Eta Production Research Articles

Influence of the nucleon spectral function in photon and electron induced reactions on nuclei

We study the influence of the nucleon spectral function on eta photo- and electroproduction on nuclei. Besides kinematical effects due to groundstate correlations, also a modification of the S11(1535) decay width is taken into account, which is caused by the possible decay into nucleons with mass smaller than the pole mass in the medium. Hence, resonances with masses below the free N eta threshold can contribute to eta production.

Chiral symmetry of baryons

Chiral symmetry aspects for baryon properties are studied. After a brief discussion on general framework, we introduce two distinctive chiral group representations for baryons: the naive and mirror assignments. Using linear sigma models, nucleon properties are studied in both representations. Finally, we propose an experiment to distinguish the two assignments in pion and eta productions.

Chiral Symmetry Aspects of Positive and Negative Parity Baryons

Chiral symmetry aspects for baryon properties are studied. After a brief discussion on general framework, we introduce two distinctive chiral group representations for baryons: the naive and mirror assignments. Using linear sigma models, nucleon properties are studied in both representations. Finally, we propose an experiment to distinguish the two assignments in the reactions of pion and eta productions.

Eta physics at threshold

The production of eta and eta-prime mesons in elementary nucleon-nucleon collisions has been investigated at the synchrotrons CELSIUS, COSY and SATURNE. The magnitude and energy dependence of the total cross section as well as the occupation distribution of the phase space serve as observables for investigating the mechanisms underlying the production processes and the interaction of mesons with nucleons. The precise data on the eta and eta-prime creation via the pp --> pp eta(eta-prime) reactions allowed to settle the general features of the eta and eta-prime meson production and revealed the sensitivity of the mentioned observables to the nucleon-nucleon-meson final state interaction. The particular production properties, like for example the determination of the dominating exchange processes which lead to the excitation of the S_11 nucleon isobar in the case of eta creation, must be established by confrontation with other observables. The present status of this investigation with an emphasis on the results of the COSY-11 collaboration is briefly presented. The available data are interpreted in view of the production mechanism and the meson-nucleon interaction.

Timothy Edward Toohig

Timothy Edward Toohig

Electromagnetic Production of η and η' Mesons

The production of eta (η) and eta-prime (η') mesons on free and bound nucleons by electromagnetic probes, i.e. real and virtual photons, is an old but very active and viable field of research. A brief summary of the present status and knowledge with a detailed reference list is given.

Eta production on carbon and tungsten targets by proton at T P <= 1500 MeV

The production of h meson induced on carbon and tungsten targets by protons of kinetic energies from 800 to 1500 MeV has been studied at the Laboratoire National Saturne at Saclay. h mesons of mean kinetic energies 55 MeV and 125 MeV have been detected by positioning the PINOT spectrometer in two different configurations. The results are presented and discussed in the framework of a recent theoretical model.

Probing Chiral Symmetry of Nucleons by Threshold Production

Double meson production of eta and pi mesons in the threshold region is investigated in order to determine chiral properties of the nucleon. The eta can be used as a probe for the negative parity nucleon N* ≡N*(1535), produced in the intermediate state. The coupling of the low energy pion in the final state is then used to extract the sign of the Yukawa coupling, gπN*N*, which distinguishes the two realizations of chiral symmetry, naive and mirror, for the nucleon.

Eta products of weight 1 and level 36

There are 65 holomorphic eta products of weight 1 on the Fricke group of level 36. We describe all Hecke theta series on imaginary quadratic fields which are linear combinations of these eta products. There are several linear relations among the eta products.

Decreased wild-type full-length Et-A and -B receptors in neuroblastoma and Ewing sarcoma cells

Endothelins and their receptors, Et-A and Et-B, play an essential role in differentiation and migration of neural crest cells. Expression of endothelin receptors has been examined in neuroblastoma and Ewing sarcoma cell lines. RNA was amplified for Et-A and Et-B by RT-PCR. Amplified products were cloned into the expression vector pLNCX, which was used to transfect CHO cells. Binding characteristics of transfected CHO cells were examined. Full-length Et-A mRNA was identified in all cell lines, in addition to a truncated Et-A product. CHO cells expressing full-length Et-A bound to endothelin, but cells expressing truncated Et-A did not. Full length Et-B mRNA was not detected, but two smaller molecular weight products were amplified. These are as yet uncharacterised. These results suggest that endothelins and their receptors may be important in the development and biology of neuroblastoma and Ewing sarcoma.

Electron and photon induced reactions on nuclei in the nucleon resonance region

We calculate total photon-nucleus and electron-nucleus cross sections γA→ X, eA→ e ′ X and several cross sections on pion and eta production in nuclei in the energy regime between the first and the third resonance region for photon virtualities Q 2⩽1.0 GeV 2 within a semi-classical BUU transport model. In both cases we discuss the varying influence of several medium modifications on the cross sections for different values of Q 2.

SU(3) chiral dynamics with coupled channels: ETA and KAON production

We identify the next-to-leading order s- and p-wave amplitudes of the SU(3) chiral meson-baryon Lagrangian with a coupled-channel potential which is iterated in a separable Lippmann-Schwinger equation. The nucleon resonance S 11(1535) emerges as a quasi-bound state of kaon and Σ-hyperon. Our approach to η- and K-meson photoproduction introduces no new parameters. By adjusting some parameters of the next-to-leading order chiral Lagrangian and a few finite range parameters we are able to describe simultaneously a large amount of low energy data. These include the cross sections for pion- and photon-induced production of η- and K-meson from nucleons.

Beam asymmetry Σ in meson photo-production at GRAAL

Meson photoproduction on the nucleon is a major tool for the investigation of nucleon resonances. In particular eta photoproduction selectively probes certain resonances which are difficult to explore with pions. The low energy behaviour of the eta production is dominated by the S 11 (1535) resonance and the influence of other resonances (like D 13(1520)) is difficult to detect. Polarization observables enhance the sensitivity to the contributions of the less pronounced resonances since they depend mostly on the interference between the dominant E 0+ multipole and the smaller ones (like E 2- + M 2- for the D 13(1520))[l].

Rescattering effects in coherent η-photoproduction on the deuteron

Rescattering effects in coherent eta photoproduction on the deuteron are studied in the region of the S 11(1535) resonance. For the elementary reaction mechanism on the nucleon, an effective isobar model with inclusion of resonances and Born terms is used. The resonance parameters are fixed by considering pion photoproduction only. The predicted cross sections for eta production on the proton are in satisfactory agreement with experiment without any additional parameter fitting. For the coherent eta production on the deuteron we have considered the impulse approximation taking into account the P 11(1440), D 13(1520), and S 11(1535) resonances, rescattering, and meson exchange currents. For the rescattering mechanism a coupled channel model including the dominant S 11(1535) resonance is used. A reduction of the total cross section up to about 50% through rescattering is found. This is considered as an upper limit for the size of rescattering effects due to uncertainties in the NS 11 transition potential. Other two-body effects like meson exchange currents appear to be small.

Novel approach to pion and eta production in proton-proton collisions near threshold

We evaluate the threshold matrix-element for the reaction $pp \to pp\pi^0$ in a fully relativistic Feynman diagrammatic approach. We employ a simple effective range approximation to take care of the S-wave $pp$ final-state interaction. The experimental value for the threshold amplitude ${\cal A} = (2.7 - i 0.3)$ fm$^4$ can be reproduced by contributions from tree level chiral (long-range) pion exchange and short-range effects related to vector meson exchanges, with $\omega$ exchange giving the largest individual contribution. Pion loop effects appear to be small. We stress that the commonly used heavy baryon formalism is not applicable in the NN-system above the pion production threshold due to the large external momentum, $|\vec p | \simeq \sqrt {Mm_\pi}$, with $M$ and $m_\pi$ the nucleon and the pion mass, respectively. We furthermore investigate the reaction $pp\to p n \pi^+$ near threshold within the same approach. We extract from the data the triplet threshold amplitude as ${\cal B}= (2.8 -i 1.5)$ fm$^4$. Its real part can be well understood from (relativistic) tree level meson-exchange diagrams. In addition, we investigate the process $pp \to pp \eta$ near threshold. We use a simple factorization ansatz for the $pp\eta$ final-state interaction and extract from the data the modulus of the threshold amplitude, $|{\cal C}| = 1.32 $fm$^4$. With $g_{\eta N}=5.3$, this value can be reproduced by (relativistic) tree level meson-exchange diagrams and $\eta$-rescattering, whose strength is fixed by the $\eta N$ scattering length. We also comment on the recent near threshold data for $\eta'$-production.

Possibility of Spontaneous Parity Violation in Hot QCD

We suggest that for QCD in the limit of a large number of colors, N, the axial U(1) symmetry of massless quarks is effectively restored at the deconfining=chiral phase transition. If the deconfining transition is of second order, then the chiral transition is weakly first order. In this case, metastable states in which parity is spontaneously broken appear at temperatures below the phase transition. The production of these metastable states would have dramatic signatures, including enhanced production of eta and eta' mesons, which can decay through parity violating decay processes such as eta -> pi^0 pi^0, and global parity odd asymmetries for charged pions. Using a nonlinear sigma model, in QCD these metastable states only appear rather near the phase transition.

Erratum:πN→ηNandηN→ηNpartial-waveTmatrices in a coupled, three-channel model [Phys. Rev. C51, 2310 (1995)

The pi N --> eta N and eta N --> eta N partial wave T-matrices for the eight lowest partial waves have been obtained in a three coupled channel model with unitarity manifestly imposed. The two physical channels are pi_N and eta_N, and the third channel, pi_pi_N is an effective, but unphysical two body channel which represents all remaining processes. The pi_N elastic phase shifts and the weighted data base of the pi- p --> eta n total and differential cross sections are chosen as the input for the fitting procedure. A model containing a single resonance in each of the three partial waves that dominates the eta production at lower energies is compared with previous analyses, based on similar assumptions. A multiresonance coupled channel model is introduced which significantly improves the agreement with all input data. Our results are compared with a complementary multiresonance coupled channel analysis that is constrained with elastic and continuum production channels. The inclusion of the fourth P11 resonance in the 1440 - 2200 MeV region further improves the agreement between the analysis and the data.

Quasi-free η photoproduction from nuclei

Quasifree $\eta$ photoproduction from nuclei is studied in the Distorted Wave Impulse Approximation (DWIA). The elementary eta production operator contains Born terms, vector meson and nucleon resonance contributions and provides an excellent description of the recent low energy Mainz measurements on the nucleon. The resonance sector includes the $S_{11}(1535)$, $P_{11}(1440)$ and $D_{13}(1520)$ states whose couplings are fixed by independent electromagnetic and hadronic data. Different models for the $\eta N$ t-matrix are used to construct a simple $\eta A$ optical potential based on a $t \rho$-approximation. We find that the exclusive $A(\gamma,\eta N)B$ process can be used to study medium modifications of the $N^*$ resonances, particularly if the photon asymmetry can be measured. The inclusive $A(\gamma, \eta)X$ reaction is compared to new data obtained on $^{12}C$, $^{40}Ca$, and is found to provide a clear distinction between different models for the $\eta N$ t-matrix.

Dielectron production in proton-proton and proton-deuteron collisions at 1-2 GeV.

Estimates of elementary cross sections for dielectron production in pN and pd reactions are presented. Throughout, we use the vector dominance model for all hadron-hadron-photon vertices. The \ensuremath{\Delta},\ensuremath{\eta} Dalitz decays and bremsstrahlung appears as dominant sources of dielectrons. We show that the recently observed strong enhancement from 2 of the ratio of dielectron production from pd to pp collision at low energy E\ensuremath{\sim}1.0--1.3 GeV may be understood by a different threshold behavior of eta production in proton-proton and proton-neutron collisions. Relying on a realistic deuteron wave function we estimate the energy dependence of the ratio and find qualitative agreement with new experimental results.

Delta excitation in nuclei and coherent meson production

We report on inclusive cross sections for reactions of the type ( 3He,t), (p,n), (p,p′) on nuclei around the Δ excitation region. Particular emphasis is made on the coherent process which leaves the nucleus in the ground state and produce pions. Coherent pion production leads to an excitation function with a Δ peak shifted towards lower excitation energies and is an important ingredient in the explanation of the shift of the total inclusive cross section. Coherent eta production is also studied and it is shown to be an excellent tool to determine the coupling g ηNN , so far unknown experimentally.