NN Threshold Research Articles

Strangeness production measured with HADES

HADES is a multi-purpose, charged-particle detector operated at the SIS18 synchrotron located at the GSI Helmholtz Center for Heavy Ion Research in Darmstadt, Germany. The provided ion beam energies of 1-2 A GeV are among the lowest of all currently running heavy-ion experiments and result in the highest baryon net-densities at freeze-out in case of Au+Au collisions. At a beam energy of 1.23 A GeV, particles containing strangeness are produced below the quasi-free NN threshold energy. Since the missing energy has to be provided by the collision system, the investigation of the production of strangeness at such energies is a very promising probe of the created medium. The data sample measured with HADES includes multi-differential analysis of charged and neutral kaons, ϕ mesons and Λ hyperons. We present the analysis method and the results.

Studying Strangeness Production with HADES

The High-Acceptance DiElectron Spectrometer (HADES) operates in the 1 - 2A GeV energy regime in fixed target experiments to explore baryon-rich strongly interacting matter in heavy-ion collisions at moderate temperatures with rare and penetrating probes. We present results on the production of strange hadrons below their respective NN threshold energy in Au+Au collisions at 1.23A GeV ([see formula in PDF] = 2.4 GeV). Special emphasis is put on the enhanced feed-down contribution of ϕ mesons to the inclusive yield of K- and its implication on the measured spectral shape of K-. Furthermore, we investigate global properties of the system, confronting the measured hadron yields and transverse mass spectra with a Statistical Hadronization Model (SHM) and a blastwave parameterization, respectively. These supplement the world data of the chemical and kinetic freeze-out temperatures.

Strange hadron production at SIS energies: an update from HADES

We present and discuss recent experimental activities of the HADES collaboration on open and hidden strangeness production close or below the elementary NN threshold. Special emphasis is put on the feed-down from ϕ mesons to antikaons, the presence of the Ξ- excess in cold nuclear matter and the comparison of statistical model rates to elementary p+p data. The implications for the interpretation of heavy-ion data are discussed as well.

Strangeness production at 1–2 [formula omitted] GeV

The production of K + and K − mesons below and at the NN threshold is summarized, based on a comparison of data with transport model calculations. K + mesons are created in associate production, together with hyperons (e.g. Λ ) in multi-step processes involving Δ resonances. These processes occur mainly during the high-density phase of the collision and this makes the K + an ideal tool to extract the stiffness of the nuclear equation of state, found to be rather soft, with a compressibility modulus K below 240 MeV. In contrast, the major part of K − mesons are produced via strangeness exchange. Most of the created K − are absorbed and the surviving ones are emitted quite late and at low densities.

Description of intermediate- and short-range NN nuclear force within a covariant effective field theory

The present paper is aimed to developing an effective field theory in the GeV region to describe consistently both elastic and inelastic NN scattering in a fully covariant way. In this development we employ our novel s-channel mechanism of NN interaction at intermediate and short ranges assuming a formation of the six-quark bag dressed with basic meson fields such as π, σ, ρ, and ω in the intermediate state. The peripheral part of the interaction is governed by t-channel one- and two-pion exchanges described fairly well by chiral perturbation theory. The σ-meson dressing should be especially important for the stabilization of the bag due to a high coupling constant of the σ-meson with the bag and also a strong interaction between the σ-field and space symmetric quark core |s 6[6]〉. This stabilization of the intermediate dressed bag (which shifts the mass of the bag downward to NN threshold) will be crucially important for the description of π, 2 π, ρ, and ω-meson production in NN collisions, and, in particular, for the correct description of π 0 production near threshold. So that, we expect that the results of the present work will lead to a deeper understanding the short- and intermediate-range nuclear forces and internucleon correlations in nuclei.

Inclusive K+-meson production in proton-nucleus interactions

The production of K^+ mesons in pA (A = D, C, Cu, Ag, Au) collisions has been investigated at the COoler SYnchrotron COSY-J\"ulich for beam energies T_p = 1.0 - 2.3 GeV. Double differential inclusive pC cross sections at forward angles theta < 12 degrees as well as the target-mass dependence of the K^+ momentum spectra have been measured with the ANKE spectrometer. Far below the free NN threshold at T_{NN}=1.58 GeV the spectra reveal a high degree of collectivity in the target nucleus. From the target-mass dependence of the cross sections at higher energies, the repulsive in-medium potential of K^+ mesons can be deduced. Using pN cross-section parameterisations from literature and our measured pD data we derive a cross-section ratio of sigma(pn -> K^+ X) / sigma(pp -> K^+ X) ~ (3-4).

ForwardK+Production in SubthresholdpACollisions at 1.0 GeV

K+-meson production in pA ( A=C,Cu,Au) collisions has been studied using the ANKE spectrometer at an internal target position of the COSY-Jülich accelerator. The complete momentum spectrum of kaons emitted at forward angles, ϑ≤12°, has been measured for a beam energy of Tp=1.0GeV, far below the free NN threshold of 1.58 GeV. The spectrum does not follow a thermal distribution at low kaon momenta and the larger momenta reflect a high degree of collectivity in the target nucleus.Received 3 April 2001DOI:https://doi.org/10.1103/PhysRevLett.87.022301©2001 American Physical Society

Total and annihilation [formula omitted] cross sections from 50 to 400 MeV/c

The recent experimental data obtained by the OBELIX Collaboration on total and annihilation cross sections are presented and analysed in terms of scattering length approximation. This analysis allowed the evaluation of the partial wave contributions to the np interaction as well as the isospin dependence in the NN total cross sections, but the most striking result is the indication of a quite unexpected, narrow structure just above the NN threshold.

The isospin 0JP=0−πNNresonance

We studied the resonance behavior of the {pi}NN system in the configuration J{sup P}=0{sup {minus}}, isospin 0. We applied nonrelativistic Faddeev equations with realistic local potentials for the pion-nucleon and nucleon-nucleon interactions in the S{sub 11} and {sup 1}S{sub 0} channels, respectively. A resonance was found in this specific state; it lies slightly above the {pi}NN threshold and has a mass of 2018 MeV and a width of 1.75 MeV. {copyright} {ital 1997} {ital The American Physical Society}

Width of NN resonances near threshold

We use an isobar model and first-order perturbation theory to determine the width of resonances that decay into a pion and two nucleons near the NN threshold. The perturbation is given in this case by the three-body unitarity cuts. As an application we study the case of the resonance observed in pionic double-charge-exchange reactions on nuclei. We consider the possible isospin assignments of this resonance and conclude that the isospin 2 assignment is the correct one.

A dedicated 0° facility for threshold particle production studies at COSY

A universal facility is in preparation for the study of particle production in proton-nucleus reactions below the nucleon-nucleon threshold. The device will be located at the internal target position TP2 at COSY-Jülich and will consist of three dipole magnets. This 0° Facility will separate ejectiles from the circulating proton beam and allow momentum and angle analysis both for positively and negatively charged particles. A major goal of the experimental program is the investigation of the K+-meson production at projectile energies below the NN threshold. The detector concept for these studies is described and also an overview over further planned experiments is given.

Description of heavy ion collisions with medium dependent forces

One of the proclaimed goals of heavy ion collisions with 100 MeV up to 2000 MeV per nucleon is the determination of the equation of state of nuclear matter, which one needs for example for neutron stars, supernova explosions and the early universe. But the situation in heavy ion collisions is quite different from thermal equilibrium with a spherical momentum distribution and a fixed temperature. The effective nucleon-nucleon interaction as determined for example by the solution of the Bethe-Goldstone equation depends through the Pauli operator and through the single particle energies on the surrounding nuclear matter. This dependence is especially pronounced since the nucleon-nucleon interacting is highly momentum dependent: It is attractive at small relative momenta and repulsive at higher values. Thus the effective nucleon-nucleon interaction in heavy ion collisions depends on the distribution of the surrounding nuclear matter in orbital and momentum space. Here results are presented using for the description of heavy ion reactions at intermediate energies Quantum Molecular Dynamics (QMD) in a non-relativistic and in a completely covariant (RQMD) form. We show that the production of gamma-rays, pions and the inclusive spectra of nucleons and light nuclei are not sensitive to the equation of states. An observable sensitive to the equation of state is the perpendicular momentum distribution in heavy ion collisions. Also sensitive is the production of heavier particles like K + and K − mesons and antiprotons below the NN threshold. These collisions can have enough energy to compress and heat up nuclear matter and since they are below the NN threshold several nucleons must cooperate.

Some new results on baryon-antibaryon nearthreshold phenomena

Polarization of antiprotons in pp-scattering at very low momenta is calculated. Obtained small polarization in pp is shown to be due to the existence of P-wave pp quasinuclear resonances. The formula connecting the nuclear scattering length and the atomic level shift is given. Nucleon electromagnetic form-factor near NN threshold is obtained. Its behaviour is shown to be dominantely defined by the NN nuclear interaction in the initial state.

H dibaryon in lattice QCD.

The mass of a doubly strange dibaryon called the H particle is calculated in lattice QCD on a 16/sup 3/ x 48 lattice with a renormalization-group--improved gauge action and Wilson's quark action. We find that the H is below the ..lambda lambda.. threshold for strong decay. Furthermore the case where the H is slightly below the NN threshold for weak decay is consistent with our numerical results and is not in conflict with the stability of the nuclei.

Hadron spectrum on a 16 3 × 48 lattice - Toward the chiral limit -

I report the results of the calculation of the hadron spectrum in the quenched approximation on a 16∗∗3 × 48 lattice with a RG improved gauge action and the Wilson quark action: i) The masses of the π and ϱ mesons are calculated up to the point m π/ m ϱ ≌ 0.38 . The results show that there exists a smooth limit K→K C for the meson masses. However, the mass of N is determined only up to the point m π/ m ϱ≌0.61 ; ii) We also calculated the masses of hadrons which are composed of different kinds of quarks. If we use, when extrapolating the results to the physical ones, phenomenological mass formula which take into account the hyperfine splitting term, all the masses of low lying flavor non-singlet hadrons agree with experimental values within one-standard deviation. In particular, the mass of σ turns out to be about 100 MeV larger than that of λ, in accord with experements; iii) We find that the H-dibaryon is below the σσ threshold for strong decay. Furthermore the case where the H is slightly below the NN threshold for weak decay is consistent with our numerical results and is not conflict with the stability of nucleus.

Possible evidence for a narrow isovectorB=2 resonance below pion production threshold

Some evidence is presented for a narrow peak at 1969±2 MeV (FWHM=9±2 MeV) in the missing mass spectrum of the3He(p, d) reaction, with 3 standard deviations. The nature of this state, the mass of which is below the NN threshold, is discussed in connection with structures found in other experiments.

Pion and γ-ray bremsstrahlung in nuclear collisions at intermediate energies

The authors apply the bremsstrahlung model for pion production to symmetric nuclear collisions at energies far below the single NN threshold. There they expect this mechanism to be dominant. Once the deceleration parameter of the theory is adjusted, all main features of recent neutral-pion measurements can be explained. The parameter is close to hydrodynamical values. Since the gamma -ray bremsstrahlung also reflects the kinematics of its source, a simultaneous measurement of pion and gamma -ray bremsstrahlung can be used to further test the collision dynamics. The results of the calculations for both kinds of radiation are presented. In addition they show that for higher bombarding energies the thermally produced pions can fill the gap between the bremsstrahlung and the experimental data.

Review of recent experiments on antinucleon-nucleon, baryonium, nucleon-nucleon and dibaryons

Many new experiments have searched narrow structures in N̄N,n̄nπ, \\ ̄ gLN,Λn̄,and Ȳ ∗N, but failed to confirm earlier findings. Searches for narrow resonances below N̄N threshold are near completion. We have not heard any positive results from them. Emphasis is shifting towards broader resonances, although narrower peaks are not entirely excluded. Broader resonances reported around 1940MeV and in T-U region have to be studied at higher precision. The first systematic measurement of the real-to-imaginary ratio in p̄p forward scattering is reported below 1GeV/c. An effective pole term is required near the NN threshold to reproduce the data. In NN, we now have accurate spin parameters from meson facilities. The pp 3F 3 resonance has been confirmed by new analyses and its effect has been seen in Πd. Preliminary data on δσ l ( I = 0) are available from ANL. New γd data are in hand. However resonances in I = 0 need further study.

Low-energy antinucleon-nucleon scattering in a coupled-channels approach

In a coupled-channels approach it is shown that the effect of NN bound states below the NN threshold can drastically reduce the imaginary component in the optical potential needed to fit NN scattering data. This results in a similar reduction of the widths of NN states. However, these calculated widths are still much larger than those suggested by experiment. Reasons for this difference are discussed.

On Reggeized Absorption in the Presence of the Shielded Fixed-Pole Pomeron

By the use of the double-partial-wave formal,ism, the structure of absorptive Regge sin­ gularities is examined in the case of the fixed simple pole Pomeron with Arnold and Barnett's shielding-cut mechanism. It is shown that the full partial wave amplitude contains a moving branch cut of (inverse-) square-root type in relation to the mr-(KK-)shielding branch cut, in addition to a fixed branch cut of logarithmic type, that the physical partial wave amplitude has the pafticle pole with the increased residue and that the asymptotic behaviour of the full Reggeonic amplitude is controlled by the input Regge pole at least in the neighbour­ hood of t=O. Phenomenological features of the recent CERN-ISR data on p-p collision seem to suggest that the Pomerania (but not the asymptopia) has approximately been attained at high energies available at the CERN-ISR, and that the possibility of an isolated, factorizable Pomeron at l= 1 should be discarded at the CERN-ISR energies. 1> Of what kind is the Pomeron which is compatible with these observations? As an example, there is a phenomenological model of the fixed simple pole Pomeron (FSPP) with Arnold and Barnett's shielding-cut (ABSC) mechanism. 2>,s> In such a scheme, of course, the increasing trend of cross section is interpretable as a transient phenomenon at non-asymptotically high energies and the totally energy-independent behaviour of diffraction scattering is predicted to appear at the asymptopia. Since asymptotically divergent cross section has not yet been established by the presently available data, it will be of physical significance to examine theoretical consequences of Reggeized absorption in the case of FSPP with ABSC mechanism. In this note, the double-partial-wave (DPW) procedure 4>'5> is applied to studying the structure of leading absorptive Regge singularities in the presence of ABSC mechanism. For the sake of brevity, our discussion is mainly confined to FSPP having just ABSC fie (t) = 1- m,.2 + t/4, which shields the nn threshold in the t channel as l tends to 1. First of all, the single-Reggeon-exchange amplit~de MR (s, t) is assumed to be parametrized by*>