Application of the Covariant Spectator Theory to the study of heavy and heavy-light mesons

As an application of the Covariant Spectator Theory (CST) we calculate the spectrum of heavy-light and heavy-heavy mesons using covariant versions of a linear confining potential, a one- gluon exchange, and a constant interaction. The CST equations possess the correct one-body limit and are therefore well-suited to describe mesons in which one quark is much heavier than the other. We find a good fit to the mass spectrum of heavy-light and heavy-heavy mesons with just three parameters (apart from the quark masses). Remarkably, the fit parameters are nearly unchanged when we fit to experimental pseudoscalar states only or to the whole spectrum. Because pseudoscalar states are insensitive to spin-orbit interactions and do not determine spin-spin interactions separately from central interactions, this result suggests that it is the covariance of the kernel that correctly predicts the spin-dependent quark-antiquark interaction

A covariant constituent-quark formalism for mesons

Using the framework of the Covariant Spectator Theory (CST) [1] we are developing a covariant model formulated in Minkowski space to study mesonic structure and spectra. Treating mesons as effective $q\bar{q}$ states, we focused in [2] on the nonrelativistic bound-state problem in momentum space with a linear confining potential. Although integrable, this kernel has singularities which are difficult to handle numerically. In [2] we reformulate it into a form in which all singularities are explicitely removed. The resulting equations are then easier to solve and yield accurate and stable solutions. In the present work, the same method is applied to the relativistic case, improving upon the results of the one-channel spectator equation (1CSE) given in [3].Comment: 6 pages, 5 figures, Presented at EEF70, Workshop on Unquenched Hadron Spectroscopy: Non-Perturbative Models and Methods of QCD vs. Experimen

Relativistic phenomenology of meson spectra with a covariant quark model in Minkowski space

In this work, we perform a covariant treatment of quark-antiquark systems. We calculate the spectra and wave functions using a formalism based on the Covariant Spectator Theory (CST). Our results not only reproduce very well the experimental data with a very small set of global parameters, but they also allow a direct test of the predictive power of covariant kernels

Taming chaos to sample rare events: The effect of weak chaos

Rare events in nonlinear dynamical systems are difficult to sample because of the sensitivity to perturbations of initial conditions and of complex landscapes in phase space. Here, we discuss strategies to control these difficulties and succeed in obtaining an efficient sampling within a Metropolis-Hastings Monte Carlo framework. After reviewing previous successes in the case of strongly chaotic systems, we discuss the case of weakly chaotic systems. We show how different types of nonhyperbolicities limit the efficiency of previously designed sampling methods, and we discuss strategies on how to account for them. We focus on paradigmatic low-dimensional chaotic systems such as the logistic map, the Pomeau-Maneville map, and area-preserving maps with mixed phase space. Published under license by AIP Publishing

Swimming abilities of temperate pelagic fish larvae prove that they may control their dispersion in coastal areas

The Sense Acuity and Behavioral (SAAB) Hypothesis proposes that the swimming capabilities and sensorial acuity of temperate fish larvae allows them to find and swim towards coastal nursery areas, which are crucial for their recruitment. To gather further evidence to support this theory, it is necessary to understand how horizontal swimming capability varies along fish larvae ontogeny. Therefore, we studied the swimming capability of white seabream Diplodus sargus (Linnaeus, 1758) larvae along ontogeny, and their relationship with physiological condition. Thus, critical swimming speed (U-crit) and the distance swam (km) during endurance tests were determined for fish larvae from 15 to 55 days post-hatching (DPH), and their physiological condition (RNA, DNA and protein contents) was assessed. The critical swimming speed of white seabream larvae increased along ontogeny from 1.1 cm s(-1) (15 DPH) to 23 cm s(-1) (50 and 55 DPH), and the distance swam by larvae in the endurance experiments increased from 0.01 km (15 DPH) to 86.5 km (45 DPH). This finding supports one of the premises of the SAAB hypothesis, which proposes that fish larvae can influence their transport and distribution in coastal areas due to their swimming capabilities. The relationship between larvae's physiological condition and swimming capabilities were not evident in this study. Overall, this study provides critical information for understanding the link between population dynamics and connectivity with the management and conservation of fish stocks.Funding Agency Portuguese Foundation for Science and Technology SFRH/BD/104209/2014 Portuguese Foundation for Science and Technology UID/Multi/04326/2019 FCT, under the Transitional Norm DL57/2016/CP[1361]/CT[CT0008 CLIMFISH project-A framework for assess vulnerability of coastal fisheries to climate change in Portuguese coast n2/SAICT/2017-SAICTinfo:eu-repo/semantics/publishedVersio

Alteration of the conserved residue tyrosine-158 to histidine renders human O6-alkylguanine-DNA alkyltransferase insensitive to the inhibitor O6- benzylguanine

The DNA repair protein O6-alkylguanine-DNA alkyltransferase (AGT) protects cells from alkylation damage. O6-Benzylguanine (BG) is a potent inactivator of human AGT (ED50 of 0.1 μM) that is currently undergoing clinical trials to enhance chemotherapy by alkylating agents. In a screen of AGT mutants randomly mutated at position glycine-160, we found that the double mutant Y158H/G160A protected Escherichia coli from killing by N- methyl-N'-nitro-N-nitrosoguanidine (MNNG) even in the presence of BG and that the AGT activity of this mutant was strongly resistant to BG (ED50 of 180 μM). Because the single mutant G160A was not resistant to BG, this suggested that the presence of the charged histidine residue at position 158 was responsible. This hypothesis was confirmed by the construction of the single mutation Y158H. The Y158H-mutant AGT was slightly less active than wild-type AGT for the repair of mcthylated DNA in vitro, but it protected E. coli from killing by MNNG even in the presence of BG and had an ED50 for the inactivation by BG of 620 μM. In contrast, mutant Y158F had an ED50 of 0.2 μM. Previous studies (M. Xu-Welliver et al., Cancer Res., 58: 1936-1945, 1998) have shown that mutant P140K is highly resistant to BG (ED50 of > 1200 μM). Models of human AGT suggest that the side chain of the lysine inserted into this mutant is close to tyrosine-158 and that the positively charged lysine side-chain may interfere with BG binding. The double mutants P140K/Y158H and P140K/Y158F resembled P140K and Y158H in being highly resistant to BG, but the use of a sensitive assay for reaction of BG with AGT indicated that their abilities to react were in the order P140K/Y158H < P140K < P140K/Y158F. These results confirm that the presence of a positively charged residue close to the active site of human AGT renders it highly resistant to BG without substantially affecting activity toward methylated DNA substrates. Such mutants may limit the value of BG therapy if they arise in malignant cells during chemotherapy, but the mutant sequences may be useful for gene therapy approaches in which BG-resistant human AGTs are used to prevent hematopoietic toxicity. At least 28 AGT sequences (from 25 species) have now been described. In 25 of these, the position equivalent to 158 in the human AGT is also a tyrosine, and in the other 3, it is a phenylalanine. The importance of an aromatic ring side chain at this position is emphasized by previous studies (S. Edara et al., Carcinogenesis, 16: 1637- 1642, 1995), which show that the replacement by alanine renders human AGT inactive. Our results show that histidine can also substitute for tyrosine at this position