157 research outputs found
Spatio-temporal dynamics of quantum-well excitons
We investigate the lateral transport of excitons in ZnSe quantum wells by
using time-resolved micro-photoluminescence enhanced by the introduction of a
solid immersion lens. The spatial and temporal resolutions are 200 nm and 5 ps,
respectively. Strong deviation from classical diffusion is observed up to 400
ps. This feature is attributed to the hot-exciton effects, consistent with
previous experiments under cw excitation. The coupled transport-relaxation
process of hot excitons is modelled by Monte Carlo simulation. We prove that
two basic assumptions typically accepted in photoluminescence investigations on
excitonic transport, namely (i) the classical diffusion model as well as (ii)
the equivalence between the temporal and spatial evolution of the exciton
population and of the measured photoluminescence, are not valid for
low-temperature experiments.Comment: 8 pages, 6 figure
Modified Fragmentation Function in Heavy Ion Collisions at RHIC via Direct photon-Jet Measurements
The presented results are the first measurements at RHIC for direct
-charged hadron azimuthal correlations in heavy ion collisions. We use
these correlations to study the color charge density of the medium through the
medium-induced modification of high-p parton fragmentation. Azimuthal
correlations of direct photons at high transverse energy (8 p 16
GeV) with away-side charged hadrons of transverse momentum (3 p 6
GeV/c) have been measured over a broad range of centrality for
collisions and collisions at = 200 GeV in the STAR
experiment. A transverse shower shape analysis in the STAR Barrel
Electromagnetic Calorimeter Shower Maximum Detector is used to discriminate
between the direct photons and photons from the decays of high p .
The per-trigger away-side yield of direct is smaller than from
trigger at the same centrality class. Within the current uncertainty
the I of direct and are similar.Comment: 5 pages, 4 figures, 3rd International Conference on Hard and Electro-
Magnetic Probes of High-Energy Nuclear Collision
Study of Decays
We report on a study of decays using
29.1 fb of annihilation data recorded at the
resonance with the Belle detector at the KEKB storage ring. Making no
assumptions about the intermediate mechanism, the branching fractions for
and are
determined to be and respectively. An analysis of candidates yields to the first observation of the color-suppressed
hadronic decay with the branching fraction . We measure the ratio of branching fractions
= 1.6 0.8.Comment: 13 pages, LaTex, 4 figures, submitted to Phys. Lett.
Measurements of Branching Fractions and Decay Amplitudes in B-> J/\psi K^* decays
The branching fractions and the decay amplitudes of B -> J/psi K^* decays are
measured in a 29.4/fb data sample collected with the Belle detector at the KEKB
electron-positron collider. The decay amplitudes of helicity states of the
J/psi K^* system are determined from the full angular distribution of the final
state particles in the transversity basis. The branching fractions are measured
to be (1.29\pm0.05\pm0.13) \times 10^{-3} for neutral mesons and
(1.28\pm0.07\pm0.14) \times 10^{-3} for charged mesons. The measured
longitudinal and transverse (perpendicular to the transversity plane)
amplitudes are |A_0|^2 = 0.62\pm0.02\pm0.03 and |A_{\perp}|^2 =
0.19\pm0.02\pm0.03, respectively. The value of |A_{\perp}|^2 shows that the CP
even component dominates in the B^0 \to J/\psi K^{*0}(K_S\pi^0) decay.Comment: 17 pages, 3 figures, 5 tables, to appear in Phys. Lett.
Measurement of the inclusive semileptonic branching fraction of B mesons and |Vcb|
We present a measurement of the electron spectrum from inclusive semileptonic
{\it B} decay, using 5.1 fb of data collected with the
Belle detector. A high-momentum lepton tag was used to separate the
semileptonic {\it B} decay electrons from secondary decay electrons. We
obtained the branching fraction, , with minimal model dependence.
From this measurement, we derive a value for the Cabibbo-Kobayashi-Maskawa
matrix element .Comment: 16 pages, 3 figures, 3 table
Evolution of the differential transverse momentum correlation function with centrality in Au+Au collisions at GeV
We present first measurements of the evolution of the differential transverse
momentum correlation function, {\it C}, with collision centrality in Au+Au
interactions at GeV. {\it C} exhibits a strong dependence
on collision centrality that is qualitatively similar to that of number
correlations previously reported. We use the observed longitudinal broadening
of the near-side peak of {\it C} with increasing centrality to estimate the
ratio of the shear viscosity to entropy density, , of the matter formed
in central Au+Au interactions. We obtain an upper limit estimate of
that suggests that the produced medium has a small viscosity per unit entropy.Comment: 7 pages, 4 figures, STAR paper published in Phys. Lett.
Percolation of Color Sources and the Shear Viscosity of the QGP in Central A-A Collisions at RHIC and LHC Energies
The Color String Percolation Model (CSPM) is used to determine the shear
viscosity to entropy ratio () of the Quark-Gluon Plasma (QGP) produced
in Au-Au collisions at = 200 GeV at RHIC and Pb-Pb at
= 2.76 TeV at LHC. The relativistic kinetic theory relation for
is evaluated using CSPM values for the temperature and the mean free
path of the QGP constituents. The experimental charged hadron transverse
momentum spectrum is used to determine the percolation density parameter
in Au-Au collisions (STAR). For Pb-Pb at = 2.76 TeV
values are obtained from the extrapolation at RHIC energy. The value of
is 0.2040.020 and 0.2620.026 at the CSPM initial
temperatures of 193.63 MeV (RHIC) and 262.2 13 MeV (LHC)
respectively. These values are 2.5 and 3.3 times the AdS/CFT conjectured lower
bound . We compare the CSPM analytic expression with weak
coupling (wQGP) and strong coupling (sQGP) calculations. This indicates that
the QGP is a strongly coupled fluid in the phase transition region.Comment: 4 pages, 3 figures Accepted for publication in European Physical
Journal C (Particles & Fields
Centrality dependence of charged particle production at large transverse momentum in Pb-Pb collisions at TeV
The inclusive transverse momentum () distributions of primary
charged particles are measured in the pseudo-rapidity range as a
function of event centrality in Pb-Pb collisions at
TeV with ALICE at the LHC. The data are presented in the range
GeV/ for nine centrality intervals from 70-80% to 0-5%.
The Pb-Pb spectra are presented in terms of the nuclear modification factor
using a pp reference spectrum measured at the same collision
energy. We observe that the suppression of high- particles strongly
depends on event centrality. In central collisions (0-5%) the yield is most
suppressed with at -7 GeV/. Above
GeV/, there is a significant rise in the nuclear modification
factor, which reaches for GeV/. In
peripheral collisions (70-80%), the suppression is weaker with almost independently of . The measured nuclear
modification factors are compared to other measurements and model calculations.Comment: 17 pages, 4 captioned figures, 2 tables, authors from page 12,
published version, figures at
http://aliceinfo.cern.ch/ArtSubmission/node/284
Measurement of \chi_{c2} Production in Two-Photon Collisions
The production of the \chi_{c2} charmonium state in two-photon collisions has
been measured with the Belle detector at the KEKB e^+e^- collider. A clear
signal for \chi_{c2} --> \gamma J/\psi, J/\psi --> l^+l^- is observed in a
32.6fb^{-1} data sample accumulated at center-of-mass energies near 10.6GeV,
and the product of its two-photon decay width and branching fraction is
determined to be \Gamma_{\gamma\gamma}(\chi_{c2})B(\chi_{c2} --> \gamma J/\psi)
B(J/\psi --> l^+l^-)= 13.5 +/- 1.3(stat.) +/- 1.1(syst.)eV.Comment: 16 pages, 5 figures, to be submitted to Phys. Lett.
Search for leptophobic Z ' bosons decaying into four-lepton final states in proton-proton collisions at root s=8 TeV
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