698 research outputs found
Improved Nonnegative Estimation of Variance Components in Balanced Multivariate Mixed Models
AbstractConsider the independent Wishart matrices S1W(Σ + λΘ,q1) and S2W(Σ, q2) where Σ is an unknown positive definite (p.d.) matrix, Θ is an unknown nonnegative definite (n.n.d.) matrix, and λ is a known positive scalar. For the estimation of Θ, a class of estimators of the form Θ̂(c,ϵ) = (c/λ){S1/q1 − ϵ(S2/q2)} (c ≥ 0, ϵ ≤ 1), uniformly better than the unbiased estimator Θ̂U = (1/λ){S1/q1 − S2/q2}, is derived (for the squared error loss function). Necessary and sufficient conditions are obtained For the existence of an n.n.d. estimator of the form Θ̂(c,ϵ) uniformly better than ΘU. It turns out that such an n.n.d. estimator exists only under restrictive conditions. However, for a suitable choice of c > 0, ϵ > 0, the estimator obtained by taking the positive part of Θ̂(c, ϵ) results in an n.n.d. estimator, say Θ̂(c, ϵ) +, that is uniformly better than Θ̂U. Numerical results indicate that in terms of mean squared error, Θ̂(c, ϵ) + performs much better than both Θ̂U and the restricted maximum likelihood estimator Θ̂REML of Θ. Similar results are also obtained for the nonnegative estimation of tr Θ and a′Θa, where a is an arbitrary nonzero vector. For estimating Σ, we have derived estimators that are claimed to be uniformly better than the unbiased estimator Σ̂U = S2/q2 under the squared error loss function and the entropy loss function. We have been able to establish the claim only in the bivariate case. Numerical results are reported showing the risk improvement of our proposed estimators of Σ
Annexes of fourth year evaluation of the IDRC / CIDA co-financed BAIF Research Foundation Rural Research Project
Contents: Fourth year evaluation of women in development aspects; Fourth year evaluation of information aspects; Fourth year evaluation of animal science aspects; Fourth year evaluation of training aspect
Lockin to Weak Ferromagnetism in TbNi2B2C and ErNi2B2C
This article describes a model in which ferromagnetism necessarily
accompanies a spin-density-wave lockin transition in the borocarbide structure
provided the commensurate phase wave vector satisfies Q = (m/n)a* with m even
and n odd. The results account for the magnetic properties of TbNi2B2C, and are
also possibly relevant also for those of ErNi2B2C.Comment: 4 page
Space-time evolution of ultra-relativistic heavy ion collisions and hadronic spectra
The space-time evolution of the hot and dense matter formed after the
collisions of heavy nuclei at ultra-relativistic energies is investigated using
(3+1) dimensional hydrodynamical models. The effects of the spectral shift of
the hadronic properties are incorporated in the equation of state (EOS) of the
evolving matter. In-medium shift of hadronic properties are considered for
Quantum Hadrodynamics (QHD) and universal scaling scenarios. It is found that
the EOS for the hadronic matter for universal scaling of hadronic masses
(except pseudoscalar mesons) is similar to the recent lattice results. We
observe that the space-time volume of the hadronic matter at the freeze-out is
considerably different from the one when medium effects on the hadrons are
ignored. The sensitivity of the results on the initial radial velocity profile
is investigated. The transverse mass spectra of pions and protons of NA49
collaboration are analyzed.Comment: Total 17 pages with 18 figure
Magnetic Phase Diagram of GdNi2B2C: Two-ion Magnetoelasticity and Anisotropic Exchange Couplings
Extensive magnetization and magnetostriction measurements were carried out on
a single crystal of GdNi2B2C along the main tetragonal axes. Within the
paramagnetic phase, the magnetic and strain susceptibilities revealed a weak
anisotropy in the exchange couplings and two-ion tetragonal-preserving
alpha-strain modes. Within the ordered phase, magnetization and
magnetostriction revealed a relatively strong orthorhombic distortion mode and
rich field-temperature phase diagrams. For H//(100) phase diagram, three
field-induced transformations were observed, namely, at: Hd(T), related to the
domain alignment; Hr(T), associated with reorientation of the moment towards
the c-axis; and Hs(T), defining the saturation process wherein the exchange
field is completely counterbalanced. On the other hand, For H//(001) phase
diagram, only two field-induced transformations were observed, namely at: Hr(T)
and Hs(T). For both phase diagrams, Hs(T) follows the relation
Hs[1-(T/Tn)^2]^(1/2)kOe with Hs(T-->0)=128.5(5) kOe and Tn(H=0)=19.5 K. In
contrast, the thermal evolution of Hr(T) along the c-axis (much simpler than
along the a-axis) follows the relation Hr[1-T/Tr]^(1/3) kOe where
Hr(T-->0)=33.5(5) kOe and Tr(H=0)=13.5 K. It is emphasized that the
magnetoelastic interaction and the anisotropic exchange coupling are important
perturbations and therefore should be explicitly considered if a complete
analysis of the magnetic properties of the borocarbides is desired
Photon interferometry and size of the hot zone in relativistic heavy ion collisions
The parameters obtained from the theoretical analysis of the single photon
spectra observed by the WA98 collaboration at SPS energies have been used to
evaluate the two photon correlation functions. The single photon spectra and
the two photon correlations at RHIC energies have also been evaluated, taking
into account the effects of the possible spectral change of hadrons in a
thermal bath. We find that the ratio for SPS and
for RHIC energy.Comment: text changed, figures adde
Particle density fluctuations
Event-by-event fluctuations in the multiplicities of charged particles and
photons at SPS energies are discussed. Fluctuations are studied by controlling
the centrality of the reaction and rapidity acceptance of the detectors.
Results are also presented on the event-by-event study of correlations between
the multiplicity of charged particles and photons to search for DCC-like
signals.Comment: Talk presented at Quark Matter 2002, Nantes, Franc
Search for DCC in 158A GeV Pb+Pb Collisions
A detailed analysis of the phase space distributions of charged particles and
photons have been carried out using two independent methods. The results
indicate the presence of nonstatistical fluctuations in localized regions of
phase space.Comment: Talk at the PANIC99 Conference, June 9-16, 199
Pion Freeze-Out Time in Pb+Pb Collisions at 158 A GeV/c Studied via pi-/pi+ and K-/K+ Ratios
The effect of the final state Coulomb interaction on particles produced in
Pb+Pb collisions at 158 A GeV/c has been investigated in the WA98 experiment
through the study of the pi-/pi+ and K-/K+ ratios measured as a function of
transverse mass. While the ratio for kaons shows no significant transverse mass
dependence, the pi-/pi+ ratio is enhanced at small transverse mass values with
an enhancement that increases with centrality. A silicon pad detector located
near the target is used to estimate the contribution of hyperon decays to the
pi-/pi+ ratio. The comparison of results with predictions of the RQMD model in
which the Coulomb interaction has been incorporated allows to place constraints
on the time of the pion freeze-out.Comment: 9 pages, 12 figure
Present Status and Future of DCC Analysis
Disoriented Chiral Condensates (DCC) have been predicted to form in high
energy heavy ion collisions where the approximate chiral symmetry of QCD has
been restored. This leads to large imbalances in the production of charged to
neutral pions. Sophisticated analysis methods are being developed to
disentangle DCC events out of the large background of events with
conventionally produced particles. We present a short review of current
analysis methods and future prospects.Comment: 12 pages, 5 figures. Invited talk presented at the 13th International
Conference on Ultrarelativistic Nucleus-Nucleus Collisions (Quark Matter 97),
Tsukuba, Japan, 1-5 Dec 199
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