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Title: A Search for Neutral Supersymmetric Higgs Bosons at DØ

Abstract

A search for Higgs bosons in multijet data from the DØ detector is reported in this thesis. The Higgs boson is the only remaining undiscovered particle in the Standard Model of particle physics, and plays an integral role in this model. It is known that this model is not a complete description of fundamental physics (it does not describe gravity, for example), and so searches for physics beyond the Standard Model are an important part of particle physics. One extension of the Standard Model, the Minimal Supersymmetric Standard Model (MSSM), predicts the existence of five Higgs bosons, two of which can show an enhanced coupling to bottom quarks. For this reason, a search in the bbb (multijet) channel is a sensitive test of Higgs boson physics. The analysis described in this thesis was conducted over 6.6 fb -1 of data. At the time of writing, the best limits on tan β (a key parameter of the MSSM) in the multijet channel were set by DØ. The new analysis described in this thesis included more data than the previous analysis in the channel, and made use of a new trigger and event-based analysis method. An improved Multivariate Analysis technique was usedmore » to separate signal and background events and produce a final discriminant for the limit setting process. These changes increased the expected sensitivity of this measurement by roughly 50% more than would be expected from the increase in the size of data sample alone.« less

Authors:
 [1]
+ Show Author Affiliations
  1. Imperial College, London (United Kingdom)
Publication Date:
Research Org.:
Fermi National Accelerator Lab. (FNAL), Batavia, IL (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1018736
Report Number(s):
FERMILAB-THESIS-2010-67
TRN: US1103448
DOE Contract Number:
AC02-07CH11359
Resource Type:
Thesis/Dissertation
Country of Publication:
United States
Language:
English
Subject:
72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS; HIGGS BOSONS; MULTIVARIATE ANALYSIS; PHYSICS; QUARKS; SENSITIVITY; STANDARD MODEL; Experiment-HEP

Citation Formats

Osman, Nicolas Ahmed. A Search for Neutral Supersymmetric Higgs Bosons at DØ. United States: N. p., 2010. Web. doi:10.2172/1018736.
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Osman, Nicolas Ahmed. A Search for Neutral Supersymmetric Higgs Bosons at DØ. United States. doi:10.2172/1018736.
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Osman, Nicolas Ahmed. Wed . "A Search for Neutral Supersymmetric Higgs Bosons at DØ". United States. doi:10.2172/1018736. https://www.osti.gov/servlets/purl/1018736.
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@article{osti_1018736,
title = {A Search for Neutral Supersymmetric Higgs Bosons at DØ},
author = {Osman, Nicolas Ahmed},
abstractNote = {A search for Higgs bosons in multijet data from the DØ detector is reported in this thesis. The Higgs boson is the only remaining undiscovered particle in the Standard Model of particle physics, and plays an integral role in this model. It is known that this model is not a complete description of fundamental physics (it does not describe gravity, for example), and so searches for physics beyond the Standard Model are an important part of particle physics. One extension of the Standard Model, the Minimal Supersymmetric Standard Model (MSSM), predicts the existence of five Higgs bosons, two of which can show an enhanced coupling to bottom quarks. For this reason, a search in the bbb (multijet) channel is a sensitive test of Higgs boson physics. The analysis described in this thesis was conducted over 6.6 fb-1 of data. At the time of writing, the best limits on tan β (a key parameter of the MSSM) in the multijet channel were set by DØ. The new analysis described in this thesis included more data than the previous analysis in the channel, and made use of a new trigger and event-based analysis method. An improved Multivariate Analysis technique was used to separate signal and background events and produce a final discriminant for the limit setting process. These changes increased the expected sensitivity of this measurement by roughly 50% more than would be expected from the increase in the size of data sample alone.},
doi = {10.2172/1018736},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Wed Sep 01 00:00:00 EDT 2010},
month = {Wed Sep 01 00:00:00 EDT 2010}
}
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Thesis/Dissertation:
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DOI:  10.2172/1018736
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  • A search for neutral supersymmetric Higgs bosons and work relating to the improvement of the b-tagging and trigger capabilities at the D0 detector during Run II of the Fermilab Tevatron collider is presented. The search for evidence of the Higgs sector in the Standard Model (SM) and supersymmetric extensions of the SM are a high priority for the D0 collaboration, and b-tagging and good triggers are a vital component of these searches. The development and commissioning of the first triggers at D0 which use b-tagging is outlined, along with the development of a new secondary vertex b-tagging tool for usemore » in the Level 3 trigger. Upgrades to the Level 3 trigger hit finding code, which have led to significant improvements in the quality and efficiency of the tracking code, and by extension the b-tagging tools, are also presented. An offline Neural Network (NN) b-tagging tool was developed, trained on Monte Carlo and extensively tested and measured on data. The new b-tagging tool significantly improves the b-tagging performance at D0, for a fixed fake rate relative improvements in signal efficiency range from ~ 40% to ~ 15%. Fake rates, for a fixed signal efficiency, are typically reduced to between a quarter and a third of their value. Finally, three versions of the search for neutral supersymmetric Higgs bosons are presented. The latest version of the analysis makes use of almost 1 fb -1 of data, the new NN b-tagger and the new b-tagging triggers, and has set one of the world's best limits on the supersymmetric parameter tanβ in the mass range 90 to 150 GeV.« less

  • Two searches for the production of Higgs bosons decaying into τ and μ leptons, using data collected with the DØ detector at the Fermilab Tevatron pmore » $$\bar{p}$$ collider, are presented. A search for the pair production of doubly charged Higgs bosons in the process q$$\bar{q}$$ → H ++H --, where H ±± decays to ττ , μμ or τμ lepton pairs, with an integrated luminosity of up to L = 7:0 fb -1, is presented. No signi cant excess of data over the expected SM background is observed and the results are used to set 95% C.L. limits on the pair production cross section of doubly charged Higgs bosons in the range 90 < M H±± < 200 GeV. A second search for the production of the Standard Model Higgs boson in the nal state ττμ +X is presented, using an integrated luminosity of L = 8:6 fb -1. Again no signi cant excess of data is observed over the background expectation and 95% C.L. limits are set on the observed cross section relative to the Standard Model prediction, in the range 100 < M H < 200 GeV.« less

  • No abstract prepared.

  • Elementary particle physics tries to find an answer to no minor question: What is our universe made of? To our current knowledge, the elementary constituents of matter are quarks and leptons, which interact via four elementary forces: electromagnetism, strong force, weak force and gravity. All forces, except gravity, can be described in one framework, the Standard Model of particle physics. The model's name reflects its exceptional success in describing all available experimental high energy physics data to high precision up to energies of about 100 GeV. An exception is given by the neutrino masses but even these can be integratedmore » into the model. The Standard Model is based on the requirement of invariance of all physics processes under certain fundamental symmetry transformations. The consideration of these symmetries leads naturally to the correct description of the electromagnetic, weak and strong forces as the exchange of interaction particles, the gauge bosons. However, this formalism has the weakness that it only allows for massless particles. In order to obey the symmetries, a way to introduce the particle masses is given by the Higgs mechanism, which predicts the existence of the only particle of the Standard Model which has yet to be observed: the Higgs boson. In spite of the success of the Standard Model, it has to be considered as a low energy approximation of a more profound theory for various reasons. For example, the underlying theory is expected to allow for an integration of gravity into the framework and to provide a valid particle candidate for the dark matter in our universe. Furthermore, a solution has to be found to the problem that the Higgs boson as a fundamental scalar is sensitive to large radiative corrections driving its mass to the Planck scale of 10 19 GeV. Several models have been proposed to address the remaining open questions of the Standard Model. Currently, the most promising extension of the Standard Model is Supersymmetry, which provides elegant solutions to the named problems by introducing a supersymmetric partner to each Standard Model particle. The superpartners of the matter particles are called squarks and sleptons, while the superpartners of the interaction particles are called gauginos. The mass eigenstates of the gauginos are referred to as charginos and neutralinos, according to their electric charge. Since the predicted supersymmetric particles have not yet been observed, Supersymmetry, if it exists in nature, has to be broken in such a way that the masses of Standard Model particles and of their superpartners differ. During the last decades, the energies accessible to experiments has steadily increased. The Tevatron Accelerator at the Fermi National Accelerator Laboratory, with the two multipurpose experiments D0 and CDF, provides currently the highest center-of-mass energy ever reached in experiments using collisions of protons and antiprotons (√s = 1.96 TeV). The study of the particle collisions allows probing of predictions of the Standard Model and its extensions, e.g. Supersymmetry.« less

  • A search for directly produced Supersymmetric Higgs Bosons has been performed in the di-tau decay channel in 86.3 ± 3.5 pb -1 of data collected by CDF during Run1b at the Tevatron. They search for events where one tau decays to an electron and the other tau decays hadronically. They perform a counting experiment and set limits on the cross section for Higgs production in the high tan β region of the m A-tan β plane. For a benchmark parameter space point where m A = 100 and tan β = 50, they set a 95% confidence level upper limitmore » at 891 pb compared to the theoretically predicted cross section of 122 pb. For events where the tau candidates are not back-to-back, they utilize a di-tau mass reconstruction technique for the first time on hadron collider data. Limits based on a likelihood binned in di-tau mass from non-back-to-back events alone are weaker than the limits obtained from the counting experiment using the full di-tau sample.« less