23,267 research outputs found
Scaling Violation through Squark and Light Gluino Production
In the light gluino scenario, squarks in the 100 GeV mass region can be
copiously produced at the Tevatron without a second heavy particle. Their
subsequent dijet decay into quark plus gluino leads to non-scaling structure in
the inclusive jet X_T distribution. The expected behavior is similar to recent
observations.Comment: 7 pages, LaTeX, 1 EPSF figur
Knight Field Enabled Nuclear Spin Polarization in Single Quantum Dots
We demonstrate dynamical nuclear spin polarization in the absence of an
external magnetic field, by resonant circularly polarized optical excitation of
a single electron or hole charged quantum dot. Optical pumping of the electron
spin induces an effective inhomogeneous magnetic (Knight) field that determines
the direction along which nuclear spins could polarize and enables nuclear-spin
cooling by suppressing depolarization induced by nuclear dipole-dipole
interactions. Our observations suggest a new mechanism for spin-polarization
where spin exchange with an electron reservoir plays a crucial role. These
experiments constitute a first step towards quantum measurement of the
Overhauser field.Comment: 5 pages, 3 figure
Shadowing Effects on the Nuclear Suppression Factor, R_dAu, in d+Au Interactions
We explore how nuclear modifications to the nucleon parton distributions
affect production of high transverse momentum hadrons in deuteron-nucleus
collisions. We calculate the charged hadron spectra to leading order using
standard fragmentation functions and shadowing parameterizations. We obtain the
d+Au to pp ratio both in minimum bias collisions and as a function of
centrality. The minimum bias results agree reasonably well with the BRAHMS data
while the calculated centrality dependence underestimates the data and is a
stronger function of p_T than the data indicate.Comment: 18 pages, 3 figures, final version, Phys. Rev. C in pres
Domain evolution of BaTiO3 ultrathin films under electric field: a first-principles study
A first-principles-derived method is used to study the morphology and
electric-field-induced evolution of stripe nanodomains in (001) BaTiO3 (BTO)
ultrathin films, and to compare them with those in (001) Pb(Zr,Ti)O3 (PZT)
ultrathin films. The BaTiO3 systems exhibit 180o periodic stripe domains at
null electric field, as in PZT ultrathin films. However, the stripes alternate
along [1-10] in BTO systems versus [010] in PZT systems, and no in-plane
surface dipoles occur in BTO ultrathin films (unlike in PZT materials).
Moreover, the evolution of the 180o stripe domains in the BaTiO3 systems, when
applying and increasing an electric field along [001], involves four regions:
Region I for which the magnitude of the down dipoles (i.e., those that are
antiparallel to the electric field) is reduced, while the domain walls do not
move; Region II in which some local down dipoles adjacent to domain walls
switch their direction, resulting in zigzagged domain walls - with the overall
stripe periodicity being unchanged; Region III in which nanobubbles are
created, then contract along [110] and finally collapse; and Region IV which is
associated with a single monodomain. Such evolution differs from that of PZT
ultrathin films for which neither Region I nor zigzagged domain walls exist,
and for which the bubbles contract along [100]. Discussion about such
differences is provided.Comment: 19 pages, 4 figures, 27 references, submitted to Phys. Rev.
Analyzing powers in inclusive pion production at high energy and the nucleon spin structure
Analyzing powers in inclusive pion production in high energy transversely
polarized proton-proton collisions are studied theoretically in the framework
of the quark recombination model. Calculations by assuming the SU(6)
spin-flavor symmetry for the nucleon structure disagree with the experiments.
We solve this difficulty by taking into account the %We overcome this
difficulty by taking into account the realistic spin distribution functions of
the nucleon, which differs from the SU(6) expectation at large , %but
coincides with a perturbative QCD constraint on the ratio of the unpolarized
valence distributions, as . We also discuss the kaon spin
asymmetry and find in the polarized proton-proton
collisions at large .Comment: 13 pages, 4 figures, late
Microchannels in conventional single-mode fibers
Microchannels are fabricated into conventional single-mode fibers by femtosecond laser processing and chemical etching. Fabrication limitations imposed by the fiber geometry are highlighted and resolved through a simple technique without compromising fabrication flexibility. A microfluidic fiber device consisting of a 4 μm wide microchannel that intersects the fiber core for refractive index sensing is further demonstrated. © 2006 Optical Society of America
Determining the strange and antistrange quark distributions of the nucleon
The difference between the strange and antistrange quark distributions,
\delta s(x)=s(x)-\sbar(x), and the combination of light quark sea and strange
quark sea, \Delta (x)=\dbar(x)+\ubar(x)-s(x)-\sbar(x), are originated from
non-perturbative processes, and can be calculated using non-perturbative models
of the nucleon. We report calculations of and using
the meson cloud model. Combining our calculations of with
relatively well known light antiquark distributions obtained from global
analysis of available experimental data, we estimate the total strange sea
distributions of the nucleon.Comment: 4 pages, 3 figures; talk given by F.-G. at QNP0
Self-consistent nonlinear kinetic simulations of the anomalous Doppler instability of suprathermal electrons in plasmas
Suprathermal tails in the distributions of electron velocities parallel to the magnetic field are found in many areas of plasma physics, from magnetic confinement fusion to solar system plasmas. Parallel electron kinetic energy can be transferred into plasma waves and perpendicular gyration energy of particles through the anomalous Doppler instability (ADI), provided that energetic electrons with parallel velocities v ≥ (ω + Ωce )/k are present; here Ωce denotes electron cyclotron frequency, ω the wave angular frequency and k the component of wavenumber parallel to the magnetic field. This phenomenon is widely observed in tokamak plasmas. Here we present the first fully self-consistent relativistic particle-in-cell simulations of the ADI, spanning the linear and nonlinear regimes of the ADI. We test the robustness of the analytical theory in the linear regime and follow the ADI through to the steady state. By directly evaluating the parallel and perpendicular dynamical contributions to j · E in the simulations, we follow the energy transfer between
the excited waves and the bulk and tail electron populations for the first time. We find that the ratio Ωce /(ωpe + Ωce ) of energy transfer between parallel and perpendicular, obtained from linear analysis, does not apply when damping is fully included, when we find it to be ωpe /(ωpe + Ωce ); here ωpe denotes the electron plasma frequency. We also find that the ADI can arise beyond the previously expected range of plasma parameters, in particular when Ωce > ωpe . The simulations also exhibit a spectral feature which may
correspond to observations of suprathermal narrowband emission at ωpe detected from low density tokamak plasmas
1.3 mm Polarized emission in the circumstellar disk of a massive protostar
We present the first resolved observations of the 1.3 mm polarized emission from the disk-like structure surrounding the high-mass protostar Cepheus A HW2. These CARMA data partially resolve the dust polarization, suggesting a uniform morphology of polarization vectors with an average position angle of 57° ± 6° and an average polarization fraction of 2.0% ± 0.4%. The distribution of the polarization vectors can be attributed to (1) the direct emission of magnetically aligned grains of dust by a uniform magnetic field, or (2) the pattern produced by the scattering of an inclined disk. We show that both models can explain the observations, and perhaps a combination of the two mechanisms produces the polarized emission. A third model including a toroidal magnetic field does not match the observations. Assuming scattering is the polarization mechanism, these observations suggest that during the first few 104 years of high-mass star formation, grain sizes can grow from1 mm to several 10s μm.Fil: Fernandez Lopez, Manuel. Provincia de Buenos Aires. Gobernación. Comision de Investigaciones Científicas. Instituto Argentino de Radioastronomía. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto Argentino de Radioastronomia; ArgentinaFil: Stephens, I. W.. Harvard-Smithsonian Center for Astrophysics; Estados Unidos. Boston University; Estados Unidos. University of Illinois; Estados UnidosFil: Girart, J. M.. Harvard-Smithsonian Center for Astrophysics; Estados Unidos. Institut de Ciències de l’Espai; EspañaFil: Looney, L.. University of Illinois; Estados UnidosFil: Curiel, S.. Universidad Nacional Autónoma de México; MéxicoFil: Segura Cox, D.. University of Illinois; Estados UnidosFil: Eswaraiah, C.. National Tsing Hua University; República de ChinaFil: Lai, S. P.. National Tsing Hua University; República de Chin
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