New possibilities for studying nucleon resonances in the production of π + π − pairs by polarized electrons on an unpolarized proton

The difference of the cross sections for double charged-pion production in the scattering of opposite-helicity electrons on an unpolarized proton is evaluated within the phenomenological model developed previously. The electromagnetic nucleon-resonance form factors and the parameters of nonresonance processes were taken from a fit to the latest data of the CLAS Collaboration on double charged-pion electroproduction. The effect of the longitudinal excitations of the P ₁₁(1440) and D ₁₃(1520) states on the difference of the helicity components of the cross section is studied. The sensitivity of this observable to the nucleon-resonance contribution opens the possibility of employing it both in extracting the Coulomb form factors and in seeking new baryon states. Signals from one of these states were possibly observed in the latest data of the CLAS Collaboration.     

Baryon Structure in AdS/QCD

We discuss baryon structure in a soft-wall anti-de Sitter/quantum chromodynamics model. From unified point of view we describe nucleon form factors and the electroproduction of the N(1440) Roper resonance. The Roper resonance is identified as the first radially excited state of the nucleon. The obtained results for helicity amplitudes of the Roper electroproduction are in good agreement with the recent results of the CLAS Collaboration at JLab.     

Evaluation of the Integrated Cross Sections of Reactions of Exclusive Pion and Kaon Electroproduction off Protons in the Resonance Region at Photon Virtualities from 5 to 12 GeV<Superscript>2</Superscript>

The integrated cross sections of the ep → eπ⁺n, ep → eπ⁰p, ep → eK⁺Λ, and ep → eK⁺Σ⁰ reactions are evaluated in the energy range of nucleon resonance excitation at photon virtualities Q² from 5 to 12 GeV². These exclusive channels will be explored at photon virtualities Q² > 5 GeV² for the first time in future experiments with the CLAS12 detector. The cross-section evaluation is based on the extrapolation of exclusive contributions to the inclusive structure functions F₁ and F₂ from a region of Q² < 5 GeV², in which the experimental data are available, to the region of higher Q². This evaluation of cross sections is of particular importance in the development of the program of experiments with the CLAS12 detector for studying the structure of the ground and excited nucleon states, which may reveal the dynamics of strong interactions in the nonperturbative regime.     

Excited baryons in the bag

We calculate the negative parityN ^* andΔ ^* states in the chiral bag model. After projecting out spurious center of mass states we reproduce the known masses reasonably well. Both the colour gluon exchange diagrams and the quark-quark spindependent forces due to the presence of Goldstone pions give important contributions to the ordering of the states. Previous calculations are examined and we conclude that the chiral bag models can sucessfully account for the negative parity states as well as the baryon ground states.     

Spectra of charmed and bottom baryons with hyperfine interaction

Up to now,the excited charmed and bottom baryon states have still not been well studied experimentally or theoretically.In this paper,we predict the mass of ?*b,the only L=0 baryon state which has not been observed,to be 6069.2 Me V.The spectra of charmed and bottom baryons with the orbital angular momentum L = 1 are studied in two popular constituent quark models,the Goldstone boson exchange(GBE) model and the one gluon exchange(OGE) hyperfine interaction model.Inserting the latest experimental data from the "Review of Particle Physics",we find that in the GBE model,there exist some multiplets(Σc(b),Ξ c(b)and ?c(b)) in which the total spin of the three quarks in their lowest energy states is 3/2,but in the OGE model there is no such phenomenon.This is the most important difference between the GBE and OGE models.These results can be tested in the near future.We suggest more efforts to study the excited charmed and bottom baryons both theoretically and experimentally,not only for the abundance of baryon spectra,but also for determining which hyperfine interaction model best describes nature.     

Rate of anomalous electroweak baryon and lepton number violation at finite temperature

Using Langer's statistical theory of the decay of metastable states we calculate the rate of the anomalous electroweak baryon and lepton number violating processes in the case that the electroweak phase transition is of second order. Our formulas are valid in a temperature range between M_w and the critical temperature T_c. We get a dissipation of the baryon and lepton number of the order of exp(4.6×10⁹).     

The spin program with CLAS at JLab

We report on the status of an extensive program to study the scattering of longitudinally polarized electrons from longitudinally polarized NH₃ and ND₃ targets using the CLAS detector at JLab. The data span a range inQ ² from 0.05–4.5 (GeV/c)² and a range inW, the γ^* N invariant mass, up to about 3 GeV. With the excellent particle identification available with the CLA, both inclusive and exclusive scattering can be studied. The experimental techniques are reviewed and some preliminary results are presented. This paper focuses on extraction of the spin structure functiongg ₁ for the proton and the deuteron.     

Recent study for multibaryon states with strangeness in pC interaction at 10 GeV/c

Strange multibaryon states with Λ-hyperon and K _s ⁰ -meson subsystems has been studied from 700 000 stereo photographs or 10⁶ inelastic interactions which was obtained from expose of 2-m propane bubble chamber (PBC) LHEP, JINR to proton beams at 10 GeV/c. The obtained results from PBC can be divided into three subjects: in-medium effects of hadronic particles; baryon spectroscopy; hyper-nucleus production. At present the experimental situation is confused; so is theory. New accelerator research complexes has unique possibility for high-statistic and 4π-geometry study of exotic states.     

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.     

Strangeness production and other recent results from Hall B at Jefferson Lab

Recent results using the CLAS detector at Jefferson Lab are discussed. We emphasize strangeness photo- and electro-production in the baryon resonance region between W = 1.6 and 2.3 GeV, where indications of s-channel structure are suggestive of high-mass baryon resonances coupling to kaons and hyperons in the final state. Other CLAS results, for the E _{1 +} /M _{1 +} transition form factor at the first -resonance, two-pion electroproduction, and photoproduction are also briefly mentioned.Received: 30 September 2002, Published online: 22 October 2003PACS: 13.30.-a Decays of baryons - 13.30.Eg Hadronic decays - 13.40.-f Electromagnetic processes and properties - 13.60.-r Photon and charged-lepton interactions with hadronsR.A. Schumacher: For the CLAS Collaboration     

Evidence for formation of a narrow <Emphasis Type="Italic">K</Emphasis><Stack><Subscript>S</Subscript><Superscript>0</Superscript></Stack><Emphasis Type="Italic">p</Emphasis> resonance with mass near 1533 MeV in neutrino interactions

A narrow baryon resonance is observed in the invariant mass of the K _S ⁰ p system formed in neutrino and antineutrino collisions with nuclei. The mass of the resonance is estimated at 1533±5 MeV. The observed width is less than 20 MeV and is compatible with being entirely due to experimental resolution. The statistical significance of the signal is near 6.7 standard deviations. Since the position of the observed resonance does not match the mass of any known Σ*⁺ states, we believe that it arises from the neutrino production of the Θ⁺ pentaquark baryon. The analysis is based on the data obtained in past neutrino experiments with big bubble chambers: WA21, WA25, WA59, E180, and E632.     

An su (6)w fit to the negative parity baryon decay rates

We present a least squares fit to the experimental data on decays of negative parity baryon resonances into a pseudoscalar meson and either a J^P = 1/2⁺ stable baryon or a J^P = 3/2⁺ decuplet member. We find that the s-waves and d-waves are separately in good agreement with the predictions of SU (6)_w ⊗ O(2)_Lz. Predictions are given regarding several as yet unobserved decay processes, and for those which concern hitherto undetected resonances, their possible detection is discussed.     

RPA description of the electric polarizability of the nucleon

Excited states of the nucleon are described as RPA configurations on a mean-field ground state taken from the MIT bag model. A residual interaction of a structure as in the Nambu-Jona-Lasinio model is used. The particlehole states are coupled to good total angular momentum and isospin. Valence excitations of particle-hole type and quark-antiquark (q $\bar q$ ) states from the Dirac-sea are included. The dependence of the baryon spectrum and multipole response functions on the coupling constantG is studied. At critical values ofG the 3q-ground state becomes degenerate with strongly collectiveq99-2 modes. The model is used to calculate the averaged electric polarizability of the neutron and the protonα. Without residual interactionα=7·10^?4 fm ³ is found. With residual interaction the value increases toα=(?11)·10^?4 fm ³. The measured value ofα is reproduced within experimental error bars.     

Potential of quasielastic meson knockout from a nucleon by high-energy electrons and pions for studying the structure of exotic excited states of a final-state baryon

The possibility of studying q ⁴ \(\bar q\) exotic baryon states by means of N(e, e’M)B reactions proceeding via an extremely simple mechanism and involving the quasielastic knockout of various mesons from a nucleon by electrons of energy in the few-GeV region is considered as a development of the previous investigations of our group. A quark microscopic formalism based on the cluster model of q ⁴ \(\bar q\) states, which makes it possible to determine momentum distributions of mesons in various channels of B → N + M virtual decays (in principle, these distributions can be compared with experimental data), is expounded by considering the example of the pentaquark (B = Θ⁺). The decay widths of the q ⁴ \(\bar q\) baryon states being discussed are governed by the degree of separation of quark clusters (this is a parameter of the model used). The electroproduction cross sections prove to be small because of kinematical constraints requiring that physically admissible values of the momentum ‖k‖ of the virtual meson M lie in the region where relevant amplitudes are suppressed substantially by form factors in pentaquark vertices. In particular, N (e, e′π ^±)B reactions involving pion knockout furnish direct information about nonstrange components of baryon B; however, the expected cross sections for such reactions are an order of magnitude smaller than their counterparts for analogous reactions leading to the production of a pentaquark Θ⁺. Because of the smallness of the electroproduction cross sections, it is reasonable to consider the production of a pentaquark and other q ⁴ \(\bar q\) exotic states in reactions characterized by quasielastic kinematics and initiated by pions of energy in the range between about 1 and 5 GeV and in similar stripping and pickup nuclear reactions.     

Semi-leptonic charm baryon decays in the relativistic spectator quark model

We calculate the exclusive semi-leptonic charm baryon decays of the lowest lying charm baryon states into the ground state strangeness baryons using the covariant spectator quark model approach. We present results on rates,q ²- andE _l -spectra as well as on the angular decay distribution in the cascade decay \(\Omega _c \to \Omega ( \to \Xi \pi ,\Lambda {\rm K})\) .     

Analysis of recent $ \eta$ photoproduction data

Recent data on h \eta -meson photoproduction off a proton target in the energy range 2￡ \le ?{s} \sqrt{{s}} ￡ \le3 GeV are analyzed with regard to their overall consistency. Results from the ELSA and CLAS measurements are compared with predictions of a Regge model whose reaction amplitude was fixed via a global fit to pre-2000 measurements of differential cross sections and polarization observables for g \gamma p ? \rightarrow h \eta p at higher energies. We find that all recent experimental results on differential cross sections for h \eta -meson photoproduction are in good agreement with each other, except for the CLAS data from 2009. However, the latter can be made consistent with the other data at the expense of introducing an energy-dependent renormalization factor. We point out that there are indications in the data for a possible excitation of baryon resonances with masses around 2.1 and 2.4GeV.     

Strange-quark matter within the Nambu-Jona-Lasinio model

The equation of state of baryon-rich quark matter is studied within the SU(3) Nambu-Jona-Lasinio model with flavor-mixing interaction. Possible bound states (strangelets) and chiral phase transitions in this matter are investigated at various values of the strangeness fraction r _s. Model predictions are very sensitive to the ratio of the vector and scalar coupling constants, ξ=G _V/G _S. At ξ=0.5 and zero temperature, the binding energy takes a maximum value of about 15 MeV per baryon at r _s?0.4. Such strangelets are negatively charged and have typical lifetimes of about 10^?7s. Calculations are performed at finite temperatures as well. According to these calculations, bound states exist up to temperatures of about 15 MeV. The model predicts a first-order chiral phase transition at finite baryon densities. The parameters of this phase transition are calculated as functions of r _s.     

Electromagnetic and Axial Form Factors of Octet and Decuplet Baryons

We report from a study of the elastic electromagnetic and axial form factors of all lowest baryon states with flavors up, down, and strange along relativistic constituent-quark models. We consider the baryons as relativistic bound states of three constituent quarks and solve the eigenvalue problem of the invariant mass operator. The corresponding eigenstates are employed to calculate manifestly covariant form factors within the point form of Poincaré-invariant quantum mechanics. The electromagnetic and axial current operators are constructed along the spectator model in point-form relativistic dynamics. We have thus obtained covariant predictions for the electroweak form factors, for momentum transfers up to Q ² ~ 4 GeV², as well as the electric radii, magnetic moments, and axial charges. The theoretical results in general agree very well with existing phenomenological data. In cases, where no experimental information is yet available, the results are well compatible with data from lattice quantum chromodynamics.