43 research outputs found
Line of Fixed Points in Gross-Neveu Theories
In the limit of many fermion flavors it is demonstrated that the sextic
Gross-Neveu theory in three dimensions displays a line of interacting UV fixed
points, characterised by an exactly marginal sextic interaction. We determine
the conformal window of UV-complete theories, universal scaling dimensions, and
the phase diagram using renormalisation group methods. Massless theories arise
naturally, and the generation of mass proceeds without the breaking of a
discrete symmetry. Striking similarities with critical scalar theories at large
are highlighted, and implications from the viewpoint of conformal field
theory and the AdS/CFT conjecture are indicated.Comment: 6 pages, 4 figures; v2: reasoning and Fig. 2 improved, accepted for
publication with Physical Review Letter
Scale Symmetry Breaking and Generation of Mass at Quantum Critical Points
We study an asymptotically free theory of relativistic Dirac fermions and
a real scalar field coupled by Yukawa and scalar self-interactions in three
dimensions using functional renormalisation. In the limit of many fermion
flavours, the cubic scalar coupling becomes exactly marginal due to quantum
fluctuations, leading to a line of strongly-coupled infrared fixed points.
Fermion mass can be generated through a quantum phase transition even if chiral
symmetry is absent. The line of fixed points terminates at a critical endpoint
due to the loss of vacuum stability. Exactly at the endpoint, scale symmetry is
broken spontaneously, leading to the generation of fermion mass. Intriguingly,
the absence of chiral symmetry is a prerequisite for the spontaneous generation
of fermion mass, and not a consequence thereof. We also highlight close
similarities between Gross-Neveu and Gross-Neveu--Yukawa theories at and away
from critical points, and establish the large- equivalence of their
functional RG flows and quantum effective actions. Further implications
including for conformal field theories are indicated.Comment: 13 pages, 5 figure
Inclined gravity currents filling basins: The influence of Reynolds number on entrainment into gravity currents
In many important natural and industrial systems, gravity currents of dense fluid feed basins. Examples include lakes fed by dense rivers and auditoria supplied with cooled air by ventilation systems. As we will show, the entrainment into such buoyancy driven currents can be influenced by viscous forces. Little work, however, has examined this viscous influence and how entrainment varies with the Reynolds number, Re. Using the idea of an entrainment coefficient, E, we derive a mathematical expression for the rise of the front at the top of the dense fluid ponding in a basin, where the horizontal cross-sectional area of the basin varies linearly with depth. We compare this expression to experiments on gravity currents with source Reynolds numbers, Res , covering the broad range 100 < Res < 1500. The form of the observed frontal rises was well approximated by our theory. By fitting the observed frontal rises to the theoretical form with E as the free parameter, we find a linear trend for E(Res ) over the range 350 < Res < 1100, which is in the transition to turbulent flow. In the experiments, the entrainment coefficient, E, varied from 4 × 10−5 to 7 × 10−2. These observations show that viscous damping can be a dominant influence on gravity current entrainment in the laboratory and in geophysical flows in this transitional regime
Discovery and Characterization of 3000+ Main-Sequence Binaries from APOGEE Spectra
We develop a data-driven spectral model for identifying and characterizing
spatially unresolved multiple-star systems and apply it to APOGEE DR13 spectra
of main-sequence stars. Binaries and triples are identified as targets whose
spectra can be significantly better fit by a superposition of two or three
model spectra, drawn from the same isochrone, than any single-star model. From
an initial sample of 20,000 main-sequence targets, we identify
2,500 binaries in which both the primary and secondary star contribute
detectably to the spectrum, simultaneously fitting for the velocities and
stellar parameters of both components. We additionally identify and fit
200 triple systems, as well as 700 velocity-variable systems in
which the secondary does not contribute detectably to the spectrum. Our model
simplifies the process of simultaneously fitting single- or multi-epoch spectra
with composite models and does not depend on a velocity offset between the two
components of a binary, making it sensitive to traditionally undetectable
systems with periods of hundreds or thousands of years. In agreement with
conventional expectations, almost all the spectrally-identified binaries with
measured parallaxes fall above the main sequence in the color-magnitude
diagram. We find excellent agreement between spectrally and dynamically
inferred mass ratios for the 600 binaries in which a dynamical mass ratio
can be measured from multi-epoch radial velocities. We obtain full orbital
solutions for 64 systems, including 14 close binaries within hierarchical
triples. We make available catalogs of stellar parameters, abundances, mass
ratios, and orbital parameters.Comment: Accepted to MNRAS with minor revisions since v1. 19 pages, 12
figures, plus Appendice
Inclined gravity currents filling basins: The influence of Reynolds number on entrainment into gravity currents
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