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Title: Search for a Standard Model Higgs boson in CMS via vector boson fusion in the $$H \to W W \to$$ lepton neutrino lepton neutrino channel and optimization of energy reconstruction in CMS using test beam 2006 data

Abstract

One of the goals of the LHC is to test the existence of the Higgs boson. This thesis presents a study of the potential to discover the Standard Model Higgs boson in the vector boson fusion (VBF) channel for the Higgs mass range 120- 200 GeV/c 2. The decay of Higgs bosons into the WW* fi state with both W -bosons decaying leptonically is considered. The main backgrounds are tt + j and W +W -jj. This study, based on a full simulation of the CMS detector at the LHC, shows that a 5σ discovery can be done with an integrated luminosity of 12 - 72 fb -1 for 130 - 200 GeV/c 2 Higgs bosons. Due to the uncertainties in the backgrounds, it is important to measure the backgrounds from data. This study shows that the major background can be measured directly to 7% with 30 fb -1. After discovering the Higgs boson, it will be crucial to probe its physical properties. A method to measure the Higgs boson mass using transverse mass template distributions is investigated in the VBF channel. iv The performance of the combined CMS electromagnetic and hadronic calorimeters (EB+HB) was measured at the H 2more » test beam at the CERN SPS during 2006 with various particles in a large momentum range, 1-350 GeV/c. Another major contribution of this thesis is developing the method to optimize the energy reconstruction for the combined EB+HB system with which the corrected responses become 100% with 6% fl and the stochastic resolution is improved from 111% to 94%.« less

Authors:
 [1]
  1. Middle East Technical Univ., Ankara (Turkey)
Publication Date:
Research Org.:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org.:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
OSTI Identifier:
1415819
Report Number(s):
FERMILAB-THESIS-2007-13; CERN-THESIS-2007-052
756034
DOE Contract Number:
AC02-07CH11359
Resource Type:
Thesis/Dissertation
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS

Citation Formats

Yazgan, Efe. Search for a Standard Model Higgs boson in CMS via vector boson fusion in the $H \to W W \to$ lepton neutrino lepton neutrino channel and optimization of energy reconstruction in CMS using test beam 2006 data. United States: N. p., 2007. Web. doi:10.2172/1415819.
Yazgan, Efe. Search for a Standard Model Higgs boson in CMS via vector boson fusion in the $H \to W W \to$ lepton neutrino lepton neutrino channel and optimization of energy reconstruction in CMS using test beam 2006 data. United States. doi:10.2172/1415819.
Yazgan, Efe. Fri . "Search for a Standard Model Higgs boson in CMS via vector boson fusion in the $H \to W W \to$ lepton neutrino lepton neutrino channel and optimization of energy reconstruction in CMS using test beam 2006 data". United States. doi:10.2172/1415819. https://www.osti.gov/servlets/purl/1415819.
@article{osti_1415819,
title = {Search for a Standard Model Higgs boson in CMS via vector boson fusion in the $H \to W W \to$ lepton neutrino lepton neutrino channel and optimization of energy reconstruction in CMS using test beam 2006 data},
author = {Yazgan, Efe},
abstractNote = {One of the goals of the LHC is to test the existence of the Higgs boson. This thesis presents a study of the potential to discover the Standard Model Higgs boson in the vector boson fusion (VBF) channel for the Higgs mass range 120- 200 GeV/c2. The decay of Higgs bosons into the WW* fi state with both W-bosons decaying leptonically is considered. The main backgrounds are tt + j and W +W -jj. This study, based on a full simulation of the CMS detector at the LHC, shows that a 5σ discovery can be done with an integrated luminosity of 12 - 72 fb-1 for 130 - 200 GeV/c2 Higgs bosons. Due to the uncertainties in the backgrounds, it is important to measure the backgrounds from data. This study shows that the major background can be measured directly to 7% with 30 fb-1. After discovering the Higgs boson, it will be crucial to probe its physical properties. A method to measure the Higgs boson mass using transverse mass template distributions is investigated in the VBF channel. iv The performance of the combined CMS electromagnetic and hadronic calorimeters (EB+HB) was measured at the H2 test beam at the CERN SPS during 2006 with various particles in a large momentum range, 1-350 GeV/c. Another major contribution of this thesis is developing the method to optimize the energy reconstruction for the combined EB+HB system with which the corrected responses become 100% with 6% fl and the stochastic resolution is improved from 111% to 94%.},
doi = {10.2172/1415819},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Fri Jun 01 00:00:00 EDT 2007},
month = {Fri Jun 01 00:00:00 EDT 2007}
}

Thesis/Dissertation:
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  • The Higgs boson is the only elementary particle predicted by the Standard Model (SM) that has not yet been observed experimentally. If it exists, it explains the spontaneous electroweak symmetry breaking and the origin of mass for gauge bosons and fermions. We test the validity of the SM by performing a search for the associated production of a Higgs boson and a W boson in the channel where the Higgs boson decays to a bottom-antibottom quark pair and the W boson decays to a charged lepton and a neutrino (the WH channel). We study a dataset of proton-antiproton collisions atmore » a centre-of-mass energy √s = 1.96 TeV provided by the Tevatron accelerator, corresponding to an integrated luminosity of 5.7 fb -1, and recorded using the Collider Detector at Fermilab (CDF).We select events consistent with the signature of exactly one charged lepton (electron or muon), missing transverse energy due to the undetected neutrino (MET) and two collimated streams of particles (jets), at least one of which is required to be identified as originating from a bottom quark. We improve the discrimination of Higgs signal from backgrounds through the use of an artificial neural network. Using a Bayesian statistical inference approach, we set for each hypothetical Higgs boson mass in the range 100-150 GeV/c 2 with 5 GeV/c 2 increments a 95% credibility level (CL) upper limit on the ratio between the Higgs production cross section times branching fraction and the SM prediction. Our main original contributions are the addition of a novel charged lepton reconstruction algorithm with looser requirements (ISOTRK) with respect the electron or muon tight criteria (TIGHT), as well as the introduction of a novel trigger-combination method that allows to maximize the event yield while avoiding trigger correlations and that is used for the ISOTRK category. The ISOTRK candidate is a high-transverse-momentum good-quality track isolated from other activity in the tracking system and not required to match a calorimeter cluster, as for a tight electron candidate, or an energy deposit in the muon detector, as for a tight muon candidate. The ISOTRK category recovers real charged leptons that otherwise would be lost in the non-instrumented regions of the detector. This allows the reconstruction of more W boson candidates, which in turn increases the number of reconstructed WH signal candidate events, and therefore improves the sensitivity of the WH search. For the TIGHT charged lepton categories, we employ charged-lepton-dedicated triggers to improve the rate of WH signal acceptance during data taking. Since there is no ISOTRK-dedicated trigger at CDF, for the ISOTRK charged lepton category we employ three MET-plus-jets-based triggers. For each trigger we first identify the jet selection where the trigger efficiency is flat with respect to jet information (transverse energy and direction of motion in the transverse plane for the two jets in the event) and then we parametrize the trigger efficiency as a function of trigger MET. On an event-by-event basis, for each trigger we compute a trigger efficiency as a function of trigger parametrization, trigger MET, jet information, trigger prescale and information about whether the trigger is defined or not. For the ISOTRK category we combine the three triggers using a novel method, which allows the combination of any number of triggers in order to maximize the event yield while avoiding trigger correlations. On an event-by-event basis, only the trigger with the largest efficiency is used. By avoiding a logical 'OR' between triggers, the loss in the yield of events accepted by the trigger combination is compensated by a smaller and easier-to-compute corresponding systematic uncertainty. The addition of the ISOTRK charged lepton category to the TIGHT category produces an increase of 33% in the WH signal yield and a decrease of 15.5% to 19.0% in the median expected 95% CL cross-section upper limits across the entire studied Higgs mass interval. The improvement in analysis sensitivity is smaller than the improvement in signal yield because the ISOTRK category has a smaller signal over background ratio than the TIGHT category, due to the looser ISOTRK reconstruction criteria. The observed (median expected) 95% CL SM Higgs upper limits on cross section times branching ratio vary between 2.39 x SM (2.73 x SM) for a Higgs mass of 100 GeV/c 2 to 31.1 x SM (31.2 x SM) for a Higgs mass of 150 GeV/c 2, while the value for a 115 GeV/c 2 Higgs boson is that of 5.08 x SM (3.79 x SM). The novel trigger combination method is already in use by several CDF analyses. It is applicable to any analysis that uses triggers based on MET and jets, such as supersymmetry searches at the ATLAS and CMS experiments at the Large Hadron Collider. In its most general form, the method can be used by any analysis that combines any number of different triggers.« less
  • This dissertation presents the first search for the standard model Higgs boson (H) in decay topologies containing a muon, an imbalance in transverse momentum (E T) and jets, using pmore » $$\bar{p}$$ collisions at √s = 1.96 TeV with an integrated luminosity of 4.3 fb -1 recorded with the D0 detector at the Fermilab Tevatron Collider. This analysis is sensitive primary to contributions from Higgs bosons produced through gluon fusion, with subsequent decay H → WW → μνjj where W represents a real or virtual W boson. In the absence of signal, limits are set at 95% confidence on the production and decay of the standard model Higgs boson for M H in the range of 115-200 GeV. For M H = 165 GeV, the observed and expected limits are factors of 11.2 larger than the standard model value. Combining this channel with eνjj final states and including earlier data to increase the integrated luminosity to 5.4 fb -1 produces observed(expected) limits of 5.5(3.8) times the standard model value.« less
  • We present the potential for discovering the Standard Model Higgs boson produced by the vector-boson fusion mechanism. We considered the decay of Higgs bosons into the W{sup +}W{sup -} final state, with both W-bosons subsequently decaying leptonically. The main background is t{bar t} with one or more jets produced. This study is based on a full simulation of the CMS detector, and up-to-date reconstruction codes. The result is that a signal of 5{sigma} significance can be obtained with an integrated luminosity of 12 - 72 fb{sup -1} for Higgs boson masses between 130 < m{sub H} < 200 GeV .more » In addition, the major background can be measured directly to 7% from the data with an integrated luminosity of 30 fb{sup -1}. In this study, we suggested a method to obtain information in Higgs mass using the transverse mass distributions.« less
  • In this dissertation, results from a search for the Standard Model (SM) Higgs boson is shown. The SM is the theoretical framework which describes particles of matter and force carrier gauge bosons. To solve the mass problem in the SM, the Higgs mechanism was introduced in 1963. The Higgs mechanism causes an electroweak symmetry breaking and a new massive scalar boson was postulated. This particle is the Higgs boson. A search for the Higgs boson has been ongoing at the Tevatron where protons and antiprotons were allowed to collide at a center-of-mass energy of 1.96 TeV. For a low mass Higgs, that is a Higgs with a mass lower than 135 GeV, the dominant decay mode is Higgs to a pair of b-quarks (H →bmore » $$\bar{b}$$ ). This work concentrated on a Higgs whose mass is in the range of 100 150 GeV, with a W vector boson produced with the Higgs boson. The final state chosen is the one which contains a lepton a neutrino and a pair of b-quarks. This study used data provided by the DZERO experiment. Results presented here are the outcome of analyzing 5.3 fb -1 of data from RunII period. The analysis used different techniques to increase the sensitivity of the study. Data were subdivided based on lepton flavor, number of jets in sample, jets identified as b-jets and dates of collected data. A multivariate analysis technique based on boosted decision trees were used to separate signal from background processes, physical and instrumental. A good agreement between data and simulated events was observed.« less
  • The question of the nature and principles of the universe and our place in it is the driving force of science since Mesopotamian astronomers glanced for the first time at the starry sky and Greek atomism has been formulated. During the last hundred years modern science was able to extend its knowledge tremendously, answering many questions, opening entirely new fields but as well raising many new questions. Particularly Astronomy, Astroparticle Physics and Particle Physics lead the race to answer these fundamental and ancient questions experimentally. Today it is known that matter consists of fermions, the quarks and leptons. Four fundamentalmore » forces are acting between these particles, the electromagnetic, the strong, the weak and the gravitational force. These forces are mediated by particles called bosons. Our confirmed knowledge of particle physics is based on these particles and the theory describing their dynamics, the Standard Model of Particles. Many experimental measurements show an excellent agreement between observation and theory but the origin of the particle masses and therefore the electroweak symmetry breaking remains unexplained. The mechanism proposed to solve this issue involves the introduction of a complex doublet of scalar fields which generates the masses of elementary particles via their mutual interactions. This Higgs mechanism also gives rise to a single neutral scalar boson with an unpredicted mass, the Higgs boson. During the last twenty years several experiments have searched for the Higgs boson but so far it escaped direct observation. Nevertheless these studies allow to further constrain its mass range. The last experimental limits on the Higgs mass have been set in 2001 at the LEP collider, an electron positron machine close to Geneva, Switzerland. The lower limit set on the Higgs boson mass is m H > 114.4 GeV/c 2 and remained for many years the last experimental constraint on the Standard Model Higgs Boson due to the shutdown of the LEP collider and the experimental challenges at hadron machines as the Tevatron. This thesis was performed using data from the D0 detector located at the Fermi National Accelerator Laboratory in Batavia, IL. Final states containing two electrons or a muon and a tau in combination with missing transverse energy were studied to search for the Standard Model Higgs boson, utilizing up to 4.2 fb -1 of integrated luminosity. In 2008 the CDF and D0 experiments in a combined effort were able to reach for the first time at a hadron collider the sensitivity to further constrain the possible Standard Model Higgs boson mass range. The research conducted for this thesis played a pivotal role in this effort. Improved methods for lepton identification, background separation, assessment of systematic uncertainties and new decay channels have been studied, developed and utilized. Along with similar efforts at the CDF experiment these improvements led finally the important result of excluding the presence of a Standard Model Higgs boson in a mass range of m H = 160-170 GeV/c 2 at 95% Confidence Level. Many of the challenges and methods found in the present analysis will probably in a similar way be ingredients of a Higgs boson evidence or discovery in the near future, either at the Tevatron or more likely at the soon starting Large Hadron Collider (LHC). Continuing to pursue the Higgs boson we are looking forward to many exciting results at the Tevatron and soon at the LHC. In Chapter 2 an introduction to the Standard Model of particle physics and the Higgs mechanism is given, followed by a brief outline of existing theoretical and experimental constraints on the Higgs boson mass before summarizing the Higgs boson production modes. Chapter 3 gives an overview of the experimental setup. This is followed by a description of the reconstruction of the objects produced in proton-antiproton collisions in Chapter 4 and the necessary calorimeter calibrations in Chapter 5. Chapter 6 follows with an explanation of the phenomenology of the proton-antiproton collisions and the data samples used. In Chapter 7 the search for the Standard Model Higgs boson using a di-electron final state is discussed, followed by the analysis of the final states using muons and hadronic decaying taus in Chapter 8. Finally a short outlook for the prospects of Higgs boson searches is given in Chapter 9.« less