Unsupervised Machine Learning of Quenched Gauge Symmetries: A Proof-of-Concept Demonstration

In condensed matter physics, one of the goals of machine learning is the classification of phases of matter. The consideration of a system's symmetries can significantly assist the machine in this goal. We demonstrate the ability of an unsupervised machine learning protocol, the Principal Component Analysis method, to detect hidden quenched gauge symmetries introduced via the so-called Mattis gauge transformation. Our work reveals that unsupervised machine learning can identify hidden properties of a model and may therefore provide new insights into the models themselves.Comment: 8 pages, 12 figure

An EPID-based method to determine mechanical deformations in a linear accelerator

Purpose: Medical linear accelerators (linac) are delivering increasingly complex treatments using modern techniques in radiation therapy. Complete and precise mechanical QA of the linac is therefore necessary to ensure that there is no unexpected deviation from the gantry's planned course. However, state-of-the-art EPID-based mechanical QA procedures often neglect some degrees of freedom (DOF) like the in-plane rotations of the gantry and imager or the source movements inside the gantry head. Therefore, the purpose of this work is to characterize a 14 DOF method for the mechanical QA of linacs. This method seeks to measure every mechanical deformation in a linac, including source movements, in addition to relevant clinical parameters like mechanical and radiation isocenters. Methods: A widely available commercial phantom and a custom-made accessory inserted in the linac's interface mount are imaged using the electronic portal imaging device (EPID) at multiple gantry angles. Then, simulated images are generated using the nominal geometry of the linac and digitized models of the phantoms. The nominal geometry used to generate these images can be modified using 14 DOF (3 rigid rotations and 3 translations for the imager and the gantry, and 2 in-plane translations of the source) and any change will modify the simulated image. The set of mechanical deformations that minimizes the differences between the simulated and measured image is found using a genetic algorithm coupled with a gradient-descent optimizer. Phantom mispositioning and gantry angular offset were subsequently calculated and extracted from the results. Simulations of the performances of the method for different levels of noise in the phantom models were performed to calculate the absolute uncertainty of the measured mechanical deformations. The measured source positions and the center of collimation were used to define the beam central axis and calculate the radiation isocenter position and radius. Results: After the simultaneous optimization of the 14 DOF, the average distance between the center of the measured and simulated ball bearings on the imager was 0.086 mm. Over the course of a full counter-clockwise gantry rotation, all mechanical deformations were measured, showing sub-millimeter translations and rotations smaller than 1° along every axis. The average absolute uncertainty of the 14 DOF (1 SD) was 0.15 mm or degree. Phantom positioning errors were determined with more than 0.1 mm precision. Errors introduced in the experimental setup like phantom positioning errors, source movements or gantry angular offsets were all successfully detected by our QA method. The mechanical deformations measured are shown to be reproducible over the course of a few weeks and are not sensitive to the experimental setup. Conclusion: This work presents of new method for an accurate mechanical QA of the linacs. It features a 14 DOF model of the mechanical deformations that is both more complete and precise than other available methods. It has demonstrated sub-millimeter accuracy through simulation and experimentation. Introduced errors were successfully detected with high precision

PP4 is a γH2AX phosphatase required for recovery from the DNA damage checkpoint

Phosphorylation of histone H2AX on Ser 139 (γH2AX) is one of the earliest events in the response to DNA double-strand breaks; however, the subsequent removal of γH2AX from chromatin is less understood, despite being a process tightly coordinated with DNA repair. Previous studies in yeast have identified the Pph3 phosphatase (the PP4C orthologue) as important for the dephosphorylation of γH2AX. By contrast, work in human cells attributed this activity to PP2A. Here, we report that PP4 contributes to the dephosphorylation of γH2AX, both at the sites of DNA damage and in undamaged chromatin in human cells, independently of a role in DNA repair. Furthermore, depletion of PP4C results in a prolonged checkpoint arrest, most likely owing to the persistence of mediator of DNA damage checkpoint 1 (MDC1) at the sites of DNA lesions. Taken together, these results indicate that PP4 is an evolutionarily conserved γH2AX phosphatase

Stability of Elastic Glass Phases in Random Field XY Magnets and Vortex Lattices in Type II Superconductors

A description of a dislocation-free elastic glass phase in terms of domain walls is developed and used as the basis of a renormalization group analysis of the energetics of dislocation loops added to the system. It is found that even after optimizing over possible paths of large dislocation loops, their energy is still very likely to be positive when the dislocation core energy is large. This implies the existence of an equilibrium elastic glass phase in three dimensional random field X-Y magnets, and a dislocation free, bond-orientationally ordered ``Bragg glass'' phase of vortices in dirty Type II superconductors.Comment: 12 pages, Revtex, no figures, submitted to Phys Rev Letter

Classical and quantum phases of the pyrochlore S = 1/2 magnet with Heisenberg and Dzyaloshinskii-Moriya interactions

We investigate the ground state and critical temperature (Tc) phase diagrams of the classical and quantum S=12 pyrochlore lattice with nearest-neighbor Heisenberg and Dzyaloshinskii-Moriya interactions (DMI). We consider ferromagnetic and antiferromagnetic Heisenberg exchange interaction as well as direct and indirect DMI. At the classical level, three ground states are found: all-in/all-out, ferromagnetic, and a locally ordered XY phase, known as Γ5, which displays an accidental classical U(1) degeneracy at the mean-field level. Quantum zero-point energy fluctuations computed to order 1/S are found to lift the classical ground-state degeneracy and select the so-called ψ3 state out of the degenerate manifold in most parts of the Γ5 regime. Likewise, thermal fluctuations treated classically at the Gaussian level entropically select the ψ3 state at T=0+. In contrast to this low-temperature state-selection behavior, classical Monte Carlo simulations find that the system orders at Tc in the noncoplanar ψ2 state of Γ5 for antiferromagnetic Heisenberg exchange and indirect DMI with a transition from ψ2 to ψ3 at a temperature TΓ5<Tc. The same method finds that the system orders via a single transition at Tc directly into the ψ3 state for most of the region with ferromagnetic Heisenberg exchange and indirect DMI. Such ordering behavior at Tc for the S=12 quantum model is corroborated by high-temperature series expansion. To investigate the T=0 quantum ground state of the model, we apply the pseudo-fermion functional renormalization group (PFFRG). The quantum paramagnetic phase of the pure antiferromagnetic S=12 Heisenberg model is found to persist over a finite region in the phase diagram for both direct or indirect DMI. Interestingly, we find that a combined ferromagnetic Heisenberg and indirect DMI, near the boundary of ferromagnetism and Γ5 antiferromagnetism, may potentially realize a T=0 quantum ground state lacking conventional magnetic order. Otherwise, for the largest portion of the phase diagram, PFFRG finds the same long-range ordered phases (all-in/all-out, ferromagnetic, and Γ5) as in the classical model

Competing Gauge Fields and Entropically-Driven Spin Liquid to Spin Liquid Transition in non-Kramers Pyrochlores

Gauge theories are powerful tools in theoretical physics, allowing complex phenomena to be reduced to simple principles, and are used in both high-energy and condensed matter physics. In the latter context, gauge theories are becoming increasingly popular for capturing the intricate spin correlations in spin liquids, exotic states of matter in which the dynamics of quantum spins never ceases, even at absolute zero temperature. We consider a spin system on a three-dimensional pyrochlore lattice where emergent gauge fields not only describe the spin liquid behaviour at zero temperature but crucially determine the system's temperature evolution, with distinct gauge fields giving rise to different spin liquid phases in separate temperature regimes. Focusing first on classical spins, in an intermediate temperature regime, the system shows an unusual coexistence of emergent vector and matrix gauge fields where the former is known from classical spin ice systems while the latter has been associated with fractonic quasiparticles, a peculiar type of excitation with restricted mobility. Upon cooling, the system transitions into a low-temperature phase where an entropic selection mechanism depopulates the degrees of freedom associated with the matrix gauge field, rendering the system spin ice like. We further provide numerical evidence that in the corresponding quantum model, a spin liquid with coexisting vector and matrix gauge fields has a finite window of stability in the parameter space of spin interactions down to zero temperature. Finally, we discuss the relevance of our findings for non-Kramers pyrochlore materials.Comment: 13 pages, 5 figure

Classical and quantum phases of the pyrochlore S=1/2S=1/2 magnet with Heisenberg and Dzyaloshinskii-Moriya interactions

We investigate the ground state and critical temperature phase diagrams of the classical and quantum $S=1/2$ pyrochlore lattice with nearest-neighbor Heisenberg and Dzyaloshinskii-Moriya interactions (DMI). We consider ferromagnetic and antiferromagnetic Heisenberg exchange as well as direct and indirect DMI. Classically, three ground states are found: all-in/all-out, ferromagnetic and a locally ordered $XY$ phase, known as $\Gamma_5$, which displays an accidental classical U(1) degeneracy. Quantum zero-point energy fluctuations are found to lift the classical ground state degeneracy and select the $\psi_3$ state in most parts of the $\Gamma_5$ regime. Likewise, thermal fluctuations treated classically, select the $\psi_3$ state at $T=0^+$. In contrast, classical Monte Carlo finds that the system orders at $T_c$ in the $\psi_2$ state of $\Gamma_5$ for antiferromagnetic Heisenberg exchange and indirect DMI with a transition from $\psi_2$ to $\psi_3$ at a temperature $T_{\Gamma_5} <T_c$. The same method finds that the system orders via a single transition at $T_c$ directly into the $\psi_3$ state for most of the region with ferromagnetic Heisenberg exchange and indirect DMI. Such ordering behavior at $T_c$ for the $S=1/2$ quantum model is corroborated by high-temperature series expansion. To investigate the $T=0$ quantum ground states, we apply the pseudo-fermion functional renormalization group (PFFRG). The quantum paramagnetic phase of the pure antiferromagnetic $S=1/2$ Heisenberg model is found to persist over a finite region in the phase diagram for both direct or indirect DMI. We find that near the boundary of ferromagnetism and $\Gamma_5$ antiferromagnetism the system may potentially realize a quantum ground state lacking conventional magnetic order. Otherwise, for the largest portion of the phase diagram, PFFRG finds the same ordered phases as in the classical model.Comment: 26 pages, 14 figure

Textural and geochemical features of freshwater microbialites from Laguna Bacalar, Quintana Roo, Mexico

Microbialites provide some of the oldest direct evidence of life on Earth. They reached their peak during the Proterozoic and declined afterward. Their decline has been attributed to grazing and/or burrowing by metazoans, to changes in ocean chemistry, or to competition with other calcifying organisms. The freshwater microbialites at Laguna Bacalar (Mexico) provide an opportunity to better understand microbialite growth in terms of interaction between grazing organisms versus calcium carbonate precipitation. The Laguna Bacalar microbialites are described in terms of their distinct mesostructures. Stromatolites display internal lamination, attributed to the precipitation of calcite and the upward migration of cyanobacteria during periods of low sedimentation. Thrombolitic stromatolites show internal lamination in addition to internal clotting. The clotting is seen as a result of binding and/or trapping of micritic peloids by cyanobacteria and attributed to periods of high sedimentation. The carbonates in both microbialites had similar C- and O-stable–isotopic signatures, both enriched in ^(13)C relative to bivalves, suggesting photosynthetic CO_2 uptake was the trigger for carbonate precipitation. This implies that the rate of microbialite growth is largely a function of ambient carbonate saturation state, while the texture is especially dependent on accretion rates and sediment deposition on their surface. Importantly, the coexistence with grazing animals suggests no significant inhibition on microbialite growth, thereby calling into question the decline of microbialite as a result of metazoan evolution. Varying sedimentation rates are likely important in controlling the distribution of thrombolite–stromatolite packages in the geological record, given the importance of this factor at Bacalar

Les activités des médecins du travail dans la prévention des TMS : ressources et contraintes

MSD represents the primary cause of occupational illness and the occupational health physician would appear to be a determining stakeholder in prevention. The studies of risk factors of MSD are important but the studies of intervention are limited. The objective of our research is double: understand the practices of occupational health physician and give the means of activity development. Theoretical framework is multidisciplinary approach about activity analysis. The problematic is to produce knowledge of activity of occupational health physicians to take into account their activity in the debate of professional evolution and pluridisciplinarity in health of work. The disciplinary skills of each equip of research produce some methods. Fifty physicians participate in our research. We have observed during 60 day with ergonomics methods, notably 200 sequences of medical consultation with protocol to record data, confirmed by institutions of protection of physician profession and protection of data. Some types of interview are realized (65): semi-directive, clarification, simple self-confrontation, cross self-confrontation. Furthermore, our research is based on the objective to develop the activity of occupational health physicians. To do this, we set up four groups of volunteer occupational health physicians. The discussion of groups is based on objective data of activity (14 discussion sessions) and on instruction of double (15 discussions sessions). All equips of our research have building a common reference to facilitate the data exploitation. Each equip has written one or some articles (in annex of report) to keep this specificity of data production based of disciplinary approach. The first result is that there are some skills of occupational health physicians to make prevention. Someone physicians prefer to realize their action in consultation while the others prefer make intervention in company. The professionals styles of physicians according to the gender building with experience and training depend on context of company, legislative prevention system, relationship between stakeholders and operators, which facilitate or slow down to built the room of maneuver to exercise these professional styles. The activity of physician can be to hamper in certain conditions. The occupational health physicians are the only one to follow the operators, the story of company and relationships of stakeholders, the evolution of health and work. They use their knowledge about work in company to help operator to understand the relation between work and health. They mobilize the words using by operators in consultation, with respected the medical secret, to modify the stakeholders representation of the interaction between work and health. In the other words, it is impossible to separate the individual approach and collective approach in prevention activity of occupational health physician. These different approaches organize the actions of physician. The discussion of research led to questioning the legislative evolution and comparing our French system with de others system like Quebec system.Les TMS constituent l'une des premières maladies professionnelles et les médecins du travail jouent un rôle important dans la prévention. Aujourd'hui les connaissances sur les facteurs de risque à l'origine de cette pathologie font l'objet de débats sur la prévention des TMS. Les recherches ont besoin d'évoluer vers une meilleure connaissance des modalités de prise en charge de ce risque par les professionnels de la prévention. L'objectif de notre recherche est donc double : comprendre les pratiques des médecins du travail et participer au développement de leur activité de prévention. Notre cadre théorique multidisciplinaire (ergonomie, clinique du travail, sociologie du travail) est celui de l'analyse du travail. Notre problématique est la production de connaissances sur l'activité réelle du médecin afin de contribuer au débat sur les évolutions de leur métier et la pluridisciplinarité en santé au travail. Plusieurs méthodologies ont été mises en œuvre selon les compétences et les disciplines de chaque équipe de recherche. Une cinquantaine de médecins volontaires ont participé à notre recherche. Nous avons fait plus de 60 jours d'observation ergonomique du travail sur le terrain, dont 200 observations de consultation avec un protocole validé par le conseil de l'ordre et la CNIL. Différentes formes d'entretiens (au total 65) ont été réalisés (semi-directif, d'explicitation, d'auto-confrontation simple et croisé). Afin de contribuer au développement de l'activité de prévention des médecins du travail, nous avons constitué quatre groupes de pairs qui ont pu échanger sur les traces de leur activité réelle (au total, 14 réunions collectives) et à partir des méthodologies d'instruction du sosie (au total, 15 réunions collectives). Nos équipes de recherche ont partagé un référentiel commun afin de faciliter l'exploitation des données. Pour ne pas perdre la spécificité de chaque discipline, chaque équipe a écrit un ou plusieurs textes (en annexes du rapport) qui constituent la base d'un document de synthèse. Premier résultat, il existe plusieurs manières et façons de faire de la prévention des TMS chez les médecins du travail. Certains médecins privilégient une action en consultation alors que d'autres sont plus dans une action en entreprise. Mais ces styles, s'ils correspondent à des postures professionnelles construites avec l'expérience et le parcours de formation, sont très dépendants du contexte de l'entreprise, des dispositifs légales de prévention et du type de relations avec les salariés, les acteurs de l'entreprise et de la prévention, qui rendent plus ou moins possibles leurs mises en œuvre. L'activité du médecin dans certaines conditions est empêchée. Les médecins du travail sont les seuls intervenants de la prévention à tenir dans la continuité l'histoire de l'entreprise, des relations, de la santé. Ils ramènent leurs connaissances sur le travail pour aider le salarié à faire les liens entre le travail et la santé. Ils mobilisent la parole des salariés issue de la consultation dans les espaces d'échanges publics de l'entreprise, tout en respectant le secret médical, afin de modifier la représentation des acteurs sur les liens travail-santé. Autrement dit, il est impossible dans l'activité de prévention du médecin de délier l'approche individuelle et collective car celles-ci sont sans arrêt articuler et combiner. Cela conduit à s'interroger sur l'évolution de la loi et à comparer les pratiques avec d'autres systèmes de prévention, comme celui du Québec