94 research outputs found
Dynamics of Tachyon and Phantom Field beyond the Inverse Square Potentials
We investigate the cosmological evolution of the tachyon and phantom-tachyon
scalar field by considering the potential parameter () as a function of another potential parameter
(), which correspondingly extends the
analysis of the evolution of our universe from two-dimensional autonomous
dynamical system to the three-dimension. It allows us to investigate the more
general situation where the potential is not restricted to inverse square
potential and .One result is that, apart from the inverse square potential,
there are a large number of potentials which can give the scaling and dominant
solution when the function equals for one or some
values of as well as the parameter satisfies
condition Eq.(18) or Eq.(19). We also find that for a class of different
potentials the dynamics evolution of the universe are actually the same and
therefore undistinguishable.Comment: 8 pages, no figure, accepted by The European Physical Journal
C(2010), online first,
http://www.springerlink.com/content/323417h708gun5g8/?p=dd373adf23b84743b523a3fa249d51c7&pi=
(Non)-singular brane-world cosmology induced by quantum effects in d5 dilatonic gravity
5d dilatonic gravity (bosonic sector of gauged supergravity) with non-trivial
bulk potential and with surface terms (boundary cosmological constant and trace
anomaly induced effective action for brane quantum matter) is considered. For
constant bulk potential and maximally SUSY Yang-Mills theory (CFT living on the
brane) the inflationary brane-world is constructed. The bulk is singular
asymptotically AdS space with non-constant dilaton and dilatonic de Sitter or
hyperbolic brane is induced by quantum matter effects. At the same time,
dilaton on the brane is determined dynamically. This all is natural realization
of warped compactification in AdS/CFT correspondence. For fine-tuned toy
example of non-constant bulk potential we found the non-singular dilatonic
brane-world where bulk again represents asymptotically AdS space and de Sitter
brane (inflationary phase of observable Universe) is induced exclusively by
quantum effects. The radius of the brane and dilaton are determined
dynamically. The analytically solvable example of exponential bulk potential
leading to singular asymptotically AdS dilatonic bulk space with de Sitter (or
hyperbolic) brane is also presented.In all cases under discussion the gravity
on the brane is trapped via Randall-Sundrum scenario. It is shown that
qualitatively the same types of brane-worlds occur when quantum brane matter is
described by dilaton coupled spinors.Comment: LaTeX file 28 pages and two eps files, few misprints are correcte
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
The unusually long duration gamma-ray burst GRB 000911: Discovery of the afterglow and host galaxy
Of all the well-localized gamma-ray bursts, GRB 000911 has the longest duration (T90 = 500 s) and ranks in the top 1% of BATSE bursts for fluence. Here we report the discovery of the afterglow of this unique burst. In order to simultaneously fit our radio and optical observations, we are required to invoke a model involving a hard electron distribution, p âź 1.5, and a jet-break time less than 1.5 days. A spectrum of the host galaxy taken 111 days after the burst reveals a single emission line, interpreted as [011] at a redshift z = 1.0585, and a continuum break that we interpret as the Balmer limit at this redshift. Despite the long 790, the afterglow of GRB 000911 is not unusual in any other way when compared to the set of afterglows studied to date. We conclude that the duration of the GRB plays little part in determining the physics of the afterglow
Observational Constraints on Dark Radiation in Brane Cosmology
We analyze the observational constraints on brane-world cosmology whereby the
universe is described as a three-brane embedded in a five-dimensional anti-de
Sitter space.
In this brane-universe cosmology, the Friedmann equation is modified by the
appearance of extra terms which derive from existence of the extra dimensions.
In the present work we concentrate on the ``dark radiation'' term which
diminishes with cosmic scale factor as .
We show that, although the observational constraints from primordial
abundances allow only a small contribution when this term is positive, a much
wider range of negative values is allowed. Furthermore, such a negative
contribution can reconcile the tension between the observed primordial \he4
and D abundances.
We also discuss the possible constraints on this term from the power spectrum
of CMB anisotropies in the limit of negligible cosmological perturbation on the
brane world. We show that BBN limits the possible contribution from dark
radiation just before the nucleosynthesis epoch to lie between -65% and
of the background photon energy density. Combining this with the CMB constraint
reduces this range to between -24% and at the confidence
level.Comment: 6 pages, 3 figures, submitted to PRD; this version includes the
referee's suggestions, updated references, and an improved treatment of BBN
model uncertaintie
The unusually long duration gamma-ray burst GRB 000911: Discovery of the afterglow and host galaxy
Of all the well-localized gamma-ray bursts, GRB 000911 has the longest duration (T90 = 500 s) and ranks in the top 1% of BATSE bursts for fluence. Here we report the discovery of the afterglow of this unique burst. In order to simultaneously fit our radio and optical observations, we are required to invoke a model involving a hard electron distribution, p âź 1.5, and a jet-break time less than 1.5 days. A spectrum of the host galaxy taken 111 days after the burst reveals a single emission line, interpreted as [011] at a redshift z = 1.0585, and a continuum break that we interpret as the Balmer limit at this redshift. Despite the long 790, the afterglow of GRB 000911 is not unusual in any other way when compared to the set of afterglows studied to date. We conclude that the duration of the GRB plays little part in determining the physics of the afterglow
Search for a W' boson decaying to a bottom quark and a top quark in pp collisions at sqrt(s) = 7 TeV
Results are presented from a search for a W' boson using a dataset
corresponding to 5.0 inverse femtobarns of integrated luminosity collected
during 2011 by the CMS experiment at the LHC in pp collisions at sqrt(s)=7 TeV.
The W' boson is modeled as a heavy W boson, but different scenarios for the
couplings to fermions are considered, involving both left-handed and
right-handed chiral projections of the fermions, as well as an arbitrary
mixture of the two. The search is performed in the decay channel W' to t b,
leading to a final state signature with a single lepton (e, mu), missing
transverse energy, and jets, at least one of which is tagged as a b-jet. A W'
boson that couples to fermions with the same coupling constant as the W, but to
the right-handed rather than left-handed chiral projections, is excluded for
masses below 1.85 TeV at the 95% confidence level. For the first time using LHC
data, constraints on the W' gauge coupling for a set of left- and right-handed
coupling combinations have been placed. These results represent a significant
improvement over previously published limits.Comment: Submitted to Physics Letters B. Replaced with version publishe
Search for the standard model Higgs boson decaying into two photons in pp collisions at sqrt(s)=7 TeV
A search for a Higgs boson decaying into two photons is described. The
analysis is performed using a dataset recorded by the CMS experiment at the LHC
from pp collisions at a centre-of-mass energy of 7 TeV, which corresponds to an
integrated luminosity of 4.8 inverse femtobarns. Limits are set on the cross
section of the standard model Higgs boson decaying to two photons. The expected
exclusion limit at 95% confidence level is between 1.4 and 2.4 times the
standard model cross section in the mass range between 110 and 150 GeV. The
analysis of the data excludes, at 95% confidence level, the standard model
Higgs boson decaying into two photons in the mass range 128 to 132 GeV. The
largest excess of events above the expected standard model background is
observed for a Higgs boson mass hypothesis of 124 GeV with a local significance
of 3.1 sigma. The global significance of observing an excess with a local
significance greater than 3.1 sigma anywhere in the search range 110-150 GeV is
estimated to be 1.8 sigma. More data are required to ascertain the origin of
this excess.Comment: Submitted to Physics Letters
Measurement of the Lambda(b) cross section and the anti-Lambda(b) to Lambda(b) ratio with Lambda(b) to J/Psi Lambda decays in pp collisions at sqrt(s) = 7 TeV
The Lambda(b) differential production cross section and the cross section
ratio anti-Lambda(b)/Lambda(b) are measured as functions of transverse momentum
pt(Lambda(b)) and rapidity abs(y(Lambda(b))) in pp collisions at sqrt(s) = 7
TeV using data collected by the CMS experiment at the LHC. The measurements are
based on Lambda(b) decays reconstructed in the exclusive final state J/Psi
Lambda, with the subsequent decays J/Psi to an opposite-sign muon pair and
Lambda to proton pion, using a data sample corresponding to an integrated
luminosity of 1.9 inverse femtobarns. The product of the cross section times
the branching ratio for Lambda(b) to J/Psi Lambda versus pt(Lambda(b)) falls
faster than that of b mesons. The measured value of the cross section times the
branching ratio for pt(Lambda(b)) > 10 GeV and abs(y(Lambda(b))) < 2.0 is 1.06
+/- 0.06 +/- 0.12 nb, and the integrated cross section ratio for
anti-Lambda(b)/Lambda(b) is 1.02 +/- 0.07 +/- 0.09, where the uncertainties are
statistical and systematic, respectively.Comment: Submitted to Physics Letters
Search for new physics in events with opposite-sign leptons, jets, and missing transverse energy in pp collisions at sqrt(s) = 7 TeV
A search is presented for physics beyond the standard model (BSM) in final
states with a pair of opposite-sign isolated leptons accompanied by jets and
missing transverse energy. The search uses LHC data recorded at a
center-of-mass energy sqrt(s) = 7 TeV with the CMS detector, corresponding to
an integrated luminosity of approximately 5 inverse femtobarns. Two
complementary search strategies are employed. The first probes models with a
specific dilepton production mechanism that leads to a characteristic kinematic
edge in the dilepton mass distribution. The second strategy probes models of
dilepton production with heavy, colored objects that decay to final states
including invisible particles, leading to very large hadronic activity and
missing transverse energy. No evidence for an event yield in excess of the
standard model expectations is found. Upper limits on the BSM contributions to
the signal regions are deduced from the results, which are used to exclude a
region of the parameter space of the constrained minimal supersymmetric
extension of the standard model. Additional information related to detector
efficiencies and response is provided to allow testing specific models of BSM
physics not considered in this paper.Comment: Replaced with published version. Added journal reference and DO
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