FR9903328 ORSAY LAL 98-08 n° d'ordre : Avril 1998 UNIVERSITE DE PARIS-SUD CENTRE D'ORSAY THESE présentée pour obtenir Le GRADE de DOCTEUR EN SCIENCES DE L'UNIVERSITE PARIS XI ORSAY par Dirk ZERWAS Energie manquante à LEP2: boson W et physique nouvelle Soutenue le 2 Avril 1998 devant la Commission d'examen M. P. BINETRUY MME. M.C. COUSINOU MM. J.F. GRIVAZ J. LEFRANÇOIS F. RICHARD K. TITTEL 30-39 Abstract In 1995 LEP, CERN's large e+e collider, increased its centre-of-mass energy be- yond the Z boson resonance up to 184 GeV in 1997. The data recorded by the ALEPH detector allow to study the parameters of the standard model and to search for new particles. The mass of the W boson can be determined at LEP via the measurement of the cross section of W pairs at the production threshold. Two selections for the final states Ivlv and ri/qq' are developed. In combination with the other decay channels, the mass of the W boson and its branching ratios are measured. The reaction e+e~ —>• Wei/ gives access to the coupling 7WW. The cross section of this process is measured and limits on the anomalous couplings (A7, K7) are determined. The non-minimal standard model with an extra scalar doublet predicts the existence of charged Higgs bosons. A selection of the final state ruqq' is developed. In absence of a signal, limits on the mass of the charged Higgs bosons are determined. In a supersymmetric theory each boson is associated to a fermion and vice versa. A search for sleptons, the supersymmetric partners of the leptons, is performed. The result is interpreted in the framework of the minimal supersymmetric extension of the standard model (MSSM). Moreover, in the MSSM a practically invisible W decay is possible. This decay can be detected if the second W decays to standard model particles. A limit on the invisible branching ratio of the W boson is deduced. Keywords : ALEPH LEP W boson charged Higgs Supersymmetry QctXarra, 'ââXarrcxl Xenophon, Contents 1 Introduction 1 2 The Standard Model 5 2.1 Particles and Interactions 5 2.2 The Properties of the W± Boson 11 2.3 The Determination of the W* Boson Mass at LEP 15 2.4 TGC at LEP 19 3 Extending the standard model 23 3.1 Extended Higgs Sector 23 3.2 Properties of the charged Higgs boson 26 3.3 Supersymmetry 29 3.4 The M.SSM Particle Spectrum 33 3.5 Supersymmetric Particles and their Properties at LEP 44 4 The Experimental Setup 55 4.1 LEP 55 4.2 ALEPH 58 4.3 Event Reconstruction 64 4.4 The N95 Technique 66 I 5 Physics Processes at LEP2 67 5.1 ff Processes 67 5.2 77 Processes 71 5.3 Four-Fermion Processes 73 6 W* Boson Physics 75 6.1 W± Boson Pair Cross Section 76 6.1.1 tvlv Selection 77 6.1.2 ri/qq' Selection 85 6.1.3 Results . 94 6.2 Single W* Boson Cross Section 98 6.2.1 Leptonic Selection 98 6.2.2 Hadronic Selection 102 6.2.3 Results 106 7 The Charged Higgs Bosons 111 7.1 The Ti/qq' Channel 112 7.2 Results 117 8 Search for Supersymmetry 121 8.1 Acoplanar Lepton Pairs 123 8.1.1 Event Selection 123 8.1.2 Results 131 8.2 Single Visible W± Bosons 134 8.2.1 Event Selection 135 8.2.2 Results 140 9 Conclusions 145 II Chapter 1 Introduction The standard model of elementary particle physics describes matter and its interactions to an unprecedented degree of precision. Matter is built of fermions (quarks and leptons) and their interactions are mediated via bosons. The photon (7) is responsible for the electromagnetic interactions, the charged vector bosons (W±) are responsible for the particles' decay (as in the neutron decay), the Z boson is the carrier of the neutral current. Masses are generated via the Higgs mechanism, leading to an additional neutral scalar particle, the Higgs boson (H). The standard model, in spite of its success, is considered to be insufficient because of theoretical prejudices, one of which can be formulated in the following way: Why is matter built only of fermions and not also of bosons and why are interactions me- diated only by bosons and not also by fermions? The supersymmetric extension of the standard model attempts to solve this apparent asymmetry by introducing a new symmetry, which effectively doubles the particle spectrum: each fermion receives a bosonic partner and each boson receives a fermionic partner. This leads to the predic- tion of several new particles and in particular, as the Higgs sector has to be extended to construct the model, charged Higgs bosons are predicted. Many of today's most precise measurements of parameters of the standard model and interesting results on searches for new physics were obtained at LEP, CERN's large e+e~ collider, where the four experiments ALEPH, DELPHI, L3 and OPAL are recording data. From its inauguration in 1989 to autumn 1995 LEP operated at centre- of-mass energies at and around the Z resonance (91 GeV). An example of the precision measurements of parameters of the standard model is the measurement of the Z boson's mass of 91.1867 ± 0.0020 GeV/c2 and its width of 2.4948 ± 0.0025 GeV/c2 [1]. Precision measurements of the standard model can be viewed in two different ways. They constitute a stringent test of the standard model, but on the other hand they can be used to constrain physics beyond the standard model via higher order effects on the 1 measured quantities. For example, from the measurement of the electroweak parame- ters one can derive a limit on the contribution of new physics or new particles to the invisible width of the Z boson, which must be less than 2.9 MeV/c2 [1]. One attempts to give the limits not in a specific framework, but in the most model independent way. While limits derived from measurements of electroweak parameters are especially well adapted as a parametrisation of a theory whose exact theoretical structure is not known, the approach is not efficient for most cases where new particles with well defined production and decay properties are predicted. The reason is that the effect of the new particles on the measured parameters can be very small and on the other hand, if the particles are light enough to be produced, they can have properties making them easily distinguishable from the known particles. In the completely decoupled environment, the search for the new particles is the only way to verify or falsify the new theory. A highly visible example of a search for new particles, where "new" in this particular case stands for "as yet unobserved", is the search for the standard model Higgs boson. The final result for LEP1 was a mass lower limit of 66 GeV/c2 [2]. No hints for the production of new particles were discovered at LEP1. Starting in late 1995, LEP's energy was increased in a first step to 130 GeV and 136 GeV and in 1996 the threshold for W pair production was reached and surpassed. In 1997 the centre-of-mass energy was once more increased to 183 GeV, passing the Z pair production threshold. Further increases of LEP's energy are foreseen with a final centre-of-mass energy of possibly 200 GeV to be reached in the year 2000. The physics goals of this high energy period have shifted from Z boson physics precision measurements to W± boson physics measurements. The mass of the W* boson is of interest because it is one of the limiting factors in the prediction of the Higgs boson mass via radiative corrections. The measurement of the triple gauge boson couplings (TGC) provides an opportunity to constrain physics beyond the standard model. The new energy regime also extends the kinematic reach of the direct search for new particles. Many of the searches for new particles performed at LEP1 had already reached the kinematic limit after only two years of running [3] due to the large cross sections at the Z boson resonance. In this thesis three ingredients of physics analyses at LEP with the ALEPH detector will be studied: measurements of the W* pair production cross section, measurement of the W* boson's branching ratio, constraining the W* boson's anomalous couplings via the measurement of the single W* cross section, i.e., e+e~ —» We^, and the search for the production of the charged Higgs boson and supersymmetric particles. The thesis is structured as follows: In chapter 2, the standard model and its inter- actions will be described briefly. The focus will be directed to the description of the 2 properties of the W* boson at LEP, i.e., its production, its decay and its couplings. The principles of the measurement of the W* mass via the measurement of the W* pair production cross section at threshold will also be described in this chapter. This will be followed by a brief discussion of the single W* process in view of the measurement of the triple gauge boson couplings. In chapter 3 two related extensions of the standard model will be described. First the Higgs sector will be extended by an additional scalar doublet, which substantially increases the number of physical Higgs bosons. In particular the properties of charged Higgs bosons, predicted by such models, will be described. In a second step the stan- dard model will be extended supersymmetrically.