534 research outputs found
Femtoscopy of Pb-Pb and pp collisions at the LHC with the ALICE experiment
We report on the results of femtoscopic analysis of Pb-Pb collisions at
sqrt(s_NN)=2.76 TeV and pp collisions at sqrt(s)=0.9, 2.76 and 7 TeV with
identical pions and kaons. Detailed femtoscopy studies in heavy-ion collisions
at SPS and RHIC have shown that emission region sizes ("HBT radii") decrease
with increasing pair transverse momentum k_T, which is understood as a
manifestation of the collective behavior of matter. The trend was predicted to
persist at the LHC. The data from Pb-Pb collisions confirm the existence of a
flowing medium and provide strict constraints on the dynamical models. Similar
analysis is carried out for pp collisions for pions and kaons and qualitative
similarities to heavy-ion data are seen, especially in collisions producing
large number of particles. The observed trends give insight into the soft
particle production mechanism in pp collisions. 3D radii were also found to
universally scale with event multiplicity in heavy-ion collisions. We extend
the range of multiplicities both upwards with the Pb-Pb data and downwards with
the pp data to test the scaling in new areas. In particular the high
multiplicity pp collisions reach particle densities comparable to the ones
measured in peripheral Cu-Cu and Au-Au collisions at RHIC. This allows for the
first time to directly compare freeze-out sizes for systems with very different
initial states.Comment: 8 pages, 5 figures, Proceedings of the Quark Matter 2011 plenary tal
Use of cumulants to quantify uncertainties in the HBT measurements of the homogeneity regions
Let us denote p(x|K) the space density of the points where identical
particles of some kind, e.g. pi+ mesons, with momentum K are produced. When
using the HBT method to determine p(x|K) one encounters ambiguities. We show
that these ambiguities do not affect the even cumulants of the distribution
p(x|K). In particular, the HBT radii of the homogeneity regions, which are
given by the second order cumulants, and the distribution of distances between
the pairs of production points for particles with momentum K can be reliably
measured. The odd cumulants are ambiguous. The are, however, correlated. In
particular, when the average position (K) is known as a function of K there
is no further ambiguity.Comment: LateX, 10 pages, no figure
Evidence for Hydrodynamic Evolution in Proton-Proton Scattering at LHC Energies
In scattering at LHC energies, large numbers of elementary scatterings
will contribute significantly, and the corresponding high multiplicity events
will be of particular interest. Elementary scatterings are parton ladders,
identified with color flux-tubes. In high multiplicity events, many of these
flux tubes are produced in the same space region, creating high energy
densities. We argue that there are good reasons to employ the successful
procedure used for heavy ion collisions: matter is assumed to thermalizes
quickly, such that the energy from the flux-tubes can be taken as initial
condition for a hydrodynamic expansion. This scenario gets spectacular support
from very recent results on Bose-Einstein correlations in scattering at
900 GeV at LHC.Comment: 11 pages, 20 figure
Nernst effect in semi-metals: the meritorious heaviness of electrons
We present a study of electric, thermal and thermoelectric transport in
elemental Bismuth, which presents a Nernst coefficient much larger than what
was found in correlated metals. We argue that this is due to the combination of
an exceptionally low carrier density with a very long electronic
mean-free-path. The low thermomagnetic figure of merit is traced to the
lightness of electrons. Heavy-electron semi-metals, which keep a metallic
behavior in presence of a magnetic field, emerge as promising candidates for
thermomagnetic cooling at low temperatures.Comment: 4 pages, including 4 figure
Optical morphology of distant RATAN-600 radio galaxies from subarcsecond resolution NOT images
We present direct imaging data of 22 ultra steep spectrum radio sources
obtained at (or near) a subarcsecond seeing. The basic sample of 40 double
radio sources was selected from the RATAN-600 catalogue. The FRII-structure has
been confirmed with VLA and preliminary optical identifications which come from
the 6 m-telescope. As the RATAN-600 flux limit at 3.9 GHz ( 10mJy) is
fainter than that of major surveys, the sample may have high- contents. This
is also suggested by the faint magnitudes in the Hubble diagram. The final aim
is to create a homogeneous sample of high- radio galaxies in a well defined
strip around the sky, with faint radio limit and subarcsecond morphology down
to .
We could confirm 16 identifications down to 24. Most of the
extended objects have multicomponent structures as expected from other surveys
of high-redshift radio galaxies. We found five unresolved objects even with a
subarcsecond seeing. Of the remaining six objects, three are extremely faint
and the other three have such a complex environment that further observations
are needed to confirm the optical identification.Comment: A&AS in pres
Polarized triplet production by circularly polarized photons
A process of the pair production by a circularly polarized photon in the
field of unpolarized atomic electron has been considered in the
Weizaecker-Williams approximation. The degree of longitudinal polarization of
positron and electron has been calculated. An exclusive cross-section as well
as a spectral distribution are obtained. We estimate the accuracy of our
calculations at the level of a few percent. We show the identity of the
positron polarization for considered process and for process of pair production
in the screened Coulomb field of nucleus.Comment: 9 pages, 3 picture
Detailed Study of the Ursa Major Supercluster of Galaxies Using the 2MASS and SDSS Catalogs
We study the infrared (K_s band) properties of clusters of galaxies in the
Ursa Major supercluster using data from 2MASS (Two-Micron All-Sky Survey) and
SDSS (Sloan Digital Sky Survey). We identified three large filaments with mean
redshifts of z = 0.051, 0.060, and 0.071. All clusters of the supercluster are
located in these filaments. We determined the total K_s-band luminosities and
masses for 11 clusters of galaxies within comparable physical regions (within a
radius R_200 close to the virial radius) using a homogeneous method. We
constructed a combined luminosity function for the supercluster in this region,
which can be described by the Schechter function with the following parameters:
M*_K = -24^m.50 and \alpha = -0.98. The infrared luminosities of the clusters
of galaxies correlate with their masses; the M/L_K ratios of the systems
increase with their masses (luminosities), with most of the Ursa Major clusters
of galaxies (particularly the richer ones) closely following the relations
derived previously for a large sample of clusters and groups of galaxies. The
total mass-to-infrared-luminosity ratio is 52 M_{\odot}/L_{\odot} for six Abell
clusters and 49 M_{\odot}/L_{\odot} for all of the clusters, except Anon2.Comment: 16 pages, 5 figure
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