Lattice QCD with Optimal Domain-Wall Fermion: Light Meson Spectroscopy
We perform lattice simulations of two flavors QCD using the optimal domain-wall fermion, in which the chiral symmetry is preserved to a good precision ($ m_{res} \sim 0.3 $ MeV) on the $ 16^3 \times 32 $ lattice ($ L \sim 2 $ fm) with inverse lattice spacing $ a^{-1} \sim 1.8 $ GeV, and $ N_s = 16 $ in the fifth dimension, for eight sea quark masses corresponding to the pion masses in the range 210-500 MeV. We present our first results of the mass and the decay constant of the pseudoscalar meson, which are in good agreement with the next-to-leading order chiral perturbation theory for $ M_\pi < 450 $ MeV, and from which we determine the low-energy constants $ f $, $ \Sigma $, $ \bar{l}_3 $ and $ \bar{l}_4 $. At the physical pion mass $ M_{\pi} = 135$ MeV, we obtain the pion decay constant $f_\pi =133(1)(2)$ MeV, and the average up and down quark mass $m_{ud}^{\bar{\rm MS}}(\mathrm{2 GeV})=4.09(7)(11)$ MeV, where the first error is statistical, and the second error is systematic due to the truncation of the higher order corrections and the uncertainty in the determination of the lattice spacing. Furthermore, we also obtain the chiral condensate $ \Sigma^{\bar{{\mathrm{MS}}}}(2 GeV) = [250(4)(7) MeV]^3 $.

title: SUSSING MERGER TREES: the influence of the halo finder
abstract: Merger tree codes are routinely used to follow the growth and merger of dark matter haloes in simulations of cosmic structure formation. Whereas in Srisawat et. al. we compared the trees built using a wide variety of such codes, here we study the influence of the underlying halo catalogue upon the resulting trees. We observe that the specifics of halo finding itself greatly influences the constructed merger trees. We find that the choices made to define the halo mass are of prime importance. For instance, amongst many potential options different finders select self-bound objects or spherical regions of defined overdensity, decide whether or not to include substructures within the mass returned and vary in their initial particle selection. The impact of these decisions is seen in tree length (the period of time a particularly halo can be traced back through the simulation), branching ratio (essentially the merger rate of subhaloes) and mass evolution. We therefore conclude that the choice of the underlying halo finder is more relevant to the process of building merger trees than the tree builder itself. We also report on some built-in features of specific merger tree codes that (sometimes) help to improve the quality of the merger trees produced.

title: RHAPSODY. I. STRUCTURAL PROPERTIES AND FORMATION HISTORY FROM A STATISTICAL SAMPLE OF RE-SIMULATED CLUSTER-SIZE HALOS
abstract: We present the rst results from the Rhapsody cluster re-simulation project: a sample of 96 \zoom-in" simulations of dark matter halos of 10 14:8 0:05 h 1 M , selected from a 1 h 3 Gpc 3 volume. This simulation suite is the rst to resolve this many halos with 5 10 6 particles per halo in the cluster mass regime, allowing us to statistically characterize the distribution of and correlation between halo properties at xed mass. We focus on the properties of the main halos and how they are aected by formation history, which we track back to z = 12, over ve decades in mass. We give particular attention to the impact of the formation history on the density proles of the halos. We nd that the deviations from the Navarro{Frenk{White (NFW) model and the Einasto model depend on formation time. Late-forming halos tend to have considerable deviations from both models, partly due to the presence of massive subhalos, while early-forming halos deviate less but still signicantly from the NFW model and are better described by the Einasto model. We nd that the halo shapes depend only moderately on formation time. Departure from spherical symmetry impacts the density proles through the anisotropic distribution of massive subhalos. Further evidence of the impact of subhalos is provided by analyzing the phase-space structure. A detailed analysis of the properties of the subhalo population in Rhapsody is presented in a companion paper.

title: TWQCD's dynamical DWF project
abstract: We present an overview of our project of simulation of unquenched lattice QCD with optimal domain-wall quarks, using a GPU cluster currently constituting of 16 units of Nvidia Tesla S1070 plus 64 graphic cards with Nvidia GTX285 (total 128 GPUs with 128 Teraflops peak), attaining sustained computing power of 15.36 Teraflops. The first production run in two-flavor QCD is on-going, using the Iwasaki gauge action on a set of lattices with sizes $ 16^3 \times (32,10,8,6,4) \times (16,32) $ at the lattice spacing $ a \sim 0.1$ fm, with eight sea quark masses down to $ m_\pi \simeq 200 $ MeV. We outline our simulation algorithm, and describe the present status of the production run. Preliminary results of pseudoscalar mass and decay constant are also presented.

title: GPU-Based Conjugate Gradient Solver for Lattice QCD with Domain-Wall Fermions
abstract: We present the first GPU-based conjugate gradient (CG) solver for lattice QCD with domain-wall fermions (DWF). It is well-known that CG is the most time-consuming part in the Hybrid Monte Carlo simulation of unquenched lattice QCD, which becomes even more computational demanding for lattice QCD with exact chiral symmetry. We have designed a CG solver for the general 5-dimensional DWF operator on NVIDIA CUDA architecture with mixed-precision, using the defect correction as well as the reliable updates algorithms. We optimize our computation by even-odd preconditioning in the 4D space-time lattice, plus several innovative techniques for CUDA kernels. For NVIDIA GeForce GTX 285/480, our CG solver attains 180/233 Gflops (sustained).

title: The Dependence of Subhalo Abundance on Halo Concentration
abstract: Hierarchical structure formation implies that the number of subhalos within a dark matter halo depends not only on halo mass, but also on the formation history of the halo. This dependence on the formation history, which is highly correlated with halo concentration, can account for the super-Poissonian scatter in subhalo occupation at a fixed halo mass that has been previously measured in simulations. Here we propose a model to predict the subhalo abundance function for individual host halos that incorporates both halo mass and concentration. We combine results of cosmological simulations with a new suite of zoom-in simulations of Milky Way-mass halos to calibrate our model. We show that the model can successfully reproduce the mean and the scatter of subhalo occupation in these simulations. The implications of this correlation between subhalo abundance and halo concentration are further investigated. We also discuss cases in which inferences about halo properties can be affected if this correlation between subhalo abundance and halo concentration is ignored; in these cases, our model would give a more accurate inference. We propose that with future deep surveys, satellite occupation in the low-mass regime can be used to verify the existence of halo assembly bias.

title: Hard Probes in CMS
abstract: This paper reviews recent experimental results on hard probes in heavy-ion collisions from the CMS Collaboration. These studies include various energy loss phenomena (jet nuclear modification factors, tagged b-quark jets, di-jet and photon-jet energy imbalance); observables characterizing jet properties like jet fragmentation function and jet shapes; and measurements of high-pT charged hadrons from jet fragmentation (nuclear modification factors up to 100 GeV/c. All of the presented results utilize the high statistics PbPb data, about 150 μb−1, collected in late 2011 at the LHC.

title: Gamma-hadrons and hadrons-hadrons correlation measurements in pp collisions at 7TeV to study jet fragmentation with ALICE at LHC

title: Measurements of jet shape in 2.76 TeV PbPb collisions with CMS
abstract: The fragmentation properties of inclusive jets produced in PbPb collisions at a center of mass energy of s N N = 2.76 TeV are characterized by measuring differential jet shapes. The jet shape is defined as the fractional jet transverse momentum carried by charged particles emitted at a distance r from the jet axis, with jets reconstructed using the anti- k T clustering algorithm. The jet shapes are measured for reconstructed jet p T , jet > 100 GeV / c and charged hardons with p T > 1 GeV / c . Different background subtraction methods are employed to remove the contribution from the underlying events. The results obtained for PbPb collisions as a function of collision centrality are compared to reference distributions from pp data collected at the same collision energy. For the most central collisions, the jet shapes are found to be modified at large distance from the jet axis.

title: Two particle correlations: A probe of the LHC QCD medium
abstract: The properties of γ-jet pairs emitted in heavy-ion collisions provide an accurate mean to perform a tomographic measurement of the medium created in the collision through the study of the medium modified jet properties. The idea is to measure the distribution of hadrons emitted on the opposite side of the direct photon. The feasibility of such measurements is studied by applying the approach on the simulation data, we have demonstrated that this method allows us to measure, with a good approximation, both the jet fragmentation and the back-to-back azimuthal alignment of the direct photon and the jet. Comparing these two observables measured in pp collisions with the ones measured in AA collisions reveals the modifications induced by the medium on the jet structure and consequently allows us to infer the medium properties. In this contribution, we discuss a first attempt of such measurements applied to real proton-proton data from the ALICE experiment.

title: Potential physics measurement with ALICE electromagnetic calorimeters
abstract: We present the two electromagnetic calorimeters of the ALICE (A Large Ion Collider Experiment) experiment at LHC (Large Hadron Collider). One is the high-resolution PHOton Spectrometer (PHOS) made of lead tungsten crystals and the other is the ElectroMagnetic Calorimeter (EMCal), a Lead-Scintillator sampling calorimeter. They are dedicated to the measurement and identification of direct photons, light neutral mesons such as π 0 , η and ω ( 782 ) , and jets emitted in proton-proton and heavy-ion collisions at the LHC energies. The PHOS is capable of precisely detecting photons with momentum range between 0.1 GeV/c and 100 GeV/c and the EMCal can extend the prompt photon and light neutral meson momentum measurement beyond 200 GeV/c. The objective of the study is to explore the physics of strongly interacting QCD matter under extreme conditions of energy density.