A. Arend

ALICE HLT High Speed Tracking on GPU

The on-line event reconstruction in ALICE is performed by the High Level Trigger, which should process up to 2000 events per second in proton-proton collisions and up to 300 central events per second in heavy-ion collisions, corresponding to an input data stream of 30 GB/s. In order to fulfill the time requirements, a fast on-line tracker has been developed. The algorithm combines a Cellular Automaton method being used for a fast pattern recognition and the Kalman Filter method for fitting of found trajectories and for the final track selection. The tracker was adapted to run on Graphics Processing Units (GPU) using the NVIDIA Compute Unified Device Architecture (CUDA) framework. The implementation of the algorithm had to be adjusted at many points to allow for an efficient usage of the graphics cards. In particular, achieving a good overall workload for many processor cores, efficient transfer to and from the GPU, as well as optimized utilization of the different memories the GPU offers turned out to be critical. To cope with these problems a dynamic scheduler was introduced, which redistributes the workload among the processor cores. Additionally a pipeline was implemented so that the tracking on the GPU, the initialization and the output processed by the CPU, as well as the DMA transfer can overlap. The GPU tracking algorithm significantly outperforms the CPU version for large events while it entirely maintains its efficiency.

ALICE HLT high speed tracking and vertexing

The on-line event reconstruction in ALICE is performed by the High Level Trigger, which should process up to 2000 events per second in proton-proton collisions and up to 200 central events per second in heavy-ion collisions, corresponding to an input data stream of 30 GB/s.

Transverse Momentum Distribution and Nuclear Modification Factor of Charged Particles in p plus Pb Collisions at root(NN)-N-s=5.02 TeV

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 modification factor R(pPb) is consistent with unity for p(T) above 2 GeV/c. This measurement indicates that the strong suppression of hadron production at high p(T) observed in Pb+Pb collisions at the LHC is not due to an initial-state effect. The measurement is compared to theoretical calculations.

Event Reconstruction Performance of the ALICE High Level Trigger for

The ALICE High Level Trigger comprises a large computing cluster, dedicated interfaces and software applications. It allows on-line event reconstruction of the full data stream of the ALICE experiment at up to 25 GByte/s. The commissioning campaign has passed an important phase since the startup of the Large Hadron Collider in November 2009. The system has been transferred into continuous operation with focus on the event reconstruction and first simple trigger applications. The paper reports for the first time on the achieved event reconstruction performance in the ALICE central barrel region.

{\rm p} + {\rm p}

Measurements of π0 and η inclusive spectra provide reference data for upcoming heavy ion runs, as well as a check on the applicability of perturbative QCD calculations at LHC energies. The high-resolution central tracking system of ALICE can be used to reconstruct π0 and η through photon conversions, as an alternative to direct measurement in the ALICE Calorimeters. Knowledge of the ALICE material budget is crucial for the extraction of the absolute yield from the conversion technique. The statistics availible from this technique are comparable to that of the photon spectrometer (PHOS), and the reconstruction method can also be applied for Pb-Pb collisions. Moreover, the implementation of a photon conversion trigger in the High Level Trigger (HLT) framework gives the possibility of identifying π0 and η candidates online, and increasing the statistics at higher momentum. The status of the π0 meson reconstruction from photon conversions from p-p collisions at = 900 GeV and = 7 TeV is presented in this article.