Search for the Higgs Boson in the ZZ --> ℓℓvv channel with CMS

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2013.Cataloged from PDF version of thesis.Includes bibliographical references (pages 171-176).The Standard Model (SM) of particle physics is a quantum field theory of the strong, weak, and electromagnetic interactions. It successfully describes a large majority of observed phenomena at microscopic scales. The key issue that remains to be addressed is the mechanism for electroweak symmetry breaking (EWSB). Without EWSB in the theory, the massive weak force mediators - the W and Z bosons, are required to be massless to satisfy gauge symmetry. To incorporate EWSB into the SM, the Higgs mechanism is invoked and introduces a scalar field, the Higgs field, that couples to the W and Z bosons and gives them mass. Additionally, the SM postulates that the fundamental fermions acquire mass through interactions with the Higgs field. Observation of the field quantum, the Higgs boson, would greatly validate our ideas on electroweak symmetry breaking and complete the table of fundamental particles predicted by the Standard Model. If it exists, the Higgs boson has eluded detection for decades. A primary objective of the Large Hadron Collider (LHC) project is to make a definitive statement about the existence of the Higgs boson. The LHC provided proton-proton collisions at [square root of]s = 7 TeV for the 2011 run. The search for the SM Higgs with the 2011 dataset represented the first major foray into the search effort by the LHC project. This thesis presents a search for the SM Higgs boson in the ZZ -->2ℓ2v channel from the analysis of 4.9 fb-1 of data collected with the Compact Muon Solenoid (CMS) detector at the LHC. Overviews of electroweak theory and of Higgs production at the LHC are given, followed by descriptions of the CMS detector and the algorithms for event reconstruction. A measurement of the inclusive cross section for Z boson production is presented as a validation on the measured efficiencies of the electron and muon reconstruction and selection requirements. Lastly, the Higgs search analysis and results are presented.by Kevin Kai Hong Sung.Ph.D

Observation of long-range, near-side angular correlations in proton-proton collisions at the LHC

Results on two-particle angular correlations for charged particles emitted in proton-proton collisions at center-of-mass energies of 0.9, 2.36, and 7 TeV are presented, using data collected with the CMS detector over a broad range of pseudorapidity (η) and azimuthal angle (ϕ). Short-range correlations in Δη, which are studied in minimum bias events, are characterized using a simple “independent cluster” parametrization in order to quantify their strength (cluster size) and their extent in η (cluster decay width). Long-range azimuthal correlations are studied differentially as a function of charged particle multiplicity and particle transverse momentum using a 980 nb[superscript −1] data set at 7 TeV. In high multiplicity events, a pronounced structure emerges in the two-dimensional correlation function for particle pairs with intermediate p [subscript T] of 1–3 GeV/c, 2.0 < |Δη| < 4.8 and Δϕ ≈ 0. This is the first observation of such a long-range, near-side feature in two-particle correlation functions in pp or p[−over]p collisions

Search for the standard model Higgs boson decaying to bottom quarks in pp collisions at √s = 7 TeV

A search for the standard model Higgs boson (H) decaying to b[bar over b] when produced in association with weak vector bosons (V) is reported for the following modes: W(μν)H, W(eν)H, Z(μμ)H, Z(ee)H and Z(νν)H. The search is performed in a data sample corresponding to an integrated luminosity of 4.7 fb[superscript −1], recorded by the CMS detector in proton–proton collisions at the LHC with a center-of-mass energy of 7 TeV. No significant excess of events above the expectation from background is observed. Upper limits on the VH production cross section times the H→b[bar over b] branching ratio, with respect to the expectations for a standard model Higgs boson, are derived for a Higgs boson in the mass range 110–135 GeV. In this range, the observed 95% confidence level upper limits vary from 3.4 to 7.5 times the standard model prediction; the corresponding expected limits vary from 2.7 to 6.7 times the standard model prediction.European Organization for Nuclear ResearchUnited States. Dept. of EnergyNational Science Foundation (U.S.)Alfred P. Sloan Foundatio

Measurement of the ratio of the inclusive 3-jet cross section to the inclusive 2-jet cross section in pp collisions at √s = 7 TeV and first determination of the strong coupling constant in the TeV range

A measurement is presented of the ratio of the inclusive 3-jet cross section to the inclusive 2-jet cross section as a function of the average transverse momentum, ⟨p[subscript T1,2]⟩, of the two leading jets in the event. The data sample was collected during 2011 at a proton–proton centre-of-mass energy of 7 TeV with the CMS detector at the LHC, corresponding to an integrated luminosity of 5.0 fb[subscript −1]. The strong coupling constant at the scale of the Z boson mass is determined to be α[subscript S](M[subscript Z])=0.1148±0.0014 (exp.)±0.0018 (PDF)±0.0050(theory), by comparing the ratio in the range 0.42<⟨p[subscript T1,2]⟩<1.39 TeV to the predictions of perturbative QCD at next-to-leading order. This is the first determination of α[subscript S](M[subscript Z]) from measurements at momentum scales beyond 0.6 TeV. The predicted ratio depends only indirectly on the evolution of the parton distribution functions of the proton such that this measurement also serves as a test of the evolution of the strong coupling constant. No deviation from the expected behaviour is observed.United States. Department of EnergyNational Science Foundation (U.S.

Measurement of the production cross section for pairs of isolated photons in pp collisions at √s = 7 TeV

The integrated and differential cross sections for the production of pairs of isolated photons is measured in proton-proton collisions at a centre-of-mass energy of 7 TeV with the CMS detector at the LHC. A data sample corresponding to an integrated luminosity of 36 pb[superscript −1] is analysed. A next-to-leading-order perturbative QCD calculation is compared to the measurements. A discrepancy is observed for regions of the phase space where the two photons have an azimuthal angle difference Δφ ≲ 2.8 rad.United States. Dept. of EnergyNational Science Foundation (U.S.)Alfred P. Sloan Foundatio

Search for microscopic black hole signatures at the Large Hadron Collider

A search for microscopic black hole production and decay in pp collisions at a center-of-mass energy of 7 TeV has been conducted by the CMS Collaboration at the LHC, using a data sample corresponding to an integrated luminosity of 35 pb[superscript −1]. Events with large total transverse energy are analyzed for the presence of multiple high-energy jets, leptons, and photons, typical of a signal expected from a microscopic black hole. Good agreement with the standard model backgrounds, dominated by QCD multijet production, is observed for various final-state multiplicities and model-independent limits on new physics in these final states are set. Using simple semi-classical approximation, limits on the minimum black hole mass are derived as well, in the range 3.5–4.5 TeV. These are the first direct limits on black hole production at a particle accelerator.European Organization for Nuclear ResearchNational Science Foundation (U.S.)United States. Dept. of Energ

Measurement of the charge asymmetry in top-quark pair production in proton–proton collisions at √s = 7 TeV

The difference in angular distributions between top quarks and antiquarks, commonly referred to as the charge asymmetry, is measured in pp collisions at the LHC with the CMS experiment. The data sample corresponds to an integrated luminosity of 1.09 fb[superscript −1] at a centre-of-mass energy of 7 TeV. Top-quark pairs are selected in the final state with an electron or muon and four or more jets. At least one jet is identified as originating from b-quark hadronization. The charge asymmetry is measured in two variables, one based on the pseudorapidities (η) of the top quarks and the other on their rapidities (y). The results A[η over C] = −0.017 ± 0.032 (stat.)[+0.025 over −0.036] (syst.) and A[y over C] = −0.013 ± 0.028 (stat.)[+0.029 over −0.031] (syst.) are consistent within uncertainties with the standard-model predictions.United States. Dept. of EnergyNational Science Foundation (U.S.)Alfred P. Sloan Foundatio

Search for the standard model Higgs boson decaying into two photons in pp collisions at √s = 7 TeV

A search for a Higgs boson decaying into two photons is described. The analysis is performed using a dataset recorded by the CMS experiment at the LHC from pp collisions at a center-of-mass energy of 7 TeV, which corresponds to an integrated luminosity of 4.8 fb[superscript −1]. Limits are set on the cross section of the standard model Higgs boson decaying to two photons. The expected exclusion limit at 95% confidence level is between 1.4 and 2.4 times the standard model cross section in the mass range between 110 and 150 GeV. The analysis of the data excludes, at 95% confidence level, the standard model Higgs boson decaying into two photons in the mass range 128 to 132 GeV. The largest excess of events above the expected standard model background is observed for a Higgs boson mass hypothesis of 124 GeV with a local significance of 3.1σ . The global significance of observing an excess with a local significance ⩾3.1σ anywhere in the search range 110–150 GeV is estimated to be 1.8σ. More data are required to ascertain the origin of this excess.United States. Dept. of EnergyNational Science Foundation (U.S.

Search for supersymmetry in pp collisions at 7 TeV in events with jets and missing transverse energy

A search for supersymmetry with R-parity conservation in proton–proton collisions at a centre-of-mass energy of 7 TeV is presented. The data correspond to an integrated luminosity of 35 pb[superscript -1] collected by the CMS experiment at the LHC. The search is performed in events with jets and significant missing transverse energy, characteristic of the decays of heavy, pair-produced squarks and gluinos. The primary background, from standard model multijet production, is reduced by several orders of magnitude to a negligible level by the application of a set of robust kinematic requirements. With this selection, the data are consistent with the standard model backgrounds, namely t [bar over t], W + jet and Z + jet production, which are estimated from data control samples. Limits are set on the parameters of the constrained minimal supersymmetric extension of the standard model. These limits extend those set previously by experiments at the Tevatron and LEP colliders.European Organization for Nuclear ResearchUnited States. Dept. of EnergyNational Science Foundation (U.S.

Measurement of the differential dijet production cross section in proton–proton collisions at √s = 7 TeV

A measurement of the double-differential inclusive dijet production cross section in proton–proton collisions at √s = 7 TeV is presented as a function of the dijet invariant mass and jet rapidity. The data correspond to an integrated luminosity of 36 pb[superscript −1], recorded with the CMS detector at the LHC. The measurement covers the dijet mass range 0.2 TeV to 3.5 TeV and jet rapidities up to |y|=2.5. It is found to be in good agreement with next-to-leading-order QCD predictions.United States. Dept. of EnergyNational Science Foundation (U.S.