Coprocessor integration for real-time event processing in particle physics detectors

Els experiments de física d’altes energies actuals disposen d’acceleradors amb més energía, sensors més precisos i formes més flexibles de recopilar les dades. Aquesta ràpida evolució requereix de més capacitat de càlcul; els processadors massivament paral·lels, com ara les targes acceleradores gràfiques, ens posen a l’abast aquesta major capacitat de càlcul a un cost sensiblement inferior a les CPUs tradicionals. L’ús d’aquest tipus de processadors requereix, però, de nous algoritmes i nous enfocaments de l’organització de les dades que són difícils d’integrar en els programaris actuals. En aquest treball s’exploren els problemes derivats de l’ús d’algoritmes paral·lels en els entorns de programari existents, orientats a CPUs, i es proposa una solució, en forma de servei, que comunica amb els diversos pipelines que processen els esdeveniments procedents de les col·lisions de partícules, recull les dades en lots i els envia als algoritmes corrent sobre els processadors massivament paral·lels. Aquest servei s’integra en Gaudí - l’entorn de software de dos dels quatre experiments principals del Gran Col·lisionador d’Hadrons. S’examina el sobrecost que el servei afegeix als algoritmes paral·lels. S’estudia un cas d´ùs del servei per fer una reconstrucció paral·lela de les traces detectades en el VELO Pixel, el subdetector encarregat de la detecció de vèrtex en l’upgrade de LHCb. Per aquest cas, s’observen les característiques del rendiment en funció de la mida dels lots de dades. Finalment, les conclusions en posen en el context dels requeriments del sistema de trigger de LHCb.La física de altas energías dispone actualmente de aceleradores con energías mayores, sensores más precisos y métodos de recopilación de datos más flexibles que nunca. Su rápido progreso necesita aún más potencia de cálculo; el hardware masivamente paralelo, como las unidades de procesamiento gráfico, nos brinda esta potencia a un coste mucho más bajo que las CPUs tradicionales. Sin embargo, para usar eficientemente este hardware necesitamos algoritmos nuevos y nuevos enfoques de organización de datos difíciles de integrarse con el software existente. En este trabajo, se investiga cómo se pueden usar estos algoritmos paralelos en las infraestructuras de software ya existentes y que están orientadas a CPUs. Se propone una solución en forma de un servicio que comunica con los diversos pipelines que procesan los eventos de las correspondientes colisiones de particulas, reúne los datos en lotes y se los entrega a los algoritmos paralelos acelerados por hardware. Este servicio se integra con Gaudí — la infraestructura del entorno de software que usan dos de los cuatro gran experimentos del Gran Colisionador de Hadrones. Se examinan los costes añadidos por el servicio en los algoritmos paralelos. Se estudia un caso de uso del servicio para ejecutar un algoritmo paralelo para el VELO Pixel (el subdetector encargado de la localización de vértices en el upgrade del experimento LHCb) y se estudian las características de rendimiento de los distintos tamaños de lotes de datos. Finalmente, las conclusiones se contextualizan dentro la perspectiva de los requerimientos para el sistema de trigger de LHCb.High-energy physics experiments today have higher energies, more accurate sensors, and more flexible means of data collection than ever before. Their rapid progress requires ever more computational power; and massively parallel hardware, such as graphics cards, holds the promise to provide this power at a much lower cost than traditional CPUs. Yet, using this hardware requires new algorithms and new approaches to organizing data that can be difficult to integrate with existing software. In this work, I explore the problem of using parallel algorithms within existing CPU-orientated frameworks and propose a compromise between the different trade-offs. The solution is a service that communicates with multiple event-processing pipelines, gathers data into batches, and submits them to hardware-accelerated parallel algorithms. I integrate this service with Gaudi — a framework underlying the software environments of two of the four major experiments at the Large Hadron Collider. I examine the overhead the service adds to parallel algorithms. I perform a case study of using the service to run a parallel track reconstruction algorithm for the LHCb experiment's prospective VELO Pixel subdetector and look at the performance characteristics of using different data batch sizes. Finally, I put the findings into perspective within the context of the LHCb trigger's requirements

Evidence for Exotic Hadron Contributions to Λb0 →j /ψpπ- Decays

A full amplitude analysis of Λ0b→J/ψpπ− decays is performed with a data sample acquired with the LHCb detector from 7 and 8 TeV pp collisions, corresponding to an integrated luminosity of 3  fb−1. A significantly better description of the data is achieved when, in addition to the previously observed nucleon excitations N→pπ−, either the Pc(4380)+ and Pc(4450)+→J/ψp states, previously observed in Λ0b→J/ψpK− decays, or the Zc(4200)−→J/ψπ− state, previously reported in B0→J/ψK+π− decays, or all three, are included in the amplitude models. The data support a model containing all three exotic states, with a significance of more than three standard deviations. Within uncertainties, the data are consistent with the Pc(4380)+ and Pc(4450)+ production rates expected from their previous observation taking account of Cabibbo suppression

Observation of Λb0→ ψ(2S)pK−and Λb0→ J/ψπ+π−pK−decays and a measurement of the Λb0baryon mass

The decays Λ b 0  → ψ(2S)pK− and Λ b 0  → J/ψπ + π −pK− are observed in a data sample corresponding to an integrated luminosity of 3 fb−1, collected in proton-proton collisions at 7 and 8 TeV centre-of-mass energies by the LHCb detector. The ψ(2S) mesons are reconstructed through the decay modes ψ(2S) → μ+μ− and ψ(2S) → J/ψπ + π −. The branching fractions relative to that of Λ b 0  → J/ψpK− are measured to be B(Λ0b→ψ(2S)pK−)B(Λ0b→J/ψpK−)=(20.70±0.76±0.46±0.37)×10−2, B(Λb0→ψ(2S)pK−)B(Λb0→J/ψpK−)=(20.70±0.76±0.46±0.37)×10−2, where the first uncertainties are statistical, the second are systematic and the third is related to the knowledge of J/ψ and ψ(2S) branching fractions. The mass of the Λ b 0 baryon is measured to be M(Λ0b)=5619.65±0.17±0.17 MeV/c2, M(Λb0)=5619.65±0.17±0.17 MeV/c2, where the uncertainties are statistical and systematic

Measurement of the branching fraction ratio B (Bc+→ψ (2 S )π+)/B (Bc+→J /ψ π+)

Using pp collision data collected by LHCb at center-of-mass energies √ s = 7 TeV and 8 TeV, corresponding to an integrated luminosity of 3 fb−1 , the ratio of the branching fraction of the B+ c → ψ(2S)π + decay relative to that of the B+ c → J/ψ π+ decay is measured to be 0.268 ± 0.032 (stat) ± 0.007 (syst) ± 0.006 (BF). The first uncertainty is statistical, the second is systematic, and the third is due to the uncertainties on the branching fractions of the J/ψ → µ +µ − and ψ(2S) → µ +µ − decays. This measurement is con

Measurement of the mass and lifetime of the Ωb- baryon

A proton-proton collision data sample, corresponding to an integrated luminosity of 3 fb−1 collected by LHCb at s√=7 and 8 TeV, is used to reconstruct 63±9 Ω−b→Ω0cπ−, Ω0c→pK−K−π+ decays. Using the Ξ−b→Ξ0cπ−, Ξ0c→pK−K−π+ decay mode for calibration, the lifetime ratio and absolute lifetime of the Ω−b baryon are measured to be τΩ−bτΞ−bτΩ−b=1.11±0.16±0.03,=1.78±0.26±0.05±0.06 ps, where the uncertainties are statistical, systematic and from the calibration mode (for τΩ−b only). A measurement is also made of the mass difference, mΩ−b−mΞ−b, and the corresponding Ω−b mass, which yields mΩ−b−mΞ−bmΩ−b=247.4±3.2±0.5 MeV/c2,=6045.1±3.2±0.5±0.6 MeV/c2. These results are consistent with previous measurements

Search for Structure in the Bs0 π± Invariant Mass Spectrum

The B 0 s π ± invariant mass distribution is investigated in order to search for possible exotic meson states. The analysis is based on a data sample recorded with the LHCb detector corresponding to 3     fb − 1 of p p collision data at √ s = 7 and 8 TeV. No significant excess is found, and upper limits are set on the production rate of the claimed X ( 5568 ) state within the LHCb acceptance. Upper limits are also set as a function of the mass and width of a possible exotic meson decaying to the B 0 s π ± final state. The same limits also apply to a possible exotic meson decaying through the chain B * 0 s π ± , B * 0 s → B 0 s γ where the photon is excluded from the reconstructed decays

Dalitz plot analysis of B0→D¯0π+π- decays

The resonant substructures of B0 → D0π +π − decays are studied with the Dalitz plot technique. In this study a data sample corresponding to an integrated luminosity of 3.0 fb−1 of pp collisions collected by the LHCb detector is used. The branching fraction of the B0 → D0π +π − decay in the region m(D0π ±) > 2.1 GeV/c2 is measured to be (8.46 ± 0.14 ± 0.29 ± 0.40) × 10−4 , where the first uncertainty is statistical, the second is systematic and the last arises from the normalisation channel B0 → D∗ (2010)−π +. The π +π − S-wave components are modelled with the Isobar and K-matrix formalisms. Results of the Dalitz plot analyses using both models are presented. A resonant structure at m(D0π −) ≈ 2.8 GeV/c2 is confirmed and its spin-parity is determined for the first time as J P = 3−. The branching fraction, mass and width of this structure are determined together with those of the D∗ 0 (2400)− and D∗ 2 (2460)− resonances. The branching fractions of other B0 → D0h 0 decay components with h 0 → π +π − are also reported. Many of these branching fraction measurements are the most precise to date. The first observation of the decays B0 → D0f0(500), B0 → D0f0(980), B0 → D0ρ(1450), B0 → D∗ 3 (2760)−π + and the first evidence of B0 → D0f0(2020) are presented

Determination of the branching fractions of B S 0 → D S ∓ K ∓ and B 0 → D S − K +

B0 → D− s K+ relative to the decays B0 s → D− s π + and B0 → D−π +, respectively. The data used correspond to an integrated luminosity of 3.0 fb−1 of proton-proton collisions. The ratios of branching fractions are B(B0 s → D∓ s K∓) B(B0 s → D − s π+) = 0.0752 ± 0.0015 ± 0.0019 and B(B0 → D− s K+) B(B0 → D−π+) = 0.0129 ± 0.0005 ± 0.0008, where the uncertainties are statistical and systematic, respectively. Keywords: Hadron-Hadron Scattering, Branching fraction, B physics, Flavor physic

Measurement of the exclusive Υ production cross-section in pp collisions at √{s}=7 TeV and 8 TeV

A study is presented of central exclusive production of Υ(nS) states, where the Υ(nS) resonances decay to the µ +µ − final state, using pp collision data recorded by the LHCb experiment. The cross-section is measured in the rapidity range 2 < y(Υ) < 4.5 where the muons are reconstructed in the pseudorapidity range 2 < η(µ ±) < 4.5. The data sample corresponds to an integrated luminosity of 2.9 fb−1 and was collected at centre-of-mass energies of 7 TeV and 8 TeV. The measured Υ(1S) and Υ(2S) production cross-sections are σ(pp → pΥ(1S)p) = 9.0 ± 2.1 ± 1.7 pb and σ(pp → pΥ(2S)p) = 1.3 ± 0.8 ± 0.3 pb, where the first uncertainties are statistical and the second are systematic. The Υ(1S) cross-section is also measured as a function of rapidity and is found to be in good agreement with Standard Model predictions. An upper limit is set at 3.4 pb at the 95% confidence level for the exclusive Υ(3S) production cross-section, including possible contamination from χb(3P) → Υ(3S)γ decays

Measurement of the forward Z boson production cross-section in pp collisions at TeV

A measurement of the production cross-section for Z bosons that decay to muons is presented. The data were recorded by the LHCb detector during pp collisions at a centre-of-mass energy of 7 TeV, and correspond to an integrated luminosity of 1.0 fb−1 . The cross-section is measured for muons in the pseudorapidity range 2.0 20 GeV/c. The dimuon mass is restricted to 60 < Mµ+µ− < 120 GeV/c2 . The measured cross-section is σZ→µ+µ− = (76.0 ± 0.3 ± 0.5 ± 1.0 ± 1.3) pb where the uncertainties are due to the sample size, systematic effects, the beam energy and the luminosity. This result is in good agreement with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics. The cross-section is also measured differentially as a function of kinematic variables of the Z boson. Ratios of the production cross-sections of electroweak bosons are presented using updated LHCb measurements of W boson production. A precise test of the Standard Model is provided by the measurement of the rati