arXiv subjects

Felipe J. Llanes-Estrada

Publications and source records attributed to Felipe J. Llanes-Estrada.

At least 145 records · Page 8Linked to original sources

arXiv2007

Illuminating the 1/x moment of parton distribution functions

The Weisberger relation, an exact statement of the parton model, elegantly relates a high-energy physics observable, the 1/x moment of parton distribution functions, to a nonperturbative low-energy observable: the dependence of the nucleon mass on the value of the quark mass or its corresponding quark condensate. We show that contemporary fits to nucleon structure functions fail to determine this 1/x moment; however, deeply virtual Compton scattering can be described in terms of a novel F_{1/x}(t) form factor which illuminates this physics. An analysis of exclusive photon-induced processes in terms of the parton-nucleon scattering amplitude with Regge behavior reveals a failure of the high Q^2 factorization of exclusive processes at low t in terms of the Generalized Parton-Distribution Functions which has been widely believed to hold in the past. We emphasize the need for more data for the DVCS process at large t in future or upgraded facilities.

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arXiv2007

Regge Exchange Contribution to Deeply Virtual Compton Scattering

Recently we have shown that exclusive QCD photon-induced reactions at low Mandelstam-t are best described by Regge exchanges in the entire scaling region, and not only for low values of Bjorken-x. In this paper we explore this crucial Regge behavior in Deeply Virtual Compton Scattering from the point of view of collinear factorization, with the proton tensor written in terms of Generalized Parton Distributions, and we reproduce this feature. Thus it appears that in the Bjorken limit, a large class of hard, low-t exclusive processes are more sensitive to the meson cloud of the proton than to its fundamental quark structure. These process will then be described most efficiently by process-dependent Regge Exclusive Amplitudes rather than by universal Generalized Parton Distributions. We introduce such Regge Exclusive Amplitudes for Deeply Virtual Compton Scattering.

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arXiv2007

Light 1-+ exotics: molecular resonances

Highlights in the search for nonconventional (non qqbar) meson states are the pi_1(1400) and pi_1(1600) exotic candidates. Should they exist, mounting theoretical arguments suggest that they are tetraquark molecular resonances excitable by meson rescattering. We report a new tetraquark calculation within a model field theory approximation to Quantum Chromodynamics in the Coulomb gauge supporting this conjecture. We also strengthen this claim by consistently contrasting results with exotic state predictions for hybrid (q qbar g) mesons within the same theoretical framework. Our findings confirm that molecular-like configurations involving two color singlets (a resonance, not a bound state) are clearly favored over hybrid or color-exotic tetraquark meson (q qbar q qbar atoms) formation. Finally, to assist needed further experimental searches we document a useful off-plane correlator for establishing the structure of these exotic systems along with similar, but anticipated much narrower, states that should exist in the charmonium and bottomonium spectra.

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arXiv2007

On the violation of the holographic viscosity versus entropy KSS bound in non relativistic systems

A computation of the quotient of shear viscosity to entropy density, or KSS number $η/s$ is performed, in the non-relativistic and classical regime, first in Chiral Perturbation Theory, and then in the $SO(g+1)/SO(g)$ Non-Linear Sigma Model in the large $g$ limit. Both are field theories stemming from a renormalizable Sigma Model but in spite of that, we explicitly calculate how one undercomes the KSS bound by increasing the number of degenerate pions sufficiently. However we argue that particle production could still keep the validity of the KSS bound in the weak sense. We also show how a large number of molecular isomers (that we estimate in terms of simple molecular properties) could undercome the bound in the strong sense. This might be possible with carbon-based molecules. We finally argue that a measurement of $η/s$ in Heavy Ion Collisions might be turned into an upper bound on the number of hadron resonances.

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arXiv2007

Fermion family recurrences in the Dyson-Schwinger formalism

We study the multiple solutions of the truncated propagator Dyson-Schwinger equation for a simple fermion theory with Yukawa coupling to a scalar field. Upon increasing the coupling constant $g$, other parameters being fixed, more than one non-perturbative solution breaking chiral symmetry becomes possible and we find these numerically. These ``recurrences'' appear as a mechanism to generate different fermion generations as quanta of the same fundamental field in an interacting field theory, without assuming any composite structure. The number of recurrences or flavors is reduced to a question about the value of the Yukawa coupling, and has no special profound significance in the Standard Model. The resulting mass function can have one or more nodes and the measurement that potentially detects them can be thought of as a collider-based test of the virtual dispersion relation $E=\sqrt{p^2+M(p^2)^2}$ for the charged lepton member of each family. This requires three independent measurements of the charged lepton's energy, three-momentum and off-shellness. We illustrate how this can be achieved for the (more difficult) case of the tau lepton.

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arXiv2007

Heat conductivity of a pion gas

We evaluate the heat conductivity of a dilute pion gas employing the Uehling-Uehlenbeck equation and experimental phase-shifts parameterized by means of the SU(2) Inverse Amplitude Method. Our results are consistent with previous evaluations. For comparison we also give results for an (unphysical) hard sphere gas.

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arXiv2006

The ratio of viscosity to entropy density in a pion gas satisfies the KSS holographic bound

We evaluate the ratio of shear viscosity to entropy density in a pion gas employing the Uehling-Uehlenbeck equation and experimental phase-shifts parameterized by means of the SU(2) Inverse Amplitude Method. We find that the ratio for this monocomponent gas stays well above the KSS 1/(4 pi) bound. We find similar results with other sets of phase shifts and conclude the bound is nowhere violated.

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arXiv2006

QCD Coulomb gauge approach to hybrid mesons

An effective Coulomb gauge Hamiltonian, H_eff, is used to calculate the light (u-ubar-g), strange (s-sbar-g) and charmed (c-cbar-g) hybrid meson spectra. For the same two parameter H_eff providing glueball masses consistent with lattice results and a good description of the observed u,d,s and c quark mesons, a large-scale variational treatment predicts the lightest hybrid has J^{PC}=0^{++} and mass 2100 MeV. The lightest exotic 1^{-+} state is just above 2200 MeV, near the upper limit of lattice and Flux Tube predictions. These theoretical formulations all indicate the observed 1^{-+} pi_1(1600) and, more clearly, pi_1(1400) are not hybrid states. The Coulomb gauge approach further predicts that in the strange and charmed sectors, respectively, the ground state hybrids have 1^{+-} with masses 2125 and 3830 MeV, while the first exotic 1^{-+} states are at 2395 and 4020 MeV. Finally, using our hybrid wavefunctions, dimensional counting rules and the Franck-Condon principle, novel experimental signatures are presented to assist light and heavy hybrid meson searches.

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arXiv2006

The BES f_0(1810): a new glueball candidate

We analyze the f_0(1810) state recently observed by the BES collaboration via radiative J/ψdecay to a resonant ϕωspectrum and confront it with DM2 data and glueball theory. The DM2 group only measured ωωdecays and reported a pseudoscalar but no scalar resonance in this mass region. A rescattering mechanism from the open flavored KKbar decay channel is considered to explain why the resonance is only seen in the flavor asymmetric ωϕbranch along with a discussion of positive C parity charmonia decays to strengthen the case for preferred open flavor glueball decays. We also calculate the total glueball decay width to be roughly 100 MeV, in agreement with the narrow, newly found f_0, and smaller than the expected estimate of 200-400 MeV. We conclude that this discovered scalar hadron is a solid glueball candidate and deserves further experimental investigation, especially in the K-Kbar channel. Finally we comment on other, but less likely, possible assignments for this state.

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arXiv2006

Renormalization Scale-Fixing for Complex Scattering Amplitudes

We show how to fix the renormalization scale for hard-scattering exclusive processes such as deeply virtual meson electroproduction by applying the BLM prescription to the imaginary part of the scattering amplitude and employing a fixed-t dispersion relation to obtain the scale-fixed real part. In this way we resolve the ambiguity in BLM renormalization scale-setting for complex scattering amplitudes. We illustrate this by computing the H generalized parton distribution at leading twist in an analytic quark-diquark model for the parton-proton scattering amplitude which can incorporate Regge exchange contributions characteristic of the deep inelastic structure functions.

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arXiv2006

Soft gluons are heavy and rowdy

We study dynamical mass generation in pure Yang-Mills theory and report on a recently developed ansatz that exactly solves the tower of Dyson-Schwinger equations in Landau gauge at low Euclidean momentum, featuring enhanced gluon-gluon vertices, a finite ghost-gluon vertex in agreement with an old argument of Taylor, and an IR suppressed gluon propagator. This ansatz reinforces arguments in favor of the concept of a gluon mass gap at low momentum (although the minimum of the gluon's dispersion relation is not at zero momentum). As an application, we have computed the spectrum of oddballs, three-gluon glueballs with negative parity and C-parity. The three body problem is variationally solved employing the color density-density interaction of Coulomb gauge QCD with a static Cornell potential. Like their even glueball counterparts, oddballs fall on Regge trajectories with similar slope to the pomeron. However their intercept at t=0 is smaller than the omega Regge trajectory and therefore the odderon may only be visible in experimental searches (for example at BNL) with higher -t than conducted to date at DESY.

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arXiv2005

Shear Viscosity in a CFL Quark Star

We compute the mean free path and shear viscosity in the color-flavor locked (CFL) phase of dense quark matter at low temperature T, when the contributions of mesons, quarks and gluons to the transport coefficients are Boltzmann suppressed. CFL quark matter displays superfluid properties, and transport phenomena in such cold regime are dominated by phonon-phonon scattering. We study superfluid phonons within thermal field theory and compute the mean free path associated to their most relevant collision processes. Small-angle processes turn out to be more efficient in slowing transport phenomena in the CFL matter, while the mean free path relevant for the shear viscosity is less sensitive to collinear scattering due to the presence of zero modes in the Boltzmann equation. In analogy with superfluid He4, we find the same T power law for the superfluid phonon damping rate and mean free path. Our results are relevant for the study of rotational properties of compact stars, and correct wrong estimates existing in the literature.

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arXiv2005

Charmonium's K2 peak

The newly reported Y(4260) becomes the second most massive state in the charmonium family. We argue that it displaces the psi(4415) as the (largely) 4s vector charmonium state, recall s-d wave interference to explain the lack of a signal in e-e+ to hadrons and suggest some further study avenues that can exclude exotic meson assignments. The absence of a J/psi KK mode can be understood, beyond phase space suppression, to be a consequence of chiral symmetry. We also provide a model calculation in this sector showing that, although forcing the fit somewhat (which suggests a small sea quark wavefunction component), the state can be incorporated in a standard scheme.

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arXiv2005

Oddballs and a Low Odderon Intercept

We report an odderon Regge trajectory emerging from a field theoretical Coulomb gauge QCD model for the odd signature JPC (P=C= -1) glueball states (oddballs). The trajectory intercept is clearly smaller than the pomeron and even the omega trajectory's intercept which provides an explanation for the nonobservation of the odderon in high energy scattering data. To further support this result we compare to glueball lattice data and also perform calculations with an alternative model based upon an exact Hamiltonian diagonalization for three constituent gluons.

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arXiv2005

Vertex functions and infrared fixed point in Landau gauge SU(N) Yang-Mills theory

The infrared behaviour of vertex functions in an SU(N) Yang-Mills theory in Landau gauge is investigated employing a skeleton expansion of the Dyson-Schwinger equations. The three- and four-gluon vertices become singular if and only if all external momenta vanish while the dressing of the ghost-gluon vertex remains finite in this limit. The running coupling as extracted from either of these vertex functions possesses an infrared fixed point. In general, diagrams including ghost-loops dominate in the infrared over purely gluonic ones.

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arXiv2004

Hyperfine meson splittings: chiral symmetry versus transverse gluon exchange

Meson spin splittings are examined within an effective Coulomb gauge QCD Hamiltonian incorporating chiral symmetry and a transverse hyperfine interaction necessary for heavy quarks. For light and heavy quarkonium systems the pseudoscalar-vector meson spectrum is generated by approximate BCS-RPA diagonalizations. This relativistic formulation includes both S and D waves for the vector mesons which generates a set of coupled integral equations. A smooth transition from the heavy to the light quark regime is found with chiral symmetry dominating the pi-rho mass difference. A reasonable description of the observed meson spin splittings and chiral quantities, such as the quark condensate and the pi mass, is obtained. Similar comparisons with TDA diagonalizations, which violate chiral symmetry, are deficient for light pseudoscalar mesons indicating the need to simultaneously include both chiral symmetry and a hyperfine interaction. The eta_b mass is predicted to be around 9400 MeV consistent with other theoretical expectations and above the unconfirmed 9300 MeV candidate. Finally, for comparison with lattice results, the J reliability parameter is also evaluated.

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arXiv2004

RPA Vector Meson Leptonic Widths

Recent J = 0 (para) baryonium interpretations of the BES narrow resonance data near the e^-e^+ -> p anti-p threshold suggests the existance of ortho baryonium. To assist future searches we study J = 1 states, especially vector meson leptonic decays, and report RPA calculations for both light and heavy mesons using a Coulomb-gauge QCD-inspired model. Since the phi(1880) is the only missing model state, other discovered J = 1 particles in this region are ortho baryonium candidates.

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arXiv2004

Semiperturbative construction for the quark-gluon vertex

We construct a model for the quark-gluon vertex of Landau gauge QCD. This is of twofold interest: on the one hand the quark-gluon interaction is at the heart of quark confinement, on the other hand it is a central element in hadron phenomenology based on QCD Greens functions. We employ the non-Abelian one-loop diagram in perturbation theory, which is of order Nc. As a novelty we replace the tree-level quark and gluon propagators in this diagram by their dressed counterparts solving the Dyson-Schwinger equations. The Nc-suppressed Abelian diagram is an order of magnitude smaller in various kinematics. We also study the effect of ghost dressing factors on the vertex obtaining a construction in good agreement with recent low-momentum lattice calculations.

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