Type of Document	Dissertation
Author	Jackson, Christopher B.
Author's Email Address	jackson@hep.fsu.edu
URN	etd-07092005-141611
Title	Higgs Boson Production with Heavy Quarks at Hadron Colliders
Degree	Doctor of Philosophy
Department	Physics, Department of
Advisory Committee	Advisor Name Title Laura Reina Committee Chair Harrison B. Prosper Committee Member Jorge Piekarewicz Committee Member Joseph F. Owens III Committee Member Philip L. Bowers Committee Member
Keywords	Hadron Colliders Higgs Physics NLO QCD
Date of Defense	2005-06-29
Availability	unrestricted
Abstract One of the remaining puzzles in particle physics is the origin of electroweak symmetry breaking. In the Standard Model (SM), a single doublet of complex scalar fields is responsible for breaking the $SU(2)_L imes U(1)_Y$ gauge symmetry, thus giving mass to the electroweak gauge bosons via the {it Higgs mechanism} and to the fermions via Yukawa couplings. The remnant of the process is a yet to be discovered scalar particle, the Higgs boson ($h$). Current and future experiments at hadron colliders hold great promise. The Fermilab Tevatron proton-antiproton ($par{p}$) collider, which is currently running, has the potential to discover a light Higgs boson with mass between 100 and 200 GeV. Starting in 2007, the CERN proton-proton ($pp$) Large Hadron Collider (LHC) will be able to produce a Higgs boson over its full mass range (up to 1 TeV) through {it multiple} processes. Of particular interest is the production of a Higgs boson in association with a pair of heavy quarks, $par{p}(pp) o Qar{Q}h$, where $Q$ can be either a top or a bottom quark. Indeed, the production of a Higgs boson with a pair of top quarks provides a very distinctive signal in hadronic collisions where background processes are formidable, and it will be instrumental in the discovery of a Higgs boson below about 130 GeV at the LHC. Also, since the Higgs boson is radiated from a top quark, this channel provides a unique opportunity to directly measure the top quark Yukawa coupling. On the other hand, the production of a Higgs boson with bottom quarks can be strongly enhanced in models of new physics beyond the SM, e.g. supersymmetric models. If this is the case, $bar{b}h$ production will play a crucial role at the Tevatron where it could provide the first signal of new physics. Given the prominent role that Higgs production with heavy quarks can play at hadron colliders, it becomes imperative to have precise theoretical predictions for total and differential cross sections. Hadronic cross sections are mainly affected by strong interaction effects which, at high energy, are described by perturbative Quantum Chromodynamics (QCD). Lowest-order predictions in perturbative QCD are often severely plagued by renormalization and factorization scale dependence. Therefore, to obtain precise results, it becomes mandatory to calculate cross sections beyond the LO. In this thesis, we report on the next-to-leading order (NLO) QCD calculation for the total and differential cross sections of $par{p}(pp) o Qar{Q}h$. The NLO cross sections exhibit drastically reduced dependence on renormalization and factorization scales and, thus, lead to increased confidence in predictions based on these results. In fact, the results presented in this thesis are currently being used in experimental simulations at both the Tevatron and the LHC. In the first part of this thesis, we outline and present detailed results for the NLO QCD calculation of $tar{t}h$ production at both the Tevatron and the LHC. This calculation involves several difficult issues due to the three massive particles in the final state, a situation which is at the frontier of radiative correction calculations in quantum field theory. For instance, the virtual one-loop corrections contain pentagon Feynman diagrams with several massive internal and external particles that pose both analytic and numerical challenges. Another difficulty arises in the calculation of the real gluon emission contribution, where one must compute a four-body phase space containing three massive particles. In this thesis, we will detail the novel techniques we have developed to deal with these challenges. In the second part of this thesis, we focus on the production of Higgs bosons with bottom quarks. The calculation of $par{p}(pp) o bar{b}h$ at NLO in QCD involves several subtle issues not encountered in the case of $par{p}(pp) o tar{t}h$. Both from an experimental and theoretical standpoint, it is important to distinguish between {it inclusive} and {it exclusive} $bar{b}h$ production. In fact, the production of a Higgs boson with a pair of $b$ quarks can be detected via: ({it i}) a fully {it exclusive} measurement, when both $b$ jets are identified; ({it ii}) a fully {it inclusive} measurement, when no $b$ jet is identified; or ({it iii}) a {it semi-inclusive} measurement, when at least one of the two $b$ jets is identified. Theoretically, different calculational approaches may be adopted when a final state $b$ quark is treated either exclusively or inclusively. In this thesis, we present results for both exclusive and inclusive production of Higgs bosons with bottom quarks, and we devote particular care to clarifying some outstanding issues concerning the inclusive production modes. Indeed, when a final state $b$ quark is not identified, the corresponding integration over its phase space gives rise to large collinear logarithms originating from the region of low transverse momentum. These collinear logarithms appear at every order in the strong coupling $alpha_s$ and, hence, could hinder the convergence of the perturbative expansion. Currently, there are two approaches to the calculation of inclusive Higgs production with bottom quarks: one can ({it i}) calculate the partonic processes $gg,qar{q} o bar{b}h$ at fixed order in $alpha_s$ with no special treatment of the collinear logarithms (the so-called {it Four Flavor Number Scheme}) or ({it ii}) introduce a bottom quark Parton Distribution Function, in which case the semi-inclusive process becomes $gb o bh$ and the inclusive one $bar{b} o h$, and resum leading and sub-leading logarithms through the Altarelli-Parisi equation (the so-called {it Five Flavor Number Scheme}). Here, we compare these two seemingly different schemes and show that they produce compatible results for the total and differential cross sections in the cases of Higgs production with zero tagged $b$ jets and one tagged $b$ jet. This comparison is made possible by having computed the NLO QCD cross section for $bar{b}h$ production.
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