Mesons In Nuclei Research Articles

Commissioning of the electron identification system for Dilepton measurement in pA collisions at J-PARC

Hadron mass spectra are expected to be modified in nuclear matter due to a partial restoration of chiral symmetry. We perform the J-PARC E16 experiment to measure the spectral modification of vector mesons in nuclei. We detect electron–positron pairs from decays of ϕ mesons, produced in 30 GeV pA reactions. It is a key for our experiment to separate electrons from huge hadronic backgrounds. To identify electrons, we developed two-stage detectors comprising hadron blind detectors (HBDs) and lead-glass electromagnetic calorimeters (LGs). These detectors were constructed and installed at the J-PARC high-momentum beamline in 2020, and commissioning runs were performed in 2020 and 2021. The HBDs and the LGs were stably operated at a high counting rate generated in 10-MHz pA interaction at targets, and successfully detected electrons. In the preliminary results, the pion rejection powers were found to be 97.6 ± 0.6% and 91.2 ± 0.7% for the HBD and LG, respectively, which are consistent with the design performance, at the operation thresholds. Further evaluation of electron efficiency is ongoing.

Υ and ηb nuclear bound states

$\mathrm{\ensuremath{\Upsilon}}$ and ${\ensuremath{\eta}}_{b}$ nuclear bound-state energies are calculated for various nuclei, neglecting any possible effects of the widths. Essential input for the calculations, namely, the medium-modified $B$ and ${B}^{*}$ meson masses, as well as the density distributions in nuclei, are calculated within the quark-meson coupling (QMC) model. The attractive potentials for the $\mathrm{\ensuremath{\Upsilon}}$ and ${\ensuremath{\eta}}_{b}$ mesons in nuclei are calculated from the mass shifts of these mesons in nuclear matter in the local density approximation. These potentials originate from the in-medium enhanced $BB$ and $B{B}^{*}$ loops in their respective self-energy. After an extensive analysis we conclude that our results suggest that the $\mathrm{\ensuremath{\Upsilon}}$ and ${\ensuremath{\eta}}_{b}$ mesons should form bound states with all the nuclei considered.

Strange mesons in nuclei and neutron stars

The present status in the field of strange mesons in nuclei and neutron stars is reviewed. In particular, the K̅N interaction, that is governed by the presence of the Λ(1405), is analyzed and the formation of the K̅NN bound state is discussed. Moreover, the properties of K̅ in dense nuclear matter are studied, in connection with strangeness production in nuclear collisions and kaon condensation in neutron stars.

K̅N interaction, p-wave terms

The inclusion of new ingredients that are expected to be specially relevant at higher energies could reveal more information about the physics behind the NLO terms of the chiral Lagrangian. In the present work, we explore the relevance of including partial waves higher than the L = 0, which is usually the only component considered in the literature to study the K̅N scattering phenomenology. In particular, we focus on the p-wave contribution, the effect of which is expected to be non-negligible, as we aim at obtaining the K−p scattering amplitudes at higher energies, necessary to describe the ηΛ, ηΣ0, K0Ξ0 and K+Ξ− production reactions. Extending the K̅N interaction to p-wave components is also relevant for studies of bound K− mesons in nuclei since their local momentum can acquire sizable values.

Measurement of Parameters of Neutral Mesons Produced in Meson–Nucleus Interactions in Hyperon-M Experiment

Measurement of Parameters of Neutral Mesons Produced in Meson–Nucleus Interactions in Hyperon-M Experiment

Data Acquisition System in the First Commissioning Run of the J-PARC E16 Experiment

J-PARC E16 is an experiment to examine the origin of hadron mass through a systematic measurement of spectral changes of vector mesons in nuclei. The measurement of e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> pairs from the decay of vector mesons will provide the information of the partial restoration of the chiral symmetry in a normal nuclear density. To resolve a pulse pile-up and achieve good discrimination of e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> and e <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> from the background of a reaction rate of an order of 10 MHz, the data acquisition (DAQ) system uses waveform sampling chips of APV25 and DRS4. The trigger rate and data rate are expected to be 1 kHz and 120-330 MiB/s, respectively. The DAQ system for readout of APV25 and DRS4 were developed, where events were synchronized by common trigger and tag data. The first commissioning in beam, called Run-0a, was performed in June 2020 with about 1/4 of the designed setup. The DAQ worked with a trigger rate of 300 Hz in the Run-0a and the main bottleneck was a large data size of APV25. Further optimization of the DAQ system will improve the performance.

Forward coherent ω and ϕ mesons production in the photonuclear reaction on nuclei

The differential cross sections of the forward coherent ω and ϕ mesons photoproduction from nuclei have been calculated using the Glauber model for the nuclear reaction. The measured cross section of the elementary reaction, i.e., γ N → ω(ϕ)N reaction, are used as input in the Glauber model. The experimentally determined free-space scattering parameters of these mesons have been used to evaluate the meson nucleus interactions. The sensitivity of the cross section to the Fermi-motion and short-range correlation of the nucleon in the nucleus are studied. The calculated results are compared with the measured cross sections.

Coherent Photoproduction of Neutral Pseudoscalar Mesons $$\boldsymbol{\pi^{0}\pi^{0}}$$ and $$\boldsymbol{\pi^{0}\eta}$$ on Light Nuclei

The coherent photoproduction of $$\pi^{0}\pi^{0}$$ and $$\pi^{0}\eta$$ on nuclei in the mass-number region of $$A\leqslant 7$$ is studied systematically over the range between the threshold and the laboratory photon energy of $$E_{\gamma}=2$$ GeV. The amplitude for photoproduction on nuclei is constructed on the basis of the impulse approximation with allowance for a number of processes responsible for meson–nucleus final-state interaction. The pion-rescattering processes $$\pi N\to\pi N$$ and $$\pi N\to\eta N$$ occurring on spectator nucleons, $$\pi^{+/-}\pi^{0}\pi^{0}$$ and $$\pi^{+/-}\pi^{0}\eta$$ production followed by $$\pi^{+/-}$$ absorption by spectator nucleons, and three-body $$\pi$$ –NN and $$\eta$$ –NN interactions are described for the first time within a unified microscopic approach.

Structure of $\eta^{\prime}$ mesonic nuclei in a relativistic mean field theory

The structure and the energy spectrum of the $\eta^{\prime}$ mesonic nuclei are investigated in a relativistic mean field theory. One expects a substantial attraction for the $\eta^{\prime}$ meson in finite nuclei due to the partial restoration of chiral symmetry in the nuclear medium. Such a hadronic scale interaction for the $\eta^{\prime}$ mesonic nuclei may provide modification of the nuclear structure. The relativistic mean field theory is a self-contained model for finite nuclei which provides the saturation property within the model, and is good to investigate the structure change of the nucleus induced by the $\eta^{\prime}$ meson. Using the local density approximation for the mean fields, we solve the equations of motion for the nucleons and the $\eta^{\prime}$ meson self-consistently, and obtain the nuclear density distribution and the $\eta^{\prime}$ energy spectrum for the $\eta^{\prime}$ mesonic nuclei. We take $^{12}$C, $^{16}$O and $^{40}$Ca for the target nuclei. We find several bound states of the $\eta^{\prime}$ meson for these nuclei thanks to the attraction for $\eta^{\prime}$ in nuclei. We also find a sufficient change of the nuclear structure especially for the $1s$ bound state of $\eta^{\prime}$. This implies that the production of the $1s$ bound state in nuclear reaction may be suppressed.

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.

Onset of η-meson binding in the He isotopes

The onset of binding η(548) mesons in nuclei is studied in the He isotopes by doing precise ηNNN and ηNNNN few-body stochastic variational method calculations for two semi-realistic NN potentials and two energy dependent ηN potentials derived from coupled-channel models of the N⁎(1535) nucleon resonance. The energy dependence of the ηN subthreshold input is treated self consistently. It is found that a minimal value of the real part of the ηN scattering length aηN close to 1 fm is required to bind η mesons in 3He, yielding then a few MeV η binding in 4He. The onset of η-meson binding in 4He requires that ReaηN exceeds 0.7 fm approximately. These results compare well with results of recent ηNNN and ηNNNN pionless effective field theory calculations. Related optical-model calculations are also discussed.

ϕ meson mass and decay width in nuclear matter and nuclei

The mass and decay width of the ϕ meson in cold nuclear matter are computed in an effective Lagrangian approach. The medium dependence of these properties are obtained by evaluating kaon–antikaon loop contributions to the ϕ self-energy, employing the medium-modified kaon masses, calculated using the quark-meson coupling model. The loop integral is regularized with a dipole form factor, and the sensitivity of the results to the choice of cutoff mass in the form factor is investigated. At normal nuclear matter density we find a downward shift of the ϕ mass by a few percent, while the decay width is enhanced by an order of magnitude. For a large variation of the cutoff mass parameter, the results for the ϕ mass and the decay width turn out to vary very little. Our results support results in the literature which suggest that one should observe a small downward mass shift and a large broadening of the decay width. In order to explore the possibility of studying the binding and absorption of ϕ mesons in nuclei, we also present the single-particle binding energies and half-widths of ϕ-nucleus bound states for some selected nuclei.

Semihadronic and hadronic decays ofρ0andωmesons coherently photoproduced in isoscalar nuclei

The interference of the $\rho^0$ and $\omega$ mesons has been studied in the hadronic and leptonic decay channels, i.e., dipion and dilepton decay channels respectively, but this interference is not studied yet in the semihadronic or semileptonic decay channel, e.g., $V \to \pi^0\gamma$. $V$ denotes either $\rho^0$ or $\omega$ meson. To look for the quoted interference in the $\pi^0\gamma$ decay channel as well as the contribution of $\rho$ meson to the cross section, the correlated $\pi^0\gamma$ invariant mass distribution spectra are calculated in the photonuclear reaction in the multi-GeV region. It is assumed that these bosons arise in the final state due to the decay of $\rho^0$ and $\omega$ mesons which are photoproduced coherently in the isoscalar nucleus. The elementary reaction in the nucleus is considered to proceed as $\gamma N \to VN$; $V \to \pi^0\gamma$. The forward propagation of the $\rho$ and $\omega$ mesons and the near forward emission of pion are considered, so that they can be described by the eikonal form. The meson nucleus interactions are evaluated using $t\varrho$ approximation. Replacing the decay vertex $V \to \pi^0\gamma$ by $V \to \pi^+\pi^-$ in the above formalism, it is also used to study the $\rho-\omega$ interference in the $\pi^+\pi^-$ decay channel.

Development of GEM trigger electronics for the J-PARC E16 experiment

At the J-PARC E16 experiment[1], we measure mass spectra of vector mesons in nuclei from the e+e- decay channel with high precision and high statistics. We have developed the trigger system consisting of newly developed ASD ASICs which can extract signals from the GEM foil used as a cathode plane of the induction gap in a GEM chamber and digital electronics using FPGAs which processes binary signals from the ASIC.

Effect of in-medium parameters of ρ meson in its photoproduction reactions on nuclei

There exist model calculations showing the modification of the hadronic parameters of $\rho$ meson in the nuclear environment. From these parameters, we extract the $\rho$ meson nucleus optical potential and show the medium effect due to this potential on the $\rho$ meson mass distribution spectra in the photonuclear reaction. The calculated results reproduced reasonably the measured $e^+e^-$ invariant mass, i.e., $\rho$ meson mass, distribution spectra in the $( \gamma, \rho^0 \to e^+e^- )$ reaction on nuclei.

Comparison of the ratios of the yields of cumulative K + and K − mesons with the predictions of the model of a collective sea of quark-antiquark pairs in nuclei

The results obtained by simulating, within the flucton model, the production and secondary interactions of cumulative K+ and K− mesons in nuclei indicate that the ratios of their yields are in acceptable agreement with the predictions of the model of a collective sea of quark-antiquark pairs in nuclei. The calculations were performed with allowance for the kaon-hadronization length according to the model of bremsstrahlung-gluon emission and the parton model.

Broadening of ρ(→ e+e−) meson produced coherently in the photonuclear reaction

The $e^+e^-$ invariant mass distribution spectra are calculated to estimate the hadron parameters of the $\rho$ meson produced coherently in the photonuclear reaction. The elementary reaction occurring in the nucleus is considered to proceed as $ \gamma N \to \rho^0 N ; ~ \rho^0 \to e^+e^- $. We describe the elementary $\rho$ meson photoproduction by the experimentally determined reaction amplitude $ f_{ \gamma N \to \rho^0 N } $. The $\rho$ meson propagator is presented by the eikonal form, and the $\rho$ meson nucleus optical potential $V_{O\rho}$ appearing in it is estimated using the "$t\varrho$" approximation. The $\rho$ meson nucleon scattering amplitude $ f_{ \rho N } $ extracted from the measurements is used to generate this potential. The calculated $e^+e^-$ invariant mass distribution spectra are compared with those measured at Jefferson Laboratory. The calculated results for the transparency ratio are also presented.

Emission of η meson with high transverse momentum in Au–Au, d–Au and p–p collisions at $$\sqrt{\mathrm{\mit s_{NN}}}=200$$ s NN = 200 GeV

Inclusive transverse momentum spectra of η meson in nucleus–nucleus, deuton–nucleus and proton–proton collisions are studied in the framework of a thermalized cylinder model. A single component distribution can only describe a narrow transverse momentum range. In the region of high transverse momentum, the considered distributions of η meson have a tail part at the maximum energy of relativistic heavy ion collider. To explain the wider transverse momentum spectra, we consider the relative importance of hard versus soft processes in the particle production mechanisms. The improved cylinder model utilizing two-component spectra is used to describe the experimental data of the PHENIX Collaboration. It is found that the two-component distribution can describe a broadened range of the transverse momentum. It can offer information about soft and hard interactions in the collisions.

Charmed Mesons in Nuclei with Heavy-Quark Spin Symmetry

We study the properties of charmed pseudoscalar and vector mesons in dense matter within a unitary meson–baryon coupled-channel model which incorporates heavy-quark spin symmetry. This is accomplished by extending the SU(3) Weinberg–Tomozawa Lagrangian to incorporate spin-flavor symmetry and implement a suitable flavor symmetry breaking. Several resonances with negative parity are generated dynamically by the s-wave interaction between pseudoscalar and vector meson multiplets with 1/2+ and 3/2+ baryons. Those states are then compared to experimental data as well as theoretical models. Next, Pauli-blocking effects and meson self-energies are introduced in a self-consistent manner to obtain the open-charm meson spectral functions in a dense nuclear environment. We finally discuss the formation of D-mesic nuclei.

D¯andBmesons in a nuclear medium

We discuss the mass modifications of $\bar{\mathrm{D}}$ and $\bar{\mathrm{D}}^{\ast}$ ($\mathrm{B}$ and $\mathrm{B}^{\ast}$) mesons in nuclear medium. The heavy quark symmetry for $\bar{\mathrm{D}}$ and $\bar{\mathrm{D}}^{\ast}$ ($\mathrm{B}$ and $\mathrm{B}^{\ast}$) mesons is adopted, and the interaction between a $\bar{\mathrm{D}}$ or $\bar{\mathrm{D}}^{\ast}$ ($\mathrm{B}$ or $\mathrm{B}^{\ast}$) meson and a nucleon is supplied from the pion exchange. We find the negative mass shifts for $\bar{\mathrm{D}}$ meson and $\mathrm{B}$ meson, and hence that the $\bar{\mathrm{D}}$ and $\mathrm{B}$ mesons are bound in the nuclear medium. As applications, we consider the atomic nuclei with a $\bar{\mathrm{D}}$ meson, $^{40}_{\bar{\mathrm{D}}}\mathrm{Ca}$ and $^{208}_{\bar{\mathrm{D}}}\mathrm{Pb}$, and investigate the energy levels of the $\bar{\mathrm{D}}$ meson in each nucleus. We also discuss the mass shifts in the isospin asymmetric nuclear medium, and present the possible phenomenon about distribution of isospin density around a $\bar{\mathrm{D}}$ or $\mathrm{B}$ meson in nuclear medium. We find that the mass shifts of $\bar{\mathrm{D}}^{\ast}$ and $\mathrm{B}^{\ast}$ mesons have large imaginary parts, which would prevent precise study of the energy levels of $\bar{\mathrm{D}}^{\ast}$ and $\mathrm{B}^{\ast}$ mesons in nuclei.