We consider the sensitivity of high luminosity neutrino experiments to light stable states, as arise in scenarios of MeV-scale dark matter. To ensure the correct thermal relic abundance, such states must annihilate to the Standard model via light mediators, providing a portal for access to the dark matter state in colliders or fixed targets. This framework implies that neutrino beams produced at a fixed target will also carry an additional “dark matter beam”, which can mimic neutrino scattering off electrons or nuclei in the detector. We therefore develop a Monte Carlo code to simulate the production of a dark matter beam at two proton fixed-target facilities with high luminosity, LSND and MiniBooNE, and with this simulation determine the existing limits on light dark matter. We find in particular that MeV-scale dark matter scenarios motivated by an explanation of the galactic 511 keV line are strongly constrained. / Graduate

La première prise des données du LHC (2010-2012) a été marquée par la découverte du boson scalaire, dit boson de Higgs. Sa masse a été mesurée avec une précision de 0.2% en utilisant ses désintégrations en deux photons et celles en deux bosons Z donnant quatre leptons dans l’état final. Les couplages ont été estimés en combinant plusieurs états finaux, tandis que la précision sur leur mesure pourra bénéficier énormément de la grande statistique qui sera accumulée pendant les prochaines périodes de prise des données au LHC (Run 2, Phase 2).Le canal H→ZZ*→4 leptons, a un rapport d'embranchement réduit mais présente un faible bruit de fond, ce qui le rend attractif pour la détermination des propriétés du nouveau boson. Dans cette thèse, l’analyse conduite pour la mise en évidence de ce mode dans l’expérience ATLAS est détaillée, avec un poids particulier porté à la mesure et au contrôle du bruit de fond réductible en présence d’électrons.Dans le cadre de la préparation de futures prises de données à très haute luminosité, prévues à partir de 2025, deux études sont menées:La première concerne l’observabilité du mode de production du boson de Higgs en association avec des quarks b. Une analyse multivariée, basée sur des données simulées, confirme un très faible signal dans le canal H→2 photons.La seconde concerne la conception et le développement d’un détecteur interne en silicium, adapté à l’environnement hostile, de haute irradiation et de taux d’occupation élevée, attendues pendant la Phase 2 du LHC. Des études concernant l’optimisation de la géométrie, l’amélioration de l’efficacité ainsi que la résistance à l’irradiation ont été menées. A travers des mesures SiMS et des simulations des procédés de fabrication, les profiles de dopage et les caractéristiques électriques attendues pour des technologies innovantes sont explorés. Des prototypes ont été testés sous faisceau et soumis à des irradiations, afin d’évaluer les performances du détecteur et celles de son électronique associée. / The discovery of a scalar boson, known as the Higgs boson, marked the first LHC data period (2010-2012). Using mainly di-photon and di-Z decays, with the latest leading to a four leptons final state, the mass of the boson was measured with a precision of <0.2%. Relevant couplings were estimated by combining several final states, while corresponding uncertainties would largely benefit from the increased statistics expected during the next LHC data periods (Run 2, Phase 2).The H→ZZ*→4l channel, in spite of its suppressed brunching ratio, benefits from a weak background, making it a prime choice for the investigation of the new boson’s properties. In this thesis, the analysis aimed to the observation of this mode with the ALTAS detector is presented, with a focus on the measurement and control of the reducible electron background.In the context of preparation for future high luminosity data periods, foreseen from 2025 onwards, two distinct studies are conducted:The first concerns the observability potential of the Higgs associated production mode in conjunction with two b-quarks. A multivariate analysis based on simulated data confirms a very weak expected signal in the H→di-photon channel.The second revolves around the conception and development of an inner silicon detector capable of operating in the hostile environment of high radiation and increased occupancy, expected during LHC Phase 2. Main studies were concentrated on improving radiation hardness, geometrical and detection efficiency. Through fabrication process simulation and SiMS measurements, doping profiles and electrical characteristics, expected for innovative technologies, are explored. Prototypes were designed and evaluated in test beams and irradiation experiments in order to asses their performances and that of associated read-out electronics.

A test-bench is created that injects digital pulses that emulate ATLAS LAr Front End Board electronic signal pulses in order to test prototypes. The prototypes are for new electronics for an upgrade to the CERN Large Hadron Collider that increases the rate of proton-proton collisions by an order of magnitude. This High-Luminosity Large Hadron Collider requires a completely new Trigger and Data Acquisition system to deal with information from detectors. One such system that is currently being developed is the Liquid Argon Signal Processor (LASP) whose architecture is based on Field Programmable Gate Arrays (FPGA). Validation of individual modules of the LASP are of key importance in the development cycle. Additionally, verification of module behaviour with real ATLAS pulses will not be available until much later in the project timeline. The injector project is implemented on an Intel Stratix 10 FPGA, using a soft-core NIOS II processor for TCP/IP communication with a workstation in order to transfer Monte Carlo simulation pulses to the FPGA, where it is then stored in a 2 GB DDR3 external memory. The pulses are then retrieved into internal memory buffers and are transmitted to the LASP at 40 MHz. The user is in complete control of the data pulses injected which is a vital property that would test LASP behaviour for different cases and possible failure modes. / Graduate

Le thème des analyses présentées dans ce document est la mesure des propriétés du boson de Higgs se désintégrant dans le mode H→ZZ→4l dans l'expérience ATLAS au CERN. Le document commence par un résumé détaillé concernant la procédure d'étalonnage des électrons: l'algorithme de combinaison trace-cluster améliore la résolution en énergie (surtout pour les électrons ayant une faible énergie transverse) en exploitant les informations du cluster et de la trace dans un ajustement par maximum de vraisemblance. L'amélioration en résolution est approximativement de 18-20% pour les désintégrations du J/Ψ en di-électrons, et 3% pour Z→ee. Par la suite, la combinaison E-p est appliquée au canal H→ZZ→4l avec électrons dans l'état final permettant d’obtenir un gain modéré sur la distribution de la masse invariante (4-5%). En deuxième lieu, la masse du boson de Higgs et sa largeur sont estimées, en particulier afin de comprendre les effets apportés par l'utilisation de l'algorithme de combinaison trace-cluster. La masse a été calculée en se servant d'un ajustement à deux dimensions appliqué sur la masse invariante m4l et un score de discrimination du signal contre le bruit de fond ZZ*. Cette discrimination est obtenue en exploitant les corrélations angulaires dont les distributions sont sensibles au spin et à la parité du boson de Higgs. L’étude sur la largeur du boson est ensuite détaillée : les résultats sont basés sur une approche qui vise à contraindre cette largeur en analysant la région de haute masse m4l où le boson de Higgs se comporte comme un propagateur. La section efficace au pic de la résonance (« on-shell ») dépend de la largeur totale du boson de Higgs, ce qui n’est pas le cas pour la production dans la région de haute masse (« off-shell »). Par conséquent, des limites indirectes sur la largeur peuvent être déterminées en combinant les régions « on-shell » et « off-shell ». Une limite à 6.7 fois la largeur Higgs ΓSMH est obtenue via le canal 4l. En combinant la mesure « on-shell » avec tous les canaux de désintégration étudiés (notamment ZZ→4l, ZZ→2l2ν and WW→lνlν), les résultats aboutissent à une limite observée (attendue) sur la largeur totale de 22.7 (33.0) MeV. La dernière partie de ce travail de thèse est consacrée à l'analyse sur la largeur du boson de Higgs en quatre leptons à haute (High-Luminosity LHC) luminosité intégrée (respectivement 300 fb⁻¹ et 3000 fb⁻¹) : il s’agit d’une étude extrapolant à √s =14 TeV les techniques utilisées pour l’analyse à 8 TeV (Run 1). / The theme of the analyses presented in this Thesis is the measurement of the Higgs boson properties in the H→ZZ→4l decay channel with the ATLAS experiment at the LHC. A detailed overview on the electron calibration process is first presented. In this regard, the track-cluster combination algorithm is found to improve the energy resolution of low ET electrons by exploiting both track and cluster information into a maximum likelihood fit. The improvement in resolution is approximately 18-20% for J/Ψ dielectron decays, and of the order of 3% for Z→ee events. In addition, the E-p combination algorithm has also been applied to the H→ZZ→4l channel with electrons in the final state resulting in a non-negligible gain on the invariant mass distribution (4-5%). Secondly, the Higgs mass and its total width are evaluated in the H→ZZ→4l channel. The Higgs mass is measured in the 4l decay channel with particular interest on the beneficial effects brought by the improved electron calibration and the track-cluster combination. The mass on the full 2011 and 2012 datasets is worked out with a 2-dimensional fit on the invariant mass of the 4 lepton final state, m4l, and on a boosted decision tree (BDT)-based output conceived against the main ZZ irreducible background and constructed on variables that are sensitive to the Higgs boson spin-parity state. Regarding the Higgs width, results are based on a relatively recent approach aimed at indirectly constraining the Higgs boson width by exploiting the m4l high-mass region where the Higgs boson acts as a propagator. The Higgs production cross section in the on-shell m4l region, where the Higgs boson is a resonance, depends on the total Higgs width, whereas this is not the case for the high mass m4l (off-shell). Limits on the Higgs width can be therefore set when merging the off-shell results with the on-shell ones. A limit of ∼ 6.7 times ΓSMH is obtained in the four lepton channel. Secondly, by combining with the on-shell measurement and using all the decay channels in the analysis, i.e. ZZ→4l, ZZ→2l2ν and WW→lνlν, the results lead to an observed (expected) 95% C.L. upper limit on the Higgs boson total width of 22.7 (33.0) MeV (4.2 MeV is the Standard Model predicted Higgs width at mH=125 GeV).The last section of the thesis is devoted to the evaluation of the Higgs width at √s=14 TeV in the high luminosity scenario (High Luminosity LHC), 300 fb⁻¹ and 3000 fb⁻¹, by employing the same techniques exploited in the previous Run 1 analysis at √s=8 TeV.

Injection in the LHC is a delicate moment, since the LHC collimation system cannot offer adequate protection during beam transfer. For this reason, a complex chain of injection protection devices has been put in place. Among them, the SPS scrapers are the multi-turn cleaning system installed in the SPS aimed at halo removal immediately before injection in the LHC. The upgrade in luminosity of the LHC foresees beams brighter than those currently available in machine, posing serious problems to the performance of the existing injection protection systems. In particular, the integrity of beam-intercepting devices is challenged by unprecedented beam parameters, leading to interactions potentially destructive. In this context, a new design of scrapers has been proposed, aimed at improved robustness and performance. This thesis compares the two scraping systems, i.e. the existing one and the one proposed for upgrade. Unlike any other collimation system for regular halo cleaning, both are "fast" systems, characterised by the variation of the relative distance between the beam and the absorbing medium during cleaning, which enhances the challenge on energy deposition values. Assets/liabilities of the two systems are highlighted by means of numerical simulations and discussed, with particular emphasis on energy deposition in the absorbing medium, time evolution of the beam current during scraping and losses in the machine. Advantages of the system proposed for upgrade over the existing one are highlighted. The analysis of the existing system takes into account present operational conditions and addresses the sensitivity to settings previously not considered, updating and extending past studies. The work carried out on the upgraded system represents the first extensive characterisation of a multi-turn cleaning system based on a magnetic bump. Results have been obtained with the Fluka-SixTrack coupling, developed during this PhD activity from its initial version to being a state-of-art tracking tool for cleaning studies in circular machines. Relevant contributions to the development involve the handling of time-varying impact conditions. An extensive benchmark against a test of the scraper blades with beam has been carried out, to verify the reliability of results. Effcts induced in the tested blades confirm the high values of energy deposition predicted by the simulation. Moreover, the comparison with the time profile of the beam intensity measured during scraping allowed the reconstruction of the actual settings of the blades during the test. Finally, the good agreement of the quantitative benchmark against readouts of beam loss monitors finally proves the quality of the analyses and the maturity of the coupling.

SuperKEKB est un collisionneur à très haute luminosité construit pour l’expérience Belle II, constitué d’un anneau de basse énergie (LER) transportant des positrons de 4 GeV et d’un anneau de haute énergie (HER) où circulent des électrons de 7 GeV. Sa mise en service -ou commissioning- se déroulera en trois phases : La phase 1, durant laquelle des faisceaux circulent sans être focalisés au point de collision, a pour but de nettoyer la chambre à vide du gaz résiduel. La seconde phase, où le détecteur Belle II sera en partie installé, permettra le réglage du système de focalisation finale des faisceaux, jusqu’à atteindre une luminosité de 10³⁴ cm⁻² s⁻¹. La troisième phase correspondra au démarrage de l’expérience Belle II avec une luminosité visée de 10³⁵ cm⁻² s⁻¹ à 8×10³⁵ cm⁻² s⁻¹. Dans ce cadre, ma thèse porte sur la conception et la mise en place d’un système permettant le monitorage rapide de la luminosité, système nécessaire pour pouvoir corriger en temps réel les instabilités des faisceaux et ainsi maintenir une luminosité optimale. Afin d’atteindre la haute précision relative souhaitée, de l’ordre de 10⁻³ en 1 ms, la mesure sera basée sur le taux de comptage des particules issues de la diffusion Bhabha radiative à angle nul, processus bien connu et dont la section efficace est importante. Ces particules seront détectées au moyen d’un capteur en diamant, matériau résistant aux radiations et permettant une acquisition très rapide du signal, situé à l’extérieur de la chambre à vide et en aval du point d’interaction. La première partie de cette thèse est consacrée à la recherche des localisations optimales pour le positionnement des capteurs diamants dans chacun des deux anneaux. Au moyen de simulations détaillées, nous avons étudié la dynamique des particules Bhabha lors de leur transport dans les anneaux ainsi que leur interaction avec la matière de la chambre à vide. Ces études ont permis d’ une part d’ identifier un emplacement à 11.9m dans le LER et un autre à 30 m dans le HER, et d’autre part de redéfinir pour l’une d’ entre elle la géométrie locale du tube à vide. La seconde partie, plus expérimentale, s’articule autour de la première phase du commissioning de SuperKEKB et des mesures réalisées au moyen des capteurs diamants que nous avons installés. Dans un premier temps, une étude détaillée des processus de perte single beam (Bremsstrahlung, effet Touschek, diffusion coulombienne) a été réalisée pour le LER en fonction des paramètres du faisceau et du collisionneur (courant, pression, taille transverse des faisceaux). Dans un deuxième temps les résultats de cette étude ont été comparés aux données que nous avons prises de février à juin 2016. Nous avons pu mettre en évidence un bon accord qualitatif et quantitatif entre nos simulations et nos mesures. Cela nous a permis d’estimer que le niveau de bruit de fond attendu dans le cadre des mesures pour le monitorage de la luminosité sera de plus de deux ordres de grandeurs inférieurs au taux du processus Bhabha radiatif à angle nul. / SuperKEKB is a very high luminosity collider dedicated to the Belle II experiment, it consists of a Low Energy Ring (LER) of 4 GeV positrons and a High Energy Ring (HER) of 7 GeV electrons. The commissioning of this machine is split into three phases: phase 1 (single-beam phase) is dedicated to vacuum scrubbing, where beams circulate without focusing at the collision point. Phase 2, for which the major part of the Belle II detector will be installed, will enable the tuning of the final focus system to achieve a luminosity of 10³⁴ cm⁻² s⁻¹. During phase 3, Belle II physics runs will start with an aimed luminosity up to 8×10³⁵ cm⁻² s⁻¹. In this context, the aim of my thesis is to develop and install a fast luminosity monitoring system, which is required for online correction of beam instabilities and maintenance of optimal luminosity. To reach the aimed relative precision of 10⁻³ in 1 ms, the measurement will be based on the radiative Bhabha process at zero photon scattering angle, whose cross-section is large and well-known. These particles will be detected using diamond sensors, resistant to radiation and enabling very fast signal acquisition, to be placed outside of the beam-pipe and downstream of the interaction point. The first part of this work is dedicated to the investigation of the best locations for the diamond sensor positioning in both rings. Using detailed simulations, we studied the dynamics of Bhabha particles during their tracking in the rings and their interaction with the beam pipe material. This led to the identification of two positions, at 11.9 m in LER and at 30 m in HER, and to considering a new geometry for the vacuum pipe in the LER. The second part is related to the phase 1 of the SuperKEKB commissioning and concerns the measurements performed with the diamond sensors that were installed. Single beam loss processes (Bremsstrahlung, Touschek, beam-gas Coulomb scattering) were studied in detail with respect to the LER beam and ring parameters (current, pressure, transverse beam sizes). The results of this study were then compared to the data collected from February to June 2016. We found good qualitative and quantitative agreement between our simulations and measurements. From this we could estimate that the level of background to be expected during luminosity monitoring will be two orders of magnitude smaller than the rate of the radiative Bhabha scattering signal.

This thesis will present results on the measurement of Z boson production in the electron channel associated with b-jets in proton-proton collision at 13 TeV, a measurement of fundamental interest in precision measurements and physics searches. This measurement represents important precision tests of perturbative QCD and provides information related to the b-quark inside of the proton. This measurement also represents one of the main sources of background in top quark production studies, Higgs precision measurements and the search for supersymmetric particles. The results on cross section measurements and kinematic distributions presented in this work have been unfolded to particle level and are compared to the four and five flavor schemes in Monte Carlo generators, showing that the predictions are consistent within the experimental uncertainties. In this thesis it will be also shown results of the development of the new event selection system proposed for high luminosity measurements. This system will be implemented during the second LHC long shutdown (2019-2020), which involves studies on the reconstruction of electrons using the concept of supercells, where the fine granularity of the cells of the ATLAS Liquid Argon detector is exploited to mitigate the problems to be found in high luminosity conditions. One of the problems is the increase of pile-up, caused by secondary collisions of protons in the same event, generating a large number of low pT jets that can be erroneously identified as electrons. The studies in this work introduce new discriminants based on the electromagnetic shower shape, showing that is possible to further reduce the rate of low pT jets maintaining a good performance on electron reconstruction. / Esta tese apresentará resultados da medida da produção de bósons Z no canal de elétrons associado com jatos b em colisões próton-próton a 13 TeV, uma medida de fundamental interesse em medidas de precisão e buscas por nova física. Essa medida representa importantes testes de precisão da QCD perturbativa e prove informações relacionadas ao quark b dentro do próton. Essa medida também representa uma das principais fontes de background na produção do quark top, medidas de precisão do Higgs e na busca de partículas supersimétricas. Os resultados na medida da seção de choque e distribuições cinemáticas apresentadas neste trabalho são comparados com os esquemas de quatro e cinco sabores nos geradores Monte Carlo, mostrando que as predições são consistentes dentro das incertezas experimentais. Nesta tese também serão apresentados estudos do desenvolvimento de um sistema de seleção de eventos proposto para medidas em alta luminosidade. Esse sistema será implementado durante o segundo LHC long shutdown (2019-2020), envolvendo estudos na reconstrução de elétrons usando o conceito de supercélulas, onde a fina granularidade das células do detector de Argônio Líquido do ATLAS será aproveitada para mitigar as dificuldades que se terão em condições de alta luminosidade. Um dos problemas é o aumento do empilhamento (pile-up), causado por colisões secundárias de prótons no mesmo evento, gerando uma grande quantidade de jatos de baixo pT que podem ser erroneamente identificados como elétrons. Os estudos neste trabalho apresentam novos discriminantes com base na forma da cascata eletromagnética, mostrando que é possível reduzir ainda mais a taxa de jatos de baixo pT mantendo uma boa reconstrução de elétrons.

This thesis will present results on the measurement of Z boson production in the electron channel associated with b-jets in proton-proton collision at 13 TeV, a measurement of fundamental interest in precision measurements and physics searches. This measurement represents important precision tests of perturbative QCD and provides information related to the b-quark inside of the proton. This measurement also represents one of the main sources of background in top quark production studies, Higgs precision measurements and the search for supersymmetric particles. The results on cross section measurements and kinematic distributions presented in this work have been unfolded to particle level and are compared to the four and five flavor schemes in Monte Carlo generators, showing that the predictions are consistent within the experimental uncertainties. In this thesis it will be also shown results of the development of the new event selection system proposed for high luminosity measurements. This system will be implemented during the second LHC long shutdown (2019-2020), which involves studies on the reconstruction of electrons using the concept of supercells, where the fine granularity of the cells of the ATLAS Liquid Argon detector is exploited to mitigate the problems to be found in high luminosity conditions. One of the problems is the increase of pile-up, caused by secondary collisions of protons in the same event, generating a large number of low pT jets that can be erroneously identified as electrons. The studies in this work introduce new discriminants based on the electromagnetic shower shape, showing that is possible to further reduce the rate of low pT jets maintaining a good performance on electron reconstruction. / Esta tese apresentará resultados da medida da produção de bósons Z no canal de elétrons associado com jatos b em colisões próton-próton a 13 TeV, uma medida de fundamental interesse em medidas de precisão e buscas por nova física. Essa medida representa importantes testes de precisão da QCD perturbativa e prove informações relacionadas ao quark b dentro do próton. Essa medida também representa uma das principais fontes de background na produção do quark top, medidas de precisão do Higgs e na busca de partículas supersimétricas. Os resultados na medida da seção de choque e distribuições cinemáticas apresentadas neste trabalho são comparados com os esquemas de quatro e cinco sabores nos geradores Monte Carlo, mostrando que as predições são consistentes dentro das incertezas experimentais. Nesta tese também serão apresentados estudos do desenvolvimento de um sistema de seleção de eventos proposto para medidas em alta luminosidade. Esse sistema será implementado durante o segundo LHC long shutdown (2019-2020), envolvendo estudos na reconstrução de elétrons usando o conceito de supercélulas, onde a fina granularidade das células do detector de Argônio Líquido do ATLAS será aproveitada para mitigar as dificuldades que se terão em condições de alta luminosidade. Um dos problemas é o aumento do empilhamento (pile-up), causado por colisões secundárias de prótons no mesmo evento, gerando uma grande quantidade de jatos de baixo pT que podem ser erroneamente identificados como elétrons. Os estudos neste trabalho apresentam novos discriminantes com base na forma da cascata eletromagnética, mostrando que é possível reduzir ainda mais a taxa de jatos de baixo pT mantendo uma boa reconstrução de elétrons.

The Large Hadron Collider (LHC) at CERN is designed to search for new physics by colliding protons with a center-of-mass energy of 13 TeV. The ATLAS detector is a multipurpose particle detector built to record these proton-proton collisions. In order to improve sensitivity to new physics at the LHC, luminosity increases are planned for 2018 and beyond. With this greater luminosity comes an increase in the number of simultaneous proton-proton collisions per bunch crossing (pile-up). This extra pile-up has adverse effects on algorithms for clustering the ATLAS detector's calorimeter cells. These adverse effects stem from overlapping energy deposits originating from distinct particles and could lead to difficulties in accurately reconstructing events. Machine learning algorithms provide a new tool that has potential to improve clustering performance. Recent developments in computer science have given rise to new set of machine learning algorithms that, in many circumstances, out-perform more conventional algorithms. One of these algorithms, convolutional neural networks, has been shown to have impressive performance when identifying objects in 2d or 3d arrays. This thesis will develop a convolutional neural network model for calorimeter cell clustering and compare it to the standard ATLAS clustering algorithm. / Graduate

Pour la deuxième période de prise des données du LHC (Run 2) de 2015 - 2022, une quatrième couche de senseurs pixels a été installée dans le détecteur de l’existence ATLAS sur un tube de faisceau de plus petit diamètre afin d’ajouter de la redondance pour améliorer la reconstruction des trajectoires des particules chargées. Ce détecteur du pixel permettra d’assurer un suivi de la qualité de l’étiquetage des mesons b haute luminosité (b-tagging). Au cours des deux dernières années plusieurs composants ont été produits et assemblés sur des structures de soutien appelées échelles. Au total, 20 échelles ont été construites et qualifiées en cardere d’un procédé d’assurance qualité définie par ATLAS au CERN. Quatorze échelles ont été intégrées sur le tube de faisceau. Ceci constitue la première partie de la thèse dédiée à la partie construction du détecteur. La deuxième partie est consacrée à la recherche de boson de Higgs issu de collisions proton proton l’énergie du 8 TeV centre de masse de, se désintégrant en particules invisibles une luminosité integrée de 20.3 fb−1 enregistrées par le détecteur ATLAS au LHC. Les résultats sont interprétés dans les modèles de matière noire Higgs portail(Higgs portal Dark Matter). / For Run 2 of the LHC a fourth, innermost Pixel Detector layer on a smaller radius beampipe has been installed in the ATLAS Detector to add redundancy against radiation damage ofthe current Pixel Detector and to ensure a high quality tracking and b-tagging performance ofthe Inner Detector over the coming years until the High Luminosity Upgrade. State of the artcomponents have been produced and assembled onto support structures known as staves overthe last two years. In total, 20 staves have been built and qualified in a designated QualityAssurance setup at CERN of which 14 have been integrated onto the beam pipe. In the secondpart, A search for a Higgs boson produced via vector-boson fusion and decaying into invisibleparticles is discussed, using 20.3 fb−1 of proton proton collision data at the centre of massenergy of 8 TeV recorded by the ATLAS detector at the LHC. For a Higgs boson with a massof 125 GeV, assuming the Standard Model production cross section, an upper bound of 0.28is set on the branching fraction of H →invisible at 90% confidence level, where the expectedupper limit is 0.31. The results are interpreted in model of Higgs portal dark matter where thebranching fraction limit is converted into upper bounds on the dark matter nucleon scatteringcross section as a function of the dark matter particle mass, and compared to results from thedirect dark matter detection experiments.