Associated production of W bosons and two jets from b quarksat the CMS experiment.

2013/2014Il Modello Standard delle particelle elementari è senza dubbio una delle teorie che hanno riscosso il maggior quantitativo di successo nella storia della fisica. La recente scoperta del bosone di Higgs, ad opera degli esperimenti ATLAS e CMS operanti presso il Large Hadron Collider (LHC) di Ginevra, è solamente l’ultima di una lunga serie di predizioni teoriche confermate dall’esperimento. Il meccanismo di produzione di quark b in associazione a bosoni vettori W e Z si inquadra all’interno della fenomenologia del Modello Standard, ma la sua predizione teorica presenta alcune interessanti sfide collegate alla complessità dei calcoli perturbativi nel campo della Cromodinamica Quantistica e alla descrizione del quark b all’interno del protone. Molti sviluppi sono stati fatti di recente in questo contesto tramite lo sviluppo di diversi modelli per la produzione del quark b. Questi risultati presuppongono diverse approssimazioni allo scopo di rendere il calcolo fattibile. La misura della sezione d’urto di produzione per questi processi costituisce quindi un ingrediente importante alla comprensioni di simili processi. Nel presente lavoro, la sezione d’urto per la produzione inclusiva di un bosone vettore W in associazione con una coppia di quark b è stata misurata sfruttando l’insieme di collisioni protone-protone prodotte da LHC ad un energia nel centro di massa pari a 8 TeV. Si è osservato il bosone vettore W tramite il suo decadimento in un elettrone o in un muone ed in un neutrino, mentre ciascun getto adronico originato da un b quark è stati identificato tramite la presenza di un adrone B associato ad un vertice di decadimento secondario. L’analisi è stata effettuata su un campione di collisioni consistente di una luminosità pari a 19.6/fb. La misura della produzione di eventi W+bb è inoltre importante in quanto essa costituisce un fondo irriducibile ad altri processi fisici di grande interesse quali la produzione di W in associazione al bosone di Higgs nel suo canale di decadimento in coppie di quark b. Altri processi del Modello Standard che producono uno stato finale simile al processo studiato nella presente tesi sono costituiti dalla produzione di coppie di quark top , quark top singoli o dalla produzione di coppie di bosoni vettori. Infine, lo stato finale del processo W+bb può costituire un fondo irriducibile per processi di nuova fisica oltre il Modello Standard, quali ad esempio la produzione di particelle supersimmetriche.The Standard Model of elementary particles is without any doubt one of the most successful theories in the history of physics. The recent discovery of the Higgs boson, thanks to the analysis of the Large Hadron Collider data performed by the ATLAS and CMS experiments, is the last and most striking of a long series of predictions of this theory confirmed by experimental evidence. The production mechanism of bottom quark pairs in association with W or Z vector bosons in proton-proton collisions is comprised within the phenomenology of the Standard Model, but nevertheless its theoretical prediction presents some interesting challenges which are connected with the complexity of perturbative calculations in the context of QCD interactions and with the description of the b quark content of the proton. Many developments have been recently made in this regard with the implementation of several models for the production of b quarks. These results are based on different approximations in order to overcome the difficulties of this calculation. A precise experimental measurement of the production cross section for these processes therefore provides an important input to the refinement of perturbative QCD calculations. In this work, the inclusive production cross section for a W boson and a pair of jets from b quarks is measured using the sample of proton-proton collisions at a center of mass energy of 8 TeV collected by the CMS experiment at the LHC. The W boson is observed via its decay to a muon or electron and a neutrino, while each b-jet is identified by the presence of a B hadron with a displaced secondary vertex. The analysis is performed on a sample of proton-proton collisions corresponding to an integrated luminosity of 19.6/fb. The measurement of the production of W+bb events is also important because this process is an irreducible background to other interesting physics processes involving the production of a Standard Model Higgs boson decaying to a b-jet pair in association with a W. Other Standard Model processes produce final states with a similar experimental signature to the one studied in this thesis, such as the production of top quark pairs, single top quarks or dibosons. Eventually, the final state of W+bb events may mimic the decay of particles predicted by new physics theories beyond the Standard Model, most notably the decay of particles predicted by Supersymmetry.XXVII Ciclo198

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

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

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

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

Measurement of transverse momentum relative to dijet systems in PbPb and pp collisions at sqrt(s_NN) = 2.76 TeV

An analysis of dijet events in PbPb and pp collisions is performed to explore the properties of energy loss by partons traveling in a quark-gluon plasma. Data are collected at a nucleon-nucleon center-of-mass energy of 2.76 TeV at the LHC. The distribution of transverse momentum (p T) surrounding dijet systems is measured by selecting charged particles in different ranges of p T and at different angular cones of pseudorapidity and azimuth. The measurement is performed as a function of centrality of the PbPb collisions, the p T asymmetry of the jets in the dijet pair, and the distance parameter R used in the anti-k T jet clustering algorithm. In events with unbalanced dijets, PbPb collisions show an enhanced multiplicity in the hemisphere of the subleading jet, with the p T imbalance compensated by an excess of low-p T particles at large angles from the jet axes

Measurement of associated W + charm production in pp collisions at sqrt(s) = 7 TeV

Measurements are presented of the associated production of a W boson and a charm-quark jet (W + c) in pp collisions at a center-of-mass energy of 7 TeV. The analysis is conducted with a data sample corresponding to a total integrated luminosity of 5 fb−1, collected by the CMS detector at the LHC. W boson candidates are identified by their decay into a charged lepton (muon or electron) and a neutrino. The W + c measurements are performed for charm-quark jets in the kinematic region pjetT > 25 GeV, |ηjet| 25 GeV) and σ(pp → W + c + X)×B(W → lν) = 84.1 ± 2.0 (stat.) ± 4.9 (syst.) pb (plT > 35 GeV), and the cross section ratios σ(pp → W+ + c ̄ ̄ + X)/σ(pp → W− + c + X) = 0.954 ± 0.025 (stat.) ± 0.004 (syst.) (plT > 25 GeV) and σ(pp → W+ + c ̄ ̄ + X)/σ(pp → W− + c + X) = 0.938 ± 0.019 (stat.) ± 0.006 (syst.) (plT > 35 GeV). Cross sections and cross section ratios are also measured differentially with respect to the absolute value of the pseudorapidity of the lepton from the W-boson decay. These are the first measurements from the LHC directly sensitive to the strange quark and antiquark content of the proton. Results are compared with theoretical predictions and are consistent with the predictions based on global fits of parton distribution functions

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

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

Search for pair production of excited top quarks in the lepton+jets final state

A search is performed for pair-produced spin-3/2 excited top quarks (t∗t ̄∗), each decaying to a top quark and a gluon. The search uses data collected with the CMS detector from pp collisions at a center-of-mass energy of s√ = 8 TeV, selecting events that have a single isolated muon or electron, an imbalance in transverse momentum, and at least six jets, of which one must be compatible with originating from the fragmentation of a b quark. The data, corresponding to an integrated luminosity of 19.5 fb−1, show no significant excess over standard model predictions, and provide a lower limit of 803 GeV at 95% confidence on the mass of the spin-3/2 t* quark in an extension of the Randall-Sundrum model, assuming a 100% branching fraction of its decay into a top quark and a gluon. This is the first search for a spin-3/2 excited top quark performed at the LHC

Search for new physics in the multijet and missing transverse momentum final state in proton-proton collisions at sqrt(s) = 8 TeV

A search for new physics is performed in multijet events with large missing transverse momentum produced in proton-proton collisions at s√ = 8 TeV using a data sample corresponding to an integrated luminosity of 19.5 fb−1 collected with the CMS detector at the LHC. The data sample is divided into three jet multiplicity categories (3-5, 6-7, and ≥8 jets), and studied further in bins of two variables: the scalar sum of jet transverse momenta and the missing transverse momentum. The observed numbers of events in various categories are consistent with backgrounds expected from standard model processes. Exclusion limits are presented for several simplified supersymmetric models of squark or gluino pair production

Sistema Integrato Multicentrico di Indicatori. Rapporto 2005. Provincia di La Spezia.

The SIMI (Integrated System of Indicators multicenter) Project contributes to the development of an integrated management of informative data streams related to drug addicted persons. This report analyzes the phenomenon of addiction on the territory of the La Spezia Province through a description of the network services that provide care and rehabilitation of those addicted. Besides the characteristics of users of local services for addictions, has been developed the analysis of the subjects reported to the prefectures for use of illegal drugs and any action taken. Standard methods of estimation were also applied to quantify the proportion of users of substances that do not relate to services and to identify certain characteristicsIl Progetto SIMI (Sistema Integrato Multicentrico di Indicatori) intende contribuire allo sviluppo di una gestione integrata e sinergica dei flussi informativi relativi ai consumatori di sostanze stupefacenti afferenti alle diverse amministrazioni dello Stato. In linea con quanto proposto dall\u27Osservatorio europeo di Lisbona, per la descrizione e analisi del fenomeno connesso all\u27uso/abuso di sostanze, risulta di fondamentale importanza la possibilit? di ottenere informazioni esaustive e comparabili sulle persone che usano e/o abusano di sostanze psicotrope. Il presente rapporto analizza il fenomeno delle dipendenze sul territorio della provincia di La Spezia attraverso la descrizione della rete dei servizi preposti alla cura e riabilitazione dei soggetti tossicodipendenti. Accanto alle caratteristiche degli utenti dei servizi territoriali per le dipendenze, ? stata sviluppata l\u27analisi dei soggetti segnalati alle Prefetture per uso di sostanze illegali e degli eventuali provvedimenti adottati. Sono state inoltre applicate metodologie standard di stima per quantificare la quota parte di utilizzatori di sostanze che non afferiscono ai servizi e per identificarne alcune caratteristiche

Combined search for the quarks of a sequential fourth generation

Results are presented from a search for a fourth generation of quarks produced singly or in pairs in a data set corresponding to an integrated luminosity of 5 inverse femtobarns recorded by the CMS experiment at the LHC in 2011. A novel strategy has been developed for a combined search for quarks of the up and down type in decay channels with at least one isolated muon or electron. Limits on the mass of the fourth-generation quarks and the relevant Cabibbo-Kobayashi-Maskawa matrix elements are derived in the context of a simple extension of the standard model with a sequential fourth generation of fermions. The existence of mass-degenerate fourth-generation quarks with masses below 685 GeV is excluded at 95% confidence level for minimal off-diagonal mixing between the third- and the fourth-generation quarks. With a mass difference of 25 GeV between the quark masses, the obtained limit on the masses of the fourth-generation quarks shifts by about +/- 20 GeV. These results significantly reduce the allowed parameter space for a fourth generation of fermions.Comment: Replaced with published version. Added journal reference and DO