119164398
QGP viscosity at RHIC and the LHC - a 2012 status report
Abstract In this article, we briefly review recent progress related to extracting the quark–gluon plasma (QGP) specific shear viscosity from the flow data measured at Relativistic Heavy-Ion Collider (RHIC) and the Large Hadron Collider (LHC).
119173058
Hadron spectra and elliptic flow for 200 A GeV Au + Au collisions from viscous hydrodynamics coupled to a Boltzmann cascade
It is shown that the recently developed hybrid code VISHNU, which couples a relativistic viscous fluid dynamical description of the quark-gluon plasma (QGP) with a microscopic Boltzmann cascade for the late hadronic rescattering stage, yields an excellent description of charged and identified hadron spectra and elliptic flow measured in 200 A GeV Au+Au collisions at the Relativistic Heavy-Ion Collider (RHIC). Using initial conditions that incorporate event-by-event fluctuations in the initial shape and orientation of the collision fireball and values {eta}/s for the specific shear viscosity of the quark-gluon plasma that were recently extracted from the measured centrality dependence of the eccentricity-scaled, p{sub T}-integrated charged hadron elliptic flow v{sub 2,ch}/{epsilon}, we obtain universally good agreement between theory and experiment for the p{sub T} spectra and differential elliptic flow v{sub 2}(p{sub T}) for both pions and protons at all collision centralities.
24865622
Energy and momentum deposited into a QCD medium by a jet shower.
For a hard parton moving through a dense QCD medium, we compute self-consistently the energy loss and the fraction deposited into the medium due to showering and rescattering of the shower, assuming weak coupling between probe and medium. The same transport coefficients thus determine both the energy loss and its deposition into the medium. This allows a parameter free calculation of the latter once the former are computed or measured. We compute them for a weakly interacting medium. Assuming a short thermalization time for the deposited energy, we determine the medium's hydrodynamical response and obtain a conical pattern that is strongly enhanced by showering.
17687299
Extracting the QGP viscosity from RHIC data—a status report from viscous hydrodynamics
We report recent progress on causal viscous hydrodynamics for relativistic heavy-ion collisions. For fixed specific shear viscosity η/s, uncertainties in the elliptic flow arising from initial conditions, equation of state, bulk viscosity and numerical viscosity, and the treatment of the highly viscous hadronic stage and freeze-out procedure are analysed. A comparison of current viscous hydrodynamic results with experimental data yields a robust upper limit .
13994103
Causal relativistic hydrodynamics for viscous fluids
We report on recent results from VISH2+1, a code that solves the relativistic Israel–Stewart equations for causal viscous hydrodynamics for heavy-ion collisions with longitudinal boost invariance. We find that even 'minimal' shear viscosity η/s = /(4π) leads to a large reduction of elliptic flow compared to ideal fluid dynamics. We explore systematically the sensitivity of this reduction to the equation of state, system size, initial conditions and the microscopic relaxation time in different formulations of the Israel–Stewart equations.
118634430
The QGP shear viscosity–elusive goal or just around the corner?
With the new viscous hydrodynamic + hadron cascade hybrid code VISHNU, a rather precise () extraction of the quark gluon plasma (QGP) shear viscosity (?/s)QGP from heavy-ion elliptic flow data is possible if the initial eccentricity of the collision fireball is known with <5% accuracy. At this point, eccentricities from initial state models differ by up to 20%, leading to an uncertainty for (?/s)QGP. It is shown that a simultaneous comparison of elliptic and triangular flow, v2 and v3, puts strong constraints on initial state models and can largely eliminate the present uncertainty in (?/s)QGP. The variation of the differential elliptic flow v2(pT) for identified hadrons between RHIC and LHC energies provides additional tests of the evolution model.
16178268
The radio lightcurve of SN 2008iz in M82 revealed by Urumqi observations
We report on a set of 5 GHz Urumqi observations of the galaxy M82, made between August 2005 and May 2009. From the resulting flux densities, we detect a strong flare, starting in March or April and peaking in June 2008. We identify this flare with supernova SN 2008iz. The time sampling of the radio light curve allows us to obtain information on the precursor mass-loss rate, the strength of the magnetic field in the radiating region, the explosion date, and the deceleration of the expanding shock. We also check the possible contribution of synchrotron self absorption to the radio light curve and compare our model with other observations of the supernova at 22 GHz.
119312762
Structure, spectra and variability of some GPS radio sources
We report the results of multifrequency-VLBI observations of GHz-Peaked-Spectrum (GPS) radio sources. The VLBI structure and component spectra of some GPS sources are presented. Our VLBI results show that about 80% of the GPS galaxies exhibit a compact double or CSO-like structure, white the GPS quasars tend to show a core-jet. The component spectra of the GPS galaxies are often steep/convex, and the core has a flat spectrum but it is usually hidden or weak. In addition, we studied the variability of GPS sources by comparing new flux density measures, acquired with the Urumqi 25-m telescope at 4.85 GHz, with previous 87GB data. The results show that 44% of the GPS quasars varied higher than 10% in passed 20 years, while the fraction is only 12% for the GPS galaxies meaning that the GPS quasars are much more variable than GPS galaxies. In total, 25% of GPS sources show > 10% variability at 4.85 GHz in our sample.
119692613
Measurement of the $B_c^{\pm}$ meson lifetime using $B_c^{\pm} \to J/\psi~\pi^{\pm}$ decays
This thesis describes a measurement of the lifetime of the B± c meson in an exclusive decay channel B± c → J/ ψ π±, where the J/ψ decays as J/ψ → μ+μ- . The measurement uses a data sample with an integrated luminosity of 6.7 fb-1 collected at CDF. This is the first measurement of the B± c meson lifetime in a hadronic channel and the measured lifetime, τ = 0.449 +0.054 -0.048(stat:) ± 0.019(syst:) ps, is in good agreement with previous results obtained from semileptonic decay channel and confirms previous measurements and theoretical predictions.
73710703
A seasonal cycle and an abrupt change in the variability characteristics of the intraday variable source S4 0954+65
The BLLac object S4 0954+65 is one of the main targets of the Urumqi monitoring program targeting intraday variable (IDV) sources. Between August 2005 and December 2009, the source was included in 41 observing sessions, carried out at a frequency of 4.8 GHz. The time analysis of the collected light curves, performed by applying both a structure function analysis and a specifically developed wavelet-based algorithm, discovered an annual cycle in the variability timescales, suggesting that there is a fundamental contribution by interstellar scintillation to the IDV pattern of the source. The combined use of the two analysis methods also revealed that there was a dramatic change in the variability characteristics of the source between February and March 2008, at the starting time of a strong outburst phase. The analysis' results suggest that the flaring state of the source coincides with the appearance of multiple timescales in its light curves, indicating that changes in the structure of the relativistically moving emitting region may strongly influence the variability observed on IDV timescales.
119209019
On the influence of the Sun on the rapid variability of compact extragalactic sources
Since December 2004, we have performed a program to monitor intraday variable sources at a frequency of 5 GHz at the Urumqi Observatory. We present our analysis of the variability characteristics of the flat-spectrum radio source AO 0235 + 164, which detects an annual cycle in the variability amplitude. This appears to correlate with the solar elongation of the source. A thorough analysis of the results of the MASIV IDV survey - which provides the variability characteristics of a large sample of compact radio sources confirms that there is a small but detectable component of the observed fractional modulation that increases with decreasing solar elongation. We discuss the hypothesis that the phenomenon is related to interplanetary scintillation.