4,934 research outputs found
Error Estimates on Parton Density Distributions
Error estimates on parton density distributions are presently based on the
traditional method of least squares minimisation and linear error propagation
in global QCD fits. We review the underlying assumptions and the various
mathematical representations of the method and address some technical issues
encountered in such a global analysis. Parton distribution sets which contain
error information are described.Comment: Latex, 12 pages, 5 figures. Needs iopart.cls and iopart12.clo.
Presented at New Trends in HERA Physics 2001, Ringberg Castle, Tegernsee,
Germany, June 17-22, 200
Enhancing the Pierre Auger Observatory to the 10^{17} to 10^{18.5} eV Range: Capabilities of an Infill Surface Array
The Pierre Auger Observatory has been designed to study the highest-energy
cosmic rays in nature (E > 10^{18.5} eV). The determination of their arrival
direction, energy and composition is performed by the analysis of the
atmospheric showers they produce. The Auger Surface Array will consist of 1600
water Cerenkov detectors placed in an equilateral triangular grid of 1.5 km
spacing. The aim of this paper is to show that the addition of a "small" area
of surface detectors at half or less the above mentioned spacing would allow a
dramatic increase of the physical scope of this Observatory, reaching lower
energies at which the transition from galactic to extragalactic sources is
expected.Comment: 21 pages, 5 figures, accepted for publication in Nucl. Instr. & Meth.
in Phys. Res.
Pair distribution function in a two-dimensional electron gas
We calculate the pair distribution function, , in a two-dimensional
electron gas and derive a simple analytical expression for its value at the
origin as a function of . Our approach is based on solving the
Schr\"{o}dinger equation for the two-electron wave function in an appropriate
effective potential, leading to results that are in good agreement with Quantum
Monte Carlo data and with the most recent numerical calculations of . [C.
Bulutay and B. Tanatar, Phys. Rev. B {\bf 65}, 195116 (2002)] We also show that
the spin-up spin-down correlation function at the origin, , is mainly independent of the degree of spin polarization of
the electronic system.Comment: 5 figures, pair distribution dependence with distance is calculate
Spermatogonial Stem Cells (SSCs) in Buffalo (Bubalus bubalis) Testis
BACKGROUND: Water buffalo is an economically important livestock species and about half of its total world population exists in India. Development of stem cell technology in buffalo can find application in targeted genetic modification of this species. Testis has emerged as a source of pluripotent stem cells in mice and human; however, not much information is available in buffalo. OBJECTIVES AND METHODS: Pou5f1 (Oct 3/4) is a transcription factor expressed by pluripotent stem cells. Therefore, in the present study, expression of POU5F1 transcript and protein was examined in testes of both young and adult buffaloes by semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) and immunohistochemical analysis. Further, using the testis transplantation assay, a functional assay for spermatogonial stem cells (SSCs), stem cell potential of gonocytes/spermatogonia isolated from prepubertal buffalo testis was also determined. RESULTS: Expression of POU5F1 transcript and protein was detected in prepubertal and adult buffalo testes. Western blot analysis revealed that the POU5F1 protein in the buffalo testis exists in two isoforms; large (∼47 kDa) and small (∼21 kDa). Immunohistochemical analysis revealed that POU5F1 expression in prepubertal buffalo testis was present in gonocytes/spermatogonia and absent from somatic cells. In the adult testis, POU5F1 expression was present primarily in post-meiotic germ cells such as round spermatids, weakly in spermatogonia and spermatocytes, and absent from elongated spermatids. POU5F1 protein expression was seen both in cytoplasm and nuclei of the stained germ cells. Stem cell potential of prepubertal buffalo gonocytes/spermatogonia was confirmed by the presence of colonized DBA-stained cells in the basal membrane of seminiferous tubules of xenotransplanted mice testis. CONCLUSION/SIGNIFICANCE: These findings strongly indicate that gonocytes/spermatogonia, isolated for prepubertal buffalo testis can be a potential target for establishing a germ stem cell line that would enable genetic modification of buffaloes
Confronting models on cosmic ray interactions with particle physics at LHC energies
Inelastic pp collisions are dominated by soft (low momentum transfer) physics
where perturbative QCD cannot be fully applied. A deep understanding of both
soft and semi-hard processes is crucial for predictions of minimum bias and
underlying events of the now coming on line pp Large Hadron Collider (LHC).
Moreover, the interaction of cosmic ray particles entering in the atmosphere is
extremely sensitive to these soft processes and consequently cannot be
formulated from first principles. Because of this, air shower analyses strongly
rely on hadronic interaction models, which extrapolate collider data several
orders of magnitude. A comparative study of Monte Carlo simulations of pp
collisions (at the LHC center-of-mass energy ~ 14 TeV) using the most popular
hadronic interaction models for ultrahigh energy cosmic ray (SIBYLL and QGSJET)
and for collider physics (the PYTHIA multiparton model) is presented. The most
relevant distributions are studied including those observables from diffractive
events with the aim of discriminating between the different models.Comment: 8 pages revtex, 8 figures, added reference
Tunneling from a correlated 2D electron system transverse to a magnetic field
We show that, in a magnetic field parallel to the 2D electron layer, strong
electron correlations change the rate of tunneling from the layer
exponentially. It results in a specific density dependence of the escape rate.
The mechanism is a dynamical Mossbauer-type recoil, in which the Hall momentum
of the tunneling electron is partly transferred to the whole electron system,
depending on the interrelation between the rate of interelectron momentum
exchange and the tunneling duration. We also show that, in a certain
temperature range, magnetic field can enhance rather than suppress the
tunneling rate. The effect is due to the magnetic field induced energy exchange
between the in-plane and out-of-plane motion. Magnetic field can also induce
switching between intra-well states from which the system tunnels, and a
transition from tunneling to thermal activation. Explicit results are obtained
for a Wigner crystal. They are in qualitative and quantitative agreement with
the relevant experimental data, with no adjustable parameters.Comment: 16 pages, 9 figure
Determination of the longitudinal structure function at HERA
Recent results from the HERA experiment H1 on the longitudinal stucture
function of the proton are presented. They include proton structure
function analyses with particular emphasis on those kinematic regions which are
sensitive to . All results can be consistently described within the
framework of perturbative QCD.Comment: 16 pages, 11 figures (requires iopart, iopams and epsfig); Talk
presented in the Intern. Workshop on New Trends in HERA Physics 2001, 17-22
June 2001, Ringberg Castle, Tegernsee, Germany; To appear in the Proceeding
- …