Professor Daniela Bortoletto

Abstract:

The performance of τ-lepton reconstruction and identification algorithms is studied using a data sample of proton-proton collisions at s = 7 TeV, corresponding to an integrated luminosity of 36 pb-1 collected with the CMS detector at the LHC. The τ leptons that decay into one or three charged hadrons, zero or more short-lived neutral hadrons, and a neutrino are identified using final-state particles reconstructed in the CMS tracker and electromagnetic calorimeter. The reconstruction efficiency of the algorithms is measured using τ leptons produced in Z-boson decays. The τ-lepton misidentification rates for jets and electrons are determined. © 2012 CERN for the benefit of the CMS collaboration, published under license by IOP Publishing Ltd and SISSA.

Abstract:

A study of dijet production in proton-proton collisions was performed at √s = 7TeV for jets with pT>35 GeV and {pipe}y{pipe}<4. 7 using data collected with the CMS detector at the LHC in 2010. Events with at least one pair of jets are denoted as "inclusive". Events with exactly one pair of jets are called "exclusive". The ratio of the cross section of all pairwise combinations of jets to the exclusive dijet cross section as a function of the rapidity difference between jets {pipe}Δy{pipe} is measured for the first time up to {pipe}Δy{pipe}=9. 2. The ratio of the cross section for the pair consisting of the most forward and the most backward jet from the inclusive sample to the exclusive dijet cross section is also presented. The predictions of the Monte Carlo event generators pythia6 and pythia8 agree with the measurements. In both ratios the herwig++ generator exhibits a more pronounced rise versus {pipe}Δy{pipe} than observed in the data. The BFKL-motivated generators cascade and hej+ariadne predict for these ratios a significantly stronger rise than observed. © 2012 CERN for the benefit of the CMS collaboration.

Abstract:

A search for the rare decays B0s → μ+μ- and B0 → μ+ μ- is performed in pp collisions at ps = 7TeV, with a data sample corresponding to an integrated luminosity of 5 fb-1 collected by the CMS experiment at the LHC. In both decays, the number of events observed after all selection requirements is consistent with the expectation from background plus standard model signal predictions. The resulting upper limits on the branching fractions are B(B0s → μ+ μ-) < 7:7 × 10-9 and B(B0 → μ+μ-) < 1:8 × 10-9 at 95% confidence level.

Abstract:

A search for the standard model Higgs boson decaying into two Z bosons with subsequent decay into a final state containing two quark jets and two leptons, H → ZZ*(→) ! qq-ℓ ℓ+ is presented. Results are based on data corresponding to an integrated luminosity of 4.6 fb-1 of proton-proton collisions at ps = 7TeV, collected with the CMS detector at the LHC. In order to discriminate between signal and background events, kinematic and topological quantities, including the angular spin correlations of the decay products, are employed. Events are further classiéd according to the probability of the jets to originate from quarks of light or heavy avor or from gluons. No evidence for the Higgs boson is found, and upper limits on its production cross section are determined for a Higgs boson of mass between 130 and 600 GeV.

Abstract:

Results are presented on a search for a light charged Higgs boson that can be produced in the decay of the top quark t → H+b and which, in turn, decays into τ+νt. The analysed data correspond to an integrated luminosity of about 2 fb-1 recorded in protonproton collisions at √s = 7 TeV by the CMS experiment at the LHC. The search is sensitive to the decays of the top quark pairs tt̄ → H±W∓bb̄ and tt̄ → H ±H∓bb̄. Various final states have been studied separately, all requiring presence of a τ lepton from H+ decays, missing transverse energy, and multiple jets. Upper limits on the branching fraction B(t → H+b) in the range of 2-4% are established for charged Higgs boson masses between 80 and 160 GeV, under the assumption that B(H+ → τ+ντ) = 1.