M. J. Vacas - Valencia University & Valencia University, IFIC

M. J. Vacas
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Name
M. J. Vacas
Affiliation
Valencia University & Valencia University, IFIC
City
Valencia
Country
Spain

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High Energy Physics - Phenomenology (41)
 
Nuclear Theory (34)
 
High Energy Physics - Lattice (8)
 
High Energy Physics - Experiment (6)
 
Nuclear Experiment (5)

Publications Authored By M. J. Vacas

The baryon electromagnetic form factors are expressed in terms of two-dimensional densities describing the distribution of charge and magnetization in transverse space at fixed light-front time. We calculate the transverse densities of the spin-1/2 flavor-octet baryons at peripheral distances b = O(M_\pi^{-1}) using methods of relativistic chiral effective field theory (\chi EFT) and dispersion analysis. The densities are represented as dispersive integrals over the imaginary parts of the form factors in the timelike region (spectral functions). Read More

Using the present upper bound on the neutron electric dipole moment, we give an estimate for the upper limit of the CP-violating couplings of the eta(eta') to the nucleon. Using this result, we then derive constraints on the CP-violating eta(eta')-pi-pi couplings, which define the two-pion CP-violating decays of the eta and eta' mesons. Our results are relevant for the running and planned measurements of rare decays of the eta and eta' mesons by the GlueX Collaboration at JLab and the LHCb Collaboration at CERN. Read More

Nuclear production experiments report missing absorption processes of the in-medium phi meson. Contributions arising from the K-bar K cloud have already been widely studied, and therefore we investigate the phi-meson properties in cold nuclear matter with the additional inclusion of resonant phi N interactions. Two models are considered which dynamically generate N*-like states close to the phi N threshold. Read More

The $\phi$-meson properties in cold nuclear matter are investigated by implementing resonant $\phi N$ interactions as described in effective approaches including the unitarization of scattering amplitudes. Several $N^*$-like states are dynamically generated in these models around $2$ GeV, in the vicinity of the $\phi N$ threshold. We find that both these states and the non-resonant part of the amplitude contribute sizably to the $\phi$ collisional self-energy at finite nuclear density. Read More

2016Jun
Affiliations: 1Beihang U. & Peking U., SKLNPT, 2Valencia U., IFIC, 3Beihang U., 4Valencia U., IFIC, 5Beihang U. & Peking U., SKLNPT & Stellenbosch U., 6Valencia U. & Valencia U., IFIC

Treating the strange quark mass as a heavy scale compared to the light quark mass, we perform a matching of the nucleon mass in the SU(3) sector to the two-flavor case in covariant baryon chiral perturbation theory. The validity of the $19$ low-energy constants appearing in the octet baryon masses up to next-to-next-to-next-to-leading order~\cite{Ren:2014vea} is supported by comparing the effective parameters (the combinations of the $19$ couplings) with the corresponding low-energy constants in the SU(2) sector~\cite{Alvarez-Ruso:2013fza}. In addition, it is shown that the dependence of the effective parameters and the pion-nucleon sigma term on the strange quark mass is relatively weak around its physical value, thus providing support to the assumption made in Ref. Read More

We calculate the neutral pion photoproduction on the proton near threshold in covariant baryon chiral perturbation theory, including the $\Delta(1232)$ resonance as an explicit degree of freedom, up to chiral order $p^{7/2}$ in the $\delta$ counting. We compare our results with recent low-energy data from the Mainz Microtron for angular distributions and photon asymmetries. The convergence of the chiral series of the covariant approach is found to improve substantially with the inclusion of the $\Delta(1232)$ resonance. Read More

Some of the recent progress in the physics of pion production induced by neutrinos on nucleons and nuclei is reviewed from a theoretical perspective. The importance of Watson's theorem to reconcile ANL and BNL data with the off-diagonal Goldberger-Treiman relation for the $\Delta(1232)$ is discussed. The disagreement between MiniBooNE data and theoretical calculations is presented in the light of the new MINERvA data. Read More

In this work, we investigate the mass corrections of the doubly charmed baryons up to $N^2LO$ in the extended-on-mass-shell (EOMS) renormalization scheme, comparing with the results of heavy baryon chiral perturbation theory. We find that the terms from the heavy baryon approach are a subset of those obtained in the EOMS scheme. By fitting the lattice data, we can determine the parameters $\tilde{m}$, $\alpha$, $c_1$ and $c_7$ from the Lagrangian, while in the heavy baryon approach no information on $c_1$ can be obtained from the baryons mass. Read More

We present a new determination of the $N\Delta$ axial form factors from neutrino induced pion production data. For this purpose, the model of Hernandez {\it et al.} [Phys. Read More

We study the neutral pion photoproduction at near-threshold energies in fully covariant chiral perturbation theory up to O(p^3). When including only nucleonic virtual states in the model, the convergence is too slow. Therefore we test the model when introducing the Delta(1232) resonance as an additional degree of freedom. Read More

The neutral pion photoproduction on the proton near threshold has a very small scattering cross section when compared to the charged channels, which in ChPT is explained by strong cancellations between the lowest order pieces. Therefore it is very sensitive to higher-order corrections of chiral perturbation theory. We perform a fully covariant calculation up to chiral order p^3 and we investigate the effect of the inclusion of the Delta(1232) resonance as an explicit degree of freedom. Read More

The strange particle production induced by (anti)neutrino off nucleon has been studied for $|\Delta S|=0$ and $|\Delta S|=1$ channels. The reactions those we have considered are for the production of single kaon/antikaon, eta and associated particle production processes. We have developed a microscopical model based on the SU(3) chiral Lagrangian. Read More

We have studied quasielastic charged current hyperon production induced by $\bar\nu_\mu$ on free nucleon and the nucleons bound inside the nucleus and the results are presented for several nuclear targets like $^{40}Ar$, $^{56}Fe$ and $^{208}Pb$. The hyperon-nucleon transition form factors are determined from neutrino-nucleon scattering and semileptonic decays of neutron and hyperons using SU(3) symmetry. The nuclear medium effects(NME) due to Fermi motion and final state interaction(FSI) effect due to hyperon-nucleon scattering have been taken into account. Read More

We have studied the strangeness changing antineutrino induced reactions $\bar{\nu}_{l} p \rightarrow l^+ \phi B $, with $\phi B = K^-p$, $\bar{K}^0n$, $\pi^0\Lambda$, $\pi^0\Sigma^0$, $\eta\Lambda$, $\eta\Sigma^0$, $\pi^+\Sigma^-$, $\pi^-\Sigma^+$, $K^+\Xi^-$ and $K^0\Xi^0$, using a chiral unitary approach. These ten coupled channels are allowed to interact strongly, using a kernel derived from the chiral Lagrangians. This interaction generates two $\Lambda(1405)$ poles, leading to a clear single peak in the $\pi \Sigma$ invariant mass distributions. Read More

We investigate the neutral pion photoproduction on the proton near threshold in covariant chiral perturbation theory with the explicit inclusion of Delta degrees of freedom. This channel is specially sensitive to chiral dynamics and the advent of very precise data from the Mainz microtron has shown the limits of the convergence of the chiral series for both the heavy baryon and the covariant approaches. We show that the inclusion of the Delta resonance substantially improves the convergence leading to a good agreement with data for a wider range of energies. Read More

RPA correlations, spectral function and 2p2h (multi-nucleon) effects on charged-current neutrino-nucleus reactions without emitted pions are discussed. We pay attention to the influence of RPA and multi-nucleon mechanisms on the MiniBooNE and MINERvA flux folded differential cross sections, the MiniBooNE flux unfolded total cross section and the neutrino energy reconstruction. Read More

The octet-baryon axial-vector charges and the g1/f1 ratios measured in the semileptonic hyperon decays are studied up to O(p^3) using the covariant baryon chiral perturbation theory with explicit decuplet contributions. We clarify the role of different low-energy constants and find a good convergence for the chiral expansion of the axial-vector charges of the baryon octet, g1(0), with O(p^3) corrections typically around 20% of the leading ones. This is a consequence of strong cancellations between different next-to-leading order terms. Read More

Fits of the p^4 covariant SU(2) baryon chiral perturbation theory to lattice QCD nucleon mass data from several collaborations for 2 and 2+1 flavors are presented. We consider contributions from explicit Delta(1232) degrees of freedom, finite volume and finite spacing corrections. We emphasize here our Nf=2+1 study. Read More

We investigate the pion-mass dependence of the nucleon mass within the covariant SU(2) baryon chiral perturbation theory up to order p4 with and without explicit Delta(1232) degrees of freedom. We fit lattice QCD data from several collaborations for 2 and 2+1 flavor ensembles. Here, we emphasize our Nf=2 study where the inclusion the Delta(1232) contributions stabilizes the fits. Read More

We obatin the ratio $F_i^A/F_i^{D}$(i=2,3, A=Be, C, Fe, Pb; D=Deuteron) in the case of weak and electromagnetic nuclear structure functions. For this, relativistic nuclear spectral function which incorporate the effects of Fermi motion, binding and nucleon correlations is used. We also consider the pion and rho meson cloud contributions and shadowing and antishadowing effects. Read More

We have studied nuclear medium effects in the weak structure functions $F^A_2(x)$ and $F^A_3(x)$ and in the extraction of weak mixing angle using Paschos Wolfenstein(PW) relation. We have modified the PW relation for nonisoscalar nuclear target. We have incorporated the medium effects like Pauli blocking, Fermi motion, nuclear binding energy, nucleon correlations, pion $\&$ rho cloud contributions, and shadowing and antishadowing effects. Read More

We have studied strange particle production off nucleons through $\Delta S =0 $ and $|\Delta S| = 1$ channels, and specifically single kaon/antikaon, eta, associated particle production for neutrino/antineutrino induced processes as well as antineutrino induced single hyperon production processes. We have developed a microscopical model based on the SU(3) chiral Lagrangians. The basic parameters of the model are $f_\pi$, the pion decay constant, Cabibbo angle, the proton and neutron magnetic moments and the axial vector coupling constants for the baryons octet. Read More

We discuss some nuclear effects, RPA correlations and 2p2h (multinucleon) mechanisms, on charged-current neutrino-nucleus reactions that do not produce a pion in the final state. We study a wide range of neutrino energies, from few hundreds of MeV up to 10 GeV. We also examine the influence of 2p2h mechanisms on the neutrino energy reconstruction. Read More

We compare our pion production results with recent MiniBooNE data measured in mineral oil. Our total cross sections lie below experimental data for neutrino energies above 1 GeV. Differential cross sections show our model produces too few high energy pions in the forward direction as compared to data. Read More

We extend to 10 GeV results from a microscopic calculation of charged-current neutrino-nucleus reactions that do not produce a pion in the final state. For the class of events coming from neutrino interactions with two nucleons producing two holes (2p2h), limiting the calculation to three-momentum transfers less than 1.2 GeV produces a two dimensional distribution in momentum and energy transfer that is roughly constant as a function of energy. Read More

We study one pion production in both charged and neutral current neutrino nucleus scattering for neutrino energies below 2 GeV. We use a theoretical model for one pion production at the nucleon level that we correct for medium effects. The results are incorporated into a cascade program that apart from production also includes the pion final state interaction inside the nucleus. Read More

We analyze the MiniBooNE muon neutrino CCQE-like d\sigma/dT_\mu/dcos\theta_\mu data using a theoretical model that, among other nuclear effects, includes RPA correlations and 2p2h (multinucleon) mechanisms. These corrections turn out to be essential for the description of the data. We find that MiniBooNE CCQE-like data are fully compatible with former determinations of the nucleon axial mass M_A ~ 1. Read More

The strange particle production off the nucleon induced by neutrinos and antineutrinos is investigated at low and intermediate energies. We develop a microscopic model based on the SU(3) chiral Lagrangian. The studied mechanisms are the main source of single kaon production for (anti)neutrino energies up to 1. Read More

The weak $\eta$-meson production off the nucleon induced by (anti)neutrinos is studied at low and intermediate energies, the range of interest for several ongoing and future neutrino experiments. We consider Born diagrams and the excitation of $N^\ast (1535)S_{11}$ and $N^\ast(1650)S_{11}$ resonances. The vector part of the N-$S_{11}$ transition form factors has been obtained from the MAID helicity amplitudes while the poorly known axial part is constrained with the help of the partial conservation of the axial current (PCAC) and assuming the pion-pole dominance of the pseudoscalar form factor. Read More

We evaluate the quasielastic and multinucleon contributions to the antineutrino nucleus scattering cross section and compare our results with the recent MiniBooNE data. We use a local Fermi gas model that includes RPA correlations and gets the multinucleon part from a systematic many body expansion of the $W$ boson selfenergy in the nuclear medium. The same model had been quite successful for the neutrino cross section and contains no new parameters. Read More

We study single kaon production off the nucleon induced by electrons (positrons) i.e. $e^-(e^+) + N \rightarrow \nu_e (\bar \nu_e) + \bar K (K) + N^\prime$ at low energies. Read More

With the aim of achieving a better and more complete understanding of neutrino interactions with nuclear targets, the coherent production of charged kaons induced by neutrinos and antineutrinos is investigated in the energy range of some of the current neutrino experiments. We follow a microscopic approach which, at the nucleon level, incorporates the most important mechanisms allowed by the chiral symmetry breaking pattern of QCD. The distortion of the outgoing (anti)kaon is taken into account by solving the Klein-Gordon equation with realistic optical potentials. Read More

We show that because of the multinucleon mechanism effects, the algorithm used to reconstruct the neutrino energy is not adequate when dealing with quasielastic-like events, and a distortion of the total flux unfolded cross section shape is produced. This amounts to a redistribution of strength from high to low energies, which gives rise to a sizable excess (deficit) of low (high) energy neutrinos. This distortion of the shape leads to a good description of the MiniBooNE unfolded CCQE-like cross sections published in Phys. Read More

The charged current antikaon production off nucleons induced by antineutrinos is studied at low and intermediate energies. We extend here our previous calculation on kaon production induced by neutrinos. We have developed a microscopic model that starts from the SU(3) chiral Lagrangians and includes background terms and the resonant mechanisms associated to the lowest lying resonance in the channel, namely, the Sigma*(1385). Read More

We study the nuclear medium effects in the weak structure functions $F_2(x,Q^2)$ and $F_3(x,Q^2)$ in the deep inelastic neutrino/antineutrino reactions in nuclei. We use a theoretical model for the nuclear spectral functions which incorporates the conventional nuclear effects, such as Fermi motion, binding and nucleon correlations. We also consider the pion and rho meson cloud contributions calculated from a microscopic model for meson-nucleus self-energies. Read More

Nuclear medium effects in the weak structure functions $F_2(x,Q^2)$ and $F_3(x,Q^2)$ have been studied for deep inelastic neutrino/antineutrino reactions in iron nucleus by taking into account Fermi motion, binding, pion and rho meson cloud contributions, target mass correction, shadowing and anti-shadowing corrections. The calculations have been performed in a local density approximation using relativistic nuclear spectral functions which include nucleon correlations. Using these structure functions we have obtained the ratio $R_{F2,F3}^A(x,Q^2)= \frac{2F_{2,3}^A(x,Q^2)}{AF_{2,3}^D(x,Q^2)}$, the differential scattering cross section $\frac{1}{E}\frac{d^2\sigma}{dxdy}$ and the total scattering cross section $\sigma$. Read More

The weak kaon production off the nucleon induced by neutrinos and antineutrinos is studied at low and intermediate energies of interest for some ongoing and future neutrino oscillation experiments. We develop a microscopical model based on the SU(3) chiral Lagrangians. The studied mechanisms are the main source of kaon production for neutrino energies up to 2 GeV for the various channels and the cross sections are large enough to be amenable to be measured by experiments such as Minerva, T2K and NO$\nu$A. Read More

The charged-current double differential neutrino cross section, measured by the MiniBooNE Collaboration, has been analyzed using a microscopical model that accounts for, among other nuclear effects, long range nuclear (RPA) correlations and multinucleon scattering. We find that MiniBooNE data are fully compatible with the world average of the nucleon axial mass in contrast with several previous analyses which have suggested an anomalously large value. We also discuss the reliability of the algorithm used to estimate the neutrino energy. Read More

We present a model for weak CC induced nuclear reactions at energies of interest for current and future neutrino oscillation experiments. This model is a natural extension of the work of Refs.[1,2], where the QE contribution to the inclusive electron and neutrino scattering on nuclei was analyzed. Read More

We review recent progress in the understanding of low-energy baryon structure by means of chiral perturbation theory. In particular, we discuss the application of this formalism to the description of the quark mass dependence of recent Lattice QCD results on the masses. We present the chiral extrapolation of those of the PACS-CS and LHP collaborations and we predict the baryonic sigma-terms. Read More

We review recent progress in the understanding of low-energy baryon structure by means of chiral perturbation theory. In particular, we discuss the application of this formalism to the description of various properties such as the baryon-octet magnetic moments, the electromagnetic structure of decuplet resonances and the hyperon vector coupling $f_1(0)$. Moreover, we present the results on the chiral extrapolation of recent lattice QCD results on the lowest-lying baryon masses and we predict the corresponding baryonic sigma-terms. Read More

We present recent results on the vector meson-vector meson and vector meson-baryon interaction using a unitary approach based on the hidden-gauge Lagrangians. For the vector-vector case we find that 11 states get dynamically generated, corresponding to poles of the scattering matrices on the second Riemann sheet. In the vector-baryon sector we also find 9 states dynamically generated from the vector-baryon octet interaction and 10 from the vector-baryon decuplet interaction. Read More

The weak kaon production off the nucleon induced by neutrinos is studied at the low and intermediate energies of interest for some ongoing and future neutrino oscillation experiments. This process is also potentially important for the analysis of proton decay experiments. We develop a microscopical model based on the SU(3) chiral Lagrangians. Read More

We present an analysis of the baryon-octet and -decuplet masses using covariant SU(3)-flavor chiral perturbation theory up to next-to-leading order. Besides the description of the physical masses we address the problem of the lattice QCD extrapolation. Using the PACS-CS collaboration data we show that a good description of the lattice points can be achieved at next-to-leading order with the covariant loop amplitudes and phenomenologically determined values for the meson-baryon couplings. Read More

We describe a model for pion production off nucleons and coherent pions from nuclei induced by neutrinos in the 1 GeV energy regime. Besides the dominant Delta pole contribution, it takes into account the effect of background terms required by chiral symmetry. Moreover, the model uses a reduced nucleon-to-Delta resonance axial coupling, which leads to coherent pion production cross sections around a factor two smaller than most of the previous theoretical estimates. Read More

We study the structure and formation of the $\phi$ mesic nuclei to investigate the in-medium modification of the $\phi$-meson spectral function at finite density. We consider (${\bar p},\phi$), ($\gamma,p$) and ($\pi^-,n$) reactions to produce a $\phi$-meson inside the nucleus and evaluate the effects of its medium modifications to the reaction cross sections. We also estimate the consequences of the uncertainties of the ${\bar K}$ selfenergy in medium to the $\phi$-nucleus interaction. Read More

The development of chiral perturbation theory in hyperon phenomenology has been troubled due to power-counting subtleties and to a possible slow convergence. Furthermore, the presence of baryon-resonances, e.g. Read More

In this talk I show recent results on how many excited baryon resonances appear as systems of one meson and one baryon, or two mesons and one baryon, with the mesons being either pseudoscalar or vectors. Connection with experiment is made including a discussion on old predictions and recent results for the photoproduction of the $\Lambda(1405)$ resonance, as well as the prediction of one $1/2^+$ baryon state around 1920 MeV which might have been seen in the $\gamma p \to K^+ \Lambda$ reaction. Read More

We study the nuclear effects in the electromagnetic structure function F2(x,Q^2) in nuclei in the deep inelastic lepton nucleus scattering process by taking into account Fermi motion, binding, pion and rho meson cloud contributions. Calculations have been done in a local density approximation using relativistic nuclear spectral functions which include nucleon correlations for nuclear matter. The ratios over deuteron structure function are obtained and compared with the recent JLAB results for light nuclei with special attention to the slope of the x distributions. Read More

We report on a recent study of the SU(3)-breaking corrections to the hyperon vector coupling $f_1(0)$ up to $\mathcal{O}(p^4)$ in covariant baryon chiral perturbation theory with dynamical octet and decuplet contributions. The decuplet contributions are taken into account for the first time in a covariant ChPT study and are found of similar or even larger size than the octet ones. We predict positive SU(3)-breaking corrections to all the four independent $f_1(0)$'s (assuming isospin symmetry), which are consistent, within uncertainties, with the latest results from large $N_c$ fits, chiral quark models, and quenched lattice QCD calculations. Read More