual nuclei from decays of hypernuclei of interests have been analysed by the FRS, and a momentum resolution, Δp/p, of 5×10−4 has been achieved. Further data analyses are to be completed. The WASA-FRS project will be continued and extended with the FRS at FAIR Phase 0, and upgrading of the WASA magnet and detectors is currently in progress. Furthermore, construction of a larger detector system with the Super-FRS at FAIR Phase 1 is also under consideration.

Search for η-mesic nuclei: a review of experimental and theoretical advances

In the mid-1980s, theoreticians predicted possible exotic states of an eta (η) meson and a nucleus bound by the strong interaction, so-called mesic nucleus. This led to extensive experimental efforts aimed at discovering such unstable states as well as understanding the underlying interactions via η meson production reactions. As experiments yielded new data, more and more sophisticated theoretical models were developed to understand the fundamental η-nucleon and η-nucleus interactions. After more than 35 years of research in this field, several signals have been identified as possible indications of hypothetical η-mesic nuclei, but there is still no general agreement or clear confirmation of their existence. In the paper, we review the study of the η-mesic nuclei with an emphasis on recent research activities.

Status of the Search for $$\eta $$-Mesic Nuclei with Particular Focus on $$\eta $$-Helium Bound States

In this paper the search for eta-mesic nuclei with particular focus on light $\eta$-He bound states is reviewed. A brief description of recent theoretical studies and experimental results is presented.

Search for η’-mesic nuclei using (p,d) reaction with FRS/Super-FRS at GSI/FAIR

We plan a semi-exclusive measurement of the 12C(p,dp) reaction to search for η’-mesic nuclei, aiming at investigating in-medium properties of the η′-meson. We employ a 2.5 GeV proton beam impinging on a carbon target to produce η′-mesic 11C nuclei via the 12C(p,d)η′⊗11C reaction. Using coincidence measurements of the forward going deuterons, important for missing-mass spectroscopy, and decay protons emitted from the η′-mesic nuclei. for event selection will provide a high experimental sensitivity to observe η’-mesic nuclei. We will perform the measurements by combining the WASA detector system with the fragment separator FRS at GSI and also with the Super-FRS at FAIR in the future. The plan of the experiments and the present status are reported.

Total and differential cross sections of the dp→He3η reaction at excess energies between 1 and 15 MeV

New high precision total and differential cross sections are reported for the $dp\to {}^3\textrm{He}\,\eta$ reaction close to threshold. The measurements were performed using the magnetic spectrometer ANKE, which is an internal fixed target facility at the COSY cooler synchrotron. The data were taken for deuteron beam momenta between $3.14641~\textrm{GeV}/c$ and $3.20416~\textrm{GeV}/c$, which corresponds to the range in excess energy $Q$ for this reaction between $1.14~\textrm{MeV}$ and $15.01~\textrm{MeV}$. The normalization was established through the measurement in parallel of deuteron-proton elastic scattering and this was checked through the study of the $dp\to {}^3\textrm{He}\,\pi^0$ reaction. The previously indicated possible change of sign of the slope of the differential cross sections near the production threshold, which could be explained by a rapid variation of the $s$- and $p$-wave interference term, is not confirmed by the new data. The energy dependence of the total cross section and the $90^{\circ}$ slope parameter are well explained by describing the final state interaction in terms of a complex Jost function and the results are significant in the discussion of $\eta$-mesic nuclei. In combination with recently published WASA-at-COSY data [P. Adlarson $et\, al.$, Phys. Lett. B 782, 297 (2018)], a smooth variation of the slope parameter is achieved up to an excess energy of $80.9~\textrm{MeV}$.

Search for the η mesic He3 in the pd→dpπ0 reaction with the WASA-at-COSY facility

The excitation function for the $pd\rightarrow$ $d p \pi^{0}$ reaction has been measured by WASA-at-COSY experiment with the aim of searching for $^{3}\hspace{-0.03cm}\mbox{He}$-$\eta$ mesic nuclei. The measurement in the vicinity of $\eta$ meson production was performed using a ramped proton beam. The data analysis and interpretation was carried out with the assumption that the $\eta$-mesic Helium decays via the formation of an intermediate N$^{*}$(1535) resonance. No direct signal of the $\eta$-mesic nucleus is observed in the excitation function. We determine a new improved upper limit for the total cross section for the bound state production and decay in the $pd\rightarrow$ $(^{3}\hspace{-0.03cm}\mbox{He}$-$\eta)_{bound} \rightarrow$ $d p \pi^{0}$ process. It varies between 13 nb to 24 nb for the bound state with width in the range $\Gamma \in (5,50)$ MeV.

Spectroscopy of $\eta ^{\prime }$-mesic Nuclei with WASA at GSI/FAIR

We plan to conduct an experimental search for η0-mesic nuclei in order to investigate in-medium properties of the η0 meson. A 2.5 GeV proton beam is employed to produce η0-mesic 11C nuclei with the 12C(p,d)η0⊗ 11C reaction. Simultaneous measurements of the forward ejected deuterons and decay protons from η0-mesic nuclei will allow us to achieve high experimental sensitivity. The experiment will be performed at GSI by making full use of the fragment separator FRS and the WASA detector system. The plan of this proposed experiment is described.

Nuclear Deformation Effects to the Formation Cross Section of $\eta '$ Mesic Nucleus

Nuclear Deformation Effects to the Formation Cross Section of $\eta '$ Mesic Nucleus

Search for η-mesic nuclei in the SRC/BM@N experiment at the Nuclotron

The SRC/BM@N experiment was carried out in the 55th run of the Nuclotron using a liquid hydrogen target and a carbon beam with a kinetic energy of about 3.1 GeV/n. We propose to analyze the experimental data to search for a quasi-bound state of η-meson and nucleons in the reaction 12C+p → η(A-1)+X → π+p+(A-2). To achieve this goal, it is necessary to identify the residual nucleus (A-2) and the proton-pion pair formed from η-nuclei decay.

Luminosity determination for the proton-deuteron reaction using pd → 3Heη channel with WASA-at-COSY detector

In this report one of the methods used for the luminosity determination for the experiment performed by WASA Collaboration to search for 3He-η mesic nuclei is presented. The method is based on the analysis of pd → 3Heη process.

$\eta$ η -nucleus interaction from the d + d reaction around the $\eta$ η production threshold

The $\eta$ mesic nucleus is considered to be one of the interesting exotic many body systems and has been studied since 1980's theoretically and experimentally. Recently, the formation of the $\eta$ mesic nucleus in the fusion reactions of the light nuclei such as $d + d \rightarrow (\eta + \alpha) \rightarrow X$ has been proposed and the experiments have been performed by WASA-at-COSY. We develop a theoretical model to evaluate the formation rate of the $\eta$ mesic nucleus in the fusion reactions and show the calculated results. We find that the $\eta$ bound states could be observed in the reactions in cases with the strong attractive and small absorptive $\eta$-nucleus interactions. We compare our results with existing data of the $d + d \rightarrow \eta + \alpha$ and the $d + d \rightarrow {^3 \rm He} + N + \pi$ reactions. We find that the analyses by our theoretical model with the existing data can provide new information on the $\eta$-nucleus interaction.

Mesic nuclei with a heavy antiquark

The binding of a hadron and a nucleus is a topic of great interest for investigating hadron properties. In the heavy-flavor region, attraction between a |$P(=\bar{D},B)$| meson and a nucleon |$N$| can appear, where |$PN$|–|$P^\ast N$| mixing plays an important role in relation to the heavy-quark spin symmetry. The attraction can produce exotic heavy mesic nuclei that are stable against strong decay. We study an exotic system where the |$\bar{D}$| (⁠|$B$|⁠) meson and nucleus are bound. The meson–nucleus interaction is given by a folding potential with single-channel |$PN$| interaction and the nucleon number distribution function. By solving the Schrödinger equations of the heavy meson and the nucleus, we obtain several bound and resonant states for nucleon number |$A=16,\ldots,208$|⁠. The results indicate the possible existence of exotic mesic nuclei with a heavy antiquark.

Search for η-mesic [formula omitted] in the [formula omitted] and [formula omitted] reactions with the WASA-at-COSY facility

The search for He4-η bound states was performed with the WASA-at-COSY facility via the measurement of the excitation function for the dd→He3nπ0 and dd→He3pπ− processes. The deuteron beam momentum was varied continuously between 2.127 GeV/c and 2.422 GeV/c, corresponding to the excess energy for the dd→He4η reaction ranging from Q=−70 MeV to Q=30 MeV. The luminosity was determined based on the dd→He3n reaction and the quasi-free proton–proton scattering via dd→ppnspectatornspectator reactions. The excitation functions, determined independently for the measured reactions, do not reveal a structure which could be interpreted as a narrow mesic nucleus. Therefore, the upper limits of the total cross sections for the bound state production and decay in dd→(He4-η)bound→He3nπ0 and dd→(He4-η)bound→He3pπ− processes were determined taking into account the isospin relation between the both of the considered channels. The results of the analysis depend on the assumptions of the N⁎(1535) momentum distribution in the anticipated mesic-He4. Assuming, as in the previous works, that this is identical with the distribution of nucleons bound with 20 MeV in He4, we determined that (for the mesic bound state width in the range from 5 MeV to 50 MeV) the upper limits at 90% confidence level are about 3 nb and about 6 nb for nπ0 and pπ− channels, respectively. However, based on the recent theoretical findings of the N⁎(1535) momentum distribution in the N⁎-He3 nucleus bound by 3.6 MeV, we find that the WASA-at-COSY detector acceptance decreases and hence the corresponding upper limits are 5 nb and 10 nb for nπ0 and pπ− channels respectively.

First Results on the Experimental Search for $\eta ^\prime $-Mesic Nuclei with the $^{12}$C($p,d$) Reaction

We performed a missing-mass spectroscopy experiment of the 12C(p, d) reaction in order to search for η′-mesic nuclei. Excitation spectrum of 11C around the η′-meson production threshold was successfully obtained with a high-statistical sensitivity and sufficiently good energy resolution. Since no peak structure associated with the formation of η′-mesic nuclei was observed, we set constraints on the formation cross sections of η′-mesic states and on the η′-nucleus interaction.

Creation of the precision magnetic spectrometer SCAN-3

The new JINR project [1] is aimed at studies of highly excited nuclear matter created in nuclei by a high-energy deuteron beam. The matter is studied through observation of its particular decay products - pairs of energetic particles with a wide opening angle, close to 180°. The new precision hybrid magnetic spectrometer SCAN-3 is to be built for detecting charged (π ± , K ± , p) and neutral (n) particles produced at the JINR Nuclotron internal target in dA collisions. One of the main and complex tasks is a study of low-energy η A interaction and a search for η -bound states (η -mesic nuclei). Basic elements of the spectrometer and its characteristics are discussed in the article.

Formation of $\eta{^\prime}$ (958) meson bound states by the 6Li( $\gamma$ ,d) reaction

We have investigated the 6Li(gamma,d) reaction theoretically for the formation of the eta'(958) mesic nucleus close to the recoilless kinematics. We have developed the theoretical formula and reported the quantitative results of the formation spectra for various cases in this article. We have found that the formation cross sections are reduced by the effects of the fragile deuteron form factor.

22pAC-8 Spectroscopy of eta' mesic nuclei with the ^ C(p, d) reaction at GSI

22pAC-8 Spectroscopy of eta' mesic nuclei with the ^ C(p, d) reaction at GSI

Formation spectra of charmed meson–nucleus systems using an antiproton beam

We investigate the structure and formation of charmed meson--nucleus systems, with the aim of understanding the charmed meson--nucleon interactions and the properties of the charmed mesons in the nuclear medium. The $\bar{D}$ mesic nuclei are of special interest, since they have tiny decay widths due to the absence of strong decays for the $\bar{D} N$ pair. Employing an effective model for the $\bar{D} N$ and $D N$ interactions and solving the Klein--Gordon equation for $\bar{D}$ and $D$ in finite nuclei, we find that the $D^{-}$-${}^{11}\rm{B}$ system has $1 s$ and $2p$ mesic nuclear states and that the $D^{0}$-${}^{11}\rm{B}$ system binds in a $1s$ state. In view of the forthcoming experiments by the PANDA and CBM Collaborations at the future FAIR facility and the J-PARC upgrade, we calculate the formation spectra of the $[D^{-}$-${}^{11}\rm{B}]$ and $[D^{0}$-${}^{11}\rm{B}]$ mesic nuclei for an antiproton beam on a ${}^{12} \rm{C}$ target. Our results suggest that it is possible to observe the $2 p$ $D^{-}$ mesic nuclear state with an appropriate experimental setup.

Search forη′ mesic nuclei by missing-mass spectroscopy of the12C(p,d) reaction

We performed a missing-mass spectroscopy experiment of the 12C(p, d) reaction at 2.5 GeV proton energy in order to search for η′ mesic nuclei. An excitation-energy spectrum of 11C was obtained around the η′ production threshold. As no significant peak structure was observed in the spectrum, upper limits of the formation cross section of η′ mesic nuclei have been determined. Preliminary results of the experiment as well as future prospects are given.