A study of tau identification with the CMS detector at the LHC

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Physics, 2008.Includes bibliographical references (p. 49-50).In this thesis I explore the identification of [tau] leptons from simulated reconstructed data that will be collected by the Compact Muon Solenoid (CMS) detector on the Large Hadron Collider (LHC) at CERN. The two components of particle identification, efficiencies of [tau] identification from generator level information, along with fake rates of the current default algorithm have been determined and analyzed for a photon plus jets background sample and QCD background sample. I propose a new [tau] lepton identification algorithm that employs a signal cone parametrized with respect to the 7 transverse energy, and an isolation cone parametrized with respect to charged particle density surrounding the [tau] jet. Using the default algorithm an efficiency of 27.7% is achieved along with a photon plus jets fake rate of 1.96%. Using the proposed algorithm and matching the efficiency of the default algorithm, an efficiency of 26.9% and a fake rate of 0.44% is achieved. Approximately matching fake rates, an efficiency of 37.4% is achieved with a fake rate of 2.36%.by by Philip James Ilten.S.B

Direct probe of the intrinsic charm content of the proton

Measurement of Z bosons produced in association with charm jets (Zc) in proton-proton collisions in the forward region provides a direct probe of a potential nonperturbative (intrinsic) charm component in the proton wave function. We provide a detailed study of the potential to measure Zc production at the LHCb experiment in Runs 2 and 3 of the LHC. The sensitivity to valence-like (sea-like) intrinsic charm is predicted to be ⟨x⟩[subscript IC] ≳ 0.3%(1%). The impact of intrinsic charm on Higgs production at the LHC, including Hc, is also discussed in detail.National Science Foundation (U.S.) (Grant PHY-1306550

Measurement of the resonant and CP components in B¯0→J/ψπ+π− decays

The resonant structure of the reaction B¯0→J/ψπ+π− is studied using data from 3  fb−1 of integrated luminosity collected by the LHCb experiment, one third at 7 TeV center-of-mass energy and the remainder at 8 TeV. The invariant mass of the π+π− pair and three decay angular distributions are used to determine the fractions of the resonant and nonresonant components. Six interfering π+π− states, ρ(770), f0(500), f2(1270), ρ(1450), ω(782) and ρ(1700), are required to give a good description of invariant mass spectra and decay angular distributions. The positive and negative charge parity fractions of each of the resonant final states are determined. The f0(980) meson is not seen and the upper limit on its presence, compared with the observed f0(500) rate, is inconsistent with a model where these scalar mesons are formed from two quarks and two antiquarks (tetraquarks) at the eight standard deviation level. In the qq¯ model, the absolute value of the mixing angle between the f0(980) and the f0(500) scalar mesons is limited to be less than 17° at 90% confidence level.National Science Foundation (U.S.)United States. Dept. of Energ

First Measurement of the Charge Asymmetry in Beauty-Quark Pair Production

The difference in the angular distributions between beauty quarks and antiquarks, referred to as the charge asymmetry, is measured for the first time in b[bar over b] pair production at a hadron collider. The data used correspond to an integrated luminosity of 1.0  fb[superscript −1] collected at 7 TeV center-of-mass energy in proton-proton collisions with the LHCb detector. The measurement is performed in three regions of the invariant mass of the b[bar over b] system. The results obtained are A[b[bar over b] over C](40 105  GeV/c[superscript 2]) = 1.6 ± 1.7 ± 0.6%, where A[b[bar over b] over C] is defined as the asymmetry in the difference in rapidity between jets formed from the beauty quark and antiquark, where in each case the first uncertainty is statistical and the second systematic. The beauty jets are required to satisfy 2 20  GeV, and have an opening angle in the transverse plane Δϕ > 2.6  rad. These measurements are consistent with the predictions of the standard model.National Science Foundation (U.S.

Measurement of charged particle multiplicities and densities in pp collisions at √s = 7 TeV in the forward region

Charged particle multiplicities are studied in proton–proton collisions in the forward region at a centre-of-mass energy of √s = 7 TeV with data collected by the LHCb detector. The forward spectrometer allows access to a kinematic range of 2.0 < η < 4.8 in pseudorapidity, momenta greater than 2 GeV/ c and transverse momenta greater than 0.2 GeV/ c. The measurements are performed using events with at least one charged particle in the kinematic acceptance. The results are presented as functions of pseudorapidity and transverse momentum and are compared to predictions from several Monte Carlo event generators.National Science Foundation (U.S.

Measurement of Υ production in pp collisions at √s = 2.76 TeV

The production of Υ(1S), Υ(2S) and Υ(3S) mesons decaying into the dimuon final state is studied with the LHCb detector using a data sample corresponding to an integrated luminosity of 3.3 pb[superscript −1] collected in proton–proton collisions at a centre-of-mass energy of √s = 2.76 TeV. The differential production cross-sections times dimuon branching fractions are measured as functions of the Υ transverse momentum and rapidity, over the ranges p[subscript T] < 15 GeV/c and 2.0 < y < 4.5. The total cross-sections in this kinematic region, assuming unpolarised production, are measured to be σ(pp → Υ(1S)X) × B(Υ(1S) → μ[superscript +]μ[superscript −]) = 1.111 ± 0.043 ± 0.044 nb, σ(pp → Υ(2S)X) × B(Υ(2S) → μ[superscript +]μ[superscript −]) = 0.264 ± 0.023 ± 0.011 nb, σ(pp → Υ(3S)X) × B(Υ(3S) → μ[superscript +]μ[superscript −]) = 0.159 ± 0.020 ± 0.007 nb, where the first uncertainty is statistical and the second systematic.National Science Foundation (U.S.

Measurement of the forward W boson cross-section in pp collisions at √s = 7 TeV

A measurement of the inclusive W → μν production cross-section using data from pp collisions at a centre-of-mass energy of √s=7 TeV is presented. The analysis is based on an integrated luminosity of about 1.0 fb[superscript −1] recorded with the LHCb detector. Results are reported for muons with a transverse momentum greater than 20 [GeV over c] and pseudorapidity between 2.0 and 4.5. The W [superscript +] and W [superscript −] production cross-sections are measured to be σW[superscript +]→μ[superscript +]ν=861.0±2.0±11.2±14.7pb, σW[superscript −]→μ[superscript −] [bar over ν] = 675.8±1.9±8.8±11.6pb, where the first uncertainty is statistical, the second is systematic and the third is due to the luminosity determination. Cross-section ratios and differential distributions as functions of the muon pseudorapidity are also presented. The ratio of W [superscript +] to W [superscript −] cross-sections in the same fiducial kinematic region is determined to be σW [superscript +] → μ [superscript +] ν [over σW [superscript −] → μ [superscript −] [bar over ν]] = 1.274±0.005±0.009, where the uncertainties are statistical and systematic, respectively. Results are in good agreement with theoretical predictions at next-to-next-to-leading order in perturbative quantum chromodynamics.National Science Foundation (U.S.

Measurement of the inelastic pp cross-section at a centre-of-mass energy of √s = 7 TeV

The cross-section for inelastic proton-proton collisions, with at least one prompt long-lived charged particle of transverse momentum p [subscript T] > 0.2GeV/c in the pseudorapidity range 2.0 < η < 4.5, is measured by the LHCb experiment at a centre-of-mass energy of √s = 7 TeV. The cross-section in this kinematic range is determined to be σ [acc over inel] = 55.0 ± 2.4 mb with an experimental uncertainty that is dominated by systematic contributions. Extrapolation to the full phase space, using Pythia 6, yields σ [subscript inel] = 66.9 ± 2.9 ± 4.4 mb, where the first uncertainty is experimental and the second is due to the extrapolation.National Science Foundation (U.S.

Measurement of ψ(2S) polarisation in pp collisions at √s = 7 TeV

The polarisation of prompt ψ(2S) mesons is measured by performing an angular analysis of ψ(2S) → μ[superscript +]μ[superscript −] decays using proton-proton collision data, corresponding to an integrated luminosity of 1.0 fb[superscript −1], collected by the LHCb detector at a centre-of-mass energy of 7 TeV. The polarisation is measured in bins of transverse momentum p[subscript T] and rapidity y in the kinematic region 3.5 < p[subscript T] < 15 GeV/c and \(2.0 , and is compared to theoretical models. No significant polarisation is observed.National Science Foundation (U.S.

Search for long-lived particles decaying to jet pairs

A search is presented for long-lived particles with a mass between 25 and 50 GeV/c[superscript 2] and a lifetime between 1 and 200 ps in a sample of proton–proton collisions at a centre-of-mass energy of √s = 7 TeV, corresponding to an integrated luminosity of 0.62 fb[superscript −1], collected by the LHCb detector. The particles are assumed to be pair-produced by the decay of a standard model-like Higgs boson. The experimental signature of the long-lived particle is a displaced vertex with two associated jets. No excess above the background is observed and limits are set on the production cross-section as a function of the long-lived particle mass and lifetime.National Science Foundation (U.S.