Search for Neutral Higgs Bosons in e+e- in Hvv- Channel with DELPHI Detector at LEP 2

The Standard Model has been tremendously successful in predicting the properties of new particles and the structure of the basic interactions. In many of its facets it has been tested at an accuracy significantly better than 0.1 percent.

The most important aspect of the Standard Model which has not yet been verified experimentally is the Higgs sector. The Standard Model without the Higgs boson is incomplete since it predicts massless fermions and gauge bosons. Furthermore, the electroweak radiative corrections to observables such as the W± and Z⁰ boson masses would be infinite if there were no Higgs boson.

In the first chapter I will show how the simplest solution given by the introduction of a SU(2)_L doublet of Higgs bosons, cures these defects.
This simple solution inside the Standard Model is fascinating, but there are a variety of nagging theoretical problems which cannot be solved without the introduction of some new physics.
There is no understanding of masses or why there are three generations of quarks and leptons.
Neither the fundamental parameters, masses and couplings, nor the symmetry pattern are accounted for: these elements are merely built into the model.
Moreover when radiative correction on Higgs mass are computed an unnatural way to suppress divergences must be introduced.
Supersymmetry is, at present, many theorists' favourite candidate for new physics beyond the Standard Model.
The Higgs sectors in Supersymmetry theory becomes more complicated and I will review it in section 1.5..1.1.
The search for Higgs boson(s) and for new physics is one of the main goals of the progressive increase of the centre-of-mass energy (√S) in the second phase of LEP (LEP2).
Since 1995 the LEP has been running at centre-of-mass energies above the Z⁰ pole. Data were collected at √S=130-136 GeV in 1995, at √S=161 GeV and √S=172 GeV in 1996, at √S=183 GeV in 1997 and at √S=189 GeV in 1998.
In the first two years of running relatively low luminosity samples (∫∼ 10 pb^-1) were collected.
A sample of about 50 pb^-1 was collected at √S=183 GeV and 158 pb^-1 at 189 GeV.
In 1999 the centre of mass energy increased till 200 GeV and the aims for the future are to go as high as possible in √S, with √S=204 GeV foreseen for the last year.

In the analysis described in this thesis the Higgs bosons is searched in the final states with neutrinos: e⁺e^-⟶Hv⊽.
They come from the Higgs-strahlung process: e⁺e^-⟶Z⁰H with Z⁰;⟶Hv⊽ or from W⁺W^- fusion process: e⁺e^-⟶W⁺W^-v_e⊽_e⟶Hv⊽. All Higgs boson decays are considered.

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This thesis is organized as follows.
In the first chapter I will provide an exhaustive description of Higgs boson sector and its properties both in Standard Model and in Minimal Supersymmetric extension of Standard Model.
Higgs production mechanism at LEP are examined.

The second chapter contains a brief description of LEP accelerator and DELPHI detector.
Since many publications exist in literature I will give only a summary. I dwell on the microvertex detector because I had the possibility to contribute to the running of it during 1999 data taking.

More emphasis is on the alignment of DELPHI detector since part of my PhD activities were dedicated to perform it.
Chapter 3 is dedicated to this subject which during my PhD constitutes a challenge in the effort of improving the performances.

The Higgs hunting done in this thesis is pursued in two steps.
The data collected during 1998 at centre of mass energy of 189 GeV are analysed.
First of all (chapter 4) a sequential series of cuts is applied in view to select a sample of events with an increase in signal over background ratio.
Then (chapter 5) the likelihood ratio method is adopted to discriminate between signal and background events.
There are two irreducible background processes: e⁺e^-⟶Z⁰Z⁰ with one⁰ decaying into neutrinos, and e⁺e^-⟶Z⁰ΥΥ with the two photons in beam pipe.

The last one (which is called the double radiative return to Z⁰) has never been studied.
Because of this, and because it represents a not negligible background in e⁺e^-⟶Hv⊽ channel I was motivated for an analysis of this process.