L. Roca - Valencia University

L. Roca
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Name
L. Roca
Affiliation
Valencia University
City
Fullerton
Country
United States

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High Energy Physics - Phenomenology (36)
 
Nuclear Theory (23)
 
High Energy Physics - Lattice (5)
 
Nuclear Experiment (2)
 
Physics - Materials Science (2)
 
High Energy Physics - Experiment (1)

Publications Authored By L. Roca

We show the important role played by the $\pi\Delta(1232)$ channel in the build up of the $N(1700)(3/2^-)$ resonance due to the non-trivial enhancement produced by a singularity of a triangular loop. The $N(1700)$ is one of the dynamically generated resonances produced by the coupled channel vector-baryon interaction. The $\pi\Delta$ channel was neglected in previous works but we show that it has to be incorporated into the coupled channel formalism due to an enhancement produced by a singularity in the triangular loop with $\rho$, nucleon and $\pi$ as internal loop lines and $\pi$ and $\Delta$ as external ones. Read More

In a previous work we presented a theoretical analysis of the $\Lambda_b \to J/\psi K^- p$ reaction from where a recent experiment by the LHCb collaboration at CERN claimed the existence of two hidden charm pentaquarks, $P_c(4380)^+$ and $P_c(4450)^+$. In that work we focused only on the $\Lambda(1405)$ and $P_c(4450)^+$ signals and discussed the possible explanation of this pentaquark state within the picture of a dynamical meson-baryon molecule made up mostly from $\bar D^* \Sigma_c$ and $\bar D^* \Sigma^*_c$ components. In the present work we improve upon the previous one by considering the total $K^- p$ and $J/\psi p$ data including all the relevant resonances contributing to the spectra, and discuss the possible nature of both $P_c(4380)^+$ and $P_c(4450)^+$. Read More

In this review we give a perspective of the theoretical work done recently on the interpretation of results from $B$, $D$, $\Lambda_b$, $\Lambda_c$ weak decays into final states that contain interacting hadrons, and how it is possible to obtain additional valuable information that is increasing our understanding of hadron interactions and the nature of many hadronic resonances. The novelty of these processes is that one begins with a clean picture at the quark level which allows one to select the basic mechanisms by means of which the process proceeds. Finally, one has a final state described in terms of quarks. Read More

In this talk we review results from studies with unconventional many hadron systems containing mesons: systems with two mesons and one baryon, three mesons, some novel systems with two baryons and one meson, and finally systems with many vector mesons, up to six, with their spins aligned forming states of increasing spin. We show that in many cases one has experimental counterparts for the states found, while in some other cases they remain as predictions, which we suggest to be searched in BESIII, Belle, LHCb, FAIR and other facilities. Read More

We perform a theoretical analysis of the $\Lambda_b \to J/\psi K^- p$ reaction from where a recent LHCb experiment extracts a $\Lambda(1405)$ contribution in the $K^- p$ spectrum close to threshold and two baryon states of hidden charm in the $J/\psi\,p$ spectrum. We recall that baryon states of this type have been theoretically predicted matching the mass, width and $J^P$ of the experiment, concretely some states built up from the $J/\psi\, N$, $\bar D^* \Lambda_c$, $\bar D^* \Sigma_c$, $\bar D \Sigma^*_c$ and $\bar D^* \Sigma^*_c$ coupled channels. We assume that the observed narrow state around 4450 MeV has this nature and we are able to describe simultaneously the shapes and relative strength of the the $K^- p$ mass distribution close to threshold and the peak of the $J/\psi\,p$ distribution, with values of the $J/\psi\, p$ coupling to the resonance in line with the theoretical ones. Read More

Understanding mechanisms in DSSCs is fundamental for their improvement; this includes the nanocrystalline semiconducting layer behaviour. Different mesoporous TiO2 layers are fabricated and analyzed for possible use in DSSC solar cells. The preparations included the addition of P123 triblock copolymer as structuring agent to the synthesized anatase sol. Read More

We calculate the shape of the $\pi\Sigma$ and $\bar K N$ invariant mass distributions in the $\Lambda_b \to J/\psi\, \pi\Sigma$ and $\Lambda_b \to J/\psi \,\bar K N$ decays that are dominated by the $\Lambda(1405)$ resonance. The weak interaction part is the same for both processes and the hadronization into the different meson-baryon channels in the final state is related by SU(3) symmetry. The most important feature is the implementation of the meson-baryon final-state interaction using two chiral unitary models from different theoretical groups. Read More

Recently, the Weinberg compositeness condition of a bound state was generalized to account for resonant states and higher partial waves. We apply this extension to the case of the $\Lambda(1520)$ resonance and quantify the weight of the meson-baryon components in contrast to other possible genuine building blocks. This resonance was theoretically obtained from a coupled channels analysis using the s-waves $\pi\Sigma^*$, $K\Xi^*$ and the d-waves $\bar{K}N$ and $\pi\Sigma$ channels applying the techniques of the chiral unitary approach. Read More

Recently we presented a successful strategy to extract the position of the two $\Lambda(1405)$ poles from experimental photoproduction data on the $\gamma p \to K^+ \pi^0 \Sigma^0 $ reaction at Jefferson Lab. Following a similar strategy, we extend the previous method to incorporate also the isospin 1 component which allows us to consider in addition the experimental data on $\gamma p \to K^+ \pi^\pm \Sigma^\mp$. The idea is based on considering a production mechanism as model independent as possible and implementing the final state interaction of the final meson-baryon pair based on small modifications of the unitary chiral perturbation theory amplitudes. Read More

We make a theoretical study of $\pi\pi$ scattering with quantum numbers $J^{PC}=1^{--}$ in a finite box. To calculate physical observables for infinite volume from lattice QCD, the finite box dependence of the potentials is not usually considered. We quantify such effects by means of two different approaches for vector-isovector $\pi\pi$ scattering based on Unitarized Chiral Perturbation Theory results: the Inverse Amplitude Method and another one based on the $N/D$ method. Read More

We present a strategy to extract the position of the two $\Lambda(1405)$ poles from experimental photoproduction data measured recently at different energies in the $\gamma p \to K^+ \pi^0 \Sigma^0 $ reaction at Jefferson Lab. By means of a chiral dynamics motivated potential but with free parameters, we solve the Bethe Salpeter equation in the coupled channels $\bar K N$ and $\pi \Sigma$ in isospin I=0 and parameterize the amplitude for the photonuclear reaction in terms of a linear combination of the $\pi \Sigma \to \pi \Sigma$ and $\bar K N \to \pi \Sigma$ scattering amplitudes in I=0, with a different linear combination for each energy. Good fits to the data are obtained with some sets of parameters, by means of which one can also predict the cross section for the $K^- p \to \pi^0 \Sigma^0 $ reaction. Read More

In2S3(1-x)O3x is known from preceding studies to have a bandgap varying continuously as a function of x, the reason why this solid solution is potentially interesting in the field of photovoltaics. In this work, we present results on oxidation of In2S3 by heating in air atmosphere to obtain the desired material. The oxidation is accompanied by a mass loss due to the substitution of S by O atoms that is studied by means of thermogravimetric analysis. Read More

The isospin violating $J/\Psi\to \phi \pi^0 \eta$ decay is thought to be dominated by the mixing of the $f_0(980)$ and $a_0(980)$ scalar resonances. We make a theoretical evaluation of the $J/\Psi\to \phi \pi^0 \eta$ decay extending our own previous model for other $J/\Psi$ decays into one vector meson and two pseudoscalars using the techniques of the chiral unitary approach. The scalar resonances are dynamically generated through the final state interaction of the pseudoscalar mesons implementing unitarity from the lowest order ChPT amplitudes. Read More

We study theoretically the effects of finite volume for pipi scattering in order to extract physical observables for infinite volume from lattice QCD. We compare three different approaches for pipi scattering (lowest order Bethe-Salpeter approach, N/D and inverse amplitude methods) with the aim to study the effects of the finite size of the box in the potential of the different theories, specially the left-hand cut contribution through loops in the crossed t,u-channels. We quantify the error made by neglecting these effects in usual extractions of physical observables from lattice QCD spectra. Read More

The production of phi mesons in proton collisions with C, Cu, Ag, and Au targets has been studied via the phi -> K+K- decay at an incident beam energy of 2.83 GeV using the ANKE detector system at COSY. For the first time, the momentum dependence of the nuclear transparency ratio, the in-medium phi width, and the differential cross section for phi meson production at forward angles have been determined for these targets over the momentum range of 0. Read More

We present a way to evaluate the scattering of unstable particles quantized in a finite volume with the aim of extracting physical observables for infinite volume from lattice data. We illustrate the method with the $\pi\rho$ scattering which generates dynamically the axial-vector $a_1(1260)$ resonance. Energy levels in a finite box are evaluated both considering the $\rho$ as a stable and unstable resonance and we find significant differences between both cases. Read More

In this talk we show recent developments on few body systems involving mesons. We report on an approach to Faddeev equations using chiral unitary dynamics, where an explicit cancellation of the two body off shell amplitude with three body forces stemming from the same chiral Lagrangians takes place. This removal of the unphysical off shell part of the amplitudes is most welcome and renders the approach unambiguous, showing that only on shell two body amplitudes need to be used. Read More

We show that the lightest pseudotensor mesons J^{PC}=2^-+ can be regarded as molecules made of a pseudoscalar (P) 0^-+ and a tensor 2^++ meson, where the latter is itself made of two vector (V) mesons. The idea stems from the fact that the vector-vector interaction in s-wave and spin 2 is very strong, to the point of generating the 2^++ tensor mesons. On the other hand the interaction of a pseudoscalar with a vector meson in s-wave is also very strong and it generates dynamically the lightest axial-vector mesons. Read More

In this talk we report on selected topics on hadrons in nuclei. The first topic is the renormalization of the width of the $\Lambda(1520)$ in a nuclear medium. This is followed by a short update of the situation of the $\omega$ in the medium. Read More

We study the interactions between the f_0(980) and a_0(980) scalar resonances and the lightest pseudoscalar mesons. We first obtain the elementary interaction amplitudes, or interacting kernels, without including any ad hoc free parameter. This is achieved by using previous results on the nature of the lightest scalar resonances as dynamically generated from the rescattering of S-wave two-meson pairs. Read More

We have studied the interactions of the scalar resonances f_0(980) and a_0(980) with the vector resonance \phi(1020) and with the lightest pseudoscalars \pi, K, \eta and \eta'. We first obtain the interaction kernels without including any new free parameter. Afterwards, the interaction kernels are unitarized and the final S-wave amplitudes result. Read More

We report on four $\Sigma$'s and three $\Lambda$'s, in the 1500 - 1800 MeV region, as two meson - one baryon S-wave $(1/2)^+$ resonances found by solving the Faddeev equations in the coupled channel approach, which can be associated to the existing $S$ = -1, $J^P= 1/2^+$ low lying baryon resonances. On the other hand we also report on a new, hidden strangeness $N^*$ state, mostly made of $K \bar{K} N$, with mass around 1920 MeV, which we think could be responsible for the peak seen in the $\gamma p \to K^+ \Lambda$ around this energy. Finally we address a very novel topic in which we show how few body systems of several $\rho$ mesons can be produced, with their spins aligned up to J=6, and how these states found theoretically can be associated to several known mesons with spins J=2,3,4,5,6. Read More

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 $\rho(770)$ 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 $\rho\rho$ and $K^*\rho$. We extend a recent work, where several resonances showed up as multi-$\rho(770)$ molecules, to the strange sector including the $K^*(892)$ into the system. Read More

In a previous work regarding the interaction of two $\rho(770)$ resonances, the $f_2(1270)$ ($J^{PC}=2^{++}$) resonance was obtained dynamically as a two-$\rho$ molecule with a very strong binding energy, 135~MeV per $\rho$ particle. In the present work we use the $\rho\rho$ interaction in spin 2 and isospin 0 channel to show that the resonances $\rho_3(1690)$ ($3^{--}$), $f_4(2050)$ ($4^{++}$), $\rho_5(2350)$ ($5^{--}$) and $f_6(2510)$ ($6^{++}$) are basically molecules of increasing number of $\rho(770)$ 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. Read More

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. Read More

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. Read More

Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz in the three isospin channels $\vec{\gamma}p\to \pi^{+}\pi^0n$, $\vec{\gamma}p\to \pi^{0}\pi^0p$ and $\vec{\gamma}p\to \pi^{+}\pi^{-}p$ . 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 $\pi^0$ mesons were detected in a $~4\pi$ electromagnetic calorimeter which combined the Crystal Ball detector with the TAPS detector. Read More

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. Read More

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. Read More

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. Read More

We report on a dispersion relation for the \gamma\gamma\to (\pi\pi)_I S-wave in isospin I emphasizing the low energy region. The f_0(980) signal that emerges in \gamma\gamma\to \pi\pi is also discussed. Our results could be used to distinguish between different \pi\pi isoscalar S-wave parameterizations. Read More

We study the behavior with the number of colors ($N_c$) of the $\Lambda(1405)$ and $\Lambda(1670)$ 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 $\Lambda(1405)$ states remains bound in the large $N_c$ limit, while the other components dissolve into the continuum. Read More

We perform a theoretical study based on dispersion relations of the reaction \gamma\gamma\to \pi^0\pi^0 emphasizing the low energy region. We discuss how the f_0(980) signal emerges in \gamma\gamma\to \pi\pi within the dispersive approach and how this fixes to a large extent the phase of the isoscalar S-wave \gamma\gamma\to \pi\pi amplitude 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 \pi\pi isoscalar S-wave parameterizations with the advent of new precise data on \gamma\gamma\to\pi^0\pi^0. Read More

We investigate the relevance of the meson loops in the $f_0(1710)$ scalar meson decay into one photon and one vector meson, ($\rho$, $\omega$ and $\phi$). 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. Read More

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. Read More

2008Feb
Affiliations: 1TU Muenchen, 2TU Muenchen, 3YITP, Kyoto Univ, 4Murcia Univ, 5RCNP, Osaka Univ

We discuss several aspects of the Lambda(1405) resonance in relation to the recent theoretical developments in chiral dynamics. We derive an effective single-channel KbarK N interaction based on chiral SU(3) coupled-channel approach, emphasizing the important role of the pi Sigma channel and the structure of the Lambda(1405) in Kbar N phenomenology. In order to clarify the structure of the resonance, we study the behavior with the number of colors (Nc) of the poles associated with the Lambda(1405), and argue the physical meaning of the renormalization procedure. Read More

We calculate the radiative decay widths of the $a_0(980)$ and $f_0(980)$ scalar mesons into $\rho\gamma$ and $\omega\gamma$ 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. Read More

In view of the recent experimental developments on the experimental side in the $\eta \to \pi^0 \gamma \gamma$ 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 $\eta$ 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. Read More

2007Dec

We study the behavior with the number of colors (Nc) of the two poles associated to the Lambda(1405) resonance obtained dynamically within the chiral unitary approach. The leading order chiral meson-baryon interaction manifests a nontrivial Nc dependence for SU(3) baryons, which gives a finite attractive interaction in some channels in the large Nc limit. As a consequence, the SU(3) singlet (Kbar N) component of the Lambda(1405) survives in the large Nc limit as a bound state, while the other components dissolve into the continuum. Read More

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. Read More

Recent studies based on unitary chiral perturbation theory (U$\chi$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 $K^-p\to K^-\pi^+\pi^- p$ at 63 GeV using the U$\chi$PT amplitudes, and show that it is in favor of the existence of two $K_1(1270)$'s [Phys. Read More

The quadratic pion scalar radius, ^\pi_s, plays an important role for present precise determinations of \pi\pi scattering. The solution of the Muskhelishvili-Omn\`es equations for the non-strange null isospin (I) pion scalar form factor determines that ^\pi_s=(0.61\pm 0. Read More

We perform a dispersive theoretical study of the reaction \gamma\gamma\to pi^0\pi^0 emphasizing the low energy region. The large source of theoretical uncertainty to calculate the \gamma\gamma\to\pi^0\pi^0 total cross section for \sqrt{s}\gtrsim 0.5 GeV within the dispersive approach is removed. Read More

The quadratic pion scalar radius, \la r^2\ra^\pi_s, plays an important role for present precise determinations of \pi\pi scattering. Recently, Yndur\'ain, using an Omn\`es representation of the null isospin(I) non-strange pion scalar form factor, obtains \la r^2\ra^\pi_s=0.75\pm 0. Read More

In this talk I present recent developments in the field of hadronic physics and hadrons in the nuclear medium. I review the unitary chiral approach to meson baryon interaction and address the topics of the two dynamically generated $\Lambda(1405)$ resonances, with experiments testing it, the $\Lambda(1520)$ and $\Delta(1700)$ resonances, plus the $\Lambda(1520)$, $\Sigma(1385)$ and $\omega$ in the nuclear medium. Read More

A previous model where the low-lying axial-vector mesons are dynamically generated, implementing unitarity in coupled channels in the vector-pseudoscalar ($VP$) meson interaction, is applied to evaluate the decay widths of the a_1(1260)$ and $b_1(1235)$ axial-vector mesons into $\pi\gamma$. Unlike the case of the $a_1$, the $b_1$ radiative decay is systematically underestimated at tree level. In this work we evaluate for the first time the loop contribution coming from an initial $VP$ vertex. Read More

The axial vector meson $K_1(1270)$ was studied within the chiral unitary approach, where it was shown that it has a two-pole structure. We reanalyze the high-statistics WA3 experiment $K^- p\to K^-\pi^+\pi^- p$ at 63 GeV, which established the existence of both $K_1(1270)$ and $K_1(1400)$, and we show that it clearly favors our two-pole interpretation. We also reanalyze the traditional K-matrix interpretation of the WA3 data and find that the good fit of the data obtained there comes from large cancellations of terms of unclear physical interpretation. Read More

We perform a non-perturbative chiral study of the masses of the lightest pseudoscalar mesons. In the calculation of the self-energies we employ the S-wave meson-meson amplitudes taken from Unitary Chiral Perturbation Theory (UCHPT) that include the lightest nonet of scalar resonances. Values for the bare masses of pions and kaons are obtained, as well as an estimate of the mass of the \eta_8. Read More

In this talk I present recent developments in chiral dynamics of hadrons and hadrons in a medium addressing the following points: interaction of the octet of pseudoscalar mesons with the octet of baryons of the nucleon, showing recent experimental evidence on the existence of two $\Lambda(1405)$ states, the interaction of the octet of pseudoscalar mesons with the decuplet of baryons of the $\Delta$, with particular emphasis on the $\Lambda(1520)$ resonance, dynamically generated by this interaction. Then I review the interaction of kaons in a nuclear medium and briefly discuss the situation around the claims of deeply bound states in nuclei. The large renormalization of the $\Lambda(1520)$ in the nuclear medium is shown as another example of successful application of the chiral unitary techniques. Read More

We study the $\Lambda(1520)$ resonance in a coupled channel approach involving the $\pi\Sigma(1385)$, $K\Xi(1530)$, $\bar{K}N$ and $\pi\Sigma$ channels. Implementing unitarity in coupled channels, we make an analysis of the relative importance of the different mechanisms which contribute to the dynamical structure of this resonance. From experimental information on some partial wave amplitudes and constraints imposed by unitarity, we get a comprehensive description of the amplitudes and hence the couplings to the different channels. Read More