492 research outputs found

    Radiation Monitoring with Diamond Sensors for the Belle-II Vertex Detector

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    The Belle II detector is currently under construction at the SuperKEKB electron-positron high-luminosity collider, that will provide an instantaneous luminosity 40 times higher than that of KEKB. There- fore the Belle-II VerteX Detector (VXD) will operate in a very harsh environment. A radiation monitoring and beam abort system is needed to safely operate the VXD detector in these conditions. This system is based on 20 single crystal CVD diamond sensors placed in 20 key positions in the vicinity of the VXD and interaction region. In this contribution we describe the system design and we present the procedures followed for the characterisation and calibration of the diamond sensors. We discuss also the performance of the prototype system during the first SuperKEKB commissioning phase in February-June 2016

    Measurement of the production cross section of a Z boson in association with exactly one or at least two b jets with the CMS experiment at LHC

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    2013/2014Il Modello Standard fornisce una delle piu’ soddisfacenti descrizioni delle proprieta’ delle particelle elementari e delle loro interazioni dato il notevole accordo esistente tra le sue predizioni ed i risultati sperimentali conseguiti negli ultimi decenni. Le predizioni teoriche di questo modello sono state verificate in maniera sempre piu’ accurata da numerosi esperimenti ma ci sono ancora alcuni aspetti che non trovano un’ adeguata spiegazione all’interno di questa teoria. Gli esperimenti a LHC si propongono, quindi, di fornire una risposta alle domande rimaste ancora aperte e ai numerosi problemi teorici irrisolti che possono rappresentare un’ indicazione per la presenza di nuova fisica. L’esplorazione di scenari di nuova fisica non puo’, comunque, prescindere da una solida conoscenza dei processi di Modello Standard che possono rappresentare un fondo importante. Per questo motivo i vasti programmi di fisica degli esperimenti di LHC includono anche misure di processi di Modello Standard. Inoltre, lo studio di questi processi e’ importante in quanto rappresentano test fondamentali per le predizioni teoriche di Cronodinamica Quantistica perturbativa. La produzione associata di bosoni Z in associazione con uno o piu’ getti provenienti dall’adronizzazione di quark pesanti fornisce un test unico per alcune predizioni teoriche. Questo lavoro di tesi verte sullo studio della produzione associata di un bosone Z con esattamente un getto di tipo b o almeno due getti di tipo b. Per eseguire questa misura sono stati usati i dati forniti nel 2012 da LHC ad una energia del centro di massa di 8 TeV e raccolti dall’esperimento CMS (corrispondente ad una luminosita’ integrata di 19.8 fb−1). La prima parte dell’analisi e’ incentrata sulla selezione degli eventi: specifiche richieste su alcune variabili vengono applicate per discriminare la segnatura Z+b. I diversi contributi dei fondi sono estratti da simulazioni Monte Carlo fatta eccezione per il fondo da quark top che e’ stato valutato dai dati. Particolare attenzione e’ stata posta per estrarre la purezza dell’evento, ovvero stimare il vero numero di getti di tipo b. Questo e’ stato fatto simultaneamente nei campioni selezionati Z + 1b e Z + 2b per poter tenere conto di possibili migrazioni di eventi da un campione all’altro a causa di inefficenze dell’algoritmo usato per l’identificazione del tipo di getto. Per poter confrontare i risultati sperimentali ottenuti con i calcoli teorici e’ necessario correggere le quantita’ misurate dagli effetti del rivelatore. Diverse sorgenti di incertezze sistematiche sulla misura sono state, infine, studiate. Le sezioni d’urto differenziali misurate in funzione di diverse osservabili sono state quindi confrontate con diverse predizioni teoriche che implementano diverse approcci nel calcolo (schema 4-flavor, schema 5-flavor).XXVII Ciclo198

    The Monitoring System of the Belle II Vertex Detector

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    The Belle II VerteX Detector (VXD) is a 6 layers silicon tracker device that will cope with an unprecedented luminosity of 8 7 10 35 cm 12 2 s 12 1 achievable by the new SuperKEKB e + e 12 collider, at the KEK laboratory (Tsukuba, Japan). All environment parameters such as temperature, humidity and radiation levels, must be constantly monitored and under certain conditions action must be promptly taken, such as interlocking the power supply or delivering an abort signal to the SuperKEKB collider. In this contribution we describe the Belle II VXD monitoring system. We also present the first results of the temperature and humidity system commissioned in a Beam Test at DESY in April 2016 and the preliminary results of the radiation monitoring achieved with a prototype system during the first SuperKEKB commissioning phase at KEK in February-June 2016

    The Belle II SVD detector

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    The Belle II VerteX Detector (VXD) is a 6 layers silicon tracker device that will cope with an unprecedented luminosity of 8 7 10 35 cm 12 2 s 12 1 achievable by the new SuperKEKB e + e 12 collider, at the KEK laboratory The Silicon Vertex Detector (SVD) is one of the main detectors in the Belle II experiment at KEK, Japan. In combination with a pixel detector, the SVD determines precise decay vertices and performs low-momentum track reconstruction. The SVD ladders are being developed at several institutes. For the development of the tracking algorithm as well as the performance estimation of the ladders, beam tests for the ladders were performed. We report an overview of the SVD development, its performance measured in the beam tests, and the prospect of its assembly and commissioning until installation

    Performance studies of the Belle II Silicon Vertex Detector with data taken at the DESY test beam in April 2016

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    Belle II is a multipurpose detector currently under construction and it will be operated at the next generation B-factory SuberKEKB in Japan. The main Belle II devices, devoted to the vertex reconstruction, are the Silicon Vertex Detector (SVD) and the Pixel Detector (PXD). In April 2016 a sector of the Belle II SVD and PXD has been tested in a beam of high energetic electrons at the test beam facility at DESY Hamburg (Germany). We report here the results for the hit efficiency estimation and the measurement of the resolution for the Belle II silicon vertex detector. Hit efficiencies are measured to be on average above 99.5% and the measured resolution is within the expectations

    The Software Framework of the Belle II Silicon Vertex Detector and its Development for the 2016 Test-Beam at DESY

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    The Silicon Vertex Detector of Belle II will be fundamental not only for the reconstruction of B meson vertices, but also for the reconstruction of neutral particles like K S , and the tracking of low- p t particles like slow pions associated with the decays of the abundant D 17 mesons. As a consequence great importance is given to the performance of the software that deals with the simulation and reconstruction of the SVD events. The Belle II experiment is the successor to Belle, one of the two experiments that first observed CP violation in the decay of B mesons. The SuperKEKB collider will deliver 40 times more luminosity than its predecessor KEKB, which requires major upgrades to the detector hardware. This also poses new challenges not only for the data collection and storage, but also for the software framework which is used to process and analyse the experimental data. In this article we present in detail the SVD software framework together with its development for the test of the Vertex Detector system, which took place at DESY in April 2016 using an electron beam

    Search for supersymmetry in electroweak production with photons and large missing transverse energy in pp collisions at sqrt(s) = 8 TeV

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    Results are reported from a search for supersymmetry with gauge-mediated supersymmetry breaking in electroweak production. Final states with photons and large missing transverse energy ( View the MathML sourceETmiss) were examined. The data sample was collected in pp collisions at View the MathML sources=8TeV with the CMS detector at the LHC and corresponds to 7.4View the MathML sourcefb−1. The analysis focuses on scenarios in which the lightest neutralino has bino- or wino-like components, resulting in decays to photons and gravitinos, where the gravitinos escape undetected. The data were obtained using a specially designed trigger with dedicated low thresholds, providing good sensitivity to signatures with photons, View the MathML sourceETmiss, and low hadronic energy. No excess of events over the standard model expectation is observed. The results are interpreted using the model of general gauge mediation. With the wino mass fixed at 10View the MathML sourceGeV above that of the bino, wino masses below 710View the MathML sourceGeV are excluded at 95% confidence level. Constraints are also set in the context of two simplified models, for which the analysis sets the lowest cross section limits on the electroweak production of supersymmetric particles

    Measurements of t t-bar spin correlations and top quark polarization using dilepton final states in pp collisions at sqrt(s) = 8 TeV

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    Measurements of the top quark-antiquark (tt¯) spin correlations and the top quark polarization are presented for tt¯ pairs produced in pp collisions at s√=8  TeV. The data correspond to an integrated luminosity of 19.5  fb−1 collected with the CMS detector at the LHC. The measurements are performed using events with two oppositely charged leptons (electrons or muons) and two or more jets, where at least one of the jets is identified as originating from a bottom quark. The spin correlations and polarization are measured from the angular distributions of the two selected leptons, both inclusively and differentially, with respect to the invariant mass, rapidity, and transverse momentum of the tt¯ system. The measurements are unfolded to the parton level and found to be in agreement with predictions of the standard model. A search for new physics in the form of anomalous top quark chromo moments is performed. No evidence of new physics is observed, and exclusion limits on the real part of the chromo-magnetic dipole moment and the imaginary part of the chromo-electric dipole moment are evaluated

    Differential cross section measurements for the production of a W boson in association with jets in proton-proton collisions at sqrt(s) = 7 TeV

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    Measurements are reported of differential cross sections for the production of a W boson, which decays into a muon and a neutrino, in association with jets, as a function of several variables, including the transverse momenta (pTpT) and pseudorapidities of the four leading jets, the scalar sum of jet transverse momenta (HTHT), and the difference in azimuthal angle between the directions of each jet and the muon. The data sample of pp collisions at a centre-of-mass energy of 7 TeV was collected with the CMS detector at the LHC and corresponds to an integrated luminosity of 5.0 fb−1. The measured cross sections are compared to predictions from Monte Carlo generators, MadGraph + pythia and sherpa, and to next-to-leading-order calculations from BlackHat + sherpa. The differential cross sections are found to be in agreement with the predictions, apart from the pTpT distributions of the leading jets at high pTpT values, the distributions of the HTHT at high-HTHT and low jet multiplicity, and the distribution of the difference in azimuthal angle between the leading jet and the muon at low values
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