Felipe José Llanes Estrada

On neutron stars in f (R) theories: Small radii, large masses and large energy emitted in a merger

Abstract In the context of f ( R ) gravity theories, we show that the apparent mass of a neutron star as seen from an observer at infinity is numerically calculable but requires careful matching, first at the star’s edge, between interior and exterior solutions, none of them being totally Schwarzschild-like but presenting instead small oscillations of the curvature scalar R ; and second at large radii, where the Newtonian potential is used to identify the mass of the neutron star. We find that for the same equation of state, this mass definition is always larger than its general relativistic counterpart. We exemplify this with quadratic R 2 and Hu–Sawicki-like modifications of the standard General Relativity action. Therefore, the finding of two-solar mass neutron stars basically imposes no constraint on stable f ( R ) theories. However, star radii are in general smaller than in General Relativity, which can give an observational handle on such classes of models at the astrophysical level. Both larger masses and smaller matter radii are due to much of the apparent effective energy residing in the outer metric for scalar–tensor theories. Finally, because the f ( R ) neutron star masses can be much larger than General Relativity counterparts, the total energy available for radiating gravitational waves could be of order several solar masses, and thus a merger of these stars constitutes an interesting wave source.

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 pipi scattering amplitude in chiral perturbation theory at Tnot equal0 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 rhopipi coupling increases. The rho behavior seems to be favored by experimental results. In the sigma case, it signals chiral symmetry restoration.

Two-meson cloud contribution to the baryon antidecuplet binding

We study the two-meson virtual cloud contribution to the self-energy of the SU(3) antidecuplet, to which the Thet (+) pentaquark is assumed to belong. This is motivated by the large branching ratio of the N(1710) decay into two pions and one nucleon. We derive effective Lagrangians that describe the N(1710) decay into N pi pi with two pions in s or p wave. We obtain increased binding for all members of the antidecuplet and a contribution to the mass splitting between states with different strangeness which is at least 20% of the empirical one. We also provide predictions for three-body decays of the pentaquark antidecuplet.

Quark Schwinger-Dyson evaluation of the l 1 , l 2 coefficients in the chiral Lagrangian

Using a systematic expansion of the quark-antiquark Bethe Salpeter wave functions in the relativistic quark model and working to O(P-4) in the chiral limit, we are able to derive theoretical expressions relating the coefficients of the chiral Lagrangian l(1),l(2) to the underlying quark-antiquark wave functions and interaction kernels. This is accomplished by using a novel technique based on a Ward identity for the quark-antiquark ladder kernel which greatly simplifies the required effort. Numerical evaluations are performed in two simple specific models.

Nonordinary light meson couplings and the 1/N_c expansion

We study the large N_c behavior of couplings among light meson states with different compositions in terms of quarks and gluons. We shortly review the most common compositions of mesons, which are of interest for the understanding of low-lying meson resonances, namely, the ordinary quark-antiquark states as well as the nonordinary glueball, tetraquark, and other states. We dedicate special attention to Jaffes generalization of the tetraquark with N_c - 1 q -q over bar q pairs, which is the only type of state we have identified whose width does not necessarily vanish with N_c, though it does decouple exponentially with N_c from the ππ channel, so that it is weakly coupled to the meson-meson system.

Hadronic aspects of exotic baryons

In this talk I look into three different topics, addressing first the possibility that the Theta(+) is a bound state of K pi N, exploiting the results of this study to find out the contribution of two meson and one baryon components in the baryon antidecuplet and in the third place I present results on a new resonant exotic baryonic state which appears as dynamically generated by the Weinberg Tomozawa Delta K interaction.

Rho and sigma mesons in unitarized thermal XPT

We present our recent results for the rho and sigma mesons considered as resonances in pion-pion scattering in a thermal bath. We use chiral perturbation theory to order p(4) for the low energy behaviour, then extend the analysis via the unitarization method of the Inverse Amplitude into the resonance region. The width of the rho broadens about twice the amount required by phase space considerations alone, its mass staying practically constant up to temperatures of order 150 MeV. The sigma meson behaves in accordance to chiral symmetry restoration expectations.