MERCÚRIO EM SISTEMAS AQUÁTICOS: FATORES AMBIENTAIS QUE AFETAM A METILAÇÃO

Methylmercury is a highly neurotoxic contaminant that accumulates in organisms and biomagnifies along the food chain. Methylmercury is formed through the transfer of a methyl group to the inorganic mercury (Hg2+). This reaction is mainly mediated by microorganisms living in anoxic environments like bottom sediments and macrophytes rhizosphere. Abiotic methylation can also occur, however in most cases with lower rates than biological methylation. Mercury methylation rates in aquatic systems are influenced by both the speciation and bioavailability of mercury. Many interrelated environmental variables such as biological activity, nutrient availability, pH, temperature, redox potential, and the presence of inorganic and organic complexing agents can also affects the net rate of methylmercury production. Which factors dominate methylmercury production is likely to differ from one ecosystem to other.O metilmercúrio é um poluente altamente neurotóxico que se acumula nos organismos e biomagnifica ao longo da cadeia trófica. O metilmercúrio é formado através de uma reação de transferência de um grupamento metil para o mercúrio inorgânico. Essa transformação, denominada metilação, é mediada principalmente por microrganismos que habitam ambientes anóxicos. A metilação pode ser abiótica como resultado de uma reação não-enzimática na transferência do grupamento metil por via fotoquímica ou interação com substâncias húmicas presentes nos corpos d'água, porém com uma taxa de metilação menor do que pela mediada por microrganismos. As taxas de metilação de mercúrio em sistemas aquáticos são influenciadas tanto pela especiação do mercúrio quanto por sua biodisponibilidade. Diversas variáveis ambientais, que se interrelacionam, tais como a atividade biológica dos microrganismos metiladores, disponibilidade de nutrientes, pH, temperatura, potencial redox, e a presença de complexos orgânicos e inorgânicos podem afetar as taxas de metilação. A importância de cada um desses fatores na produção de metilmercúrio pode variar em diferentes ecossistemas

Direct observation of the dead-cone effect in quantum chromodynamics

At particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons (known as partons) whose evolution is governed by the strong force, as described by the theory of quantum chromodynamics (QCD) [1]. The vacuum is not transparent to the partons and induces gluon radiation and quark pair production in a process that can be described as a parton shower [2]. Studying the pattern of the parton shower is one of the key experimental tools in understanding the properties of QCD. This pattern is expected to depend on the mass of the initiating parton, through a phenomenon known as the dead-cone effect, which predicts a suppression of the gluon spectrum emitted by a heavy quark of mass m and energy E, within a cone of angular size m/E around the emitter [3]. A direct observation of the dead-cone effect in QCD has not been possible until now, due to the challenge of reconstructing the cascading quarks and gluons from the experimentally accessible bound hadronic states. Here we show the first direct observation of the QCD dead-cone by using new iterative declustering techniques [4, 5] to reconstruct the parton shower of charm quarks. This result confirms a fundamental feature of QCD, which is derived more generally from its origin as a gauge quantum field theory. Furthermore, the measurement of a dead-cone angle constitutes the first direct experimental observation of the non-zero mass of the charm quark, which is a fundamental constant in the standard model of particle physics.The direct measurement of the QCD dead cone in charm quark fragmentation is reported, using iterative declustering of jets tagged with a fully reconstructed charmed hadron.In particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons (known as partons) whose evolution is governed by the strong force, as described by the theory of quantum chromodynamics (QCD). These partons subsequently emit further partons in a process that can be described as a parton shower which culminates in the formation of detectable hadrons. Studying the pattern of the parton shower is one of the key experimental tools for testing QCD. This pattern is expected to depend on the mass of the initiating parton, through a phenomenon known as the dead-cone effect, which predicts a suppression of the gluon spectrum emitted by a heavy quark of mass $m_{\rm{Q}}$ and energy $E$, within a cone of angular size $m_{\rm{Q}}$/$E$ around the emitter. Previously, a direct observation of the dead-cone effect in QCD had not been possible, owing to the challenge of reconstructing the cascading quarks and gluons from the experimentally accessible hadrons. We report the direct observation of the QCD dead cone by using new iterative declustering techniques to reconstruct the parton shower of charm quarks. This result confirms a fundamental feature of QCD. Furthermore, the measurement of a dead-cone angle constitutes a direct experimental observation of the non-zero mass of the charm quark, which is a fundamental constant in the standard model of particle physics