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A. Dobado

Publications and source records attributed to A. Dobado.

At least 55 records · Page 3Linked to original sources
arXiv2002

Thermal Meson properties within Chiral Perturbation Theory

We report on our recent work about the description of a meson gas below the chiral phase transition within the framework of Chiral Perturbation Theory. As an alternative to the standard treatment, we present a calculation of the quark condensate which combines the virial expansion and the meson-meson scattering data. We have also calculated the full one-loop elastic pion scattering amplitude at finite temperature and we have unitarized the amplitude using the Inverse Amplitude Method in order to reproduce the temperature effects on the mass and width of the $ρ$ and $σ$ resonances. Our results show a clear increase of the thermal $ρ$ width, as expected from previous analysis. The results for the $σ$ are consistent with Chiral Symmetry Restoration. We comment on the relevance of our results within the context of Relativistic Heavy Ion Collisions.

hep-ph↗
arXiv2002

Thermal rho and sigma mesons from chiral symmetry and unitarity

We study the temperature evolution of the rho and sigma mass and width, using a unitary chiral approach. The one-loop pion-pion scattering amplitude in Chiral Perturbation Theory at finite temperature is unitarized via the Inverse Amplitude Method. Our results predict a clear increase with T of both the rho and sigma widths. The masses decrease slightly for high T, while the rho-pion-pion coupling increases. The rho behavior seems to be favored by experimental results. In the sigma case, it signals chiral symmetry restoration.

hep-ph↗
arXiv2001

Chiral Perturbation Theory and the f2(1270) resonance

Within Chiral Perturbation Theory, we study elastic pion scattering in the I=0, J=2, channel, whose main features are the f2(1270) resonance and the vanishing of the lowest order. By means of a chiral model that includes an explicit resonance coupled to pions, we describe the data and calculate the resonance contribution to the O(p^4) and O(p^6) chiral parameters. We also generalize the Inverse Amplitude Method to higher orders, which allows us to study channels with vanishing lowest order. In particular, we apply it to the I=0,J=2 case, finding a good description of the f2(1270) resonance, as a pole in the second Riemann sheet.

hep-ph↗
arXiv2001

Brane-skyrmions and wrapped states

In the context of a brane world and including an induced curvature term in the brane action, we obtain the effective lagrangian for the Goldstone bosons (branons) associated with the spontaneous breaking of the translational invariance in the bulk. In addition to the branons, this effective action has Skyrmion-like solitonic states which can be understood as holes in the brane. We study their main properties such as mass and size, the Skyrmion-branon interaction and their possible fermionic quantization. We also consider states where the brane is wrapped around the extra dimensions and their relation with the brane-skyrmions. Finally, we extend our results to higher-dimensional branes, such as those appearing in M-theory, where brane-skyrmions could also be present.

hep-ph↗
arXiv2001

Goldstone bosons and solitons on the brane

In this work we compute the effective action describing the low-energy brane Goldstone-boson dynamics. The Goldstone bosons or branons appear because the brane spontaneously breaks the translational invariance along the extra dimensions. We discuss the Higgs-like mechanism in which the Kaluza-Klein gauge fields absorb the Goldstone bosons and acquire mass. We also present the explicit form of the effective action describing the low-energy interactions between the three-brane Goldstone bosons and the Standard Model fields. In addition to the branons, two new kinds of states arise corresponding to Skyrmion-like solutions and wrapped states. We study the main properties of these states, their geometrical interpretation and the relations between them.

hep-ph↗
arXiv2000

LHC sensitivity to the resonance spectrum of a minimal strongly interacting electroweak symmetry breaking sector

We present a unified analysis of the two main production processes of vector boson pairs at the LHC, VV-fusion and qqbar annihilation, in a minimal strongly interacting electroweak symmetry breaking sector. Using a unitarized electroweak chiral Lagrangian formalism and modeling the final V_L V_L strong rescattering effects by a form factor, we describe qqbar annihilation processes in terms of the two chiral parameters that govern elastic V_L V_L scattering. Depending on the values of these two chiral parameters, the unitarized amplitudes may present resonant enhancements in different angular momentum-isospin channels. Scanning this two parameter space, we generate the general resonance spectrum of a minimal strongly interacting electroweak symmetry breaking sector and determine the regions that can be probed at the LHC.

hep-ph↗
arXiv1999

Chiral lagrangians and the QCD string

We propose a method to derive the low-energy efective action of QCD assuming that the long-distance properties of strong interactions can be described by a string theory. We bypass the usual problems related to the existence of the tachyon and absence of the adequate Adler zero by using a sigma model approach where excitations above the correct (chirally non-invariant) QCD vacuum are included. Two-dimensional conformal invariance then implies the vanishing of the O(p^4) effective lagrangian coefficients. We discuss ways to go beyond this limit.

hep-th↗
arXiv1999

The SM as the Quantum Low Energy Effective Theory of the MSSM

In the framework of the Minimal Supersymmetric Standard Model we compute the one-loop effective action for the electroweak bosons obtained after integrating out the different sleptons, squarks, neutralinos and charginos, and present the result in terms of the physical sparticle masses. In addition we study the asymptotic behaviour of the two, three and four point Green's functions with external electroweak bosons in the limit where the physical sparticle masses are very large in comparison with the electroweak scale. We find that in this limit all the effects produced by the supersymmetric particles can be, either absorbed in the Standard Model parameters and gauge bosons wave functions, or else they are supressed by inverse powers of the supersymmetric particle masses. This work, therefore, completes the proof of decoupling of the heavy supersymmetric particles from the standard ones in the electroweak bosons effective action and in the sense of the Appelquist-Carazzone Theorem that we started in a previous work. From the point of view of effective field theories this work can be seen as a (partial) proof that the SM can be obtained indeed from the MSSM as the quantum low energy effective theory of this latter when the SUSY spectra is much heavier than the electroweak scale.

hep-ph↗
arXiv1998

Strong WW interactions at the LHC

We present a brief pedagogical introduction to the Effective Electroweak Chiral Lagrangians, which provide a model independent description of the WW interactions in the strong regime. When it is complemented with some unitarization or a dispersive approach, this formalism allows the study of the general strong scenario expected at the LHC, including resonances.

hep-ph↗
arXiv1998

Decoupling of supersymmetric particles

The possibility of a heavy supersymmetric spectrum at the Minimal Supersymmetric Standard Model is considered and the decoupling from the low energy electroweak scale is analyzed in detail. The formal proof of decoupling of supersymmetric particles from low energy physics is stated in terms of the effective action for the particles of the Standard Model that results by integrating out all the sparticles in the limit where their masses are larger than the electroweak scale. The computation of the effective action for the standard electroweak gauge bosons W^{+-}, Z and γis performed by integrating out all the squarks, sleptons, charginos and neutralinos to one-loop. The Higgs sector is not considered in this paper. The large sparticle masses limit is also analyzed in detail. Explicit analytical formulae for the two-point functions of the electroweak gauge bosons to be valid in that limit are presented. Finally, the decoupling of sparticles in the S, T and U parameters is studied analitically. A discussion on how the decoupling takes place in terms of both the physical sparticle masses and the non-physical mass parameters as the μ-parameter and the soft-breaking parameters is included.

hep-ph↗
arXiv1998

On the Size of the Chiral Condensate, Generalized Chiral Perturbation Theory and the DIRAC experiment

In the near future, the DIRAC collaboration will measure pion-pion scattering lengths with great precision. Those measurements are likely to shed some light on the problem of the size of the chiral quark-antiquark condensate. Although it is usually assumed to be as a large as (-225 MeV)^3, in the last years a more general approach, has been developed to accommodate either a large or a small alternative (-100 MeV)^3. Such a low value would also modify the standard temperature estimate at which the chiral phase transition occurs. In this work we briefly review the basic theoretical ideas related to this issue as well as the experiment that could help to establish any of the two scenarios.

hep-ph↗
arXiv1998

Do heavy sfermions decouple from low energy Standard Model?

We explore analytically how does the Standard Model emerge as the quantum low energy effective theory of the Minimal Supersymmetric Standard Model (MSSM) in the decoupling limit where the sparticles are much heavier than the electroweak scale. In this work we integrate the sfermions to one-loop and compute their contributions to the effective action for standard electroweak gauge bosons. A proof of decoupling of sfermions is performed by analyzing the resulting effective action in the asymptotic limit $m_{\tilde f} \gg m_z$. A discussion on how the decoupling takes place in terms of both the sparticle physical masses and the non-physical MSSM parameters is included.

hep-ph↗
arXiv1998

Chiral symmetry and the pion gas virial expansion

In this work we study the thermodynamic properties of the pion gas starting from the realistic elastic scattering phase-shifts obtained from Chiral Perturbation Theory and using the virial expansion. In particular we study the state equation and the behavior of the quark condensate as a function of the temperature and the density.

hep-ph↗
arXiv1998

Non-local gravitational effective action and particle production

We study the effective action for gravity obtained after the integration of scalar matter fields, using the local momentum representation based on the Riemann normal coordinates expansion. By considering this expansion around different space-time points, we also compute the non-local terms together with the more usual divergent ones. We discuss the applicability of our results to the calculation of particle production rates in cosmological backgrounds and compare this method with the traditional Bogolyubov transformations.

gr-qc↗
arXiv1997

Decoupling of Supersymmetric Particles in the MSSM

A heavy supersymmetric spectrum at the Minimal Supersymmetric Standard Model is considered and the decoupling from the low energy electroweak scale is analyzed. A formal and partial proof of decoupling of supersymmetric particles in the limit where their masses are larger than the electroweak scale is performed by integrating out all the sparticles to one loop and by evaluating the effective action for the standard electroweak gauge bosons $W^{\pm}, Z$ and $γ$. The Higgs sector is not considered here. Analytical results for the two-point functions of the electroweak gauge bosons and the $S, T$ and $U$ parameters, to be valid in that limit, are also presented. A discussion on how the decoupling takes place in terms of both the physical sparticle masses and the non-physical mass parameters as the $μ$-parameter and the soft-breaking parameters is included.

hep-ph↗
arXiv1997

Applicability constraints of the Equivalence Theorem

In this work we study the applicability of the Equivalence Theorem, either for unitary models or within an effective lagrangian approach. There are two types of limitations: the existence of a validity energy window and the use of the lowest order in the electroweak constants. For the first kind, we consider some methods, based on dispersion theory or the large $N$ limit, that allow us to extend the applicability. For the second, we have obtained numerical estimates of the effect of neglecting higher orders in the perturbative expansion.

hep-ph↗
arXiv1997

The inverse amplitude method in Chiral Perturbation Theory

Based on a dispersive approach, we apply the inverse amplitude method to unitarize one-loop SU(2) and SU(3) Chiral Perturbation Theory. Numerically, we find that this unitarization technique yields the correct complex analytic structure in terms of cuts and poles. Indeed, using the chiral parameter estimates obtained from low energy experiments we obtain the poles associated with the rho(770) and K^*(982) resonances. Just by fixing their actual masses we obtain a parametrization of the pi-pi and pi-K phase shifts in eight different channels. With this fit we have then calculated several low-energy phenomenological parameters estimating their errors. Among others, we have obtained the chiral parameters and scattering lengths, which can be relevant for future experiments.

hep-ph↗