Sussing merger trees: the Merger Trees Comparison Project
Merger trees follow the growth and merger of dark-matter haloes over cosmic history. As well as giving important insights into the growth of cosmic structure in their own right, they provide an essential backbone to semi-analytic models of galaxy formation. This paper is the first in a series to arise from the Sussing Merger Trees Workshop in which 10 different tree-building algorithms were applied to the same set of halo catalogues and their results compared. Although many of these codes were similar in nature, all algorithms produced distinct results. Our main conclusions are that a useful merger-tree code should possess the following features: (i) the use of particle IDs to match haloes between snapshots; (ii) the ability to skip at least one, and preferably more, snapshots in order to recover subhaloes that are temporarily lost during merging; (iii) the ability to cope with (and ideally smooth out) large, temporary fluctuations in halo mass. Finally, to enable different groups to communicate effectively, we defined a common terminology that we used when discussing merger trees and we encourage others to adopt the same language. We also specified a minimal output format to record the results.

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: 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: 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: 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: 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: Two particle correlations with photon and pi0 triggers in ALICE at LHC
abstract: Comparing the measurements of the hadronic final state from partonic showers in proton-proton and heavy-ion collisions will reveal the modifications generated by the medium on partons produced in hard scatterings. This can be achieved by selecting the hard processes in which there is a direct photon in the final state. The experimental technique consists in tagging events with a well identified high energy direct photon and in measuring the azimuthal angle correlation with charged hadrons. To establish a reference measurement for heavy-ion collisions, proton-proton collision data collected with ALICE have been analyzed. Preliminary results are presented together with photon and $\pi^{0}$-charged hadrons correlations showing the characteristic di-jet pattern from where the partonic momentum $k_{T}$ is extracted.

title: Transverse momentum spectra of charged particles in proton-proton collisions at sqrt(s) = 900 GeV with ALICE at the LHC
abstract: The inclusive charged particle transverse momentum distribution is measured in proton-proton collisions at $\sqrt{s} = 900$~GeV at the LHC using the ALICE detector. The measurement is performed in the central pseudorapidity region $(|\eta|_{\rm INEL}=0.483\pm0.001$~(stat.)~$\pm0.007$~(syst.)~GeV/$c$ and $\left_{\rm NSD}=0.489\pm0.001$~(stat.)~$\pm0.007$~(syst.)~GeV/$c$, respectively. The data exhibit a slightly larger $\left$ than measurements in wider pseudorapidity intervals. The results are compared to simulations with the Monte Carlo event generators PYTHIA and PHOJET

title: CHARM QUARK ENERGY LOSS IN DENSE MATTER WITHIN THE LIGHT-CONE PATH INTEGRAL APPROACH
abstract: The energy loss of heavy quarks traversing color dense matter is calculated with an analytical formula derived within the light-cone path integral (LCPI) approach. We find that the energy loss mechanism is dominated by gluon radiation and induces a suppression pattern of charm quark different from the suppression of light quarks. We find also that this radiative energy-loss is proportional to L2 for energetic quarks, but changes to a L dependance with decreasing quark energy, where L is the length of the traversed medium.

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: Photon — jet correlations measurement with ALICE at LHC: a feasibility study in proton-proton collisions
abstract: The feasibility to measure the imbalance distribution between direct isolated photons and charged hadrons is discussed within the context of the ALICE experiment at LHC. The case of proton-proton collisions at $\sqrt s $ TeV is studied with Monte-Carlo simulations. Performances for prompt photon identification and rejection of the π0 decay photons are presented. Such a measurement will offer the possibilty to probe the medium formed in AA collisions with jets of energies below 50 GeV, which cannot be reconstructed in the heavy-ion environment.