1,721,472 research outputs found
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Search for Dark Matter at CMS
No full textThe CMS collaboration searched for Dark Matter (DM) particles produced in pairs in proton-proton (pp) collisions performed by the LHC at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.7 /fb. The following signatures were investigated a single boosted object and a significant transverse momentum imbalance; associated production of DM with a pair of top quarks; decay of a Higgs boson (H) into a pair of DM particles. Exclusion limits were set on the cross sections of DM production and interaction cross sections, as a function of the DM particle mass
CMS Level-1 electron/photon trigger performance
No full textSince March 2010 the Large Hadron Collider (LHC) has provided high energy proton collisions with an instantaneous luminosity that has risen by several orders of magnitude to around 4e33 cm-2 s-1 at the end of 2011 corresponding to millions of collisions per second. With this unprecedented collision rate, efficient triggering on electrons and photons has become a major challenge for LHC experiments. The Compact Muon Solenoid (CMS) experiment uses a two-level trigger system. The first level (L1) is based on coarse information coming from calorimeters and muon detectors, accepting up to 100kHz of events per second. A High-Level Trigger (HLT) then combines fine-grain information from all sub-detectors to reduce this rate further to about 200-300Hz. At L1 the electron/photon trigger is based upon information from the Electromagnetic Calorimeter (ECAL), a high resolution detector comprising 75848 lead tungstate (PbWO4) crystals in a "barrel" and two "endcaps". The optimization and performance of this system in terms of electron and photon triggering efficiency are presented
Electron selection and search for the Higgs boson decaying into tau leptons pairs with the CMS detector at the LHC
No full textCette thèse s'inscrit dans le contexte des premières années d'exploitation du Large Hadron Collider (LHC). Cet appareil monumental a été construit dans le but d'explorer la physique de l'infiniment petit à l'échelle du TeV. Un des objectifs majeurs du LHC est la recherche du boson de Higgs. Sa découverte validerait le mécanisme de brisure de symétrie électrofaible, au travers duquel les bosons W et Z acquièrent leur masse. L'expérience Compact Muon Solenoid (CMS) analyse les collisions de protons du LHC. Leur fréquence élevée (20 MHz) permet d'observer des phénomènes rares, comme la production et la désintégration d'un boson de Higgs, mais elle nécessite alors une sélection rapide des collisions intéressantes, par un système de déclenchement. Les ressources informatiques disponibles pour le stockage et l'analyse des données imposent une limite au taux de déclenchement : la bande passante, répartie entre les différents signaux physiques, doit donc être optimisée. Dans un premier temps, j'ai étudié le déclenchement sur les électrons : ils constituent une signature claire dans l'environnement hadronique intense du LHC et permettent à la fois des mesures de haute précision et la recherche de signaux rares. Ils font partie des états finaux étudiés par un grand nombre d'analyses (Higgs, électrofaible, etc.). Dès les premières collisions en 2010, la présence de signaux anormaux dans l'électronique de lecture du calorimètre électromagnétique (ECAL) constituait une source d'augmentation incontrôlée du taux de déclenchement. En effet, leur taux de production augmentait avec l'énergie et l'intensité des collisions : ils étaient susceptibles de saturer la bande passante dès 2011, affectant gravement les performances de physique de CMS. J'ai optimisé l'algorithme d'élimination de ces signaux en conservant une excellente efficacité de déclenchement sur les électrons, pour les prises de données en 2011. D'autre part, l'intensité croissante des collisions au LHC fait perdre leur transparence aux cristaux du ECAL, induisant une inefficacité de déclenchement. La mise en place de corrections hebdomadaires de l'étalonnage du système de déclenchement a permis de compenser cette inefficacité. Dans un second temps, j'ai participé à la recherche du boson de Higgs dans son mode de désintégration en deux leptons tau. Cette analyse est la seule qui puisse actuellement vérifier le couplage du boson de Higgs aux leptons. Le lepton tau se désintégrant soit en lepton plus léger (électron ou muon), soit en hadrons, six états finaux sont possibles. Je me suis concentré sur les états finaux semi-leptoniques (électron/muon et hadrons), où la signification statistique du signal est maximale. Les algorithmes de déclenchement dédiés à cette analyse sélectionnent un lepton (électron ou muon) et un « tau hadronique » d'impulsions transverses élevées. Cependant, cette sélection élimine la moitié du signal, ce qui a motivé la mise en place d'algorithmes sélectionnant des leptons de basse impulsion, incluant une coupure sur l'énergie transverse manquante. Celle-ci limite le taux de déclenchement et sélectionne des évènements contenant des neutrinos, caractéristiques des désintégrations du lepton tau. Les distributions de masse invariante des processus de bruit de fond et de signal permettent de quantifier la compatibilité entre les données et la présence ou l'absence du signal. La combinaison de l'ensemble des états finaux conduit à l'observation d'un excès d'évènements sur un large intervalle de masse. Sa signification statistique vaut 3,2 déviations standard à 125 GeV ; la masse du boson mesurée dans ce canal vaut 122 ± 7 GeV. Cette mesure constitue la toute première évidence d'un couplage entre le boson de Higgs et le lepton tau.This thesis fits into the first operating years of the Large Hadron Collider. This monumental machine was built to explore the infinitesimal structure of matter at the multi-TeV scale. The LHC aimed primarily at searching for the Higgs boson, the discovery of which would confirm the electroweak symmetry breaking model. This mechanism, which provides W and Z bosons with a mass, describes the transition from a unified electroweak interaction to a weak interaction (short range) and an electromagnetic interaction (infinite range). The LHC's proton collisions, operated at a 50 ns period, are analysed by 4 large detectors, including the Compact Muon Solenoid (CMS). This small period allows to observe very rare phenomena, such as the Higgs boson production and decay, but it requires a fast online selection of the interesting collisions: the trigger system. The computing resources available for the data's storage and analysis set a limit to the trigger rate. Therefore the bandwidth, which is split into several physics signals, must be optimised. Firstly, I studied the electron trigger: electrons are a clear signature in the intense hadronic environment within the LHC and allow a high measurement accuracy, as well as a search for rare signals. Besides, they are part of the final states investigated by a large number of analyses (Higgs, electroweak, etc). From the first collisions in 2010, anomalous signals in the CMS electromagnetic calorimeter (ECAL) were a source of uncontrolled trigger rate increase. Indeed, their production rate increased along with the collisions' energy and intensity: they were likely to saturate the bandwidth as early as 2011, crippling drastically the CMS physics performances. I optimised the anomalous signal rejection algorithm, while conserving an excellent electron triggering efficiency, as regards the data collected in 2011. Moreover, the increasing intensity of the LHC collisions causes a loss of transparency in the ECAL crystals. The setting-up of weekly corrections to the ECAL trigger calibration helped make up for the inefficiency caused by this loss of transparency. Secondly, I contributed to the search for the Higgs boson decaying to 2 tau leptons. So far, this analysis proved to be the only possible method to check the coupling of the Higgs boson to leptons. The tau lepton decays either into lighter leptons (electron or muon), or into hadrons: hence the study of six final states. I focused on the semileptonic final states, in which the expected signal is the most statistically significant. The trigger algorithms dedicated to this analysis select a lepton and a hadronic tau, with high transverse momenta. However, this selection removes half of the signal, which motivated the elaboration of new algorithms selecting low momenta leptons, including a cut on the missing transverse energy. This cut helps controlling the trigger rate and selects events containing neutrinos, which are a distinguishing feature of the tau lepton decay. The invariant mass distributions for all background and signal processes allow to quantify the compatibility between the acquired data and the presence of a signal. The combination of all final states leads to the observation of an excess of events over a large mass range. Its statistical significance is 3,2 standard deviations at 125 GeV ; the boson mass measured in this channel is 122 ± 7 GeV. This measurement is the first evidence for a coupling between the Higgs boson and the tau lepton
Sélection des électrons et recherche du boson de Higgs se désintégrant en paires de leptons tau avec l'expérience CMS au LHC
No full textThis thesis fits into the first operating years of the Large Hadron Collider. Thismonumental machine was built to explore the infinitesimal structure of matter at themulti-TeV scale. The LHC aimed primarily at searching for the Higgs boson, the discoveryof which would confirm the electroweak symmetry breaking model. This mechanism, whichprovides W and Z bosons with a mass, describes the transition from a unified electroweakinteraction to a weak interaction (short range) and an electromagnetic interaction (infiniterange). The LHC’s proton collisions, operated at a 50 ns period, are analysed by 4 largedetectors, including the Compact Muon Solenoid (CMS). This small period allows toobserve very rare phenomena, such as the Higgs boson production and decay, but it requiresa fast online selection of the interesting collisions : the trigger system. The computingresources available for the data’s storage and analysis set a limit to the trigger rate.Therefore the bandwidth, which is split into several physics signals, must be optimised.Firstly, I studied the electron trigger : electrons are a clear signature in the intensehadronic environment within the LHC and allow a high measurement accuracy, as well asa search for rare signals. Besides, they are part of the final states investigated by a largenumber of analyses (Higgs, electroweak, etc).From the first collisions in 2010, anomalous signals in the CMS electromagneticcalorimeter (ECAL) were a source of uncontrolled trigger rate increase. Indeed, theirproduction rate increased along with the collisions’ energy and intensity : they were likelyto saturate the bandwidth as early as 2011, crippling drastically the CMS physics performances. I optimised the anomalous signal rejection algorithm, while conserving anexcellent electron triggering efficiency, as regards the data collected in 2011. Moreover,the increasing intensity of the LHC collisions causes a loss of transparency in the ECALcrystals. The setting-up of weekly corrections to the ECAL trigger calibration helpedmake up for the inefficiency caused by this loss of transparency.Secondly, I contributed to the search for the Higgs boson decaying to 2 tau leptons. Sofar, this analysis proved to be the only possible method to check the coupling of the Higgsboson to leptons. The tau lepton decays either into lighter leptons (electron or muon),or into hadrons : hence the study of six final states. I focused on the semileptonic finalstates, in which the expected signal is the most statistically significant.The trigger algorithms dedicated to this analysis select a lepton and a hadronic tau,with high transverse momenta. However, this selection removes half of the signal, whichmotivated the elaboration of new algorithms selecting low momenta leptons, including acut on the missing transverse energy. This cut helps controlling the trigger rate and selectsevents containing neutrinos, which are a distinguishing feature of the tau lepton decay.The invariant mass distributions for all background and signal processes allow toquantify the compatibility between the acquired data and the presence of a signal. Thecombination of all final states leads to the observation of an excess of events over a largemass range. Its statistical significance is 3,2 standard deviations at 125 GeV ; the bosonmass measured in the H → τ τ channel is 122 ± 7 GeV. This measurement is the firstevidence for a coupling between the Higgs boson and the tau lepton.Keywords : LHC, CMS, Higgs, tau lepton, electron, trigger
Dark matter searches in CMS
No full textThe CMS collaboration searched for Dark Matter (DM) particles produced in pairs in proton-proton (pp) collisions performed by the LHC at a centre-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 19.7 /fb. The following signatures were investigated: a single boosted object and a significant transverse momentum imbalance, associated production of DM with a pair of top quarks, decay of a Higgs boson (H) into a pair of DM particles. Exclusion limits were set on the cross sections of DM production and interaction cross sections, as a function of the DM particle mass
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