Baryon Resonances Research Articles

Evidence for Δ(2200)7/2− from photoproduction and consequence for chiral-symmetry restoration at high mass

Abstract We report a partial-wave analysis of new data on the double-polarization variable E for the reactions γ p → π + n and γ p → π 0 p and of further data published earlier. The analysis within the Bonn–Gatchina (BnGa) formalism reveals evidence for a poorly known baryon resonance, the one-star Δ ( 2200 ) 7 / 2 − . This is the lowest-mass Δ ⁎ resonance with spin-parity J P = 7 / 2 − . Its mass is significantly higher than the mass of its parity partner Δ ( 1950 ) 7 / 2 + which is the lowest-mass Δ ⁎ resonance with spin-parity J P = 7 / 2 + . It has been suggested that chiral symmetry might be restored in the high-mass region of hadron excitations, and that these two resonances should be degenerate in mass. Our findings are in conflict with this prediction.

Polarization and dilepton anisotropy in pion–nucleon collisions

Hadronic polarization and the related anisotropy of the dilepton angular distribution are studied for the reaction $\pi N \rightarrow Ne^+ e^-$. We employ consistent effective interactions for baryon resonances up to spin-5/2, where non-physical degrees of freedom are eliminated, to compute the anisotropy coefficients for isolated intermediate baryon resonances. It is shown that the spin and parity of the intermediate baryon resonance is reflected in the angular dependence of the anisotropy coefficient. We then compute the anisotropy coefficient including the $N(1520)$ and $N(1440)$ resonances, which are essential at the collision energy of the recent data obtained by the HADES collaboration on this reaction. We conclude that the anisotropy coefficient provides useful constraints for unravelling the resonance contributions to this process.

Bulk viscosity for pion and nucleon thermal fluctuation in the hadron resonance gas model

We have calculated microscopically bulk viscosity of hadronic matter, where equilibrium thermodynamics for all hadrons in medium are described by Hadron Resonance Gas (HRG) model. Considering pions and nucleons as abundant medium constituents, we have calculated their thermal widths, which inversely control the strength of bulk viscosities for respective components and represent their in-medium scattering probabilities with other mesonic and baryonic resonances, present in the medium. Our calculations show that bulk viscosity increases with both temperature and baryon chemical potential, whereas viscosity to entropy density ratio decreases with temperature and with baryon chemical potential, the ratio increases first and then decreases. The decreasing nature of the ratio with temperature is observed in most of the earlier investigations with few exceptions. We find that the temperature dependence of bulk viscosity crucially depends on the structure of the relaxation time. Along the chemical freeze-out line in nucleus-nucleus collisions with increasing collision energy, bulk viscosity as well as the bulk viscosity to entropy density ratio decreases, which also agrees with earlier references. Our results indicate the picture of a strongly coupled hadronic medium.

Cross sections forνμandν¯μinduced pion production on hydrocarbon in the few-GeV region using MINERvA

Separate samples of charged-current pion production events representing two semi-inclusive channels $\nu_\mu$-CC($\pi^{+}$) and $\bar{\nu}_{\mu}$-CC($\pi^{0}$) have been obtained using neutrino and antineutrino exposures of the MINERvA detector. Distributions in kinematic variables based upon $\mu^{\pm}$-track reconstructions are analyzed and compared for the two samples. The differential cross sections for muon production angle, muon momentum, and four-momentum transfer $Q^2$, are reported, and cross sections versus neutrino energy are obtained. Comparisons with predictions of current neutrino event generators are used to clarify the role of the $\Delta(1232)$ and higher-mass baryon resonances in CC pion production and to show the importance of pion final-state interactions. For the $\nu_\mu$-CC($\pi^{+}$) ($\bar{\nu}_{\mu}$-CC($\pi^{0}$)) sample, the absolute data rate is observed to lie below (above) the predictions of some of the event generators by amounts that are typically 1-to-2 $\sigma$. However the generators are able to reproduce the shapes of the differential cross sections for all kinematic variables of either data set.

Towards a realistic description of hadron resonances

We report on our attempts of treating excited hadron states as true quantum resonances. Hitherto the spectroscopy of mesons, usually considered as quark-antiquark systems, and of baryons, usually considered as three-quark systems, has been treated through excitation spectra of bound states (namely, confined few-quark systems), corresponding to poles of the quantum-mechanical resolvent at real negative values in the complex energy plane. As a result the wave functions, i.e. the residua of the resolvent, have not exhibited the behaviour as required for hadron resonances with their multiple decay modes. This has led to disturbing shortcomings in the description of hadronic resonance phenomena. We have aimed at a more realistic description of hadron resonances within relativistic constituent-quark models taking into account explicitly meson-decay channels. The corresponding coupled-channels theory is based on a relativistically invariant mass operator capable of producing hadron ground states with real energies and hadron resonances with complex energies, the latter corresponding to poles in the lower half-plane of the unphysical sheet of the complex energy plane. So far we have demonstrated the feasibility of the coupled-channels approach to hadron resonances along model calculations producing indeed the desired properties. The corresponding spectral properties will be discussed in this contribution. More refined studies are under way towards constructing a coupled-channels relativistic constituent-quark model for meson and baryon resonances.

Pion induced reaction with carbon and polyethylene targets obtained by HADES-GSI in 2014

In the summer of 2014, HADES was conducting measurements with secondary pion-beam using different targets. The program was devoted to measure dilepton radiation from baryonic resonances. In particular we investigated a sub-threshold coupling of ρ to baryonic resonances in the second resonance region (N(1520), N(1535)). Most of the beam time was dedicated to measurement of e+e- production from Polyethylene target at pion beam momentum of 0.69 GeV/c. In addition we run part of the time with pure carbon target. This allow us to study exclusive π- + p → ne-e+ channel. The normalization of spectra has been done using elastic scattering of pion on proton and carbon. The simulations of dilepton yields for π0, Δ and N(1520) Dalitz decay using PLUTO was carried out.

Study of decuplet baryon resonances from lattice QCD

A lattice QCD study of the strong decay width and coupling constant of decuplet baryons to an octet baryon - pion state is presented. The transfer matrix method is used to obtain the overlap of lattice states with decuplet baryon quantum numbers on the one hand and octet baryon-pion quantum numbers on the other as an approximation to the matrix element of the corresponding transition. By making use of leading order effective field theory, the coupling constants, as well as the widths for the various decay channels are determined. The transitions studied are $ \Delta \to \pi \,N$, $\Sigma^* \to \Lambda\,\pi$, $\Sigma^* \to \Sigma\,\pi$ and $\Xi^* \to \Xi\,\pi$. We obtain results for two ensembles of $N_f=2+1$ dynamical fermion configurations, one using domain wall valence quarks on a staggered sea at a pion mass of $350\,\mathrm{MeV}$ and a box size of $3.4\,\mathrm{fm}$ and a second one using domain wall sea and valence quarks at pion mass $180\,\mathrm{MeV}$ and box size $4.5\,\mathrm{fm}$.

Correlations ofπNpartial waves for multireaction analyses

In the search for missing baryonic resonances, many analyses include data from a variety of pion- and photon-induced reactions. For elastic $\pi N$ scattering, however, usually the partial waves of the SAID or other groups are fitted, instead of data. We provide the partial-wave covariance matrices needed to perform correlated $\chi^2$ fits, in which the obtained $\chi^2$ equals the actual $\chi^2$ up non-linear and normalization corrections. For any analysis relying on partial waves extracted from elastic pion scattering, this is a prerequisite to assess the significance of resonance signals and to assign any uncertainty on results. The influence of systematic errors is also considered.

Photoproduction ofΛandΣ0hyperons using linearly polarized photons

Background: Measurements of polarization observables for the reactions $\vec{\gamma} p \rightarrow K^+ \Lambda$ and $\vec{\gamma} p \rightarrow K^+ \Sigma^0$ have been performed. This is part of a programme of measurements designed to study the spectrum of baryon resonances. Purpose: The accurate measurement of several polarization observables provides tight constraints for phenomenological fits. Beam-recoil observables for the $\vec{\gamma} p \rightarrow K^+ \Sigma^0$ reaction have not been reported before now. Method: The measurements were carried out using linearly polarized photon beams and the CLAS detector at the Thomas Jefferson National Accelerator Facility. The energy range of the results is 1.71\,GeV $<W<$ 2.19\,GeV, with an angular range $-0.75 < \cos\theta^{\star}_K < +0.85$. Results: The observables extracted for both reactions are beam asymmetry $\Sigma$, target asymmetry $T$, and the beam-recoil double polarization observables $O_x$ and $O_z$. Conclusions: Comparison with theoretical fits indicates that in the regions where no previous data existed, the new data contain significant new information, and strengthen the evidence for the set of resonances used in the latest Bonn-Gatchina fit.

TandFasymmetries inπ0photoproduction on the proton

The γp→π0p reaction was studied at laboratory photon energies from 425 to 1445 MeV with a transversely polarized target and a longitudinally polarized beam. The beam-target asymmetry F was measured for the first time and new high precision data for the target asymmetry T were obtained. The experiment was performed at the photon tagging facility of the Mainz Microtron (MAMI) using the Crystal Ball and TAPS photon spectrometers. The polarized cross sections were expanded in terms of associated Legendre functions and compared to recent predictions from several partial-wave analyses. The impact of the new data on our understanding of the underlying partial-wave amplitudes and baryon resonance contributions is discussed.

Compositeness of baryonic resonances: Application to theΔ(1232),N(1535), andN(1650)resonances

We present a formulation of the compositeness for baryonic resonances in order to discuss the meson-baryon molecular structure inside the resonances. For this purpose, we derive a relation between the residue of the scattering amplitude at the resonance pole position and the two-body wave function of the resonance in a sophisticated way, and we define the compositeness as the norm of the two-body wave functions. As applications, we investigate the compositeness of the $\Delta (1232)$, $N (1535)$, and $N (1650)$ resonances from precise $\pi N$ scattering amplitudes in a unitarized chiral framework with the interaction up to the next-to-leading order in chiral perturbation theory. The $\pi N$ compositeness for the $\Delta (1232)$ resonance is evaluated in the $\pi N$ single-channel scattering, and we find that the $\pi N$ component inside $\Delta (1232)$ in the present framework is nonnegligible, which supports the previous work. On the other hand, the compositeness for the $N (1535)$ and $N (1650)$ resonances is evaluated in a coupled-channels approach, resulting that the $\pi N$, $\eta N$, $K \Lambda$ and $K \Sigma$ components are negligible for these resonances.

Hamiltonian Effective Field Theory Study of the N^{*}(1535) Resonance in Lattice QCD.

Drawing on experimental data for baryon resonances, Hamiltonian effective field theory (HEFT) is used to predict the positions of the finite-volume energy levels to be observed in lattice QCD simulations of the lowest-lying J^{P}=1/2^{-} nucleon excitation. In the initial analysis, the phenomenological parameters of the Hamiltonian model are constrained by experiment and the finite-volume eigenstate energies are a prediction of the model. The agreement between HEFT predictions and lattice QCD results obtained on volumes with spatial lengths of 2 and 3fm is excellent. These lattice results also admit a more conventional analysis where the low-energy coefficients are constrained by lattice QCD results, enabling a determination of resonance properties from lattice QCD itself. Finally, the role and importance of various components of the Hamiltonian model are examined.

Investigations on the I(JP) = 0(3+) hidden color hexaquark states

Here we investigate the energies of hidden color hexaquark states with quantum numbers \(I(J^{p})=0(3^{+})\) within the framework of the constituent quark model. The hyperfine interaction between quarks is taken to be the instanton-induced interaction, and mixing between different hexaquark configurations caused by hyperfine interaction between quarks is considered. Numerical results show that energies of several obtained hexaquark states are very close to the mass of the experimentally observed \( d^{\ast}\) resonance, it indicates that these states may be the dominant components of \( d^{\ast}\) baryon resonance, if \( d^{\ast}\) is a genuine hexaquark dominated baryon state.

The Ξ* and Ωη Interaction Within a Chiral Unitary Approach**Supported by the National Basic Research Program (973 Program Grant No. 2014CB845406), and the National Natural Science Foundation of China under Grant Nos. 11475227, 11265014, and the Open Project Program of State Key Laboratory of Theoretical Physics, Institute of Theoretical Physics, Chinese Academy of Sciences, China No. Y5KF151CJ1

In this work we study the interaction of the coupled channels Ωη and within the chiral unitary approach. The systems under consideration have total isospins 0, strangeness S = −3, and spin 3/2. We study the s wave interaction which implies that the possible resonances generated in the system can have spin-parity JP = 3/2−. The unitary amplitudes in coupled channels develop poles that can be associated with some known baryonic resonances. We find there is a dynamically generated 3/2− Ω state with mass around 1800 MeV, which is in agreement with the predictions of the five-quark model.

Double polarisation experiments in meson photoproduction

One of the remaining challenges within the standard model is to gain a good understanding of QCD in the non-perturbative regime. A key step towards this aim is baryon spectroscopy, investigating the spectrum and the properties of baryon resonances. To gain access to resonances with small $\pi N$ partial width, photoproduction experiments provide essential information. Partial wave analyses need to be performed to extract the contributing resonances. Here, a complete experiment is required to unambiguously determine the contributing amplitudes. This involves the measurement of carefully chosen single and double polarisation observables. In a joint endeavour by MAMI, ELSA, and Jefferson Laboratory, a new generation of experiments with polarised beams, polarised proton and neutron targets, and $4\pi$ particle detectors have been performed in recent years. Many results of unprecedented quality were recently published by all three experiments, and included by the various partial wave analysis groups in their analyses, leading to substantial improvements, e.g. a more precise determination of resonance parameters. An overview of recent results is given, with an emphasis on results from the CBELSA/TAPS experiment, and their impact on our understanding of the nucleon excitation spectrum is discussed.

Studyingρ–N couplings with HADES in pion-induced reactions

The High-Acceptance Di-Electron Spectrometer (HADES) operates at the GSI Helmholtzzentrum fur Schwerionenforschung in Darmstadt with pion, proton and heavyion beams provided by the synchrotron SIS18. In summer 2014, HADES took data using a pion beam on carbon and polyethylene targets. A large part of the data was taken at a pion beam momentum of 0.69 GeV/c in order to explore di-pion and di-electron production in the second resonance region and the sub-threshold coupling of the ρ to baryonic resonances. In this contribution lepton identification will be discussed as well the purity of reconstructed e + e − . Finally we will show the preliminary di-electon raw spectra.

Exotic Baryon Resonances in Photoproduction

Exotic Baryon Resonances in Photoproduction

Production and decay of baryonic resonances in pion induced reactions

Pion induced reactions give unique opportunities for an unambiguous description of baryonic resonances and their coupling channels. A systematic energy scan and high precision data, in conjunction with a partial wave analysis, allow for the study of the excitation function of the various contributions. A review of available world data unravels strong need for modern facilities delivering measurements with a pion beam. Recently, HADES collaboration collected data in pion-induced reactions on light ( 12 C) and heavy ( 74 W) nuclei at a beam momentum of 1.7 GeV/c dedicated to strangeness production. It was followed by a systematic scan at four different pion beam momenta (0.656, 0.69, 0.748 and 0.8 GeV/c) in π − − p reaction in order to tackle the role of N (1520) resonance in conjunction with the intermediate ρ production. First results on exclusive channels with one pion ( π − p ) and two pions ( n π + π − , p π − π 0 ) in the final state are discussed.

A New Method for Obtaining the Baryons Mass under the Killingbeck Plus Isotonic Oscillator Potentials

The spectrum of ground state and excited baryons (N, Δ, Λ, Σ,Ξ, and Ω particles) has been investigated by using nonrelativistic quantum mechanics under the Killingbeck plus isotonic oscillator potentials. Using the Jacobi coordinates, anzast method, and generalized Gürsey Radicati (GR) mass formula the three-body-wave equation is solved to calculate the different states of the considered baryons. A comparison between our calculations and the available experimental data shows that the position of the Roper resonances of the nucleon, the ground states, and the excited multiplets up to three GeV are in general well reproduced. Also one can conclude that the interaction between the quark constituents of baryon resonances could be described adequately by using the combination of Killingbeck and isotonic oscillator potentials form.

Inclusive reconstruction of hadron resonances in elementary and heavy-ion collisions with HADES

The unambiguous identification of hadron modifications in hot and dense QCD matter is one of the important goals in nuclear physics. In the regime of 1 - 2 GeV kinetic energy per nucleon, HADES has measured rare and penetrating probes in elementary and heavy-ion collisions. The main creation mechanism of mesons is the excitation and decay of baryonic resonances throughout the fireball evolution. The reconstruction of shortlived (≈ 1 fm/c) resonance states through their decay products is notoriously difficult. We have developed a new iterative algorithm, which builds the best hypothesis of signal and background by distortion of individual particle properties. This allows to extract signals with signal-to-background ratios of <1%.