The electromagnetic pion form-factor is calculated in a Bethe-Salpeter approach which accounts for pion rescattering. In the scattering kernel the pion-pion contact interaction from lowest-order chiral perturbation theory is considered together with an optional vector meson in the s-channel. Correspondingly the virtual photon couples to a two-pion state and optionally to the vector meson. It is shown that for reasonable ranges of input parameters the experimentally observed pion form-factor cannot be described by the iterated pion-pion contact interaction alone, i.e. without an elementary vector meson. The inclusion of an elementary vector meson allows for an excellent description. This completes a recent study (``Information on the structure of the a_1 from tau decay'') where it has been shown that the a_1 meson can be well understood as a rescattering process of rho meson and pion. Here it is demonstrated that within the same formalism the rho meson cannot be understood as a pion-pion rescattering process. This suggests that the chiral partners a_1 and rho are not only different in mass, but also different in nature.
The Kroll-Lee-Zumino renormalizable Abelian quantum field theory of pionic strong interactions is used to compute the rho-meson propagator at the two-loop level.
Considering analyticity, unitarity and an influence of coupled channels, experimental data on the isovector P-wave of pion-pion scattering was analyzed to study rho-like mesons below 1900 MeV. The analysis indicates evidently that in the energy range 1200--1800 MeV, there are three rho-like mesons: rho(1250), rho(1450) and rho(1600), unlike the PDG tables. The obtained P-wave pion-pion scattering length (a_1^1=33.9+- 2.02 [10^{-3}m_{pi^+}^{-3}]) most matches to the one calculated in the local Nambu--Jona-Lasinio model.
In the approach, based on analyticity and unitarity and assuming an influence of coupled channels, experimental data on the isovector P-wave of pion-pion scattering have been analyzed to study rho-like mesons below 1.9 GeV.
We show that AdS/QCD generates predictions for the rate of diffractive rho meson electroproduction that are in agreement with data collected at the HERA electron-proton collider.
The investigation in the work of the reaction electron-positron to omega and pi0 mesons in the 3P0 nonrelativistic quark model reveals that the reaction electron-positron to omega and pi0 mesons process at the energy region from the omega and pi mesons threshold to 2.0 GeV is dominated by the two-step process in which the primary quark-antiquark pair first forms rho and rho' mesons and then the vector mesons decay into omega and pi. With rho(1450) and rho(1700) mainly in 2S and 1D states respectively, the experimental data for the cross section of the reaction electron-positron to omega and pi…
Two-photon decays of rho and omega mesons and strong decays rho -> eta pi and omega -> pi pi are described in the quark Nambu-Jona-Lasinio(NJL) model in dense medium. Decays of rho-meson are especially interesting because they contain intermediate a_0-meson which became a sharp resonance in the vicinity of phase transition of hadron matter into quark-gluon plasma. This leads to increasing rho decay width and, as a result, two-photon decay of rho-meson in medium became comparable with two-photon decays of scalar and pseudoscalar mesons in vacuum. Similar effect take place in the strong decay rh…
We study photoproduction of rho mesons in a model of hidden local symmetry. The rho meson is introduced as a hidden gauge boson and a phenomenological rho meson-nucleon Lagrangian is constructed respecting chiral symmetry. It is shown that the sigma exchange interaction is needed for neutral rho meson photoproduction to reproduce the experimental cross sections. For charged rho meson photoproduction, the model takes into account the rho meson magnetic moments from the three-point vertex in the kinetic terms. We show that the magnetic moment of the charged rho meson has a significant effect on …
We study medium modifications of the dilepton mass spectrum in coherent photo- and electroproduction of the Rho(1S)- and Rhoprim(2S)-meson resonances on nuclear targets. The analysis is performed within the coupled Rho, Rhoprim... channel formalism in which nuclear modifications derive from the off-diagonal rescatterings. We find that the effect of off-diagonal rescatterings on the shape of the dilepton mass spectrum in the Rho(1S)-meson mass region is only marginal but is very important in the Rhoprim mass region. The main off-diagonal contribution in the Rhoprim mass region comes from the se…
We investigate the pion electromagnetic charge radii and mass dropping of the rho meson at finite density. We first calculate the pion charge radii within the framework of the nonlocal chiral quark model from the instanton vacuum both at zero and finite densities. In order to relate the change of the pion charge radius to that of the rho-meson mass at finite density, we employ the vector-meson dominance for the pion electromagnetic form factor. It turns out that the pion charge radius is getting larger as the quark chemical potential increases. As a result, the rho-meson mass falls off as the …
The magnetic moment $μ$ of the "rho" meson is studied in QCD light cone sum rules, and it is found that "mu = (2.3 pm 0.5)". A comparison of our result on the magnetic moment of the "rho" meson with the predictions of the other approaches, is presented.
A chiral SU(2) x SU(2) Lagrangian containing, besides the usual meson fields, their first radial excitations is considered. The Lagrangian is derived by bosonization of the Nambu-Jona-Lasinio quark model with separable non-local interactions, with form factors corresponding to 3-dimensional ground and excited state wave functions. The spontaneous breaking of chiral symmetry is governed by the NJL gap equation. The first radial excitations of the pions, rho-and omega-mesons are described with the help of two form factors. The weak decay constant F_pi' is calculated. The values for the decay wid…
Valence quark distributions in transversally and longitudinally polarizied rho mesons in the region of intermediate x are obtained by generalizied QCD sum rules. Power corrections up to d=6 are taken into account. Comparison of the results for pi and rho mesons shows, that polarization effects are very significant and SU(6) symmetry of distribution functions is absent. The strong suppression of quark and gluon sea distributions in longitudinally polarizied rho mesons is found.
We present predictions for rho-meson form factors obtained from the analysis of QCD sum rules in next-to-leading order of perturbation theory. The radiative corrections turn out to be sizeable and should be taken into account in rigorous theoretical analysis.
The rho-meson magnetic moment is calculated in the framework of QCD sum rules in external fields. Bare loop calculations (parton model) give 2.0. The contribution of the operators of dimension 6 reduces this value to 1.5.
We report on the results of our recent research published in [1] that shows that AdS/QCD generates predictions for the rate of diffractive ρ-meson electroproduction that are in agreement with data collected at the HERA electron-proton collider [2,3]. Preliminary results of this research were presented in [4].
We calculate the branching ratio of omega --> pi^+ pi^- gamma decay in a phenomenological framework in which the contributions of VMD, chiral loops, sigma-meson intermediate state amplitudes and the effects of omega-rho mixing are considered. We conclude that the sigma-meson intermediate state amplitude and omega-rho mixing make substantial contribution to the branching ratio.
We extract the light-cone wavefunctions of the rho meson using the HERA data on diffractive rho photoproduction. We find good agreement with predictions for the distribution amplitude based on QCD sum rules and from the lattice. We also find that the data prefer a transverse wavefunction with enhanced end-point contributions.
The excitation curve of e+e- annihilation into four charged pions in the rho(770) region is calculated using three existing models with rho mesons and pions in intermediate states supplemented by Feynman diagrams with the a_1(1260)pi intermediate states. A two-term phenomenological Lagrangian of the a_1-rho-pi interaction is used. The mixing angle is determined by fitting the e+e- --> pi+pi-pi+pi- cross section data of the Novosibirsk CMD-2 collaboration and also its combination with the low-energy part of the BaBar collaboration data. It is shown that the inclusion of the a_1-pi intermediate …
We have shown that the lowest order QCD amplitude, i.e. the quark exchange contribution, to the forward production of a pair of longitudinally polarized rmesons in the scattering of two virtual photons gamma (Q1) gamma (Q2) rho rho factorizes in two different ways: the part with transverse photons is described by the QCD factorization formula involving the generalized distribution amplitude of two final rho mesons, whereas the part with longitudinally polarized photons takes the QCD factorized form with the gamma rho transition distribution amplitude. Perturbative expressions forthese, in gene…
We present new models of the rho-meson leading-twist light-cone distribution amplitudes based on the QCD sum rule approach with nonlocal condensates. Their shapes differ noticeably from that known in the literature. The phenomenological consequences for physically important process B --> rho e nu are discussed in the framework of the light-cone sum rules. The results are compared with those found recently by P.Ball and V.M.Braun (1997).
The constituent quark rho meson electromagnetic form-factors are calculated, with covariant and null-plane approaches with the same model. The null-plane formalism produces the breakdown of the rotational symmetry for the one-body current operador, wich is investigated by comparing the numerical results in both approaches. This allows to choose the appropriate null-plane prescriptio, among the several ones, to evaluate the rho meson form-factors.
We study the radiative $ρ^0\to π^+π^-γ$ and $ρ^0\to π^0π^0γ$ decays and we calculate their branching ratios using a phenomenological approach by adding to the amplitude calculated within the framework of chiral perturbation theory and vector meson dominance the amplitude of $σ$-meson intermediate state. Our results for the branching ratios are in good agreement with the experimental values.
The magnetic moment of the rho-meson is calculated in the framework of QCD sum rules in external fields. Perturbative corrections are taken into account. Bare loop calculations (parton model) give the value 2.0. The nonperturbative effects (operators of dimension 6) result in negative contribution, whereas perturbative corrections -- positive one. The final answer is 1.8.
A systematic multiple scattering formalism for vector meson electroproduction from nuclei is developed, with emphasis on the formation, propagation and hadronization scales for quark-gluon fluctuations of the virtual high-energy photon. The theory is compared to HERMES measurements of rho electroproduction on 14N. The nuclear transparency as a function of the vector meson propagation length is well reproduced.
The second moments of the quark structure functions of the longitudinally and transversally polarized rho-mesons are calculated in the framework of QCD sum rules in external fields. Operators of dimension 4 and 6 are taken into account. The results are: 0.84 for the longitudinal polarization, 0.5 for the transversal polarization. The large difference between these values indicates that the rho-meson polarization strongly influences the internal structure. In particular, in the case of the longitudinally polarized rho-meson the gluon sea is found to be strongly suppressed, it is less than 0.20 …
It is shown that both the universal coupling of the rho-meson and the Kawarabayashi-Suzuki-Riadzuddin-Fayyazuddin expression for the magnitude of its coupling constant follow from the requirement that chiral perturbation theory of pions, nucleons, and rho-mesons is a consistent effective field theory. The prerequisite of the derivation is that all ultraviolet divergences can be absorbed in the redefinition of fields and the available parameters of the most general effective Lagrangian.
Rho meson propagation in hot hadronic matter is studied in a model with coupling to $ππ$ states. Medium modifications are induced by a change of the pion dispersion relation through collisions with nucleons and $Δ's$ in the fireball. Maintaining gauge invariance dilepton production is calculated and compared to the recent data of the CERES collaboration in central S+Au collisions at 200 GeV/u. The observed enhancement of the rate below the rho meson mass can be largely accounted for.
The DDrho form factor is evaluated in a QCD sum rule calculation for both D and rho off-shell mesons. We study the double Borel sum rule for the three point function of two pseudoscalar and one vector meson currents. We find that the momentum dependence of the form factors is very different if the D or the rho meson is off-shell, but they lead to the same coupling constant in the DDrho vertex. We discuss two different approaches to extract the DDrho coupling constant.
We study the p(e,e' pi+)n reaction in the framework of an effective Lagrangian approach including nucleon, pi and rho meson degrees of freedom and show the importance of the rho-meson t-pole contribution to sigmaT, the transverse part of cross section. We test two different field representations of the rho meson, vector and tensor, and find that the tensor representation of the rho meson is more reliable in the description of the existing data. In particular, we show that the rho-meson t-pole contribution, including the interference with an effective non-local contact term, sufficiently improv…
We report on a successful extraction of the twist-2 and twist-3 Distribution Amplitudes (DAs) of the rho meson using the HERA data on diffractive rho photoproduction. We extract these DAs using several Colour Glass Condensate (CGC) inspired and a Regge inspired dipole models. All our extracted DAs are consistent with Sum Rules and lattice predictions.
We explore the range of values of the in-medium width of a $ρ$-meson at rest which is compatibale with the QCD sum rule approach in a nuclear medium assuming vector meson dominance and a Brown-Rho scaling law of the $ρ$-meson mass with the chiral condensate. The lower and upper bounds for the in-medium width are found to be strongly increasing with the decreasing mass of the $ρ$-meson (increasing nuclear density). We also study the bounds for the in-medium width in models not satisfying the Brown-Rho scaling law. It is shown that the in-medium width depends on how rapidly the mass decreases in…
Aiming at the calculation of the properties of rho-mesons, non-perturbative QCD-based methods are discussed concerning their potentials as well as their short-comings. The latter are overcome by combining these techniques. The utilized methods are (i) the chiral constituent quark model deduced from the instanton vacuum model and large-N_c arguments, (ii) chiral perturbation theory unitarized by the inverse amplitude method and (iii) QCD sum rules. Advantages of the combination of these methods are especially the absence of un-physical quark-production thresholds and parameter-free results. Alr…
We investigate the electromagnetic form factors of the (rho) meson in light cone QCD sum rules. We find that the ratio of the magnetic and charge form factors is larger than two at all values of Q^2, (Q^2 >= 0.5 GeV^2). The values of the individual form factors at fixed values of Q^2 predicted by the light cone QCD sum rules are quite different compared to the results of other approaches. These results can be checked in future, when more precise data on (rho) meson form factors is available.
We describe hard exclusive processes involving a transversally polarized rho meson in the twist 3 approximation, in a framework based on the Taylor expansion of the amplitude around the dominant light-cone direction.
We calculate the amplitudes of J^{PC}=3^{--} meson production in diffractive DIS within the k_t-factorization approach, with a particular attention paid to the rho_3(1690) meson. We find that at all Q^2 the rho_3(1690) production cross section is 2-5 times smaller than the rho(1700) production cross section, which is assumed to be a pure D-wave state. Studying sigma_L and sigma_T separately, we observe domination of rho_3 in sigma_L and domination of rho(1700) in sigma_T and offer an explanation of this behavior in simple terms. We also find very strong contributions -- sometimes even dominati…
We use an AdS/QCD holographic wavefunction to generate predictions for the rate of diffractive ρ-meson electroproduction that are in reasonable agreement with data collected at the HERA electron-proton collider.
We derive an explicit analytical expression for the magnetic dipole moment of the rho meson, mu_rho, in a relativistic constituent-quark model. We adopt our relativistic approach to composite systems, modified instant-form (mIF) Relativistic Quantum Mechanics (RQM), that we used particularly to construct a unified pi&rho model (Phys. Rev. D93, 036007 (2016) = arXiv:1602.00907) describing electroweak properties of light mesons. This model provides a parameter-free calculation to give mu_rho=(2.16+-0.03) [e/2M_rho] which is in accordance with the conventional experimental data. The magnetic, qua…
We study inelastic meson-meson scattering that is governed by quark-antiquark annihilation and creation involving a quark and an antiquark annihilating into a gluon, and subsequently the gluon creating another quark-antiquark pair. The resultant hadronic reactions include for I=1: pion + pion to rho + rho, kaon + antikaon to kaon* + antikaon*, kaon + antikaon* to kaon* + antikaon*, kaon* + antikaon to kaon* + antikaon*, as well as pion + pion to kaon + antikaon, pion + rho to kaon + antikaon*, pion + rho to kaon* + antikaon, and kaon + antikaon to rho + rho. In each reaction, one or two Feynma…
We compute the difference in decay widths of charged and neutral ρ(770) vector mesons. The isospin breaking arising from mass differences of neutral and charged πand ρmesons, radiative corrections to ρ-> ππ, and the ρ-> ππγdecays are taken into account. It is found that the width difference ΔΓ_rho is very sensitive ot the isospin breaking in the $ρ$ meson mass Δm_ρ. This result can be useful to test the correlations observed between the values of these parameters extracted from experimental data.
In this paper we improve and extend a recent analysis which showed that the rho-omega mixing amplitude cannot be unambiguously extracted from the pion electromagnetic form-factor in a model independent way. In particular, we focus on the argument that the extraction is sensitive to the presence of any intrinsic omega_I -> pi pi coupling. Our extended analysis confirms the original conclusion, with only minor, quantitative differences. The extracted mixing amplitude is shown to be sensitive to both the intrinsic coupling omega_I -> pi pi and to the value assumed for the mass of the rho^0-meson.
A realistic model for the free rho meson with coupling to two-pion states is employed to calculate the rho propagator in a hot and dense hadron gas. The medium modifications are based on hadronic rescattering processes: intermediate two-pion states are renormalized through interactions with surrounding nucleons and deltas, and rho meson scattering is considered off nucleons, deltas, pions and kaons. Constraints from gauge invariance as well as the full off-shell dynamics of the interactions are accounted for. Within the vector dominance model we apply the resulting in-medium rho spectral funct…
We evaluate the form factors governing the exclusive decays B->rho l nu, B->a_1 l nu, by using an effective quark-meson lagrangian. The model is based on meson-quark interactions, and the computation of the mesonic transition amplitudes is performed by considering diagrams with heavy mesons attached to loops containing heavy and light constituent quarks. This approach was successfully employed to compute the Isgur-Wise form factors and other hadronic observables for negative and positive parity heavy mesons and is presently used for exclusive heavy-to-light weak transitions. We also evaluate a…
We use perturbative QCD and parton-hadron duality to estimate the cross sections for the diffractive electroproduction of rhoprime(1-) and rho(3-) resonances which occur in the 1.3-1.8 GeV mass interval. We present the cross sections and the ratios sigma_L/sigma_T as a function of Q^2. We compare the predictions with those for the diffractive electroproduction of rho mesons. We show how such diffractive electroproduction measurements at HERA can probe features of the perturbative QCD 'Pomeron'.
Brown-Rho scaling, which has been strongly discussed after recent NA60 data was presented, is investigated within a nonequilibrium field theoretical description that includes quantum mechanical memory. Dimuon yields are calculated by application of a model for the fireball, and strong modifications are found in the comparison to quasi-equilibrium calculations, which assume instantaneous adjustment of all meson properties to the surrounding medium. In addition, we show results for the situation of very broad excitations.
We calculate the Born order cross-section for the exclusive production of rho meson pairs in e+e- scattering in the Regge limit of QCD and we show the feasibility of the measurement of this process at the ILC. We also investigate the leading and next-to-leading order BFKL evolution, making this process a very clean test of the BFKL resummation effects.
An outstanding mystery of charmonium physics is why the J/ψdecays prominently to pseudoscalar plus vector meson channels, such as J/ψ-> ρπand J/ψ-> K^*K, whereas the ψ'(2S) does not. We show that such decays of J/ψand their suppression for ψ'(2S) follow naturally from the existence of intrinsic charm |qbar q cbar c> Fock components of the light vector mesons.
Using a holographic dual model of QCD, we compute the pion electromagnetic form factor F_pi(Q^2) in the spacelike momentum transfer region, as well as pion couplings to vector mesons g_rho^(n) pi pi. Spontaneous and explicit chiral symmetry breaking are intrinsic features of this particular holographic model. We consider variants with both ``hard-wall'' and ``soft-wall'' infrared cutoffs, and find that the F_pi(Q^2) data tend to lie closer to the hard-wall model predictions, although both are too shallow for large Q^2. By allowing the parameters of the soft-wall model (originally fixed by obse…
We extract the leading twist-2 and subleading twist-3 Distribution Amplitudes (DAs) of the rho meson using the HERA data on diffractive rho photoproduction. We do so using several Colour Glass Condensate (CGC) inspired and a Regge inspired dipole models. We find that our extracted twist-2 DA is not much model dependent and is consistent with QCD Sum Rules and lattice predictions. The extracted twist-3 DA is more model dependent but is still consistent with the Sum Rules prediction.
The leading-twist distribution amplitude for the longitudinal rho-meson was studied using QCD Sum Rules with nonlocal condensates and a spectral density which includes next-to-leading order radiative corrections. The obtained profile is compared with results from standard QCD sum rules, lattice QCD, holographic QCD, a light-front quark model, and the instanton liquid model. Preliminary estimates for the first two moments of the transverse $ρ$-meson distribute amplitude are also given.
Some properties of the rho vector meson are calculated within the Nambu-Jona-Lasinio model, including processes that go beyond the random phase approximation. To classify the higher order contributions, we adopt $1/N_c$ as expansion parameter. In particular, we evaluate the leading order contributions to the $ρ\rightarrow ππ$ decay width, obtaining the value $Γ= 118$ MeV, and to the shift of the rho mass which turns out to be lowered by 64 MeV with respect to its RPA value. A set of model parameters is determined accordingly.
Mass distributions of pi^+ pi^- pi^0 and omega pi^0 systems in the e^+ e^- annihilation obtained by the several experiments are analyzed. We obtain indication of light-vector mesons omega'(1.3) and rho'(1.3) in the lower mass region. Those states are expected to exist in the U~(12)-classification scheme.
In this talk, we introduce an effective chiral action, derived from the instanton QCD vacuum configuration, in order to study the modification of mesonic properties in medium. We present our recent theoretical results on the electromagnetic from factor and rho-meson mass shift at finite density. We observe that the rho-meson mass is dropped by about 10%: m*_rho/m_rho ~ 1-0.12 mu_B/mu_0, which is compatible with that estimated by the QCD sum rule as well as by Brown and Rho.
The couplings and decay widths of the proceses (1^+,2^+)->(0^-,1^-)+rho are studied with ligh-cone QCD sum rules in the leading order of heavy quark effective theory. These processes are employed to estimate the two pion transition widths of the (1^+,2^+) doublet through the low mass tail of the rho resonance. Our calculation shows that the two pion decay widths are much smaller than the single pion decay widths. However, B_1 and B_2^* mesons should also have significant two pion decay widths around 1.5 MeV.
We study the temperature dependence of the pion decay constant and rho-meson mass in the hidden local symmetry model at one loop. Using the standard imaginary time formalism, we include the thermal effect of rho meson as well as that of pion. We show that the pion gives a dominant contribution to the pion decay constant and rho-meson contribution slightly decreases the critical temperature. The rho-meson pole mass increases as T^4/m_ρ^2 at low temperature dominated by the pion-loop effect. At high temperature, although the pion-loop effect decreases the rho-meson mass, the rho-loop contributio…
Using effective meson Lagrangians we study the interaction of rho mesons in a hot baryon-free system. Various mesonic resonances in direct $s$-channel reactions are investigated employing standard self-energy techniques, including new reactions that have up to now not been considered in a self-consistent approach at finite temperature. The importance of subthreshold resonances, which are readily accounted for through off-shell effects within our framework, is emphasized. Special care is taken in reproducing radiative decay widths, as they provide valuable constraints on the evaluation of dilep…
In order to reveal possible mass shifts of the vector mesons in a dense strongly interacting medium presumably created in high energy heavy ion collisions it is necessary to know their free masses reliably. The rho(770) mass quoted in the last two editions of the Review of Particle Physics is significantly larger than the values quoted in previous editions. The new value is mostly influenced by the results of recent experiments CMD-2 and SND at the VEPP-2M e+e- collider at Novosibirsk. We show that the values of the mass and width of the rho(770) meson measured in the e+e- -> pi+pi- annihilati…
We present a calculation of the polarized rho meson cloud in a nucleon using time-ordered perturbation theory in two different variants advocated in the literature. We calculate the induced difference between the distributions Delta d^bar(x) and Delta u^bar(x). We use a recent lattice calculation to motivate an ansatz for the polarized valence quark distribution of the rho meson. Our calculations show that the two theoretical approaches give vastly different results. We conclude that Delta d^bar(x) - Delta u^bar(x) can be of relevant size with important consequences for the combined fits of po…
