arXiv subjects

A. Gomez Nicola

Publications and source records attributed to A. Gomez Nicola.

49 records · Page 3Linked to original sources

arXiv2001

Unitarization of the complete meson-meson scattering at one loop in Chiral Perturbation Theory

We report on our one-loop calculation of all the two meson scattering amplitudes within SU(3) Chiral Perturbation Theory, i.e. with pions, kaons and etas. Once the amplitudes are unitarized with the coupled channel Inverse Amplitude Method, they satisfy simultaneously the correct low-energy chiral constraints and unitarity. We obtain a remarkable description of meson-meson scattering data up to 1.2 GeV including the scattering lengths and seven light resonances.

hep-ph↗

arXiv2001

Pion production in nonequilibrium Chiral perturbation Theory

We apply the formalism of Chiral Perturbation Theory out of thermal equilibrium to describe explosive production of pions via the parametric resonance mechanism. To lowest order the lagrangian is that of the Nonlinear Sigma Model where the pion decay constant becomes a time-dependent function. This model alows for a consistent nonequilibrium formulation within the framework of the closed time path method, where one-loop effects can be systematically accounted for and renormalized. We work in the narrow resonance regime where there is only one resonant band. The present approach is limited to remain below the back-reaction time, although it accounts for nearly all the pion production during the typical plasma lifetime. The space and time components of the pion decay constant are also analyzed.

hep-ph↗

arXiv2000

Exploiting duality in a toy model of QCD at non-zero temperature and chemical potential: the massive Thirring model, sine-Gordon model and Coulomb gases

We focus on the massive Thirring model in 1+1 dimensions at finite temperature and non-zero chemical potential, and comment on some parallels between this model and QCD. In QCD, calculations of physical quantities such as transport coefficients are extremely difficult. In the massive Thirring model, similar calculations are greatly simplified by exploiting the duality which exists with the sine-Gordon model and its relation, at high temperature, to the exactly solvable classical Coulomb gas on the line.

hep-th↗

arXiv2000

Unitarized pion-nucleon scattering within Heavy Baryon Chiral Perturbation Theory

By means of the Inverse Amplitude Method we unitarize the elastic pion-nucleon scattering amplitudes obtained from Heavy Baryon Chiral Perturbation Theory to O(q^3). Within this approach we can enlarge their applicability range and generate the Delta(1232) resonance. We can find a reasonable description of the pion nucleon phase shifts with (q^2) parameters in agreement with the resonance saturation hypothesis. However, the uncertainties in the analysis of the low energy data as well as the large number of chiral parameters, which can have strong correlations, allow us to obtain very good fits with rather different sets of chiral constants.

hep-ph↗

arXiv1999

Nonequilibrium chiral perturbation theory and disoriented chiral condensates

We analyse the extension of Chiral Perturbation Theory to describe a meson gas out of thermal equilibrium. For that purpose, we let the pion decay constant be a time-dependent function and work within the Schwinger-Keldysh contour technique. A useful connection with curved space-time QFT allows to consistently renormalise the model, introducing two new low-energy constants in the chiral limit. We discuss the applicability of our approach within a Relativistic Heavy-Ion Collision environment. In particular, we investigate the formation of Disoriented Chiral Condensate domains in this model, via the parametric resonance mechanism.

hep-ph↗

arXiv1999

The Inverse Amplitude Method and Heavy Baryon Chiral Perturbation Theory applied to pion-nucleon scattering

We report on our present work, where by means of the Inverse Amplitude Method we unitarize the elastic pion nucleon scattering amplitudes of Heavy Barion Chiral Perturbation Theory at O(q^3). We reproduce the scattering up to the inelastic thresholds including the Delta(1232) resonance. The fitted chiral constants are rather different from those obtained by fitting the extrapolated threshold parameters for the non-unitarized theory.

hep-ph↗

arXiv1999

Chiral Symmetry restoration in the massive Thirring model at finite T and $μ$: Dimensional reduction and the Coulomb gas

We show that in certain limits the (1+1)-dimensional massive Thirring model at finite temperature $T$ is equivalent to a one-dimensional Coulomb gas of charged particles at the same $T$. This equivalence is then used to explore the phase structure of the massive Thirring model. For strong coupling and $T>>m$ (the fermion mass) the system is shown to behave as a free gas of "molecules" (charge pairs in the Coulomb gas terminology) made of pairs of chiral condensates. This binding of chiral condensates is responsible for the restoration of chiral symmetry as $T\to\infty$. In addition, when a fermion chemical potential $μ\neq 0$ is included, the analogy with a Coulomb gas still holds with $μ$ playing the role of a purely imaginary external electric field. For small $T$ and $μ$ we find a typical massive Fermi gas behaviour for the fermion density, whereas for large $μ$ it shows chiral restoration by means of a vanishing effective fermion mass. Some similarities with the chiral properties of low-energy QCD at finite $T$ and baryon chemical potential are discussed.

hep-th↗

arXiv1999

Nonequilibrium chiral perturbation theory

We explore the extension of chiral perturbation theory out of thermal equilibrium. The pion decay constant becomes then a time-dependent function and we work within the Schwinger-Keldysh contour technique. A useful connection with curved space-time QFT allows to renormalise the model. We discuss the applicability of our approach within a relativistic heavy-ion collision environment.

hep-ph↗

arXiv1998

Nonequilibrium chiral perturbation theory and pion decay functions

We extend chiral perturbation theory to study a meson gas out of thermal equilibrium. Assuming that the system is initially in equilibrium below the critical temperature and working within the Schwinger-Keldysh contour technique, we define consistently the time-dependent pion decay functions, the counterparts of the pion decay constants, and calculate them up to next to leading order. The link with curved space-time QFT allows to establish nonequilibrium renormalisation. The short-time behaviour and the applicability to a heavy-ion collision plasma are also discussed in this work.

hep-ph↗

arXiv1998

Kinks versus fermions or the 2D sine-Gordon versus massive Thirring models, at nonzero temperature and chemical potential

We study bosonisation in the massive Thirring and sine-Gordon models at finite temperature and nonzero fermion chemical potential. Both canonical operator and path integral approaches are used to prove the equality of the partition functions of the two models at finite $T$ and zero chemical potential, as it has been recently shown. This enables the relationship between thermal normal ordering and path-integral renormalisation to be specified. Furthermore, we prove that thermal averages of zero-charge operators can also be identified. At nonzero chemical potential and temperature we show, in perturbation theory around the massless case, that the bosonised theory is the sine-Gordon model plus an additional topological term, accounting for the existence of zero charge excitations (the fermions or the kinks) in the thermal bath. This result is the 2D version of the low-energy lagrangian at finite baryon density.

hep-th↗

arXiv1998

Thermal bosonisation in the sine-Gordon and massive Thirring models

We study bosonisation in the massive Thirring and sine-Gordon models at finite temperature T and nonzero fermion chemical potential $μ$. For that purpose we use both canonical operator and path integral approaches, paying particular attention to the issues of thermal normal ordering and renormalisation. At T>0 and $μ=0$, the massive Thirring model bosonises to the sine-Gordon model with the same T=0 identification between coupling constants. We prove that not only the partition functions of the two models coincide, as was recently shown, but also that thermal averages of zero-charge operators can be identified. In particular, analysis of the point-split regularised fermion current then leads to the thermal equivalence between sine-Gordon kinks and Thirring fermions. At $μ\neq 0$, T>0 and working in perturbation theory about the massless Thirring model, we show that the bosonised theory is the sine-Gordon model plus an additional topological term which accounts for the existence of net fermion charge excitations (the fermions or the kinks) in the thermal bath. This result generalises one recently obtained for the massless case, and it is the two-dimensional version of the low-energy QCD chiral Lagrangian at finite baryon density.

hep-ph↗

arXiv1994

One loop calculations on the Wess-Zumino-Witten anomalous functional at finite temperature

We analyze the finite temperature (T) extension of the Wess-Zumino- Witten functional, discussed in a previous work, to one loop in chiral perturbation theory. As a phenomenological application, we calculate finite temperature corrections to the amplitude of the $π^0$ decay into two photons. This calculation is performed in three limits : i)$T/M_π<<1$, ii)the chiral limit at finite T and iii)$T/M_π>>1$ ($M_π$ being the pion mass). The $T$-corrections tend to vanish in the chiral limit, where only the kaon contribution remains (although it is exponentially suppressed).

hep-ph↗

arXiv1993

On the Wess-Zumino-Witten anomalous functional at finite temperature

We discuss the finite temperature extension of the anomalous Wess-Zumino -Witten lagrangian. The finite temperature S^1\times S^3 compactification makes a structure in disconnected sectors, corresponding to different baryon numbers appear naturally. The consistency of the anomalous functional is proved for arbitrary baryon number configurations. The anomalous behavior of the functional is shown to be consistent with the absence of finite temperature corrections to chiral anomalies in QCD, for each baryon number sector.

hep-ph↗