5,900 research outputs found
Width of exotics from QCD sum rules : tetraquarks or molecules?
We investigate the widths of the recently observed charmonium like resonances
X(3872), Z(4430) and Z_2(4250) using QCD sum rules. Extending previous analyses
regarding these states as diquark-antiquark states or molecules of D mesons, we
introduce the Breit-Wigner function in the pole term. We find that introducing
the width increases the mass at small Borel window region. Using the OPE up to
dimension eight, we find that the sum rules based on interpolating current with
molecular components give a stable Borel curve from which both the masses and
widths of these resonances can be well obtained. Thus the QCD sum rule approach
strongly favors the molecular description of these states.Comment: 6 pages, 6 figure
Haul Truck Tires Recycling
The Ball Hogs competed in Task 2 - Haul Truck Tires in the 2016 Waste-Management Education and Research Consortium (WERC) International Environmental Design Contest. This task required designing a solution to recycle discarded haul truck tires from mining sites. Our solution was to use the treads of these haul truck tires to construct liners for ball mills. Recycling haul truck tires in this manner would save the mining companies a substantial amount of money and would result in a decrease of carbon emissions
A Zero-Parameter Extension of General Relativity with Varying Cosmological Constant
We provide a new extension of general relativity (GR) which has the
remarkable property of being more constrained than GR plus a cosmological
constant, having one less free parameter. This is implemented by allowing the
cosmological constant to have a consistent space-time variation, through coding
its dynamics in the torsion tensor. We demonstrate this mechanism by adding a
`quasi-topological' term to the Einstein action, which naturally realizes a
dynamical torsion with an automatic satisfaction of the Bianchi identities.
Moreover, variation of the action with respect to this dynamical
fixes it in terms of other variables, thus providing a scenario with less
freedom than general relativity with a cosmological constant. Once matter is
introduced, at least in the homogeneous and isotropic reduction, is
uniquely determined by the field content of the model. We make an explicit
construction using the Palatini formulation of GR and describe the striking
properties of this new theory. We also highlight some possible extensions to
the theory. A companion paper [1] explores the Friedmann--Robertson--Walker
reduction for cosmology, and future work will study Solar System tests of the
theory.Comment: Companion paper to arXiv:1905.10382. Minor updates to match published
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The cosmology of minimal varying Lambda theories
Inserting a varying Lambda in Einstein's field equations can be made
consistent with the Bianchi identities by allowing for torsion, without the
need to add scalar field degrees of freedom. In the minimal such theory, Lambda
is totally free and undetermined by the field equations in the absence of
matter. Inclusion of matter ties Lambda algebraically to it, at least when
homogeneity and isotropy are assumed, i.e. when there is no Weyl curvature. We
show that Lambda is proportional to the matter density, with a proportionality
constant depending on the equation of state. Unfortunately, the proportionality
constant becomes infinite for pure radiation, ruling out the minimal theory
prima facie despite of its novel internal consistency. It is possible to
generalize the theory still without the addition of kinetic terms, leading to a
new algebraically-enforced proportionality between Lambda and the matter
density. Lambda and radiation may now coexist in a form consistent with Big
Bang Nucleosynthesis, though this places strict constraints on the single free
parameter of the theory, . In the matter epoch Lambda behaves just like
a dark matter component. Its density is proportional to the baryonic and/or
dark matter, and its presence and gravitational effects would need to be
included in accounting for the necessary dark matter in our Universe. This is a
companion paper to Ref. [1] where the underlying gravitational theory is
developed in detail.Comment: Companion paper to arXiv:1905.10380. Minor updates to match published
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