Quark Cluster Model Study of Isospin-Two Dibaryons

Based on a quark cluster model for the non-strange sector that reproduces reasonably well the nucleon-nucleon system and the excitation of the $\Delta$ isobar, we generate a nucleon-$\Delta$ interaction and present the predictions for the several isospin two channels. The only attractive channels are $0^+$ and $0^-$, but not attractive enough to generate a resonance. If a resonance is artificially generated and is required to have the observed experimental mass, then our model predicts a width that agrees with the experimental result.Comment: 12 pages, 5 poscript figures available under request. To appear in Phys. Rev.

ΛNN\Lambda NN and ΣNN\Sigma NN systems at threshold: II. The effect of D waves

Using the two-body interactions obtained from a chiral constituent quark model we study all $\Lambda NN$ and $\Sigma NN$ states with I=0,1,2 and J=1/2,3/2 at threshold, taking into account all three-body configurations with S and D wave components. We constrain further the limits for the $\Lambda N$ spin-triplet scattering length a_{1/2,1}. Using the hypertriton binding energy we find a narrow interval for the possible values of the $\Lambda N$ spin-singlet scattering length a_{1/2,0}. We found that the $\Sigma NN$ system has a quasibound state in the (I,J) = (1,1/2) channel very near threshold with a width of about 2.1 MeV.Comment: 19 pages, 4 tables, 6 figures. Accepted for publication in Phys. Rev.

The confined hydrogen atom with a moving nucleus

We study the hydrogen atom confined to a spherical box with impenetrable walls but, unlike earlier pedagogical articles on the subject, we assume that the nucleus also moves. We obtain the ground-state energy approximately by means of first--order perturbation theory and by a more accurate variational approach. We show that it is greater than the one for the case in which the nucleus is clamped at the center of the box. Present approach resembles the well-known treatment of the helium atom with clamped nucleus

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

CCharPPI web server: computational characterization of protein–protein interactions from structure

The atomic structures of protein–protein interactions are central to understanding their role in biological systems, and a wide variety of biophysical functions and potentials have been developed for their characterization and the construction of predictive models. These tools are scattered across a multitude of stand-alone programs, and are often available only as model parameters requiring reimplementation. This acts as a significant barrier to their widespread adoption. CCharPPI integrates many of these tools into a single web server. It calculates up to 108 parameters, including models of electrostatics, desolvation and hydrogen bonding, as well as interface packing and complementarity scores, empirical potentials at various resolutions, docking potentials and composite scoring functions.The research leading to these results has received funding from the People Programme (Marie Curie Actions) of the European Unions Seventh Framework Programme (FP7/2007- 2013) under REA grant agreement PIEF-GA-2012-327899 and grant BIO2013-48213-R from Spanish Ministry of Economy and Competitiveness.Peer ReviewedPostprint (published version

A family of complex potentials with real spectrum

We consider a two-parameter non hermitean quantum-mechanical hamiltonian that is invariant under the combined effects of parity and time reversal transformation. Numerical investigation shows that for some values of the potential parameters the hamiltonian operator supports real eigenvalues and localized eigenfunctions. In contrast with other PT symmetric models, which require special integration paths in the complex plane, our model is integrable along a line parallel to the real axis.Comment: Six figures and four table