397 research outputs found
String Theory has no Isotropic Solution for the Modified Einstein's Equations without the Dilaton
We investigate the modification to Einstein's vacuum field equations which is
imposed by string theory when the dilaton field is ignored. Including the
cosmological constant in all calculations, we prove that such a theory of
gravity admits no static isotropic solution. We then show that any isotropic
solution of the equations in question must necessarily be static, therefore
proving that no isotropic solution exists for this stringy modification to
gravity
Modification to Einstein\u27s field equations imposed by string theory and consequences for the classical tests of general relativity
String theory imposes slight modifications to Einstein\u27s equations of general relativity (GR). In (4), the authors claim that the gravitational field equations in empty space, which in GR are just R [subscript greek letters mu nu ] = 0, should hold one extra term which is first order in the string constant [alpha\u27] and proportional to the Riemann curvature tensor squared. They do admit, however, that this simple modification is just schematic. In (1) the authors use modified equations which are coupled to the dilation field. We show that equations given in (4) do not admit an isotropic solution; justification of these equations would require sacrificing isotropy. We thus investigate the consequences of the coupled equations from (1) and the black-hole solution they give there. We calculate the additional perihelion precession of Mercury, the added deflection of photons by the sun, and the extra gravitational redshift which should be present if these equations hold. We determine that additional effects due to string theory in each of these cases are quite minuscule
JUNIPR: a Framework for Unsupervised Machine Learning in Particle Physics
In applications of machine learning to particle physics, a persistent
challenge is how to go beyond discrimination to learn about the underlying
physics. To this end, a powerful tool would be a framework for unsupervised
learning, where the machine learns the intricate high-dimensional contours of
the data upon which it is trained, without reference to pre-established labels.
In order to approach such a complex task, an unsupervised network must be
structured intelligently, based on a qualitative understanding of the data. In
this paper, we scaffold the neural network's architecture around a
leading-order model of the physics underlying the data. In addition to making
unsupervised learning tractable, this design actually alleviates existing
tensions between performance and interpretability. We call the framework
JUNIPR: "Jets from UNsupervised Interpretable PRobabilistic models". In this
approach, the set of particle momenta composing a jet are clustered into a
binary tree that the neural network examines sequentially. Training is
unsupervised and unrestricted: the network could decide that the data bears
little correspondence to the chosen tree structure. However, when there is a
correspondence, the network's output along the tree has a direct physical
interpretation. JUNIPR models can perform discrimination tasks, through the
statistically optimal likelihood-ratio test, and they permit visualizations of
discrimination power at each branching in a jet's tree. Additionally, JUNIPR
models provide a probability distribution from which events can be drawn,
providing a data-driven Monte Carlo generator. As a third application, JUNIPR
models can reweight events from one (e.g. simulated) data set to agree with
distributions from another (e.g. experimental) data set.Comment: 37 pages, 24 figure
Casimir Meets Poisson: Improved Quark/Gluon Discrimination with Counting Observables
Charged track multiplicity is among the most powerful observables for
discriminating quark- from gluon-initiated jets. Despite its utility, it is not
infrared and collinear (IRC) safe, so perturbative calculations are limited to
studying the energy evolution of multiplicity moments. While IRC-safe
observables, like jet mass, are perturbatively calculable, their distributions
often exhibit Casimir scaling, such that their quark/gluon discrimination power
is limited by the ratio of quark to gluon color factors. In this paper, we
introduce new IRC-safe counting observables whose discrimination performance
exceeds that of jet mass and approaches that of track multiplicity. The key
observation is that track multiplicity is approximately Poisson distributed,
with more suppressed tails than the Sudakov peak structure from jet mass. By
using an iterated version of the soft drop jet grooming algorithm, we can
define a "soft drop multiplicity" which is Poisson distributed at
leading-logarithmic accuracy. In addition, we calculate the
next-to-leading-logarithmic corrections to this Poisson structure. If we allow
the soft drop groomer to proceed to the end of the jet branching history, we
can define a collinear-unsafe (but still infrared-safe) counting observable.
Exploiting the universality of the collinear limit, we define generalized
fragmentation functions to study the perturbative energy evolution of
collinear-unsafe multiplicity.Comment: 38+10 pages, 21 figures; v2: discussions added to match JHEP versio
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