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Register a new userCasimir force measurements in Au-Au and Au-Si cavities at low temperature
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We report on measurements of the Casimir force in a sphere-plane geometry using a cryogenic force microscope to move the force probe in situ over different materials. We show how the electrostatic environment of the interacting surfaces plays an important role in weak force measurements and can overcome the Casimir force at large distance. After minimizing these parasitic forces, we measure the Casimir force between a gold-coated sphere and either a gold-coated or a heavily doped silicon surface in the 100-400 nm distance range. We compare the experimental data with theoretical predictions and discuss the consequence of a systematic error in the scanner calibration on the agreement between experiment and theory. The relative force over the two surfaces compares favorably with theory at short distance, showing that this Casimir force experiment is sensitive to the dielectric properties of the interacting surfaces.
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We report on temperature measurements of nuclear systems formed in the Au+Au collisions at incident energies of 50, 100, 150, 200 and 1000 A MeV. The target spectator matter was studied at the highest energy and the interacting zone (participants) at the lower ones.The temperature deduced from the isotope ratios was compared with the one deduced via the excited states population. An unexpected disagreement was found between the two measurements.
Flow harmonics ($v_n$) in the Fourier expansion of the azimuthal distribution of particles are widely used to quantify the anisotropy in particle emission in high-energy heavy-ion collisions. The symmetric cumulants, $SC(m,n)$, are used to measure the correlations between different orders of flow harmonics. These correlations are used to constrain the initial conditions and the transport properties of the medium in theoretical models. In this Letter, we present the first measurements of the four-particle symmetric cumulants in Au+Au collisions at $sqrt{s_{NN}}$ = 39 and 200 GeV from data collected by the STAR experiment at RHIC. We observe that $v_{2}$ and $v_{3}$ are anti-correlated in all centrality intervals with similar correlation strengths from 39 GeV Au+Au to 2.76 TeV Pb+Pb (measured by the ALICE experiment). The $v_{2}$-$v_{4}$ correlation seems to be stronger at 39 GeV than at higher collision energies. The initial-stage anti-correlations between second and third order eccentricities are sufficient to describe the measured correlations between $v_{2}$ and $v_{3}$. The best description of $v_{2}$-$v_{4}$ correlations at $sqrt{s_{NN}}$ = 200 GeV is obtained with inclusion of the systems nonlinear response to initial eccentricities accompanied by the viscous effect with $eta/s$ $>$ 0.08. Theoretical calculations using different initial conditions, equations of state and viscous coefficients need to be further explored to extract $eta/s$ of the medium created at RHIC.
When gold is deposited on Si(553), the surface self-assembles to form a periodic array of steps with nearly perfect structural order. In scanning tunneling microscopy these steps resemble quasi-one-dimensional atomic chains. At temperatures below ~50 K the chains develop tripled periodicity. We recently predicted, on the basis of density-functional theory calculations at T=0, that this tripled periodicity arises from the complete polarization of the electron spin on every third silicon atom along the step; in the ground state these linear chains of silicon spins are antiferromagnetically ordered. Here we explore, using ab-initio molecular dynamics and kinetic Monte Carlo simulations, the behavior of silicon spin chains on Si(553)-Au at finite temperature. Thermodynamic phase transitions at T>0 in one-dimensional systems are prohibited by the Mermin-Wagner theorem. Nevertheless we find that a surprisingly sharp onset occurs upon cooling---at about 30 K for perfect surfaces and at higher temperature for surfaces with defects---to a well-ordered phase with tripled periodicity, in good agreement with experiment.
We report measurements of charmed hadron production from hadronic ($D^{0}rightarrow Kpi$) and semileptonic ($mu$ and $e$) decays in 200 GeV Au+Au collisions at RHIC. Analysis of the spectra indicates that charmed hadrons have a different radial flow pattern from light or multi-strange hadrons. Charm cross sections at mid-rapidity are extracted by combining the three independent measurements, covering the transverse momentum range that contributes to $sim$90% of the integrated cross section. The cross sections scale with number of binary collisions of the initial nucleons, a signature of charm production exclusively at the initial impact of colliding heavy ions. The implications for charm quark interaction and thermalization in the strongly interacting matter are discussed.
Identified charged particle spectra of $pi^{pm}$, $K^{pm}$, $p$ and $pbar$ at mid-rapidity ($|y|<0.1$) measured by the $dedx$ method in the STAR-TPC are reported for $pp$ and d+Au collisions at $snn = 200$ GeV and for Au+Au collisions at 62.4 GeV, 130 GeV, and 200 GeV. ... [Shortened for arXiv list. Full abstract in manuscript.]
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