79 research outputs found

### Prospects for measuring quark polarization and spin correlations in $b\bar b$ and $c\bar c$ samples at the LHC

Polarization and spin correlations have been studied in detail for top quarks
at the LHC, but have been explored very little for the other flavors of quarks.
In this paper we consider the processes $pp\to q\bar{q}$ with $q = b$, $c$ or
$s$. Utilizing the partial preservation of the quark's spin information in
baryons in the jet produced by the quark, we examine possible analysis
strategies for ATLAS and CMS to measure the quark polarization and spin
correlations. We find polarization measurements for the $b$ and $c$ quarks to
be feasible, even with the currently available datasets. Spin correlation
measurements for $b\bar b$ are possible using the CMS Run 2 parked data, while
such measurements for $c\bar c$ will become possible with higher integrated
luminosity. For the $s$ quark, we find the measurements to be challenging with
the standard triggers. We also provide leading-order QCD predictions for the
polarization and spin correlations expected in the $b\bar b$ and $c\bar c$
samples with the cuts envisioned for the above analyses. Apart from
establishing experimentally the existence of spin correlations in $b\bar b$ and
$c\bar c$ systems produced in $pp$ collisions, the proposed measurements can
provide new information on the polarization transfer from quarks to baryons and
might even be sensitive to physics beyond the Standard Model.Comment: 50 pages, 5 figure

### Asymmetric implied market volatility and terrorist attacks

This paper studies the impact of terrorism on implied volatility in the U.S. financial market via an event study methodology. We decompose the options-based and forward looking VIX index into its negative (VIXâˆ’) and positive (VIX+) components, extracted only from put options and call options, respectively. This decomposition of the VIX index allows us to better investigate the asymmetric impact of terrorist attacks on implied volatility from the puts and calls channels separately. Our study finds evidence of a greater impact of terror detected for the puts channel of VIX, namely VIXâˆ’. We further show that events that occur within the U.S. appear to impact both VIX and VIXâˆ’ in a similar way, whereas international terrorist attacks show a greater impact on the puts component, VIXâˆ’. The calls component, VIX+, is found to be mainly detached from terrorist attacks

### Cosmology and Astrophysical Constraints of Gauss-Bonnet Dark Energy

Cosmological consequences of a string-motivated dark energy scenario
featuring a scalar field coupled to the Gauss-Bonnet invariant are
investigated. We study the evolution of the universe in such a model,
identifying its key properties. The evolution of the homogeneous background and
cosmological perturbations, both at large and small scales, are calculated. The
impact of the coupling on galaxy distributions and the cosmic microwave
background is examined. We find the coupling provides a mechanism to viably
onset the late acceleration, to alleviate the coincidence problem, and
furthermore to effectively cross the phantom divide at the present while
avoiding a Big Rip in the future. We show the model could explain the present
cosmological observations, and discuss how various astrophysical and
cosmological data, from the Solar system, supernovae Ia, cosmic microwave
background radiation and large scale structure constrain it.Comment: 6 pages, 3 figures. References added. Accepted for publication in
Phys. Lett.

### Inhomogeneous Gravity

We study the inhomogeneous cosmological evolution of the Newtonian
gravitational 'constant' G in the framework of scalar-tensor theories. We
investigate the differences that arise between the evolution of G in the
background universes and in local inhomogeneities that have separated out from
the global expansion. Exact inhomogeneous solutions are found which describe
the effects of masses embedded in an expanding FRW Brans-Dicke universe. These
are used to discuss possible spatial variations of G in different regions. We
develop the technique of matching different scalar-tensor cosmologies of
different spatial curvature at a boundary. This provides a model for the linear
and non-linear evolution of spherical overdensities and inhomogeneities in G.
This allows us to compare the evolution of G and \dot{G} that occurs inside a
collapsing overdense cluster with that in the background universe. We develop a
simple virialisation criterion and apply the method to a realistic lambda-CDM
cosmology containing spherical overdensities. Typically, far slower evolution
of \dot{G} will be found in the bound virialised cluster than in the
cosmological background. We consider the behaviour that occurs in Brans-Dicke
theory and in some other representative scalar-tensor theories.Comment: 15 pages, 15 figures. Submitted to MNRAS. References adde

### Varying Alpha in a More Realistic Universe

We study the space-time evolution of the fine structure constant, $\alpha$,
inside evolving spherical overdensities in a lambda-CDM Friedmann universe
using the spherical infall model. We show that its value inside virialised
regions will be significantly larger than in the low-density background
universe. The consideration of the inhomogeneous evolution of the universe is
therefore essential for a correct comparison of extragalactic and solar system
limits on, and observations of, possible time variation in $\alpha$ and other
constants. Time variation in $\alpha$ in the cosmological background can give
rise to no locally observable variations inside virialised overdensities like
the one in which we live, explaining the discrepancy between astrophysical and
geochemical observations.Comment: 5 pages, 5 figures. An extra section added describing the spherical
collapse model. References added. Accepted for publication in Phys. Lett.

### Local non-Gaussianity from inflation

The non-Gaussian distribution of primordial perturbations has the potential
to reveal the physical processes at work in the very early Universe. Local
models provide a well-defined class of non-Gaussian distributions that arise
naturally from the non-linear evolution of density perturbations on
super-Hubble scales starting from Gaussian field fluctuations during inflation.
I describe the delta-N formalism used to calculate the primordial density
perturbation on large scales and then review several models for the origin of
local primordial non-Gaussianity, including the cuvaton, modulated reheating
and ekpyrotic scenarios. I include an appendix with a table of sign conventions
used in specific papers.Comment: 21 pages, 1 figure, invited review to appear in Classical and Quantum
Gravity special issue on non-linear and non-Gaussian cosmological
perturbation

### Cosmology of a Scalar Field Coupled to Matter and an Isotropy-Violating Maxwell Field

Motivated by the couplings of the dilaton in four-dimensional effective
actions, we investigate the cosmological consequences of a scalar field coupled
both to matter and a Maxwell-type vector field. The vector field has a
background isotropy-violating component. New anisotropic scaling solutions
which can be responsible for the matter and dark energy dominated epochs are
identified and explored. For a large parameter region the universe expands
almost isotropically. Using that the CMB quadrupole is extremely sensitive to
shear, we constrain the ratio of the matter coupling to the vector coupling to
be less than 10^(-5). Moreover, we identify a large parameter region,
corresponding to a strong vector coupling regime, yielding exciting and viable
cosmologies close to the LCDM limit.Comment: Refs. added, some clarifications. Published in JHEP10(2012)06

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