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"CALCULATIONS"
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Evidence for quark-matter cores in massive neutron stars
The theory governing the strong nuclear force—quantum chromodynamics—predicts that at sufficiently high energy densities, hadronic nuclear matter undergoes a deconfinement transition to a new phase of quarks and gluons
1
. Although this has been observed in ultrarelativistic heavy-ion collisions
2
,
3
, it is currently an open question whether quark matter exists inside neutron stars
4
. By combining astrophysical observations and theoretical ab initio calculations in a model-independent way, we find that the inferred properties of matter in the cores of neutron stars with mass corresponding to 1.4 solar masses (
M
⊙
) are compatible with nuclear model calculations. However, the matter in the interior of maximally massive stable neutron stars exhibits characteristics of the deconfined phase, which we interpret as evidence for the presence of quark-matter cores. For the heaviest reliably observed neutron stars
5
,
6
with mass
M
≈ 2
M
⊙
, the presence of quark matter is found to be linked to the behaviour of the speed of sound
c
s
in strongly interacting matter. If the conformal bound
c
s
2
≤
1
/
3
(ref.
7
) is not strongly violated, massive neutron stars are predicted to have sizable quark-matter cores. This finding has important implications for the phenomenology of neutron stars and affects the dynamics of neutron star mergers with at least one sufficiently massive participant.
The cores of neutron stars could be made of hadronic matter or quark matter. By combining first-principles calculations with observational data, evidence for the presence of quark matter in neutron star cores is found.
Journal Article
Numerical methods for engineers
The fifth edition of 'Numerical Methods for Engineers' includes challenging problems drawn from all engineering disciplines, of which 80% are new or revised.
Book
Cyclohexane Vibronic States
Funding Information: This research was funded by the Brazilian agencies Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and the European Community’s Seventh Framework Program (FP7/2007-2013) under grant agreement nº 226716 (ELISA) and Fundação para a Ciência e a Tecnologia (FCT) CEFITEC (UID/00068).
In this work, we provide results from a joint experimental and theoretical study of the vibronic features of cyclohexane (C6H12) in the photon energy range of 6.8–10.8 eV (182–115 nm). The high-resolution vacuum ultraviolet (VUV) photoabsorption measurements, together with quantum chemical calculations at the time-dependent density functional theory (TDDFT) level, have helped to assign the major electronic excitations to mixed valence–Rydberg and Rydberg transitions. The C6H12 photoabsorption spectrum shows fine structure which has been assigned to CH2 scissoring, ′3(1), CH2 rocking, ′4(1), C–C stretching, ′5(1), and CCC bending/CC torsion, ′24(), modes. Molecular structure calculations at the DFT level for the neutral and cationic electronic ground-states have shown the relevant structural changes that are operative in the higher-lying electronic states. Photolysis lifetimes in the Earth’s atmosphere are shown to be irrelevant, while the main atmospheric sink mechanism is the reaction with the •OH radical. Potential energy curves have been obtained at the TDDFT level of theory, showing the relevance of interchange character mainly involving the CH2 scissoring, ′3(1), and CH2 rocking, ′4(1), modes, while Jahn–Teller distortion yields weak vibronic coupling involving the non-totally symmetric CCC bending/CC torsion, ′24(), mode.
Journal Article
Calculated values : finance, politics, and the quantitative age
Modern political culture features a deep-seated faith in the power of numbers to find answers, settle disputes, and explain how the world works. Whether evaluating economic trends, measuring the success of institutions, or divining public opinion, we are told that numbers don't lie. But numbers have not always been so revered. Calculated Values traces how numbers first gained widespread public authority in one nation, Great Britain. Into the seventeenth century, numerical reasoning bore no special weight in political life. Complex calculations were often regarded with suspicion, seen as the narrow province of navigators, bookkeepers, and astrologers, not gentlemen. This changed in the decades following the Glorious Revolution of 1688. Though Britons' new quantitative enthusiasm coincided with major advances in natural science, financial capitalism, and the power of the British state, it was no automatic consequence of those developments, William Deringer argues. Rather, it was a product of politics--ugly, antagonistic, partisan politics. From Parliamentary debates to cheap pamphlets, disputes over taxes, trade, and national debt were increasingly conducted through calculations. Some of the era's most pivotal political moments, like the 1707 Union of England and Scotland and the 1720 South Sea Bubble, turned upon calculative conflicts. As Britons learned to fight by the numbers, they came to believe, as one calculator wrote in 1727, that \"facts and figures are the most stubborn evidences.\" Yet the authority of numbers arose not from efforts to find objective truths that transcended politics, but from the turmoil of politics itself.-- Provided by publisher
Book
Origin of enhanced water oxidation activity in an iridium single atom anchored on NiFe oxyhydroxide catalyst
The efficiency of the synthesis of renewable fuels and feedstocks from electrical sources is limited, at present, by the sluggish water oxidation reaction. Single-atom catalysts (SACs) with a controllable coordination environment and exceptional atom utilization efficiency open new paradigms toward designing high-performance water oxidation catalysts. Here, using operando X-ray absorption spectroscopy measurements with calculations of spectra and electrochemical activity, we demonstrate that the origin of water oxidation activity of IrNiFe SACs is the presence of highly oxidized Ir single atom (Ir5.3+) in the NiFe oxyhydroxide under operating conditions. We show that the optimal water oxidation catalyst could be achieved by systematically increasing the oxidation state and modulating the coordination environment of the Ir active sites anchored atop the NiFe oxyhydroxide layers. Based on the proposed mechanism, we have successfully anchored Ir single-atom sites on NiFe oxyhydroxides (Ir0.1/Ni₉Fe SAC) via a unique in situ cryogenic–photochemical reduction method that delivers an overpotential of 183 mV at 10 mA · cm−2 and retains its performance following 100 h of operation in 1 M KOH electrolyte, outperforming the reported catalysts and the commercial IrO₂ catalysts. These findings open the avenue toward an atomic-level understanding of the oxygen evolution of catalytic centers under in operando conditions.
Journal Article
An observation on Feynman diagrams with axial anomalous subgraphs in dimensional regularization with an anticommuting γ 5
Abstract Through the calculation of the matrix element of the singlet axial-current operator between the vacuum and a pair of gluons in dimensional regularization with an anti-commuting γ 5 defined in a Kreimer-scheme variant, we find that additional renormalization counter-terms proportional to the Chern-Simons current operator are needed starting from O 𝓞 ( α s 2 α_(s)² ) in QCD. This is in contrast to the well-known purely multiplicative renormalization of the singlet axial-current operator defined with a non-anticommuting γ 5. Consequently, without introducing compensation terms in the form of additional renormalization, the Adler-Bell-Jackiw anomaly equation does not hold automatically in the bare form in this kind of schemes. We determine the corresponding (gauge-dependent) coefficient to O 𝓞 ( α s 3 α_(s)³ ) in QCD, using a variant of the original Kreimer prescription which is implemented in our computation in terms of the standard cyclic trace together with a constructively-defined γ 5. Owing to the factorized form of these divergences, intimately related to the axial anomaly, we further performed a check, using concrete examples, that with γ 5 treated in this way, the axial-current operator needs no more additional renormalization in dimensional regularization but only for non-anomalous amplitudes in a perturbatively renormalizable theory. To be complete, we provide a few additional ingredients needed for a proposed extension of the algorithmic procedure formulated in the above analysis to potential applications to a renormalizable anomaly-free chiral gauge theory, i.e. the electroweak theory.
Journal Article
The MLIP package: moment tensor potentials with MPI and active learning
The subject of this paper is the technology (the 'how') of constructing machine-learning interatomic potentials, rather than science (the 'what' and 'why') of atomistic simulations using machine-learning potentials. Namely, we illustrate how to construct moment tensor potentials using active learning as implemented in the MLIP package, focusing on the efficient ways to automatically sample configurations for the training set, how expanding the training set changes the error of predictions, how to set up ab initio calculations in a cost-effective manner, etc. The MLIP package (short for Machine-Learning Interatomic Potentials) is available at https://mlip.skoltech.ru/download/.
Journal Article
Three loop master integrals for$$\\mathcal{O}\\left(\\alpha {\\alpha }_{s}^{2}\\right)$$corrections to quark form factor
We consider the three-loop mixed strong-electroweak$$\\left(\\mathcal{O}\\left(\\alpha {\\alpha }_{s}^{2}\\right)\\right)$$corrections to the quark form factor. We compute the master integrals which are appearing in the Feynman diagrams containing a single massive boson in the loop. We use the state-of-the-art method of differential equations to compute all 303 of them, expressing the results in terms of generalized polylogarithms. We encounter multiple square roots that cannot be simultaneously rationalized using a single transformation. Applying concurrent transformations allows us to express the results through generalized polylogarithms with a simple alphabet, but with multiple interdependent arguments.
Journal Article
NNLO+PS Monte Carlo simulation of photon pair production with MiNNLO PS
Abstract We present a NNLO QCD accurate event generator for direct photon pair production at hadron colliders, based on the MiNNLO PS formalism, within the Powheg Box Res framework. Despite the presence of the photons requires the use of isolation criteria, our generator is built such that no technical cuts are needed at any stage of the event generation. Therefore, our predictions can be used to simulate kinematic distributions with arbitrary fiducial cuts. Furthermore, we describe a few modifications of the MiNNLO PS formalism in order to allow for a setting of the renormalization and factorization scales more similar to that of a fixed-order computation, thus reducing the numerical impact of higher-order terms beyond the nominal accuracy. Finally, we show several phenomenological distributions of physical interest obtained by showering the generated events with Pythia8, and we compare them with the 13 TeV data from the ATLAS Collaboration.
Journal Article