72 research outputs found
Experimental determination of the H(n=3) density matrix for 80-keV H+ on He
The density matrix is determined for H(n=3) atoms produced in axially symmetric electron-transfer collisions of 80-keV protons on helium. In the experiment axial or transverse electric fields with respect to the proton beam are applied to the collision region. The intensity and polarization of Balmer-α radiation emitted by the H(n=3) atoms are measured as a function of the strength of the external electric field. Detailed analysis of the measured optical signals, taking into account the time evolution of the H(n=3) atoms in the applied electric field, makes it possible to extract the complete density matrix of the H(n=3) atoms at the moment of their formation, averaged over all impact parameters. Significant improvements in the experimental technique and in the data analysis associated with the fit of the density matrix to the optical signals have eliminated systematic effects that were present in our previous work [Phys. Rev. A 33, 276 (1986)].
The improvements in the apparatus are as follows: application of electric fields using electrodes with a simple geometry for the axial and transverse orientations that allows accurate calculation of the spatial variation of the electric field inside the collision chamber; use of high-quality optical elements and a rotatable, single-unit design for the polarimeter; automated gas handling for background subtraction; and full computer control of the electric fields, polarimeter, gas handling, and data acquisition. The analysis incorporates the following improvements: hyperfine structure of the H(n=3) manifold; cascade from the H(n=4) manifold; nonuniform detection efficiency over the viewing region; and modeling of the nonuniform electric fields, the nonuniform gas density, and the exponential decrease of the proton beam current in the gas cell due to electron transfer. With these improvements the results from axial electric field measurements are in good agreement with results obtained independently from transverse electric fields. Moreover, the extracted density-matrix elements are found to be within their physically meaningful bounds. The major results from 80-keV collisions are that the H(n=3) density matrix has an average coherence of 81%±1%, an electric dipole moment of 3.50±0.09 a.u., and a first-order moment of the electron current density distribution ă(LĂA)z,să of -0.13±0.02 a.u. Results from a recent calculation show qualitative agreement with the experiment
Primordial black holes in braneworld cosmologies: Formation, cosmological evolution and evaporation
We consider the population evolution and evaporation of primordial black
holes in the simplest braneworld cosmology, Randall-Sundrum type II. We
demonstrate that black holes forming during the high-energy phase of this
theory (where the expansion rate is proportional to the density) have a
modified evaporation law, resulting in a longer lifetime and lower temperature
at evaporation, while those forming in the standard regime behave essentially
as in the standard cosmology. For sufficiently large values of the AdS radius,
the high-energy regime can be the one relevant for primordial black holes
evaporating at key epochs such as nucleosynthesis and the present. We examine
the formation epochs of such black holes, and delimit the parameter regimes
where the standard scenario is significantly modified.Comment: 9 pages RevTeX4 file with four figures incorporated, minor changes to
match published versio
Nonlinear multidimensional cosmological models with form fields: stabilization of extra dimensions and the cosmological constant problem
We consider multidimensional gravitational models with a nonlinear scalar
curvature term and form fields in the action functional. In our scenario it is
assumed that the higher dimensional spacetime undergoes a spontaneous
compactification to a warped product manifold. Particular attention is paid to
models with quadratic scalar curvature terms and a Freund-Rubin-like ansatz for
solitonic form fields. It is shown that for certain parameter ranges the extra
dimensions are stabilized. In particular, stabilization is possible for any
sign of the internal space curvature, the bulk cosmological constant and of the
effective four-dimensional cosmological constant. Moreover, the effective
cosmological constant can satisfy the observable limit on the dark energy
density. Finally, we discuss the restrictions on the parameters of the
considered nonlinear models and how they follow from the connection between the
D-dimensional and the four-dimensional fundamental mass scales.Comment: 21 pages, LaTeX2e, minor changes, improved references, fonts include
Cosmological bounds on large extra dimensions from non-thermal production of Kaluza-Klein modes
The existing cosmological constraints on theories with large extra dimensions
rely on the thermal production of the Kaluza-Klein modes of gravitons and
radions in the early Universe. Successful inflation and reheating, as well as
baryogenesis, typically requires the existence of a TeV-scale field in the
bulk, most notably the inflaton. The non-thermal production of KK modes with
masses of order 100 GeV accompanying the inflaton decay sets the lower bounds
on the fundamental scale M_*. For a 1 TeV inflaton, the late decay of these
modes distort the successful predictions of Big Bang Nucleosynthesis unless
M_*> 35, 13, 7, 5 and 3 TeV for 2, 3, 4, 5 and 6 extra dimensions,
respectively. This improves the existing bounds from cosmology on M_* for 4, 5
and 6 extra dimensions. Even more stringent bounds are derived for a heavier
inflaton.Comment: 17 pages, latex, 4 figure
Electroweak Symmetry Breaking via UV Insensitive Anomaly Mediation
Anomaly mediation solves the supersymmetric flavor and CP problems. This is
because the superconformal anomaly dictates that supersymmetry breaking is
transmitted through nearly flavor-blind infrared physics that is highly
predictive and UV insensitive. Slepton mass squareds, however, are predicted to
be negative. This can be solved by adding D-terms for U(1)_Y and U(1)_{B-L}
while retaining the UV insensitivity. In this paper we consider electroweak
symmetry breaking via UV insensitive anomaly mediation in several models. For
the MSSM we find a stable vacuum when tanbeta < 1, but in this region the top
Yukawa coupling blows up only slightly above the supersymmetry breaking scale.
For the NMSSM, we find a stable electroweak breaking vacuum but with a chargino
that is too light. Replacing the cubic singlet term in the NMSSM superpotential
with a term linear in the singlet we find a stable vacuum and viable spectrum.
Most of the parameter region with correct vacua requires a large superpotential
coupling, precisely what is expected in the ``Fat Higgs'' model in which the
superpotential is generated dynamically. We have therefore found the first
viable UV complete, UV insensitive supersymmetry breaking model that solves the
flavor and CP problems automatically: the Fat Higgs model with UV insensitive
anomaly mediation. Moreover, the cosmological gravitino problem is naturally
solved, opening up the possibility of realistic thermal leptogenesis.Comment: 27 pages, 3 figures, 1 tabl
Large Extra Dimensions and Decaying KK Recurrences
We suggest the possibility that in ADD type brane-world scenarios, the higher
KK excitations of the graviton may decay to lower ones owing to a breakdown of
the conservation of extra dimensional ``momenta'' and study its implications
for astrophysics and cosmology. We give an explicit realization of this idea
with a bulk scalar field , whose nonzero KK modes acquire vacuum
expectation values. This scenario helps to avoid constraints on large extra
dimensions that come from gamma ray flux bounds in the direction of nearby
supernovae as well as those coming from diffuse cosmological gamma ray
background. It also relaxes the very stringent limits on reheat temperature of
the universe in ADD models.Comment: 16 pages, late
Experimental Probes of Localized Gravity: On and Off the Wall
The phenomenology of the Randall-Sundrum model of localized gravity is
analyzed in detail for the two scenarios where the Standard Model (SM) gauge
and matter fields are either confined to a TeV scale 3-brane or may propagate
in a slice of five dimensional anti-deSitter space. In the latter instance, we
derive the interactions of the graviton, gauge, and fermion Kaluza-Klein (KK)
states. The resulting phenomenological signatures are shown to be highly
dependent on the value of the 5-dimensional fermion mass and differ
substantially from the case where the SM fields lie on the TeV-brane. In both
scenarios, we examine the collider signatures for direct production of the
graviton and gauge KK towers as well as their induced contributions to
precision electroweak observables. These direct and indirect signatures are
found to play a complementary role in the exploration of the model parameter
space. In the case where the SM field content resides on the TeV-brane, we show
that the LHC can probe the full parameter space and hence will either discover
or exclude this model if the scale of electroweak physics on the 3-brane is
less than 10 TeV. We also show that spontaneous electroweak symmetry breaking
of the SM must take place on the TeV-brane.Comment: 62 pages, Latex, 22 figure
Constraints on the mass spectrum of primordial black holes and braneworld parameters from the high-energy diffuse photon background
We investigate the spectral shape of a high-energy diffuse photon emitted by
evaporating primordial black holes (PBHs) in the Randall-Sundrum type II (RS2)
braneworld. In their braneworld scenario, the nature of small PBHs is
drastically modified from the ordinary four-dimensional case for the following
two reasons. (i) dropping Hawking temperature, which equivalently lengthens the
lifetime of the individual PBH due to the change of space-time topology and
(ii) the effective increase of the total amount of PBHs caused by accretion
during the earliest part of the radiation-dominated epoch, the brane
high-energy phase. From studies of the expected spectral shape and its
dependence on braneworld parameters, we obtain two qualitatively distinctive
possibilities of constraints on the braneworld PBHs from the observations of
diffuse high-energy photon background. If the efficiency of accretion in the
high-energy phase exceeds a critical value, the existence of the extra
dimension gives a more stringent upper bound on the abundance of PBHs than the
4D case and a small length scale for the extra dimension is favored. On the
contrary, in the case below the critical accretion efficiency, we find that the
constraint on the PBH abundance can be relaxed by a few orders of magnitude in
exchange for the existence of the large extra dimension; its size may be even
bounded in the region above 10^{19} times 4D Planck length scale provided the
rest mass energy density of the PBHs relative to energy density of radiation is
actually larger than 10^{-27} (4D upper bound) at their formation time. The
above analytical studies are also confirmed numerically, and an allowed region
for braneworld parameters and PBH abundance is clearly obtained.Comment: 16 pages, 8 figures, REVTeX4; version published in PR
Hierarchically Acting Sterile Neutrinos
We propose that a hierarchical spectrum of sterile neutrinos (eV, keV,
GeV) is considered to as the explanations for MiniBooNE and LSND
oscillation anomalies, dark matter, and baryon asymmetry of the universe (BAU)
respectively. The scenario can also realize the smallness of active neutrino
masses by seesaw mechanism.Comment: 4 pages, 1 tabl
Pion contamination in the MICE muon beam
The international Muon Ionization Cooling Experiment (MICE) will perform a systematic investigation of ionization cooling with muon beams of momentum between 140 and 240\,MeV/c at the Rutherford Appleton Laboratory ISIS facility. The measurement of ionization cooling in MICE relies on the selection of a pure sample of muons that traverse the experiment. To make this selection, the MICE Muon Beam is designed to deliver a beam of muons with less than 1\% contamination. To make the final muon selection, MICE employs a particle-identification (PID) system upstream and downstream of the cooling cell. The PID system includes time-of-flight hodoscopes, threshold-Cherenkov counters and calorimetry. The upper limit for the pion contamination measured in this paper is at 90\% C.L., including systematic uncertainties. Therefore, the MICE Muon Beam is able to meet the stringent pion-contamination requirements of the study of ionization cooling.Department of Energy and National Science Foundation (U.S.A.), the Instituto Nazionale di Fisica Nucleare (Italy), the Science and Technology Facilities Council (U.K.), the European Community under the European Commission Framework Programme 7 (AIDA project, grant agreement no. 262025, TIARA project, grant agreement no. 261905, and EuCARD), the Japan Society for the Promotion of Science and the Swiss National Science Foundation, in the framework of the SCOPES programme
- âŠ