52 research outputs found
Nearly scale-invariant curvature modes from entropy perturbations during the graceful exit phase
In this paper we describe how a spectrum of entropic perturbations generated during a period of slow contraction can source a nearly scale-invariant spectrum of curvature perturbations on length scales larger than the Hubble radius during the transition from slow contraction to a classical nonsingular bounce (the "graceful exit"phase). The sourcing occurs naturally through higher-order scalar field kinetic terms common to classical (nonsingular) bounce mechanisms. We present a concrete example in which, by the end of the graceful exit phase, the initial entropic fluctuations have become negligible and the curvature fluctuations have a nearly scale-invariant spectrum with an amplitude consistent with observations
Sourcing curvature modes with entropy perturbations in non-singular bouncing cosmologies
The observed temperature fluctuations in the cosmic microwave background can
be traced back to primordial curvature modes that are sourced by adiabatic
and/or entropic matter perturbations. In this paper, we explore the entropic
mechanism in the context of non-singular bouncing cosmologies. We show that
curvature modes are naturally generated during `graceful exit,' i.e., when the
smoothing slow contraction phase ends and the universe enters the bounce stage.
Here, the key role is played by the kinetic energy components that come to
dominate the energy density and drive the evolution towards the cosmological
bounce.Comment: 17 page
Elliptic flow of charged particles in Pb-Pb collisions at 2.76 TeV
We report the first measurement of charged particle elliptic flow in Pb-Pb
collisions at 2.76 TeV with the ALICE detector at the CERN Large Hadron
Collider. The measurement is performed in the central pseudorapidity region
(||<0.8) and transverse momentum range 0.2< < 5.0 GeV/. The
elliptic flow signal v, measured using the 4-particle correlation method,
averaged over transverse momentum and pseudorapidity is 0.087 0.002
(stat) 0.004 (syst) in the 40-50% centrality class. The differential
elliptic flow v reaches a maximum of 0.2 near = 3
GeV/. Compared to RHIC Au-Au collisions at 200 GeV, the elliptic flow
increases by about 30%. Some hydrodynamic model predictions which include
viscous corrections are in agreement with the observed increase.Comment: 10 pages, 4 captioned figures, published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/389
Alignment of the ALICE Inner Tracking System with cosmic-ray tracks
37 pages, 15 figures, revised version, accepted by JINSTALICE (A Large Ion Collider Experiment) is the LHC (Large Hadron Collider) experiment devoted to investigating the strongly interacting matter created in nucleus-nucleus collisions at the LHC energies. The ALICE ITS, Inner Tracking System, consists of six cylindrical layers of silicon detectors with three different technologies; in the outward direction: two layers of pixel detectors, two layers each of drift, and strip detectors. The number of parameters to be determined in the spatial alignment of the 2198 sensor modules of the ITS is about 13,000. The target alignment precision is well below 10 micron in some cases (pixels). The sources of alignment information include survey measurements, and the reconstructed tracks from cosmic rays and from proton-proton collisions. The main track-based alignment method uses the Millepede global approach. An iterative local method was developed and used as well. We present the results obtained for the ITS alignment using about 10^5 charged tracks from cosmic rays that have been collected during summer 2008, with the ALICE solenoidal magnet switched off.Peer reviewe
Transverse momentum spectra of charged particles in proton-proton collisions at GeV with ALICE at the LHC
The inclusive charged particle transverse momentum distribution is measured
in proton-proton collisions at GeV at the LHC using the ALICE
detector. The measurement is performed in the central pseudorapidity region
over the transverse momentum range GeV/.
The correlation between transverse momentum and particle multiplicity is also
studied. Results are presented for inelastic (INEL) and non-single-diffractive
(NSD) events. The average transverse momentum for is (stat.) (syst.) GeV/ and
\left_{\rm NSD}=0.489\pm0.001 (stat.) (syst.)
GeV/, respectively. The data exhibit a slightly larger than measurements in wider pseudorapidity intervals. The results are
compared to simulations with the Monte Carlo event generators PYTHIA and
PHOJET.Comment: 20 pages, 8 figures, 2 tables, published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/390
From Models of the Early Universe to Developing Experiments for Axionic Dark Matter Detection
Modern physical cosmology is thriving with abundant data and new observations from the James Webb Space Telescope, multiple CMB experiments, and new surveys of galaxies from the ground and space. This thesis uses theoretical and experimental approaches to discuss two important topics of cosmology: early universe curvature perturbations and dark matter.
One of the critical questions posed to theoretical cosmology is to explain the origin of temperature fluctuations seen on the cosmic microwave background. In the first part of the thesis, we explore a new entropic mechanism for generating curvature perturbations in the context of non-singular bouncing cosmologies. A gauge-invariant hydrodynamic and a microphysical approach are used to demonstrate the mechanism. We show that the curvature perturbations can be generated from the entropic ones in a cosmological model with two kinetically coupled scalar fields, an arbitrary potential, and a higher-order (quartic) kinetic term. We also present a concrete example of a cosmological scenario in which a nearly scale-invariant spectrum of curvature perturbations is generated on super-Hubble scales at the end of the phase of the universe contraction in accordance with the current observations.
In the second part of the thesis, we discuss an experimental search for dark matter. Multiple pieces of indirect evidence suggest that about one-fourth of the universe’s energy budget consists of dark matter, but it has yet to be observed. In particular, we focus on sub-ueV axion as one of the most promising dark matter candidates. We discuss experimental approaches in the search for sub-ueV axions and argue that developing a high-Q superconducting lumped-element resonator is one of the key elements of the upcoming dark matter search experiments. We then demonstrate our current development of a new state-of-the-art resonator with a Q factor of 710,000 at 301 kHz, the highest value recorded near this frequency
Evaluating Release Kinetics from Alginate Beads Coated with Polyelectrolyte Layers for Sustained Drug Delivery
Charged-Particle Multiplicity Density at Midrapidity in Central Pb-Pb Collisions at root s(NN)=2.76 TeV
The first measurement of the charged-particle multiplicity density at midrapidity in Pb-Pb collisions at a center-of-mass energy per nucleon pair root s(NN) = 2.76 TeV is presented. For an event sample corresponding to the most central 5% of the hadronic cross section, the pseudorapidity density of primary charged particles at midrapidity is 1584 +/- 4(stat) +/- 76(syst), which corresponds to 8.3 +/- 0.4(syst) per participating nucleon pair. This represents an increase of about a factor 1.9 relative to pp collisions at similar collision energies, and about a factor 2.2 to central Au-Au collisions at root s(NN) = 0.2 TeV. This measurement provides the first experimental constraint for models of nucleus-nucleus collisions at LHC energies
Centrality Dependence of the Charged-Particle Multiplicity Density at Midrapidity in Pb-Pb Collisions at root s(NN)=2.76 TeV
The centrality dependence of the charged-particle multiplicity density at midrapidity in Pb-Pb collisions at root s(NN) = 2: 76 TeV is presented. The charged-particle density normalized per participating nucleon pair increases by about a factor of 2 from peripheral (70%-80%) to central (0%-5%) collisions. The centrality dependence is found to be similar to that observed at lower collision energies. The data are compared with models based on different mechanisms for particle production in nuclear collisions
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