Studies of azimuthal dihadron correlations in ultra-central PbPb collisions at $ \sqrt{{{s_{NN }}}} $ =2.76 TeV
A bstractAzimuthal dihadron correlations of charged particles have been measured in PbPb collisions at $ \sqrt{{{s_{NN }}}} $ = 2.76TeV by the CMS collaboration, using data from the 2011 LHC heavy-ion run. The data set includes a sample of ultra-central (0-0.2% centrality) PbPb events collected using a trigger based on total transverse energy in the hadron forward calorimeters and the total multiplicity of pixel clusters in the silicon pixel tracker. A total of about 1.8 million ultra-central events were recorded, corresponding to an integrated luminosity of 120 μb − 1. The observed correlations in ultra-central PbPb events are expected to be particularly sensitive to initial-state fluctuations. The single-particle anisotropy Fourier harmonics, from v2 to v6, are extracted as a function of particle transverse momentum. At higher transverse momentum, the v2 harmonic becomes significantly smaller than the higher-order vn (n ≥ 3). The pT-averaged v2 and v3 are found to be equal within 2%, while higher-order vn decrease as n increases. The breakdown of factorization of dihadron correlations into single-particle azimuthal anisotropies is observed. This effect is found to be most prominent in the ultra-central PbPb collisions, where the initial-state fluctuations play a dominant role. A comparison of the factorization data to hydrodynamic predictions with event-by-event fluctuating initial conditions is also presented.

title: High-$p_T$ triggered dihadron correlations with $v_n$ background subtraction by STAR
abstract: STAR measurements of dihadron correlations with high-pt trigger partcles in Au+Au collisions at 200 GeV are presented with subtraction of v2, v3, and v4 backgrounds. The vn azimuthal anisotropies were measured by the two-particle cumulant method with \eta\ gap to reduce nonflow. The dihadron correlations relative to the v2 as well as the v3 harmonic planes are also presented. Implications of the results are discussed.

title: Analysis method for jet-like three-particle azimuthal correlations
abstract: Jet-like three-particle azimuthal correlations can discriminate various physical scenarios that have been proposed to explain the observed strong modification to two-particle azimuthal correlations. The three-particle correlation analysis is notoriously difficult in heavy-ion collisions due to the large combinatorial backgrounds. We describe the general idea behind the jet-like three-particle azimuthal correlation analysis, with emphasis on the subtraction of the combinatorial backgrounds. We discuss in detail various sources of systematic uncertainties in such an analysis.

title: Effects of nuclear absorption on the antiLambda/antiproton ratio in relativistic heavy ion collisions
abstract: An enhanced antiLambda/antiproton ratio in heavy-ion relative to p+p collisions has been proposed as one of the signatures for the Quark-Gluon Plasma (QGP) formation. A significantly large (antiLambda+antiSigma0+1.1*antiSigma-)/antiproton ratio of 3.5 has been observed in the mid-rapidity and low transverse momentum region in central Au+Au collisions at the nucleon-nucleon center-of-mass energy of 4.9 GeV at the Alternating Gradient Synchrotron (AGS). This is an order of magnitude larger than the values in peripheral Au+Au collisions and p+p collisions at the corresponding energy. By using the Ultra-relativistic Quantum Molecular Dynamics (UrQMD) transport model, we demonstrate that the observed large ratio can be explained by strong absorption of antiprotons (~99.9%) and antiLambdas (~99%) in dense nuclear matter created in central collisions. We find within the model that the initial antiLambda/antiproton ratio, mainly from string fragmentation, does not depend on the collision centrality, and is consistent with that observed in p+p collisions. This suggests that the observed large (antiLambda+antiSigma0+1.1*antiSigma-)/antiproton ratio at the AGS does not necessarily imply the formation of the QGP. We further study the excitation function of the ratio in UrQMD, which may help in the search and study of the QGP.

title: Measurement of WZ and ZZ production in pp collisions at $$\sqrt{s} = 8\,\text {TeV} $$s=8TeV in final states with b-tagged jets
abstract: Measurements are reported of the WZ and ZZ production cross sections in proton-proton collisions at $$\sqrt{s} = 8$$s=8$$\,\text {TeV}$$TeV in final states where one Z boson decays to b-tagged jets. The other gauge boson, either W or Z, is detected through its leptonic decay (either $$\mathrm {W}\rightarrow \mathrm {e}\nu $$W→eν, $$\mathrm {\mu }\nu $$μν or $$\mathrm {Z}\rightarrow \mathrm {e}^+\mathrm {e}^-$$Z→e+e-, $$\mathrm {\mu ^+}\mathrm {\mu ^-}$$μ+μ-, or $${\nu }\overline{\nu ). The results are based on data corresponding to an integrated luminosity of 18.9 fb$$^{-1}$$-1 collected with the CMS detector at the Large Hadron Collider. The measured cross sections, $$\sigma (\mathrm {p}\mathrm {p}\rightarrow \mathrm {W}\mathrm {Z}) = 30.7 \pm 9.3\,\text {(stat.)} \pm 7.1\,\text {(syst.)} \pm 4.1\,\text {(th.)} \pm 1.0\,\text {(lum.)} \,\text {pb} $$σ(pp→WZ)=30.7±9.3(stat.)±7.1(syst.)±4.1(th.)±1.0(lum.)pb and $$\sigma (\mathrm {p}\mathrm {p}\rightarrow \mathrm {Z}\mathrm {Z}) = 6.5 \pm 1.7\,\text {(stat.)} \pm 1.0\,\text {(syst.)} \pm 0.9\,\text {(th.)} \pm 0.2\,\text {(lum.)} \,\text {pb} $$σ(pp→ZZ)=6.5±1.7(stat.)±1.0(syst.)±0.9(th.)±0.2(lum.)pb, are consistent with next-to-leading order quantum chromodynamics calculations.

title: Effects of cluster particle correlations on local parity violation observables
abstract: We investigate effects of cluster particle correlations on two- and three-particle azimuth correlator observables sensitive to local strong parity violation. We use two-particle angular correlation measurements as inputs and estimate the magnitudes of the effects with straightforward assumptions. We found that the measurements of the azimuth correlator observables in the STAR experiment can be entirely accounted for by cluster particle correlations together with a reasonable range of cluster anisotropy in nonperipheral collisions. Our result suggests that new physics, such as local strong parity violation, may not be required to explain the correlator data.

title: Unconventional behavior of Dirac fermions in three-dimensional gauge theory
abstract: We study the unconventional behavior of massless Dirac fermions due to interaction with a U(1) gauge field in two spatial dimensions. At zero chemical potential, the longitudinal and transverse components of gauge interaction are both long-ranged. There is no fermion velocity renormalization since the system respects Lorentz invariance. At finite chemical potential, the Lorentz invariance is explicitly broken by the finite Fermi surface. The longitudinal gauge interaction is statically screened and becomes unimportant, whereas the transverse gauge interaction remains long-ranged and leads to singular renormalization of fermion velocity. The anomalous dimension of fermion velocity is calculated by means of the renormalization group method. We then examine the influence of singular velocity renormalization on several physical quantities, and show that they exhibit different behavior at zero and finite chemical potential.

title: On the minimum mass ratio of W UMa binaries
abstract: Using Eggleton’s stellar evolution code, we study the minimum mass ratio (qmin) of W Ursae Majoris (W UMa) binaries that have different primary masses. It is found that the minimum mass ratio of W UMa binaries decreases with increasing mass of the primary if the primary’s mass is less than about 1.3M⊙, and above this mass the ratio is roughly constant. By comparing the theoretical minimum mass ratio with the observational data, it is found that the existence of low-q systems can be explained by the different structure of the primaries with different masses. This suggests that the dimensionless gyration radius (k 1 ) and thus the structure of the primary is very important in determining the minimum mass ratio. In addition, we investigate the mass loss during the merging process of W UMa systems and calculate the rotation velocities of the single stars formed by the merger of W UMa binaries due to tidal instability. It is found that in the case of the conservation of mass and angular momentum, the merged single stars rotate with a equatorial velocity of about 588 ∼ 819 km s, which is much larger than their break-up velocities (vb). This suggests that the merged stars should extend to a very large radius (3.7∼5.3 times the radii of the primaries) or W UMa systems would lose a large amount of mass (21∼33 per cent of the total mass) during the merging process. If the effect of magnetic braking is considered, the mass loss decreases to be 12∼18 per cent of their total masses. This implies that the significant angular momentum and mass might be lost from W UMa systems in the course of the merging process, and this kind of mass and angular momentum

title: Measurements of spin correlation in top-antitop quark events from proton-proton collisions at √s =7 TeV using the ATLAS detector
abstract: Measurements of spin correlation in top quark pair production are presented using data collected with the ATLAS detector at the LHC with proton-proton collisions at a center-of-mass energy of 7 TeV, corresponding to an integrated luminosity of 4.6 fb(-1). Events are selected in final states with two charged leptons and at least two jets and in final states with one charged lepton and at least four jets. Four different observables sensitive to different properties of the top quark pair production mechanism are used to extract the correlation between the top and antitop quark spins. Some of these observables are measured for the first time. The measurements are in good agreement with the Standard Model prediction at next-to-leading-order accuracy.

title: A neural network clustering algorithm for the ATLAS silicon pixel detector
abstract: A novel technique to identify and split clusters created by multiple charged particles in the ATLAS pixel detector using a set of artificial neural networks is presented. Such merged clusters are a common feature of tracks originating from highly energetic objects, such as jets. Neural networks are trained using Monte Carlo samples produced with a detailed detector simulation. This technique replaces the former clustering approach based on a connected component analysis and charge interpolation. The performance of the neural network splitting technique is quantified using data from proton-proton collisions at the LHC collected by the ATLAS detector in 2011 and from Monte Carlo simulations. This technique reduces the number of clusters shared between tracks in highly energetic jets by up to a factor of three. It also provides more precise position and error estimates of the clusters in both the transverse and longitudinal impact parameter resolution.

title: A compact RF power coupler for the NLC linac
abstract: A high power RF coupler connecting WR90 rectangular waveguide to the disc loaded waveguide accelerating structure of NLC is described. The coupler design is symmetric and free of beam deflecting field components. It makes an efficient electrical match between the rectangular waveguide power feed and the accelerator structure while keeping electrical surface field enhancement low. Electrical and mechanical design of the coupler are presented as well as quantitative comparisons between numerical simulation and low power cold test models.