Rho Meson Research Articles

Rho(0) meson production in the pp-->pppi(+)pi(-) reaction at 3.67 GeV/c.

Total and differential cross sections for the exclusive reaction pp-->pp rho observed via the pi(+)pi(-) decay channel have been measured at p(beam)=3.67 GeV/c. The observed total meson production cross section is determined to be (23.4+/-0.8+/-8) mu b and is significantly lower than typical cross sections used in model calculations for heavy-ion collisions. The differential cross sections measured indicate a strong anisotropy (approximately cos(theta(CM)(rho)) in the rho(0) meson production.

Aspects of meson properties in dense nuclear matter

We investigate the modification of meson spectral densities in dense nuclear matter at zero temperature. These effects are studied in a fully relativistic mean field model which goes beyond the linear density approximation and also includes baryon resonances. In particular, the role of N*(1520) and N*(1720) on the rho meson spectral density is highlighted. Even though the nucleon-nucleon loop and the nucleon-resonance loop contribute with the opposite sign, an overall reduction of rho meson mass is still observed at high density. Importantly, it is shown that the resonances cause substantial broadening of the rho meson spectral density in matter and also induces non-trivial momentum dependence. The spectral density of the a0 meson is also shown. We study the dispersion relations and collective oscillations induced by the rho meson propagation in nuclear matter together with the influence of the mixing of rho with the a0 meson. The relevant expression for the plasma frequency is also recovered analytically in the appropriate limit.

Vector meson photoproduction at high-t and comparison to HERA data

We explore QCD calculations for the process gamma p -> V X where V is a vector meson, in the region s >> -t and -t >> Lambda_QCD^2. We compare our calculations for the J/psi, phi and rho mesons with data from the ZEUS Collaboration at HERA and demonstrate that the BFKL approach is consistent with the data even for light mesons, whereas the two-gluon exchange approach is inadequate. We also predict the differential cross-sections for the Upsilon and omega for which no data are currently available.

Charm meson production from meson–nucleon scattering

Using an effective hadronic Lagrangian with physical hadron masses and coupling constants determined either empirically or from SU(4) flavor symmetry, we study the production cross sections of charm mesons from pion and rho meson interactions with nucleons. With a cutoff parameter of 1 GeV at interaction vertices as usually used in studying the cross sections for J/ψ absorption and charm meson scattering by hadrons, we find that the cross sections for charm meson production have values of a few tenth of mb and are dominated by the s channel nucleon pole diagram. Relevance of these reactions to charm meson production in relativistic heavy ion collisions is discussed.

Scale of corrections to the t-channel pole approximation in quasielastic pion knockout from a nucleon by high-energy electrons

An investigation of a dominant role of the simplest t-channel pole diagrams in pion electroproduction on nucleons for quasielastic-knockout kinematics at an electron energy of a few GeV is completed-namely, the competition between the t-channel pion and rho-meson pole amplitudes, on one hand, and the s-channel pole amplitude (tree diagram), on the other hand, is considered. When the virtual-photon mass is sufficiently large [Q2>2(GeV/c)2], the last amplitude does not make significant contributions to relevant cross sections, either the longitudinal (dσL/dt) or the transverse (dσT/dt) one. At Q2=0.7(GeV/c)2, the term associated with the interference between the t-channel pion-pole amplitude and the s-channel pole amplitude is still noticeable in the longitudinal cross section. The vertex functions g ρ NN(t) as obtained from the cross section for the quasielastic knockout of rho mesons and from the cross section for pion photoproduction are compared. Their disagreement must give impetus to a further development of the gauge-invariant theory of pion photoproduction.

Chiral approach to the rho meson in nuclear matter

In this work, the properties of the ρ meson at rest in cold symmetric nuclear matter are studied. We make use of a chiral unitary approach to pion–pion scattering in the vector–isovector channel, calculated from the lowest-order Chiral Perturbation Theory ( χPT) Lagrangian including explicit resonance fields. Low-energy chiral constraints are considered by matching our expressions to those of one-loop χPT. To account for the medium corrections, the ρ couples to ππ pairs which are properly renormalized in the nuclear medium, accounting for both p–h and Δ–h excitations. The terms where the ρ couples directly to the hadrons in the p–h or Δ–h excitations are also accounted for. In addition, the ρ is also allowed to couple to N ∗ (1520)–h components.

Temperature dependence of rho and a1-meson masses and mixing of vector and axial-vector correlators.

Within a chiral model which provides a good description of the properties of rho and a1 mesons in vacuum, it is shown that, to order T2, the rho- and a1-meson masses remain constant in the chiral limit, even if at tree level they are proportional to the chiral condensate, sigma0. Numerically, the temperature dependence of the masses turns out to be small also for realistic parameter sets and high temperatures. The weak temperature dependence of the masses is consistent with the Eletsky-Ioffe mixing theorem, and traces of mixing effects can be seen in the spectral function of the vector correlator at finite temperature.

Couplings between pion and charmed mesons

We compute the couplings $DD^*\pi$,$D_1D^*\pi$,$D^*D^*\pi$, $D_1 D_1\pi$ using QCD sum rules. These couplings are important inputs in the meson exchange model calculations used to estimate the amount of $J/\psi$ absorption due to pions and rho mesons in heavy ion collisions. Our sum rules are constructed at the first order in the pion momentum $p_\mu$, which give the couplings that are not trivially related to the soft-pion theorem. Our calculated couplings, which somewhat depend upon the values of the heavy meson decay constants, are $g_{DD^*\pi}=8.2 \pm 0.1$, $g_{D_1 D^* \pi} = 15.8 \pm 2 {\rm GeV}$, $g_{D^* D^* \pi}= 0.3 \pm 0.03$ and $g_{D_1 D_1 \pi}= 0.17 \pm 0.04$.

D and ρ mesons: who resolves whom?

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.

Photoproduction of the rho(0) meson on the proton at large momentum transfer.

The differential cross section, d sigma/dt, for rho(0) meson photoproduction on the proton above the resonance region was measured up to a momentum transfer -t = 5 GeV2 using the CLAS detector at the Thomas Jefferson National Accelerator Facility. The rho(0) channel was extracted from the measured two charged-pion cross sections by fitting the pi(+)pi(-) and p pi(+) invariant masses. The low momentum transfer region shows the typical diffractive pattern expected from Reggeon exchange. The flatter behavior at large -t cannot be explained solely in terms of QCD-inspired two-gluon exchange models. The data indicate that other processes, like quark interchange, are important to fully describe rho photoproduction.

Chiral behavior of the rho meson in lattice QCD

In order to guide the extrapolation of the mass of the rho meson calculated in lattice QCD with dynamical fermions, we study the contributions to its self-energy which vary most rapidly as the quark mass approaches zero; from the processes $\rho \to \omega \pi$ and $\rho \to \pi \pi$. It turns out that in analysing the most recent data from CP-PACS it is crucial to estimate the self-energy from $\rho \to \pi \pi$ using the same grid of discrete momenta as included implicitly in the lattice simulation. The correction associated with the continuum, infinite volume limit can then be found by calculating the corresponding integrals exactly. Our error analysis suggests that a factor of 10 improvement in statistics at the lowest quark mass for which data currently exists would allow one to determine the physical rho mass to within 5%. Finally, our analysis throws new light on a long-standing problem with the J-parameter.

Nucleon–nucleon effective potential in dense matter including rho-meson exchange

We obtain the RPA summed one-meson exchange potential between nucleons in symmetric nuclear matter at zero temperature, from a model which includes ρ, σ, ω and π mesons. The behavior of rho mesons inside the medium is first discussed using different schemes to extract a finite contribution from the vacuum polarization. These schemes give qualitatively different results for the in-medium rho mass. The results are discussed in connection with the nonrenormalizability of the model. We next study the modified potential as density increases. In the intermediate-distance range, it is qualitatively modified by matter and vacuum effects. In the long-distance range ( r>2 fm), one observes the presence of oscillations, which are not present in free space. Features on this distance range are insensitive to the renormalization scheme.

Probing the mesonic structure of the nucleon by means of exclusive quasielastic pion knockout in (e, e′π) reactions at high energies

By including the Z diagram in an analysis performed in the laboratory frame (instantaneous form of dynamics), the notion of quasielastic pion knockout by protons and electrons [(p, 2p) and (e, e′p) reactions treated in terms of the relevant pole diagrams] is generalized to the relativistic case where a meson is quasielastically knocked out of a nucleon by an electron having an energy of a few GeV. The concept of the wave function is introduced for the pion (and for other mesons), and its relation to the vertex constant G πNN and the vertex function g πNN(k 2) is indicated. The spectroscopic factor S N Bℳ is defined as the normalization of the wave function for the meson ℳ. It is shown by two methods that, under the kinematical conditions of quasielastic knockout that include the condition E π≫m π (E π is the energy of the knock-on pion) and the condition that the square Q 2 of the virtual pion mass is large, the competing tree diagram is suppressed in relation to the pole diagram (this is not so in the case of pion photoproduction). From data of a p(e, e′π +)n experiment involving longitudinal virtual photons γ L * , the momentum distribution |Ψ p nπ (k)|2 of pions in the nucleon is extracted for the first time over the entire range of significant momenta k, and this result is used to determine the cutoff constant Λπ and the value of S p nπ ≈0.2. The momentum distribution of positive rho mesons in the soft section of the spectrum is determined from experimental data on the process p(e, e′π +)n proceeding through the mechanism ρ ++γ T * → π + involving transverse photons. A way to determine the momentum distribution of omega mesons through data on the process p(e, e′π 0)p is indicated. Two forms of dynamics—instantaneous form and that of light-front dynamics (the latter does not involve the Z diagram)—are compared for the example where the calculations are performed for the spectroscopic factor S N Bℳ .

Extended Gari-Krümpelmann model fits to nucleon electromagnetic form factors

Nucleon electromagnetic form factor data (including recent data) is fitted with models that respect the confinement and asymptotic freedom properties of QCD. Gari-Krumpelmann (GK) type models, which include the major vector meson pole contributions and at high momentum transfer conform to the predictions of perturbative QCD, are combined with Hohler-Pietarinen (HP) models, which also include the width of the rho meson and the addition of higher mass vector meson exchanges, but do not evolve into the explicit form of PQCD at high momentum transfer. Different parameterizations of the GK model's hadronic form factors, the effect of including the width of the rho meson and the addition of the next (in mass) isospin 1 vector meson are considered. The quality of fit and the consistency of the parameters select three of the combined HP/GK type models. Projections are made to the higher momentum transfers which are relevant to electron-deuteron experiments. The projections vary little for the preferred models, removing much of the ambiguity in electron-nucleus scattering predictions.

Mesons on a transverse lattice

The meson eigenstates of the light-cone Hamiltonian in a coarse transverse lattice gauge theory are investigated. Building upon previous work in pure gauge theory, the Hamiltonian and its Fock space are expanded in powers of dynamical fields. In the leading approximation, the couplings appearing in the Hamiltonian are renormalised by demanding restoration of space-time symmetries broken by the cut-off. Additional requirements from chiral symmetry are discussed and difficulties in imposing them from first principles in the leading approximation are noted. A phenomenological calculation is then performed, in which chiral symmetry in spontaneously broken form is modelled by imposing the physical pion-rho mass splitting as a constraint. The light-cone wavefunctions of the resulting Hamiltonian are used to compute decay constants, form factors and quark momentum and spin distributions for the pion and rho mesons. Extensions beyond leading order, and the implications for first principles calculations, are briefly discussed.

Charm meson scattering cross sections by pion and rho meson

Using the local flavor SU(4) gauge invariance in the limit of vanishing vector-meson masses, we extend our previous study of charm-meson scattering cross sections by pion and rho meson, which is based only on the pseudoscalar–pseudoscalar–vector meson couplings, to include also contributions from the couplings among three vector mesons and among four particles. We find that diagrams with light-meson exchanges usually dominate the cross sections. For the processes considered previously, the additional interactions lead only to diagrams involving charm-meson exchanges and contact interactions, and the cross sections for these processes are thus not much affected. Nevertheless, these additional interactions introduce new processes with light-meson exchanges and increase significantly the total scattering cross sections of charm mesons by pion and rho meson.

QCD sum rules for ρ mesons in nuclear matter

We investigate QCD sum rules for vector currents in the rho meson channel in the nuclear medium. For increased sensitivity, we subtract out the vacuum contributions. With a saturation scheme often considered in the literature, we find these subtracted sum rules to be unstable. It indicates the importance of other contributions neglected in the saturation scheme. These include more Landau singularities at low energy, additional operators in the medium, and possibly the in-medium width of the rho meson.

Low energy pion-hyperon interaction

We study the low energy pion-hyperon interaction considering effective nonlinear chiral invariant Lagrangians including pions, \ensuremath{\rho} mesons, \ensuremath{\sigma} mesons, hyperons, and corresponding resonances. Then we calculate the S- and P-wave phase shifts, total cross sections, angular distributions, and polarizations for the momentum in the center-of-mass frame up to $k=400$ MeV. With these results we discuss the $\mathrm{CP}$ violation in the $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\Xi}}\ensuremath{\Lambda}\ensuremath{\pi}$ and $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\Omega}}\ensuremath{\Xi}\ensuremath{\pi}$ weak decays.

Search for Rare and Forbidden Charm Meson DecaysD0→Vℓ+ℓ−andhhℓℓ

We report results of a search for flavor-changing neutral current (FCNC), lepton flavor, and lepton-number violating decays of the D0 (and its antiparticle) into three and four bodies. Using data from Fermilab charm hadroproduction experiment E791, we examine modes with two leptons (muons or electrons) and a rho(0), K( *0), or straight phi vector meson or a nonresonant pi(pi), Kpi, or KK pair of pseudoscalar mesons. No evidence for any of these decays is found. Therefore, we present branching-fraction upper limits at 90% confidence level for the 27 decay modes examined (18 new).

Vector manifestation of chiral symmetry.

We propose "vector manifestation" (VM) of the Wigner realization of chiral symmetry in which the symmetry is restored at the critical point by the massless degenerate pion (and its flavor partners) and the rho meson (and its flavor partners) as the chiral partner, in sharp contrast to the traditional manifestation á la the linear sigma model where the symmetry is restored by the degenerate pion and the scalar meson. The application to the chiral phase transition of large N(f) QCD is performed using the hidden local symmetry Lagrangian. Combined with the Wilsonian matching proposed recently, VM determines the critical number of massless flavors N(f) approximately equal to 5 without much ambiguity.