Rene Brun

CERN | CERN · Physics Department (PH)

Any attempt to predict any future subject includes in general two components: extrapolations of the current trends and discussions about possible surprises. The extrapolation of the trends based on the latest function derivatives seems to be the easiest one. However, one has to be careful with fashionable trends and associated hype peaks. When one...

The development of the GeantV Electromagnetic (EM) physics package has evolved following two necessary paths towards code modernization. A first phase required the revision of the main electromagnetic physics models and their implementation. The main objectives were to improve their accuracy, extend them to the new high-energy frontier posed by the...

SIMD acceleration can potentially boost by factors the application throughput. Achieving efficient SIMD vectorization for scalar code with complex data flow and branching logic, goes however way beyond breaking some loop dependencies and relying on the compiler. Since the refactoring effort scales with the number of lines of code, it is important t...

In the fall 2016, GeantV went through a thorough community evaluation of the project status and of its strategy for sharing the R&D results with the LHC experiments and with the HEP simulation community in general. Following this discussion, GeantV has engaged onto an ambitious 2-year road-path aiming to deliver a beta version that has most of the...

GeantV is a complex system based on the interaction of different modules needed for detector simulation, which include transport of particles in fields, physics models simulating their interactions with matter and a geometrical modeler library for describing the detector and locating the particles and computing the path length to the current volume...

The recent progress in parallel hardware architectures with deeper vector pipelines or many-cores technologies brings opportunities for HEP experiments to take advantage of SIMD and SIMT computing models. Launched in 2013, the GeantV project studies performance gains in propagating multiple particles in parallel, improving instruction throughput an...

An intensive R&D and programming effort is required to accomplish new challenges posed by future experimental high-energy particle physics (HEP) programs. The GeantV project aims to narrow the gap between the performance of the existing HEP detector simulation software and the ideal performance achievable, exploiting latest advances in computing te...

The GeantV project aims to research and develop the next-generation simulation software describing the passage of particles through matter. While the modern CPU architectures are being targeted first, resources such as GPGPU, Intel© Xeon Phi, Atom or ARM cannot be ignored anymore by HEP CPU-bound applications. The proof of concept GeantV prototype...

The recent emergence of hardware architectures characterized by many-core or accelerated processors has opened new opportunities for concurrent programming models taking advantage of both SIMD and SIMT architectures. GeantV, a next generation detector simulation, has been designed to exploit both the vector capability of mainstream CPUs and multi-t...

Detector simulation is consuming at least half of the HEP computing cycles, and even so, experiments have to take hard decisions on what to simulate, as their needs greatly surpass the availability of computing resources. New experiments still in the design phase such as FCC, CLIC and ILC as well as upgraded versions of the existing LHC detectors w...

The recent emergence of hardware architectures characterized by many-core or accelerated processors has opened new opportunities for concurrent programming models taking advantage of both SIMD and SIMT architectures. The GeantV vector prototype for detector simulations has been designed to exploit both the vector capability of mainstream CPUs and m...

Thread-parallelisation and single-instruction multiple data (SIMD) "vectorisation" of software components in HEP computing has become a necessity to fully benefit from current and future computing hardware. In this context, the Geant-Vector/GPU simulation project aims to re-engineer current software for the simulation of the passage of particles th...

The GeantV project is focused on the R&D of new particle transport techniques to maximize parallelism on multiple levels, profiting from the use of both SIMD instructions and co-processors for the CPU-intensive calculations specific to this type of applications. In our approach, vectors of tracks belonging to multiple events and matching different...

A framework is presented for obtaining combined experimental results through joint fits of datasets from several experiments. The JFIT framework allows such fits to be performed keeping the data separated, in its original format, and using independent fitting environments, thus simplifying the process with respect to data access policies. It is bas...

ALICE (A Large Ion Collider Experiment) is studying the physics of strongly interacting matter, and in particular the properties of the Quark-Gluon Plasma (QGP), using proton-proton, proton-nucleus and nucleus-nucleus collisions at the CERN LHC (Large Hadron Collider). The ALICE Collaboration is preparing a major upgrade of the experimental apparat...

High Energy Physics code has been known for making poor use of high performance computing architectures. Efforts in optimising HEP code on vector and RISC architectures have yield limited results and recent studies have shown that, on modern architectures, it achieves a performance between 10% and 50% of the peak one. Although several successful at...

CHEP Conferences are dedicated to a quite specific scientific computing domain, and deal with rather specialised software, developed for the needs of the High Energy Physics community. The sheer size of this community created an environment which until recently has been to a large extent isolated from the mainstream computing. There is however an e...

One of the current challenges in HEP computing is the development of particle propagation algorithms capable of efficiently use all performance aspects of modern computing devices. The Geant-Vector project at CERN has recently introduced an approach in this direction. This paper describes the implementation of a similar workflow using the Intel(r)...

High Energy Physics has traditionally been a technology-limited science that has pushed the boundaries of both the detectors collecting the information about the particles and the computing infrastructure processing this information. However, since a few years the increase in computing power comes in the form of increased parallelism at all levels,...

Born after World War II, large-scale experimental high-energy physics (HEP) has found itself limited ever since by available accelerator, detector and computing technologies. Accordingly, HEP has made significant contributions to the development of these fields, more often than not driving their innovations. The invention of the World Wide Web at C...

Inclusive J/psi production has been studied with the ALICE detector in p-Pb collisions at the nucleon-nucleon center of mass energy = 5.02 TeV at the CERN LHC. The measurement is performed in the center of mass rapidity domains 2.03 < y (cms) < 3.53 and -4.46 < y (cms) < -2.96, down to zero transverse momentum, studying the mu (+) mu (-) decay mode...

The production of and baryons and their anti-particles in Pb–Pb collisions at has been measured using the ALICE detector. The transverse momentum spectra at mid-rapidity ( ) for charged Ξ and Ω hyperons have been studied in the range and , respectively, and in several centrality intervals (from the most central 0–10% to the most peripheral 60–80% c...

In this Letter, comprehensive results on pi(+/-), K-+/-, K-S(0), p((p) over bar) and A((A) over bar) production at mid-rapidity (0 < y(CMS) < 0.5) in p-Pb collisions at root s(NN) = 5.02 TeV, measured by the ALICE detector at the LHC, are reported. The transverse momentum distributions exhibit a hardening as a function of event multiplicity, which...

The average transverse momentum 〈pT〉 versus the charged-particle multiplicity Nch was measured in p-Pb collisions at a collision energy per nucleon-nucleon pair sNN=5.02TeV and in pp collisions at collision energies of s=0.9,2.76, and 7TeV in the kinematic range 0.15<pT<10.0GeV/c and |η|<0.3 with the ALICE apparatus at the LHC. These data are compa...

The directed flow of charged particles at midrapidity is measured in Pb-Pb collisions at sqrt[s_{NN}]=2.76 TeV relative to the collision symmetry plane defined by the spectator nucleons. A negative slope of the rapidity-odd directed flow component with approximately 3 times smaller magnitude than found at the highest RHIC energy is observed. This s...

Among the components contributing to particle transport, geometry navigation is an important consumer of CPU cycles. The tasks performed to get answers to "basic" queries such as locating a point within a geometry hierarchy or computing accurately the distance to the next boundary can become very computing intensive for complex detector setups. So...

The ALICE measurement of K(S)(0) and Λ production at midrapidity in Pb-Pb collisions at √(s(NN))=2.76 TeV is presented. The transverse momentum (p(T)) spectra are shown for several collision centrality intervals and in the p(T) range from 0.4 GeV/c (0.6 GeV/c for Λ) to 12 GeV/c. The p(T) dependence of the Λ/K(S)(0) ratios exhibits maxima in the vic...

The ALICE measurement of KS0 and Λ production at midrapidity in Pb-Pb collisions at √sNN=2.76 TeV is presented. The transverse momentum (pT) spectra are shown for several collision centrality intervals and in the pT range from 0.4 GeV/c (0.6 GeV/c for Λ) to 12 GeV/c. The pT dependence of the Λ/KS0 ratios exhibits maxima in the vicinity of 3 GeV/c,...

The ALICE Collaboration at the LHC has measured the J/ψ and ψ′ photoproduction at mid-rapidity in ultra-peripheral Pb–Pb collisions at \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \be...

We report on the first measurement of inclusive J/ψ elliptic flow v_{2} in heavy-ion collisions at the LHC. The measurement is performed with the ALICE detector in Pb-Pb collisions at sqrt[s_{NN}]=2.76 TeV in the rapidity range 2.5<y<4.0. The dependence of the J/ψ v_{2} on the collision centrality and on the J/ψ transverse momentum is studied in th...

Angular correlations between unidentified charged trigger particles and various species of charged associated particles (unidentified particles, pions, kaons, protons and antiprotons) are measured by the ALICE detector in p-Pb collisions at a nucleon-nucleon centre-of-mass energy of 5.02 TeV in the transverse-momentum range 0.3 < p(T) < 4 GeV/c. Th...

Azimuthally anisotropic distributions of D^{0}, D^{+}, and D^{*+} mesons were studied in the central rapidity region (|y|<0.8) in Pb-Pb collisions at a center-of-mass energy sqrt[s_{NN}]=2.76 TeV per nucleon-nucleon collision, with the ALICE detector at the LHC. The second Fourier coefficient v_{2} (commonly denoted elliptic flow) was measured in t...

Differential cross sections of charged particles in inelastic pp collisions as a function of p_T have been measured at sqrt(s) = 0.9, 2.76 and 7 TeV at the LHC. The p_T spectra are compared to NLO-pQCD calculations. Though the differential cross section for an individual sqrt(s) cannot be described by NLO-pQCD, the relative increase of cross sectio...

The ratios of yields of anti-baryons to baryons probes the mechanisms of baryon-number transport. Results for (p) over bar /p, (Lambda) over bar/Lambda, (Xi) over bar (+)/(Xi) over bar (-) and (Omega) over bar (+)/(Omega) over bar (-) in pp collisions at root s = 0.9, 2.76 and 7 TeV, measured with the ALICE detector at the LHC, are reported. Within...

In high-energy heavy-ion collisions, the correlations between the emitted particles can be used as a probe to gain insight into the charge creation mechanisms. In this Letter, we report the first results of such studies using the electric charge balance function in the relative pseudorapidity (Delta eta) and azimuthal angle (Delta phi) in Pb-Pb col...

Measurements of cross sections of inelastic and diffractive processes in proton-proton collisions at LHC energies were carried out with the ALICE detector. The fractions of diffractive processes in inelastic collisions were determined from a study of gaps in charged particle pseudorapidity distributions: for single diffraction (diffractive mass M X...

The ALICE Collaboration at the CERN Large Hadron Collider reports the first measurement of the inclusive differential jet cross section at mid-rapidity in pp collisions at root s = 2.76 TeV, with integrated luminosity of 13.6 nb(-1). Jets are measured over the transverse momentum range 20 to 125 GeV/c and are corrected to the particle level. Calcul...

We report the first measurement of the net-charge fluctuations in Pb-Pb collisions at sqrt[s_{NN}]=2.76 TeV, measured with the ALICE detector at the CERN Large Hadron Collider. The dynamical fluctuations per unit entropy are observed to decrease when going from peripheral to central collisions. An additional reduction in the amount of fluctuations...

The production cross section of electrons from semileptonic decays of beauty hadrons was measured at mid-rapidity ( ) in the transverse momentum range with the ALICE experiment at the CERN LHC in pp collisions at a center of mass energy using an integrated luminosity of . Electrons from beauty hadron decays were selected based on the displacement o...

Correlations of two charged identical kaons (KchKch) are measured in pp collisions at root s = 7 TeV by the ALICE experiment at the Large Hadron Collider (LHC). One-dimensional (KKch)-K-ch correlation functions are constructed in three multiplicity and four transverse momentum ranges. The (KKch)-K-ch femtoscopic source parameters R and lambda are e...

The inclusive transverse momentum (p(T)) distributions of primary charged particles are measured in the pseudo-rapidity range vertical bar eta vertical bar <0.8 as a function of event centrality in Pb-Pb collisions at root S-NN = 2.76 TeV with ALICE at the LHC. The data are presented in the p(T) range 0.15 < p(T) <50 GeV/c for nine centrality inter...

The transverse momentum (p_{T}) distribution of primary charged particles is measured in minimum bias (non-single-diffractive) p+Pb collisions at sqrt[s_{NN}]=5.02 TeV with the ALICE detector at the LHC. The p_{T} spectra measured near central rapidity in the range 0.5<p_{T}<20 GeV/c exhibit a weak pseudorapidity dependence. The nuclear modificatio...

Angular correlations between charged trigger and associated particles are measured by the ALICE detector in p-Pb collisions at a nucleon-nucleon centre-of-mass energy of 5.02 TeV for transverse momentum ranges within 0.5 < P-T,P-assoc < P-T,P-trig < 4 GeV/c. The correlations are measured over two units of pseudorapidity and full azimuthal angle in...

The ALICE Collaboration has made the first measurement at the LHC of J/psi photoproduction in ultra-peripheral Pb-Pb collisions at root s(NN) = 2.76 TeV. The J/psi is identified via its dimuon decay in the forward rapidity region with the muon spectrometer for events where the hadronic activity is required to be minimal. The analysis is based on an...

The charged-particle pseudorapidity density measured over four units of pseudorapidity in nonsingle-diffractive p+Pb collisions at a center-of-mass energy per nucleon pair sqrt[s_{NN}]=5.02 TeV is presented. The average value at midrapidity is measured to be 16.81±0.71 (syst), which corresponds to 2.14±0.17 (syst) per participating nucleon, calcula...

Measurements of charge-dependent azimuthal correlations with the ALICE detector at the LHC are reported for Pb-Pb collisions at sqrt[s_{NN}]=2.76 TeV. Two- and three-particle charge-dependent azimuthal correlations in the pseudorapidity range |η|<0.8 are presented as a function of the collision centrality, particle separation in pseudorapidity, and...

In this Letter we report the first results on π^{±}, K^{±}, p, and p[over ¯] production at midrapidity (|y|<0.5) in central Pb-Pb collisions at sqrt[s_{NN}]=2.76 TeV, measured by the ALICE experiment at the LHC. The p_{T} distributions and yields are compared to previous results at sqrt[s_{NN}]=200 GeV and expectations from hydrodynamic and thermal...

The first measurement of neutron emission in electromagnetic dissociation of ^{208}Pb nuclei at the LHC is presented. The measurement is performed using the neutron zero degree calorimeters of the ALICE experiment, which detect neutral particles close to beam rapidity. The measured cross sections of single and mutual electromagnetic dissociation of...

The ALICE Collaboration has measured inclusive J/psi production in pp collisions at a center-of-mass energy root s = 2.76 TeV at the LHC. The results presented in this Letter refer to the rapidity ranges vertical bar y vertical bar < 0.9 and 2.5 < y <4 and have been obtained by measuring the electron and muon pair decay channels, respectively. The...

Detector simulation is one of the most CPU intensive tasks in modern High Energy Physics. While its importance for the design of the detector and the estimation of the efficiency is ever increasing, the amount of events that can be simulated is often constrained by the available computing resources. Various kind of “fast simulations” have been deve...

The paper reviews the evolution of the software in High Energy Physics from the time of expensive mainframes to grids and clouds systems using thousands of multi-core processors. It focuses on the key parameters or events that have shaped the current software infrastructure.

Identical neutral kaon pair correlations are measured in root s = 7 TeV pp collisions in the ALICE experiment. One-dimensional (KsKs0)-K-0 correlation functions in terms of the invariant momentum difference of kaon pairs are formed in two multiplicity and two transverse momentum ranges. The femtoscopic parameters for the radius and correlation stre...

The ALICE experiment has measured the inclusive J/ψ production in Pb-Pb collisions at √s(NN) = 2.76 TeV down to zero transverse momentum in the rapidity range 2.5 < y < 4. A suppression of the inclusive J/ψ yield in Pb-Pb is observed with respect to the one measured in pp collisions scaled by the number of binary nucleon-nucleon collisions. The nuc...

The ALICE Collaboration has measured the inclusive production of muons from heavy-flavor decays at forward rapidity, 2.5<y<4, in pp and Pb-Pb collisions at sqrt[s(NN)]=2.76 TeV. The p(t)-differential inclusive cross section of muons from heavy-flavor decays in pp collisions is compared to perturbative QCD calculations. The nuclear modification fact...

The production of the prompt charm mesons D0, D+, D*+, and their antiparticles, was measured with the ALICE detector in Pb-Pb collisions at the LHC, at a centre-of-mass energy $ \sqrt{{{s_{\mathrm{NN}}}}}=2.76\;\mathrm{TeV} $ per nucleon-nucleon collision. The p t-differential production yields in the range 2 < p t < 16 GeV/c at central rapidity,...

Measurements of the sphericity of primary charged particles in minimum bias proton-proton collisions at root s = 0.9, 2.76 and 7 TeV with the ALICE detector at the LHC are presented. The observable is measured in the plane perpendicular to the beam direction using primary charged tracks with p(T) > 0.5 GeV/c in vertical bar eta vertical bar < 0.8....

The effect of event background fluctuations on charged particle jet reconstruction in Pb-Pb collisions at $ \sqrt {{{s_{\text{NN}}}}} = {2}{.76}\,{\text{TeV}} $ has been measured with the ALICE experiment. The main sources of non-statistical fluctuations are characterized based purely on experimental data with an unbiased method, as well as by us...

We present measurements of Underlying Event observables in pp collisions at $ \sqrt {s} = 0.9 $ and 7TeV. The analysis is performed as a function of the highest charged-particle transverse momentum p T,LT in the event. Different regions are defined with respect to the azimuthal direction of the leading (highest transverse momentum) track: Toward,...

A measurement of the multi-strange Ξ- and Ω- baryons and their antiparticles by the ALICE experiment at the CERN Large Hadron Collider (LHC) is presented for inelastic proton-proton collisions at a centre-of-mass energy of 7 TeV. The transverse momentum (pT) distributions were studied at mid-rapidity (|y|<0.5) in the range of 0.6<pT<8.5 GeV/c for Ξ...

The ALICE Collaboration reports the measurement of the relative J/ψ yield as a function of charged particle pseudorapidity density dNch/dη in pp collisions at s=7 TeV at the LHC. J/ψ particles are detected for pt>0, in the rapidity interval |y|<0.9 via decay into e+e-, and in the interval 2.5<y<4.0 via decay into μ+μ- pairs. An approximately linear...