Bulk viscosity and the conformal anomaly in the pion gas

We calculate the bulk viscosity of the massive pion gas within Unitarized Chiral Perturbation Theory. We obtain a low temperature peak arising from explicit conformal breaking due to the pion mass and another peak near the critical temperature, dominated by the conformal anomaly through gluon condensate terms. The correlation between bulk viscosity and conformal breaking supports a recent QCD proposal. We discuss the role of resonances, heavier states and large-$N_c$ counting.Comment: Revised version accepted in Phys.Rev.Lett. 4 pages, 3 figure

Transport properties of a meson gas

We present recent results on a systematic method to calculate transport coefficients for a meson gas (in particular, we analyze a pion gas) at low temperatures in the context of Chiral Perturbation Theory. Our method is based on the study of Feynman diagrams with a power counting which takes into account collisions in the plasma by means of a non-zero particle width. In this way, we obtain results compatible with analysis of Kinetic Theory with just the leading order diagram. We show the behavior with temperature of electrical and thermal conductivities and shear and bulk viscosities, and we discuss the fundamental role played by unitarity. We obtain that bulk viscosity is negligible against shear viscosity near the chiral phase transition. Relations between the different transport coefficients and bounds on them based on different theoretical approximations are also discussed. We also comment on some applications to heavy-ion collisions.Comment: 4 pages, 4 figures, IJMPE style. Contribution to the International Workshop X Hadron Physics (2007), Florianopolis, Brazil. Accepted for publication in IJMPE; 1 typo correcte

The Inverse Amplitude Method and Adler Zeros

The Inverse Amplitude Method is a powerful unitarization technique to enlarge the energy applicability region of Effective Lagrangians. It has been widely used to describe resonances from Chiral Perturbation Theory as well as for the Strongly Interacting Symmetry Breaking Sector. In this work we show how it can be slightly modified to account also for the sub-threshold region, incorporating correctly the Adler zeros required by chiral symmetry and eliminating spurious poles. These improvements produce negligible effects on the physical region.Comment: 17 pages, 4 figure

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 $\pi^0$ decay into two photons. This calculation is performed in three limits : i)$T/M_{\pi}<<1$, ii)the chiral limit at finite T and iii)$T/M_{\pi}>>1$ ($M_{\pi}$ 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).Comment: Latex, 13 pages and 3 figures avalaible upon reques

Pion scattering poles and chiral symmetry restoration

Using unitarized Chiral Perturbation Theory methods, we perform a detailed analysis of the $\pi\pi$ scattering poles $f_0(600)$ and $\rho(770)$ behaviour when medium effects such as temperature or density drive the system towards Chiral Symmetry Restoration. In the analysis of real poles below threshold, we show that it is crucial to extend properly the unitarized amplitudes so that they match the perturbative Adler zeros. Our results do not show threshold enhancement effects at finite temperature in the $f_0(600)$ channel, which remains as a pole of broad nature. We also implement T=0 finite density effects related to chiral symmetry restoration, by varying the pole position with the pion decay constant. Although this approach takes into account only a limited class of contributions, we reproduce the expected finite density restoration behaviour, which drives the poles towards the real axis, producing threshold enhancement and $\pi\pi$ bound states. We compare our results with several model approaches and discuss the experimental consequences, both in Relativistic Heavy Ion Collisions and in $\pi\to \pi\pi$ and $\gamma\to \pi\pi$ reactions in nuclei.Comment: 17 pages, 9 figures, final version to appear in Phys.Rev.D, added comments and reference

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 non-zero 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 ≠ 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

Neutral pion decay in dense skyrmion matter

We study the density dependence of the decay $\pi^0\to \gamma \gamma$ using the Skyrme Lagrangian to describe simultaneously both the matter background and mesonic fluctuations. Pion properties such as mass and decay constant are modified by the medium. This leads to large suppression at high density of both photo-production from the neutral pion and the reverse process. The in-medium effective charge of $\pi^{\pm}$ are also discussed in the same framework.Comment: 8 pages, 4 figures. Corrections in light of referee comment