The water cycle already bears the fingerprint of human activity

Human activities are increasingly intertwined with the way water is distributed across the continents, according to a new study that brings the extent of that influence into sharper focus. The work addresses a long-standing open question: to what extent the continental water cycle has ceased to be a purely natural process and how far human intervention is reshaping its distribution. The study follows a line of research aimed at separating natural variability in the hydrological cycle from anthropogenic perturbations, a difficult problem because both effects overlap in the same observable variables. The novelty, according to the available description, lies in the greater precision with which the human fingerprint on continental water distribution can be isolated and visualised. The work thus provides a sharper framework for assessing how water is shared out among the continents and the degree to which that distribution already responds to human activity. Delimiting that influence is relevant for water resource management and for interpreting observed trends in the water cycle. The methodological detail and quantitative results of the study, as well as their contrast with independent observations, remain to be seen in order to gauge the exact scope of the human fingerprint described.

First observation of weak magnetism in neutron decay

Weak magnetism, predicted by Murray Gell-Mann nearly seven decades ago as a direct consequence of the conserved vector current (CVC) hypothesis, has been observed for the first time in the decay of the free neutron. This is the simplest β-decay system that exists, yet until now the key observable, the β asymmetry, could not disentangle the weak-magnetism form factor g_M from the axial-to-vector ratio λ. By combining β-asymmetry measurements with the neutron lifetime and the Ft value of superallowed 0⁺→0⁺ transitions, the authors resolve that degeneracy. Current data yield g_M = 4.47(87), in good agreement with the CVC prediction of 4.71. The hypothesis of no weak magnetism at all is excluded at the 5.1σ level. The robustness of the result is striking: the significance remains between 5.0σ and 6.1σ across all considered variations of the neutron dataset, despite the well-known internal tensions that these measurements carry. The work also assesses how far a detailed analysis of the energy dependence of the β asymmetry could refine the extraction of g_M. The conclusion is that, under present experimental conditions, that route would improve the determination by only a small margin, just a few percent. The more promising path forward, therefore, lies not in dissecting the shape of the spectrum but in improving the precision of the quantities that feed the global combination. The result matters on two counts. On the one hand, it confirms a pillar of the Standard Model —the structure of the hadronic weak current and its conservation— in the cleanest possible nuclear setting. On the other, it closes a chapter open since 1958, when Gell-Mann formulated the prediction. The measurement of g_M also becomes a useful tool to cross-check future determinations of the ratio g_A/g_V and of the neutron lifetime, two quantities that continue to fuel debate in nuclear and particle physics.

General master equation for two accelerated detectors and their entanglement

A team of theoretical physicists has derived a general Gorini-Kossakowski-Lindblad-Sudarshan (GKLS) master equation describing the joint behaviour of two co-located two-level detectors undergoing uniform acceleration and weakly coupled to scalar fields in the Minkowski vacuum. The formulation retains both coherent and dissipative contributions, and holds for arbitrary detector orientations, any field mass, and any spacetime dimension. The work belongs to a research line investigating how accelerated motion and the structure of the quantum vacuum affect coherence and entanglement between detectors, a terrain where the Unruh effect and quantum thermalisation phenomena have been hotly debated. Until now, available descriptions were largely restricted to simplified configurations —specific orientations, massless fields, or particular dimensions— preventing a unified analysis of the role of geometry and field mass in the system dynamics. For the specific case of two detectors with equal orientation coupled to massless scalar fields in four dimensions, the authors explicitly determine the stationary states and quantify their entanglement via concurrence. The result depends markedly on the type of coupling considered: with symmetric coupling, concurrence varies with acceleration, whereas with rotating-wave coupling the concurrence turns out to be independent of acceleration. The discrepancy between the two couplings underscores that entanglement dynamics in the vacuum is not universal but depends critically on how the detectors are bound to the field. The general master equation paves the way to exploring more realistic configurations —arbitrary relative orientations, massive fields, and spacetime dimensions other than four— as well as to examining whether entanglement can be robustly extracted from the vacuum in experimental scenarios of accelerated detectors.

Two-loop renormalization constants of Feynman gauge QED computed analytically

A new analytic calculation solves the renormalization constants Z_m and Z₁ = Z₂ of quantum electrodynamics in Feynman gauge, within the on-shell scheme and using dimensional regularization. The results are obtained in analytic form at two loops, closing a technical problem that until now was handled only numerically or through partial approximations. In addition to the main values, the work provides analytic expressions for the expansion coefficients in the dimensional regularization parameter ε, from order 1/ε² through order ε⁵. This expansion range is far wider than what single-loop calculations usually require, and responds to the demands of future higher-order corrections in QED. The value of these high-order coefficients is twofold. On the one hand, they will be needed for more precise multi-loop calculations in quantum electrodynamics, where the ε expansions must be carried several terms beyond the simple pole. On the other, they provide a useful check for multi-loop calculations in quantum chromodynamics (QCD), whose renormalization structure shares techniques and divergences with those of QED. The availability of closed-form analytic results also facilitates the detection of errors in future automatic implementations of Feynman integrals, a field in which independent checks are increasingly necessary as the number of loops considered at colliders grows.

Elliptic flow in heavy-ion collisions departs from linear response

Fluctuations of the elliptic flow v₂ in heavy-ion collisions are usually explained as a linear response to the initial anisotropy ε₂, with a small cubic correction. Now, an analysis based on event-by-event hydrodynamic simulations reveals significant deviations from this scheme. The authors decompose the response order by order in the cumulant expansion of the ε₂ and v₂ fluctuations, working in the intrinsic frame where the magnitude and direction of the impact parameter are fixed. The results show two unexpected effects. On the one hand, the asymmetry of the v₂ fluctuations turns out to be opposite to that of the ε₂ fluctuations. On the other, the response to ε₂ fluctuations is approximately 10-20% larger than the response to the average geometry. This second effect is not explained by the cubic response and increases the splitting between the cumulants v₂{2} and v₂{4}, two observables measured with precision in Pb+Pb collisions at the LHC. Collective flow is the most prominent signature of the hydrodynamic evolution of the quark-gluon plasma created at the LHC, and its event-by-event fluctuations encode information about the early stages of the collision. Understanding precisely the relation between the initial geometry and the final flow is essential to extract properties of the medium, such as its viscosity, from the data. The described deviation suggests that the framework of linear response plus cubic correction is not sufficient to describe the full range of observed fluctuations. The work paves the way to refine the hydrodynamic models and analysis chains used to interpret flow measurements at the LHC. If the effect is confirmed, it could affect the determination of quark-gluon plasma parameters and the comparison between theory and data from ATLAS, CMS and ALICE.