1,842 research outputs found

    Search for new physics contributions entering the tWb electroweak vertex at 13 TeV with the CMS experiment

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    The Standard Model (SM) of Particles Physics is the current framework in which the electromagnetic, weak, and strong interactions successfully find an explanation. The SM is verified with high precision for most processes it describes, and all particles it predicted have been now been observed, after the discoveries of the Higgs boson at the LHC in 2012 and of the top quark at the TeVatron in 1994. Since then, intense studies have been performed to measure the features and the properties of such particles and their couplings. The top quark in particular holds a special place in the Standard Model, as it is the most massive elementary particle ever discovered. As a consequence of its mass, it is also characterised by a very short lifetime: the decay occurs before it can hadronise, implying that most of the top quark properties can be directly inferred from the its decay products, as if it was a free particle. Another notable feature of top quark physics is that a distinct hierarchy can be identified in top quark decays: it almost exclusively decays in a W boson and a b quark. Decay modes in a W boson and a d or s quarks are allowed, but they are strongly suppressed, so much that up to now they have never been studied directly. This feature stems from the fact that there is a distinct preference of top quarks for couplings with b quarks via electroweak charged current interactions. The mixing among families is regulated in the SM by a matrix of fundamental parameters named the Cabibbo-Kobayashi-Maskawa (CKM) matrix. Through the study of top quark decays and electroweak couplings it is possible to extract the magnitude of the CKM matrix elements related to the third row, which, according to constraints from low energy measurements, should exhibit a remarkable hierarchical structure. Such hierarchical structure might also be an indication of a new more fundamental underlying physics model. In order to precisely study this sector of the SM, the best tool currently available is the Large Hadron Collider (LHC). The LHC is a circular accelerator designed to provide proton-proton collisions with a luminosity of 1034^{34} cm−2^{−2}s−1^{−1} and a centre-of-mass energy of sqrt(s) = 14 TeV. The LHC is equipped with four main experiments: ALICE, ATLAS, CMS, and LHCb. The aim of LHC is to provide further proof on the validity of the SM and to give clues of new physics at the TeV scale. The first goal of the present thesis work is the first direct measurement of the CKM matrix elements |Vtb|,|Vts| and |Vtd| in events where single top quarks are produced via electroweak charged-current interactions. The main mechanisms for electroweak charged current production of single top quarks is the t−channel. The data analysed correspond to an integrated luminosity of 35.9 fb−1^{−1} of proton-proton collisions at a centre-of-mass energy of sqrt(s) = 13 TeV collected with the CMS experiment at the LHC during 2016. The performed analysis explicitly probes in great detail the left-handed only interactions at the tWb vertex at energy regimes around the electroweak scale. New physics might rise in right-handed couplings, or manifest at higher energy regimes, both indirectly or directly via new resonances. There is experimental evidence, both in particle physics and in astrophysics observations, suggesting that the SM in not the ultimate fundamental theory. Many physics models have been proposed to extend it in a more general picture, so to provide an explanation to such phenomena, and several of them predict new particles that could have a mass larger than the one of the top quark. The top quark could therefore have a privileged relationship with new physics particles and play a crucial role in their discovery. The LHC is the perfect tool to perform direct searches for new particles thanks to the high-energy collisions and the large number of top quark produced. In the present work a search for a beyond the SM Wâ€Č boson decaying in tb quarks in leptonic final states is presented. Data from proton-proton collisions with a centre-of-mass energy of sqrt(s) = 13 TeV, corresponding to 137.2 fb−1^{−1}, collected by the CMS experiment at the LHC from 2016 to 2018 are analysed. An upgrade of the LHC accelerator complex in the next years will allow to significantly increase the collision rate, further improving the results obtained so far and effectively extending the physics reach of the machine. The CMS experiment foresees a series of detector upgrades to cope with the new challenging conditions. Part of this work documents the testing and validation of the Gas Electron Multiplier chambers that are part of the Muon System upgrade

    Machine Learning Applications for Jet Tagging in the CMS Experiment

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    The fundamental physics research at the frontier accessible by today's particle accelerators such as the CERN Large Hadron Collider pose unique challenges in terms of complexity and abundance of data to analyse. In this context, it is of paramount importance to develop algorithms capable of dealing with multivariate problems to enhance humans' ability to interpret data and ultimately increase the discovery potential of the experiments. Machine learning techniques therefore assume an increasingly important role in the experiments at the LHC. In this work, we give an overview of the latest developments in this field, with a particular focus on the algorithms developed and used within the CMS Collaboration. The review follows this structure: (1) Introduction presents the CMS Experiment at LHC and the most common methods used in particle physics; (2) Jet Flavour Tagging briefly describes the main algorithms used to reconstruct heavy-flavour jets; (3) Jet Substructure and Deep Tagging focuses on the identification of heavy-particle decay in boosted jets; (4) Analysis Applications gives examples of applying the algorithm in physics analyses; and (5) Conclusions summarises the state-of-the-art and gives indications for future studies

    Refinement of a 0D Turbulence Model to Predict Tumble and Turbulent Intensity in SI Engines. Part II: Model Concept, Validation and Discussion

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    As known, reliable information about underlying turbulence intensity is a mandatory pre-requisite to predict the burning rate in quasi-dimensional combustion models. Based on 3D results reported in the companion part I paper, a quasi-dimensional turbulence model, embedded under the form of "user routine" in the GT-Power\u2122 software, is here presented in detail. A deep discussion on the model concept is reported, compared to the alternative approaches available in the current literature. The model has the potential to estimate the impact of some geometrical parameters, such as the intake runner orientation, the compression ratio, or the bore-to-stroke ratio, thus opening the possibility to relate the burning rate to the engine architecture. Preliminarily, a well-assessed approach, embedded in GT-Power commercial software v.2016, is utilized to reproduce turbulence characteristics of a VVA engine. This test showed that the model fails to predict tumble intensity for particular valve strategies, such LIVC, thus justifying the need for additional refinements. The model proposed in this work is conceived to solve 3 balance equations, for mean flow kinetic energy, tumble vortex momentum, and turbulent kinetic energy (3-eq. concept). An extended formulation is also proposed, which includes a fourth equation for the dissipation rate, allowing to forecast the integral length scale (4-eq. concept). The impact of the model constants is parametrically analyzed in a first step, and a tuning procedure is advised. Then, a comparison between the 3- and the 4-eq. concepts is performed, highlighting the advantages of the 3-eq. version, in terms of prediction accuracy of turbulence speed-up at the end of the compression stroke. An extensive 3-eq. model validation is then realized according to different valve strategies and engine speeds. The user-model is then utilized to foresee the effects of main geometrical parameters analyzed in part I, namely the intake runner orientation, the compression ratio, and the bore-to-stroke ratio. A two-valve per cylinder engine is also considered. Temporal evolutions of 0D- and 3D-derived mean flow velocity, turbulent intensity, and tumble velocity present very good agreements for each investigated engine geometry and operating condition. The model, particularly, exhibits the capability to accurately predict the tumble trends by varying some geometrical parameter of the engine, which is helpful to estimate the related impact on the burning rate. Summarizing, the developed 0D model well estimates the in-cylinder turbulence characteristics, without requiring any tuning constants adjustment with engine speed and valve strategy. In addition, it demonstrates the capability to properly take into account the intake duct orientation and the compression ratio without tuning adjustments. Some minor tuning variation allows predicting the effects of bore-to-stroke ratio, as well. Finally, the model is verified to furnish good agreements also for a two-valve per cylinder engine, and with reference to two different high-performance engines

    Role of prophylactic central compartment lymph node dissection in clinically N0 differentiated thyroid cancer patients: Analysis of risk factors and review of modern trends

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    In the last years, especially thanks to a large diffusion of ultrasound-guided FNBs, a surprising increased incidence of differentiated thyroid cancer (DTC), "small" tumors and microcarcinomas have been reported in the international series. This led endocrinologists and surgeons to search for "tailored" and "less aggressive" therapeutic protocols avoiding risky morbidity and useless "overtreatment". Considering the most recent guidelines of referral endocrine societies, we analyzed the role of routine or so-called prophylactic central compartment lymph node dissection (RCLD), also considering its benefits and risks. Literature data showed that the debate is still open and the surgeons are divided between proponents and opponents of its use. Even if lymph node metastases are commonly observed, and in up to 90 % of DTC cases micrometastases are reported, the impact of lymphatic involvement on long-term survival is subject to intensive research and the best indications of lymph node dissection are still controversial. Identification of prognostic factors for central compartment metastases could assist surgeons in determining whether to perform RLCD. Considering available evidence, a general agreement to definitely reserve RCLD to "high-risk" cases was observed. More clinical researches, in order to identify risk factors of meaningful predictive power and prospective long-term randomized trials, should be useful to validate this selective approach

    Performance of CMS muon reconstruction in pp collision events at sqrt(s) = 7 TeV

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    The performance of muon reconstruction, identification, and triggering in CMS has been studied using 40 inverse picobarns of data collected in pp collisions at sqrt(s) = 7 TeV at the LHC in 2010. A few benchmark sets of selection criteria covering a wide range of physics analysis needs have been examined. For all considered selections, the efficiency to reconstruct and identify a muon with a transverse momentum pT larger than a few GeV is above 95% over the whole region of pseudorapidity covered by the CMS muon system, abs(eta) < 2.4, while the probability to misidentify a hadron as a muon is well below 1%. The efficiency to trigger on single muons with pT above a few GeV is higher than 90% over the full eta range, and typically substantially better. The overall momentum scale is measured to a precision of 0.2% with muons from Z decays. The transverse momentum resolution varies from 1% to 6% depending on pseudorapidity for muons with pT below 100 GeV and, using cosmic rays, it is shown to be better than 10% in the central region up to pT = 1 TeV. Observed distributions of all quantities are well reproduced by the Monte Carlo simulation.Comment: Replaced with published version. Added journal reference and DO

    Search for the standard model Higgs boson in the H to ZZ to 2l 2nu channel in pp collisions at sqrt(s) = 7 TeV