BABAR measurement of baryon time-like form factors via initial state radiation

BABAR has measured with unprecedented accuracy the e+e-→p(p-)and e+e-Λ(Λ-)cross sections by means of the initial state radiation technique,which has the advantages of good efficiency,good energy resolution and full angular acceptance,even exactly at threshold.A peculiar feature of these cross sections is their non-vanishing values at threshold.In the case of charged baryons,this phenomenon is expected according to the Coulomb interaction between the outgoing baryon and antibaryon.Once this Coulomb enhancement factor is taken into account,the striking result is achieved that the proton form factor at threshold is |GP(4M2p)|=1,that is what is expected for pointlike fermion pairs,in spite of the proton structure.However a Coulomb enhancement factor is not expected for neutral fermions,likely in contradiction with the BABAR data.Qualitatively this behaviour is consistent with Coulomb interactions at the valence quark level.     

BABAR measurement of baryon time-like form factors via initial state radiation

BABAR has measured with unprecedented accuracy the e^＋e^- → pp and e^＋e^- →∧∧ cross sections by means of the initial state radiation technique, which has the advantages of good efficiency, good energy resolution and full angular acceptance, even exactly at threshold. A peculiar feature of these cross sections is their non-vanishing values at threshold. In the case of charged baryons, this phenomenon is expected according to the Coulomb interaction between the outgoing baryon and antibaryon. Once this Coulomb enhancement factor is taken into account, the striking result is achieved that the proton form factor at threshold is ｜GP（4Mp^2）｜ = 1, that is what is expected for pointlike fermion pairs, in spite of the proton structure. However a Coulomb enhancement factor is not expected for neutral fermions, likely in contradiction with the BABAR data. Qualitatively this behaviour is consistent with Coulomb interactions at the valence quark level.     

Doubly heavy-quark baryon spectroscopy and semileptonic decay

Working in the framework of a nonrelativistic quark model we evaluate the spectra and semileptonic decay widths for the ground state of doubly heavy Ξ and Ω baryons. We solve the three-body problem using a variational ansatz made possible by the constraints imposed by heavy-quark spin symmetry. In order to check the dependence of our results on the inter-quark interaction, we have used five different quark-quark potentials which include Coulomb and hyperfine terms coming from one-gluon exchange, plus a confining term. Our results for the spectra are in good agreement with a previous calculation done using a Faddeev approach. For the semileptonic decay our results for the total decay widths are in good agreement with the ones obtained within a relativistic quark model in the quark-diquark approximation.     

Monopoles,baryon decay and charge conservation

The mechanism of baryon decay via monopoles is analyzed. For this purpose we quantize isodoublet fermion fields in the presence of a 't Hooft-Polyakov monopole. When the electromagnetic interactions are switched off, we find a condensation of a fermion pair \(\bar \psi _ - ^{(i)} \gamma ^0 \gamma ^5 \psi _ + ^{(i)} \) as well as that of \(\bar \psi _ - ^{(i)} \gamma ^0 \gamma ^5 \psi _ + ^{(i)} \bar \psi _ + ^{(j)} \gamma ^0 \gamma ^5 \psi _ - ^{(j)} \) . Here, the indices ± stand for the electric charge and (i,j) for the flavour. Hence, the charge symmetry is spontaneously broken. However, when the Coulomb interactions are switched on, it is proved that all fermion condensates carrying non-zero electric charges are removed; the condensates carrying zero electric charge, which induce baryon decay in the standardSU (5) model, are not removed by switching on the Coulomb interactions. In these analyses, the key element is the charge mixing boundary condition imposed on the fermion wave-function at the monopole center; the chiral anomaly does not play any role.     

SU(2N f) ⊗O(3) light diquark symmetry and current-induced heavy baryon transition form factors

We study the current-induced bottom baryon to charm baryon transitions in the Heavy Quark Symmetry limit asm _q→∞. Our discussion involvess-wave tos-wave as well ass-wave top-wave transitions. Using a constituent quark model picture for the light diquark system with an underlyingSU(2N _f)?O(3) symmetry and the heavy quark symmetry we arrive at a number of new predictions for the reduced form factors that describe these transitions.     

SU (2N f) ⊗ O(3) light diquark symmetry and current-induced heavy baryon transition form factors

We study the current-induced bottom baryon to charm baryon transitions in the Heavy Quark Symmetry limit as m_q → ∞. Our discussion involves s-wave to s-wave as well as s-wave to p-wave transitions. Using a constituent quark model picture for the light diquark system with an underlying SU (2N_f) ? O(3) symmetry and the heavy quark symmetry we arrive at a number of new predictions for the reduced form factors that describe these transitions.     

Electromagnetic form factors of the baryon octet in the perturbative chiral quark model

We apply the perturbative chiral quark model at one loop to analyze the electromagnetic form factors of the baryon octet. The analytic expressions for baryon form factors, which are given in terms of fundamental parameters of low-energy pion-nucleon physics (weak pion decay constant, axial nucleon coupling, strong pion-nucleon form factor), and the numerical results for baryon magnetic moments, charge and magnetic radii are presented. Our results are in good agreement with experimental data.Received: 7 January 2003, Revised: 4 November 2003, Published online: 15 April 2004PACS: 12.39.Ki Relativistic quark model - 13.40.Gp Electromagnetic form factors - 14.20.Dh Protons and neutrons - 14.20.Jn Hyperons     

Heavy to light baryon weak form factors in the lightcone constituent quark model

Heavy to light baryon weak form factors are investigated in a lightcone constituent quark model. In a SU(4) symmetry broken scheme, both charged and neutral weak current-induced form factors are calculated at theq ² = 0 point including the leading relativistic effects in the spin composition of baryons. The corresponding semileptonic decays are described by assuming dipole dependence of form factors onq ².     

Weak form factors for semileptonic octet baryon decays in the chiral quark model

We study the weak vector and axial-vector form factors of first- and second-class currents for the semileptonic octet baryon decays in the spirit of the chiral quark model. Our results for the weak magnetism form factors are consistent with the conserved vector current (CVC) results. The induced pseudotensor form factors, which are highly model dependent, are small. The overall performance of the chiral quark model is quite good and in general agreement with existing experimental data. Received: 9 March 1998 / Published online: 12 August 1998     

QCD bound state calculation of Dalitz decay form factors

We present calculations of the cross-sections and the forward-backward asymmetry forp \(\bar p\) →?⁺ ?^? +X in the framework of anSU(2)×U(1)×G model with two neutral gauge bosons [G=U(1) orSU(2)]. It is shown that a peak in the cross-section associated with a large asymmetry will characterize a deviation from the standard model.     

Generalized heavy baryon chiral perturbation theory

Standard SU(2) Heavy Baryon Chiral Perturbation Theory is extended in order to include the case of small or even vanishing quark condensate. The effective lagrangian is given to in its most general form and to in the scalar sector. A method is developed to efficiently construct the relativistic baryonic effective lagrangian for chiral SU(2) to all orders in the chiral expansion. As a first application, mass- and wave-function renormalization as well as the scalar form factor of the nucleon is calculated to . The result is compared to a dispersive analysis of the nucleon scalar form factor adopted to the case of a small quark condensate. In this latter analysis, the shift of the scalar form factor between the Cheng-Dashen point and zero momentum transfer is found to be enhanced over the result assuming strong quark condensation by up to a factor of two, with substantial deviations starting to be visible for . Received: 22 July 1998 / Revised version: 2 September 1998 / Published online: 2 November 1998     

One-gluon-exchange effects on semileptonic decay of baryon octet

We study one-gluon-exchange (OGE) effects ong _A /g _V of weak semileptonic decay processes of theSU(3) baryon octet. We especially investigate the systematic behavior of OGE effects on all the possible decay processes and search for better overall agreements between theory and experiment. We adopt the MIT bag model to describe quarks confined in a baryon. We also use a formulation of the perturbative QCD in which gluon field is also confined in the bag. Our calculations respect the boundary conditions which achieve the confinement of each field within the bag. The magnitudes of our corrections tog _A /g _V have a variation of 1 ～30% depending on the decay, processes. They have process-dependence quite different from that of the uncorrected values ofg _A /g _V primarily governed by theSU(6) symmetry. These OGE effects ong _A /g _V reflectSU(6) asymmetric spin-spin correlation between quarks due to the color magnetic interactions. Especially, the OGE corrections tog _A /g _V are very important in the processes, \(\Sigma ^ - \to n + l^ - + \bar v_l ,\Sigma ^ - \to p + l^ - + \bar v_l \) and \(\Xi ^ - \to \Xi ^0 + e^ - + \bar v_e \) . Consequently, we clarify that the OGE effects can sizably improve theoretical values ofg _A /g _V in favor of overall agreements with the present experimental data of semileptonic decays of the baryon octet, at least, in the MIT bag model.     

Microscopic form factors for transfers between heavy ions

Static cluster wave functions are constructed by translations of shell-model wave functions. Waves in the continuum are expanded on this cluster basis. A DWBA transition amplitude is derived, where the form factor is given by a microscopic formula.     

Charmed baryon decay asymmetry in e~+e~- annihilation

We propose to measure the decay asymmetry parameters in the hadronic weak decays of singly charmed baryons, such as Λ_c~+ →Λπ~+, Σ~0π~+, pK_0, Ξ0c →Ξ-π+and ?0c →?-π+. The joint angular formulae for these processes are presented, and are used to extract the asymmetry parameters in e+e-annihilation data. Base on the currentΛ+c data set collected at BESIII, we estimate the experimental sensitivities to measure the parameters αΛπ+forΛ+c →Λπ+, αΣ+π0for Λ+c →Σ+π0and αΣ0π+for Λ+c →Σ0π+.     

Towards an understanding of heavy baryon spectroscopy

The recent observation at CDF and D0 of Σ _b , Σ _b ^* and Ξ _b baryons opens the door to the advent of new states in the bottom baryon sector. The states measured provide sufficient constraints to fix the parameters of phenomenological models. One may therefore consistently predict the full bottom baryon spectra. For this purpose we have solved exactly the three-quark problem by means of the Faddeev method in momentum space. We consider our guidance may help experimentalists in the search for new bottom baryons and their findings will help in constraining further the phenomenological models. We identify particular states whose masses may allow to discriminate between the dynamics for the light quark pairs predicted by different phenomenological models. Within the same framework we also present results for charmed, doubly charmed, and doubly bottom baryons. Our results provide a restricted possible assignment of quantum numbers to recently reported charmed-baryon states. Some of them are perfectly described by D-wave excitations with J ^P = 5/2⁺, as the Λ _c (2880), Λ _c (3055), and Λ _c (3123). Communicated by V. Vento