3,757 research outputs found

### Covariant Bardeen Perturbation Formalism

In a previous work we obtained a set of necessary conditions for the linear
approximation in cosmology. Here we discuss the relations of this approach with
the so called covariant perturbations. It is often argued in the literature
that one of the main advantages of the covariant approach to describe
cosmological perturbations is that the Bardeen formalism is coordinate
dependent. In this paper we will reformulate the Bardeen approach in a
completely covariant manner. For that, we introduce the notion of pure and
mixed tensors, which yields an adequate language to treat both perturbative
approaches in a common framework. We then stress that in the referred covariant
approach one necessarily introduces an additional hyper-surface choice to the
problem. Using our mixed and pure tensors approach, we were able to construct a
one-to-one map relating the usual gauge dependence of the Bardeen formalism
with the hyper-surface dependence inherent to the covariant approach. Finally,
through the use of this map, we define full non-linear tensors that at first
order correspond to the three known gauge invariant variables $\Phi$, $\Psi$
and $\Xi$, which are simultaneously foliation and gauge invariant. We then
stress that the use of the proposed mixed tensors allows one to construct
simultaneously gauge and hyper-surface invariant variables at any order.Comment: 15 pages, no figures, revtex4-1, accepted for publication in PRD,
typos fixed, improved discussion about higher order gauge and foliation
invarianc

### Modified Gravity Away from a $\Lambda$CDM Background

Within the effective field theory approach to cosmic acceleration, the
background expansion can be specified separately from the gravitational
modifications. We explore the impact of modified gravity in a background
different from a cosmological constant plus cold dark matter ($\Lambda$CDM) on
the stability and cosmological observables, including covariance between
gravity and expansion parameters. In No Slip Gravity the more general
background allows more gravitational freedom, including both positive and
negative Planck mass running. We examine the effects on cosmic structure
growth, as well as showing that a viable positive integrated Sachs-Wolfe effect
crosscorrelation easily arises from this modified gravity theory. Using current
data we constrain parameters with a Monte Carlo analysis, finding a maximum
running $|\alpha_M|\lesssim 0.03$. We provide the modified {\tt hi\_class} code
publicly on GitHub, now enabling computation and inclusion of the redshift
space distortion observable $f\sigma_8$ as well as the No Slip Gravity
modifications.Comment: 14 pages, 13 figures. Matches published version in JCAP, LCDM
discussion adde

### Lorentz invariance relations and Wandzura-Wilczek approximation

A complete list of the so-called Lorentz invariance relations between parton
distribution functions is given and some of their consequences are discussed,
such as the Burkhardt-Cottingham sum rule. The violation of these relations is
considered in a model independent way. It is shown that several Lorentz
invariance relations are not violated in a generalized Wandzura-Wilczek
approximation, indicating that numerically their violation may be small.Comment: 10 pages; Proceedings of the workshop "Recent Advances in
Perturbative QCD and Hadronic Physics", July 20-24, 2009, at ECT*, Trento
(Italy), in honor of Anatoli V. Efremov on the occasion of his 75th birthday;
to appear in Mod. Phys. Lett.

### Q^2 dependence of azimuthal asymmetries in semi-inclusive deep inelastic scattering and in Drell-Yan

We study several azimuthal asymmetries in semi-inclusive deep inelastic
scattering and in Drell-Yan, interpreting them in the framework of the
formalism of the quark correlator, with a particular reference to T-odd
functions. The correlator contains an undetermined energy scale, which we fix
on the basis of a simple and rather general argument. We find a different value
than the one assumed in previous treatments of T-odd functions. This implies
different predictions on the Q^2 dependence of the above mentioned asymmetries.
Our result about the azimuthal asymmetry of unpolarized Drell-Yan agrees with
presently available data, contrary to the alternative assumption on the scale.
Predictions on other azimuthal asymmetries could be tested against data of
planned experiments on Drell-Yan and semi-inclusive deep inelastic scattering.Comment: 37 pages, 7 figure

### Monte-Carlo simulation of events with Drell-Yan lepton pairs from antiproton-proton collisions

The complete knowledge of the nucleon spin structure at leading twist
requires also addressing the transverse spin distribution of quarks, or
transversity, which is yet unexplored because of its chiral-odd nature.
Transversity can be best extracted from single-spin asymmetries in fully
polarized Drell-Yan processes with antiprotons, where valence contributions are
involved anyway. Alternatively, in single-polarized Drell-Yan the transversity
happens convoluted with another chiral-odd function, which is likely to be
responsible for the well known (and yet unexplained) violation of the Lam-Tung
sum rule in the corresponding unpolarized cross section. We present Monte-Carlo
simulations for the unpolarized and single-polarized Drell-Yan $\bar{p}
p^{(\uparrow)} \to \mu^+ \mu^- X$ at different center-of-mass energies in both
configurations where the antiproton beam hits a fixed proton target or it
collides on another proton beam. The goal is to estimate the minimum number of
events needed to extract the above chiral-odd distributions from future
measurements at the HESR ring at GSI. It is important to study the feasibility
of such experiments at HESR in order to demonstrate that interesting spin
physics can be explored already using unpolarized antiprotons.Comment: Deeply revised text with improved discussion of kinematics and
results; added one table; 12 figures. Accepted for publication in Phys. Rev.

### Quantum Cosmological Perturbations of Generic Fluids in Quantum Universes

In previous works, it was shown that the Lagrangians and Hamiltonians of
cosmological linear scalar, vector and tensor perturbations of homogeneous and
isotropic space-times with flat spatial sections containing a perfect fluid can
be put in a simple form through the implementation of canonical transformations
and redefinitions of the lapse function, without ever using the background
classical equations of motion. In this paper, we generalize this result to
general fluids, which includes entropy perturbations, and to arbitrary
spacelike hyper-surfaces through a new method together with the Faddeev-Jackiw
procedure for the constraint reduction. A simple second order Hamiltonian
involving the Mukhanov-Sasaki variable is obtained, again without ever using
the background equations of motion.Comment: 19 pages, revtex4-1, submitted to PRD, expanded discussion about our
method, fixed typo

### Formal analogies between gravitation and electrodynamics

We develop a theoretical framework that allows us to compare electromagnetism
and gravitation in a fully covariant way. This new scenario does not rely on
any kind of approximation nor associate objects with different operational
meaning as it's sometime done in the literature. We construct the
electromagnetic analogue to the Riemann and Weyl tensors and develop the
equations of motion for these objects. In particular, we are able to identify
precisely how and in what conditions gravity can be mapped to electrodynamics.
As a consequence, many of the gemometrical tools of General Relativity can be
applied to Electromagnetism and vice-versa. We hope our results would shed new
light in the nature of electromagnetic and gravitational theories.Comment: 9pages, submitted to General Relativity and Gravitatio

### Effects of azimuth-symmetric acceptance cutoffs on the measured asymmetry in unpolarized Drell-Yan fixed target experiments

Fixed-target unpolarized Drell-Yan experiments often feature an acceptance
depending on the polar angle of the lepton tracks in the laboratory frame.
Typically leptons are detected in a defined angular range, with a dead zone in
the forward region. If the cutoffs imposed by the angular acceptance are
independent of the azimuth, at first sight they do not appear dangerous for a
measurement of the cos(2\phi)-asymmetry, relevant because of its association
with the violation of the Lam-Tung rule and with the Boer-Mulders function. On
the contrary, direct simulations show that up to 10 percent asymmetries are
produced by these cutoffs. These artificial asymmetries present qualitative
features that allow them to mimic the physical ones. They introduce some
model-dependence in the measurements of the cos(2\phi)-asymmetry, since a
precise reconstruction of the acceptance in the Collins-Soper frame requires a
Monte Carlo simulation, that in turn requires some detailed physical input to
generate event distributions. Although experiments in the eighties seem to have
been aware of this problem, the possibility of using the Boer-Mulders function
as an input parameter in the extraction of Transversity has much increased the
requirements of precision on this measurement. Our simulations show that the
safest approach to these measurements is a strong cutoff on the Collins-Soper
polar angle. This reduces statistics, but does not necessarily decrease the
precision in a measurement of the Boer-Mulders function.Comment: 13 pages, 14 figure

### Extraction of the pion distribution amplitude from polarized muon pair production

We consider the production of muon pairs from the scattering of pions on
longitudinally polarized protons. We calculate the cross section and the single
spin asymmetry for this process, taking into account pion bound state effects.
We work in the kinematic region where the photon has a large longitudinal
momentum fraction, which allows us to treat the bound state problem
perturbatively. Our predictions are directly proportional to the pion
distribution amplitude. A measurement of the polarized Drell-Yan cross section
thus allows the determination of the shape of the pion distribution amplitude.Comment: 13 pages, using revtex, two figures added separately as one uuencoded
Z-compressed fil

### Intrinsic transverse momentum and the polarized Drell-Yan process

In this paper we study the cross section at leading order in $1/Q$ for
polarized Drell-Yan scattering at measured lepton-pair transverse momentum
$Q_T$. We find that for a hadron with spin $1/2$ the quark content at leading
order is described by six distribution functions for each flavor, which depend
on both the lightcone momentum fraction $x$, and the quark transverse momentum
\bbox{k}_T^2. These functions are illustrated for a free-quark ensemble. The
cross sections for both longitudinal and transverse polarizations are expressed
in terms of convolution integrals over the distribution functions.Comment: 25 pages, REVTEX 3.0 (3 figures included in separate LATEX file using
feynman.tex), NIKHEF-94-P1 (Revised version

- …