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L. Roca

Publications and source records attributed to L. Roca.

At least 55 records · Page 3Linked to original sources
arXiv2010

On the nature of the K*2(1430), K*3(1780), K*4(2045), K*5(2380) and K*6 as K*--multi-rho states

We show that the $K^*_2(1430)$, $K^*_3(1780)$, $K^*_4(2045)$, $K^*_5(2380)$ and a not yet discovered $K_6^*$ resonance are basically molecules made of an increasing number of $ρ(770)$ and one $K^*(892)$ mesons. The idea relies on the fact that the vector-vector interaction in s-wave with spins aligned is very strong both for $ρρ$ and $K^*ρ$. We extend a recent work, where several resonances showed up as multi-$ρ(770)$ molecules, to the strange sector including the $K^*(892)$ into the system. The resonant structures show up in the multi-body scattering amplitudes, which are evaluated in terms of the unitary two-body vector-vector scattering amplitudes by using the fixed center approximation to the Faddeev equations.

hep-ph↗
arXiv2010

A description of the f2(1270), rho3(1690), f4(2050), rho5(2350) and f6(2510) resonances as multi-rho(770) states

In a previous work regarding the interaction of two $ρ(770)$ resonances, the $f_2(1270)$ ($J^{PC}=2^{++}$) resonance was obtained dynamically as a two-$ρ$ molecule with a very strong binding energy, 135~MeV per $ρ$ particle. In the present work we use the $ρρ$ interaction in spin 2 and isospin 0 channel to show that the resonances $ρ_3(1690)$ ($3^{--}$), $f_4(2050)$ ($4^{++}$), $ρ_5(2350)$ ($5^{--}$) and $f_6(2510)$ ($6^{++}$) are basically molecules of increasing number of $ρ(770)$ particles. We use the fixed center approximation of the Faddeev equations to write the multi-body interaction in terms of the two-body scattering amplitudes. We find the masses of the states very close to the experimental values and we get an increasing value of the binding energy per $ρ$ as the number of $ρ$ mesons is increased.

hep-ph↗
arXiv2010

The a1(1260) as a rho pi resonance in nuclear matter

We present a theoretical study of the properties of the a1(1260) axial-vector resonance in a cold nuclear medium. In the vacuum, the a1(1260) resonance is generated dynamically from the interactions of a pseudoscalar and vector meson (rho pi and K barK*) in a coupled channel chiral unitary approach. Medium effects are implemented through the modification of the rho and pi propagators at finite nuclear density from well established microscopic many-body calculations. The in-medium pion spectral function accounts for the coupling to N-hole and Delta-hole excitations including short range correlations, whereas the in-medium rho incorporates modifications of its virtual pion cloud as well as direct resonance-hole excitations. The resulting in-medium a1(1260) exhibits significant broadening with increasing density as reflected in the rho pi scattering amplitude. The possible relation of our results with partial restoration of chiral symmetry in nuclear matter is discussed in terms of in-medium Weinberg sum rules.

nucl-th↗
arXiv2009

Asymmetry observables in e+ e- --> pi+ pi- gamma in the phi region within a chiral unitary approach

We make a theoretical study of the charge and forward-backward pion asymmetries in the e+e- --> pi+ pi- gamma process on and off the phi resonance energy. These observables are rather sensitive to the inner details of the theoretical models to describe the reaction. In addition to the standard implementation of the initial state radiation (ISR) and the Bremsstrahlung contribution to the final state radiation (FSR), we use the techniques of the chiral unitary approach to evaluate the contribution from the mechanisms of phi decay into pi+pi-gamma. This contribution involves the implementation of final state interaction from direct chiral loops, the exchange of vector and axial-vector resonances and the final state interaction through the consideration of the meson-meson unitarized amplitudes, which where found important in a previous work describing the phi --> pi pi gamma. We find a good reproduction of the experimental data from KLOE for the forward-backward asymmetry, both at the phi peak and away from it. We also make predictions for the angular distributions of the charge asymmetry and show that this observable is very sensitive to the chiral loops involved in phi radiative decay.

hep-ph↗
arXiv2009

Beam-Helicity Asymmetries in Double Pion Photoproduction off the Proton

Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz in the three isospin channels $\vecγp\to π^{+}π^0n$, $\vecγp\to π^{0}π^0p$ and $\vecγp\to π^{+}π^{-}p$ . The circularly polarized photons, produced from bremsstrahlung of longitudinally polarized electrons, were tagged with the Glasgow magnetic spectrometer. Charged pions and the decay photons of $π^0$ mesons were detected in a $~4π$ electromagnetic calorimeter which combined the Crystal Ball detector with the TAPS detector. The precisely measured asymmetries are very sensitive to details of the production processes and are thus key observables in the modeling of the reaction dynamics.

nucl-ex↗
arXiv2008

Nature of the axial-vector mesons from their Nc behavior within the chiral unitary approach

By describing within the chiral unitary approach the s-wave interaction of the vector meson nonet with the octet of pseudoscalar Goldstone Bosons, we find that the main component of the axial vector mesons b_1(1235), h_1(1170), h_1(1380), a_1(1260), f_1(1285) and the two states associated to the K_1(1270) does not follow the QCD dependence on the number of colors for ordinary qqbar mesons.

hep-ph↗
arXiv2008

Hidden gauge formalism for the radiative decays of axial-vector mesons

The radiative decay of the axial-vector resonances into a pseudoscalar meson and a photon is studied using the vector meson Lagrangian obtained from the hidden gauge symmetry (HGS) formalism. The formalism is well suited to study this problem since it deals with pseudoscalar and vector mesons in a unified way, respecting chiral invariance. We show explicitly the gauge invariance of the set of diagrams that appear in the approach and evaluate the radiative decay width of the a1(1260) and b1(1235) axial vector meson resonances into pi gamma. We also include the contribution of loops involving anomalous couplings and compare the results to those obtained previously within another formalism.

hep-ph↗
arXiv2008

Chiral Unitary Dynamics of Hadrons and Hadrons in a Nuclear Medium

In this talk I summarize recent findings around the description of axial vector mesons as dynamically generated states from the interaction of pseudoscalar mesons and vector mesons, dedicating some attention to the two $K_1(1270)$ states. Then I review the generation of open and hidden charm scalar and axial states, and how some recent experiment supports the existence of the new hidden charm scalar state predicted. I present recent results showing that the low lying $1/2^+$ baryon resonances for S=-1 can be obtained as bound states or resonances of two mesons and one baryon in coupled channels. Then show the differences with the S=0 case, where the $N^*(1710)$ appears also dynamically generated from the two pion one nucleon system, but the $N^*(1440)$ does not appear, indicating a more complex structure of the Roper resonance. Finally I shall show how the state X(2175), recently discovered at BABAR and BES, appears naturally as a resonance of the $ϕK \bar{K}$ system.

nucl-th↗
arXiv2008

S-wave γγ\to ππand f_0(980)\to ππ

We report on a dispersion relation for the γγ\to (ππ)_I S-wave in isospin I emphasizing the low energy region. The f_0(980) signal that emerges in γγ\to ππis also discussed. Our results could be used to distinguish between different ππisoscalar S-wave parameterizations. We also calculate the width of the σresonance to γγand obtain the value Γ(σ\toγγ)=(1.68\pm 0.15) KeV. Finally, we elaborate on the size of the f_0(980) coupling to ππand show that its smallness compared to the K\bar{K} one is not related to the OZI rule.

hep-ph↗
arXiv2008

On the nature of the $Λ(1405)$ and $Λ(1670)$ from their $N_c$ behavior in chiral dynamics

We study the behavior with the number of colors ($N_c$) of the $Λ(1405)$ and $Λ(1670)$ resonances obtained dynamically within the chiral unitary approach. The leading order meson-baryon interaction, used as the kernel of the unitarization procedure, manifests a nontrivial $N_c$ dependence of the flavor SU(3) representation for baryons. As a consequence, the SU(3) singlet (or $\bar{K}N$) component of the $Λ(1405)$ states remains bound in the large $N_c$ limit, while the other components dissolve into the continuum. Introducing explicit SU(3) breaking, we obtain the $N_c$ dependence of the excitation energy, masses and widths of the physical $Λ(1405)$ and $Λ(1670)$ resonance. The $N_c$ behavior of the decay width is found to be different from the general counting rule for a $qqq$ state, indicating the dynamical origin of these resonances.

hep-ph↗
arXiv2008

Two photons into π^0π^0

We perform a theoretical study based on dispersion relations of the reaction γγ\to π^0π^0 emphasizing the low energy region. We discuss how the f_0(980) signal emerges in γγ\to ππwithin the dispersive approach and how this fixes to a large extent the phase of the isoscalar S-wave γγ\to ππamplitude above the K\bar{K} threshold. This allows us to make sharper predictions for the cross section at lower energies and our results could then be used to distinguish between different ππisoscalar S-wave parameterizations with the advent of new precise data on γγ\toπ^0π^0. We compare our dispersive approach with an updated calculation employing Unitary Chiral Perturbation Theory (U\chiPT). We also pay special attention to the role played by the σresonance in γγ\toππand calculate its coupling and width to gammaγ, for which we obtain Γ(σ\toγγ)=(1.68\pm 0.15) KeV.

hep-ph↗
arXiv2008

On the role of meson loops in the $f_0(1710)$ decay into $Vγ$

We investigate the relevance of the meson loops in the $f_0(1710)$ scalar meson decay into one photon and one vector meson, ($ρ$, $ω$ and $ϕ$). In particular we estimate the size of the loops coming from the decay of the $f_0(1710)$ into two pseudoscalar mesons, containing three pseudoscalar mesons in the loop or two pseudoscalar and one vector meson. The results, despite having large uncertainties, manifest that the contribution of the meson loops to these radiative decays is quite relevant and should be taken into account by the theoretical calculations which use this observables as a test of the possible glueball nature of this resonance.

hep-ph↗
arXiv2008

Radiative decay into $γP$ of the low lying axial-vector mesons

We evaluate the radiative decay into a pseudoscalar meson and a photon of the whole set of the axial-vector mesons dynamically generated from the vector-pseudoscalar meson ($VP$) interaction. We take into account tree level and loop diagrams coming from the axial-vector decay into a vector and a pseudoscalar meson. We find a large span for the values of the radiative widths of the different axial-vector mesons. In particular, we evaluate the radiative decay into $K γ$ of the two $K_1(1270)$ states, recently claimed theoretically, and discuss the experimental values quoted so far on the assumption of only one state.

hep-ph↗
arXiv2008

Meson loops in the $f_0(980)$ and $a_0(980)$ radiative decays into $ρ$, $ω$

We calculate the radiative decay widths of the $a_0(980)$ and $f_0(980)$ scalar mesons into $ργ$ and $ωγ$ considering the dynamically generated nature of these scalar resonances within the realm of the Chiral Unitary Approach. The main ingredient in the evaluation of the radiative width of the scalar mesons are the loops coming from the decay into their constituent pseudoscalar-pseudoscalar components and the subsequent radiation of the photon. The dominant diagrams with only pseudoscalar mesons in the loops are found to be convergent while the divergence of those with a vector meson in the loop are written in terms of the two meson loop function easily regularizable. We provide results for all the possible charge channels and obtain results, with uncertainties, which differ significantly from quark loops models and some version of vector meson dominance.

hep-ph↗
arXiv2008

$η\to π^0 γγ$ decay within a chiral unitary approach revisited

In view of the recent experimental developments on the experimental side in the $η\to π^0 γγ$ decay, and the fact that the Particle Data Group in the on line edition of 2007 reports sizable changes of the radiative decay widths of vector mesons used as input in the theoretical calculations of \cite{eta}, a reevaluation of the decay width of the $η$ in this channel has been done, reducing its uncertainty by almost a factor of two. The new input of the PDG is used and invariant mass distributions and total widths are compared with the most recent results from AGS, MAMI and preliminary ones of KLOE. The agreement of the theory with the AGS and MAMI data is very good, both for the total rates as well as for the invariant mass distributions of the two photons.

hep-ph↗
arXiv2007

Recent Developments in Chiral Unitary Dynamics of Resonances

In this talk I summarize recent findings made on the description of axial vector mesons as dynamically generated states from the interaction of peseudoscalar mesons and vector mesons, dedicating some attention to the two $K_1(1270)$ states. Then I review the generation of open and hidden charm scalar and axial states. Finally, I present recent results showing that the low lying $1/2^+$ baryon resonances for S=-1 can be obtained as bound states or resonances of two mesons and one baryon in coupled channels dynamics.

nucl-th↗
arXiv2007

The WA3 data and the two $K_1(1270)$ resonances

Recent studies based on unitary chiral perturbation theory (U$χ$PT) found that the low-lying axial vector mesons can be dynamically generated due to the interaction of the pseudoscalar octet of the pion and the vector nonet of the rho. In particular, two poles in the second Riemann sheet have been associated to the nominal $K_1(1270)$ resonance. In this talk, we present a recent analysis of the WA3 data on $K^-p\to K^-π^+π^- p$ at 63 GeV using the U$χ$PT amplitudes, and show that it is in favor of the existence of two $K_1(1270)$'s [Phys. Rev. D 75, 014017 (2007)].

hep-ph↗
arXiv2007

Scalar radius of the pion and two photons into two pions. Strong S-wave final state interactions

The quadratic pion scalar radius, ^π_s, plays an important role for present precise determinations of ππscattering. The solution of the Muskhelishvili-Omnès equations for the non-strange null isospin (I) pion scalar form factor determines that ^π_s=(0.61\pm 0.04) fm^2. However, by using an Omnès representation of this form factor, Ynduráin recently obtains ^π_s=(0.75\pm 0.07) fm^2. A large discrepancy between both values, given the precision, then results. We show that Ynduráin's method is indeed compatible with the determinations from the Muskhelishvili-Omnès equations once a zero in the scalar form factor for some S-wave I=0 T-matrices is considered. Once this is accounted for, the resulting value is ^π=(0.63\pm 0.05) fm^2. On the other hand, we perform a theoretical study of the reaction γγ\to π^0π^0 based on dispersion relations. The large source of uncertainty for \sqrt{s}\gtrsim 0.5 GeV, due to variations in the phase used in the Omnès function above the K\bar{K} threshold, is removed by taking one more subtraction in the dispersion relation. This allows us to make sharper predictions for the cross section so that one could use this reaction to distinguish between different low energy ππparameterizations, once independent experiments are available. We also study the role played by the σor f_0(600) meson in this reaction and determine its width to two photons.

hep-ph↗