Aachen, Techn. Hochsch., Diss., 2002. / Computerdatei im Fernzugriff.

Aachen, Techn. Hochsch., Diss., 2002. / Computerdatei im Fernzugriff.

Berlin, Humboldt-University, Diss., 2001.

Universiẗat, Diss., 2000--Freiburg (Breisgau).

La these porte sur les corrections mixtes QCD-EW au niveau NNLO a la productionDrell-Yan de bosons Z et W. Le processus Drell-Yan est un processus fondamentalpermettant de tester avec precision le Modele Standard (MS) de physique des partic-ules au sein de collisionneurs hadroniques, car ce dernier presente une section ecaceimportante, une signature experimentale tres propre, ainsi qu'une tres haute sensi-bilite aux proprietes des bosons de jauge. Pour toutes ces raisons, une prediction theorique precise et able, siginant ici que l'on garde sous contr^ole lestermes provenant des corrections perturbatives d'ordre superieur de la section ecaceet des distributions du mecanisme de production de Drell-Yan, est exigee pour menera bien des etudes de physique au niveau de collisionneurs hadroniques.Dans cette thèse , nous étudions les corrections QCD mixtes - EW à Drell - Yan traite à la NNLO . D'un point de vue technique , le calcul d'un tel ensemble de corrections impliquerait le cal-tion de diagrammes de Feynman très compliquées , La plus grande contribution provient des diagrammes dans lesquels la particule de décomposition ( Z ou boson W ) est presque sur - coquille.En utilisant les règles Cutkosky , nous pouvons ré-écrire l'intégration sur l'espace de phase de latermes d'interférence ( une boucle 2 à 2 diagrammes interféré avec le niveau arbre 2 à 2 etarbre 2 ou 3 diagrammes carré ) en termes de combinaison des intégrales de propagationteurs ayant la prescription et propagateurs de causalité droite avec une face .Ces intégrales peuvent être traités de la même manière que les corrections virtuelles . Cette réduction se fait en utilisant l' algorithme Laporta \ " , sur la base del'intégration par parties identités . Le calcul de l' IM est réalisée en utilisant la méthode de la différenceéquations. En conséquence , nous obtenons l' IM exprimée en série de Laurent ,où D est la dimension de l'espace - temps , la multiplication d'un facteur qui prend entenir compte de la limite souple de l'intégrale en D dimensions . / The thesis concerns the NNLO mixed QCD-EW corrections to the Drell-Yan (DY)production of Z andW bosons, via the following reactions: pp(p) Z+X to l + Xand pp to W + X to l + X. This is a fundamental process for an accurate testof the Standard Model (SM) at hadron colliders, since it has a large cross section, aclean experimental signature. In particular, the Drell-Yan production of Ws is important for an accuratedetermination (via transverse mass and pT distributions) of the W mass, mW, aninput parameter of the model. Because of all these reasons, an accurate and reliable theoretical prediction forthe cross section and the distributions of the Drell-Yan production mechanism, thatmeans control on the higher-order perturbative corrections, is demanded for physicsstudies at hadron colliders. In this thesis, we study the mixed QCD-EW corrections to Drell-Yan processes at the NNLO. From a technical point of view, the calculation of such a set of corrections would involve the calcu-lation of very complicated Feynman diagrams, The biggest contribution comes from the diagrams in which the decaying particle(Z or W boson) is nearly on-shell. Using the Cutkosky rules, we can re-write the integration over the phase-space of theinterference terms (one-loop 2 to 2 diagrams interfered with the tree-level 2 to 2 andtree 2 to 3 diagrams squared) in terms of a combination of integrals with propaga-tors having the right causality prescription and propagators with the opposite one.These integrals can be treated in the same way as the virtual corrections. This reduction is done using the \Laporta Algorithm", based onthe Integration-by-Parts Identities. The calculation of the MIs is performed using the method of differentialequations. As a result, we get the MIs expressed as a Laurent series ,where D is the dimension of the space-time, multiplying a factor which takes intoaccount the soft limit of the integral in D dimensions.

For the first time, the Z0 boson angular distribution in the center-of-momentum frame is measured in proton-proton collisions at sqrt(S) = 7 TeV at the CERN LHC. The data sample, recorded with the CMS detector, corresponds to an integrated luminosity of approximately 36 pb-1. Events in which there is a Z0 and at least one jet, with a jet transverse momentum threshold of 20 GeV and absolute jet rapidity less than 2.4, are selected for the analysis. Only the Z0’s muon decay channel is studied. Within experimental and theoretical uncertainties, the measured angular distribution is in agreement with next-to-leading order perturbative QCD predictions.

No description available.

We present in this thesis a detailed analysis of Higgs boson interactions at future colliders. In particular we examine, in a model independent way, the sensitivity of an Linear Collider in probing the interaction of Higgs boson with a pair of vector bosons with/without the use of polarized initial beams and/or the information on final state fermion polarization. We devise several observables which have definite transformation properties under discrete symmetry operations to constrain the different anomalous parts of the Higgs boson interactions having the same transformation properties. We also investigate effects of initial state radiation (ISR) and beamstrahlung on probes of anomalous Higgs boson couplings at higher center of mass energies. We begin the first chapter with an introduction of the Standard Model (SM) of particle physics. We mainly focus on the Higgs sector of the SM. In this chapter we review the electroweak (EW) symmetry breaking mechanism, viz. the Higgs mechanism, responsible for generating masses of all the particles in the SM. We briefly summarize the high precision tests of the SM. We discuss constraints on the mass of the SM Higgs boson derived from theoretical considerations such as stability of the electroweak vacuum, unitarity in scattering amplitudes, perturbativity of the Higgs self-coupling and no fine-tuning in the radiative corrections in the Higgs sector. Next we present the experimental bounds on the mass of the SM Higgs boson obtained from the direct searches of the Higgs boson at LEP and from the electroweak high precision measurements. We then discuss the importance of a general model independent approach to study properties of the Higgs boson and to verify the uniqueness of the SM. In the context of low energy effective theory, this analysis can be made by using the effective Lagrangian that contains higher dimensional operators. We conclude this chapter giving examples of dimension-6 operators which can contribute to the anomalous Higgs boson interactions that we analyze in this thesis. Second chapter contains the dominant Higgs boson production processes at an collider.In a model independent analysis we consider the effects of the most general ¯ (V = W Z) vertex, consistent with Lorentz invariance, for the process where f is any light fermion. This vertex also includes the possibility of CP violation and can be written as: where ki denote the momenta of the two W’s (Z’s), ǫναβis the antisymmetric tensor with ǫ0123 = 1. Previous studies showed that the squared matrix element of the process e+e−ZH does not include all the anomalous parts of a general ZZH vertex. Also it is obvious that one cannot analyse anomalous WWH couplings using this process. Hence we consider the full process e+e−ffH to probe all the anomalous parts of the VVH vertex. We devise a general and very elegant procedure to probe these couplings at an e+e−collider. We construct various combinations by taking dot and scalar triple product of momenta of initial and final state particles. These combinations have definite transformation properties under CP and naive time reversal (T˜)transformations. Hence the corresponding observables constructed using expectation value of sign of these combinations can probe a specific part of the anomalous VVH couplings whose coefficient in the effective Lagrangian has same transformation properties. We investigate the possible sensitivity to which the anomalous VVH couplings can be probed at a Linear Collider using these observables in the process e+e−ffH for unpolarized beams [1, 2]. We consider the case of a Linear Collider, operating at center of mass energy of 500 GeV, with an integrated luminosityof 500 fb−1 and assume a Higgs boson of mass 120 GeV. We impose various kinematical cuts on different final state particles to reduce backgrounds ¯and consider the events where H decays into bb with branching ratio 0.68. We can enhance or suppress the effect of the s-channel, Z-exchange diagram by imposing cut on the ¯invariant mass of the ff system. We use b-tagging efficiency to be 70%; a value expected to be possible in the collider environment. We first consider asymmetries involving either the polar or azimuthal angular distributions. Then we combine these informations to construct combined polar-azimuthal asymmetries in order to enhance the sensitivity. We obtain strong constraints on most of the anomalous parts of the ZZH vertex using cross section and these asymmetries. The process e+eν¯ −νH has two missing ν’s in the final state. Hence their momenta are not available to construct any observables. Therefore, direct probes for T˜-odd WWH couplings viz. ℑ(bW), ℜ(˜bW), cannot be constructed and only weak, indirect bounds are possible. Further, without using polarized beams the contamination from the ZZH vertex cannot be eliminated in the determination of WWH couplings. In the third chapter we analyze use of linearly polarized e+/e−beams and/or information on final state lepton polarization in probingthe interaction of the Higgs boson with a pair of vector bosons[3, 4]. We make several combinations of different particle momenta and spins. We then define observables as expectation values of signs of these combinations for longitudinally polarized beams and/or for production of final state τ’s with a definite helicity state. Use of polarization allows us to devise more observables as compared to the unpolarized case. We list the observables for which use of polarization affords a distinct gain in sensitivity. In our analysis we divide the total luminosity of 500 fb−1 equally among different polarization states of initial state e−/e+ and take the values 80% and 60% for e−/e+ respectively, foreseen at the ILC. We construct numerical combinations of various linearly polarized cross sections to enhance the contribution of ℜ(bZ) while getting rid of ΔaZand vice versa. It is necessary to construct such combinations of cross section as ℜ(bZ), ΔaZhave same CP and T˜transformation properties and hence there are no asymmetries that can be constructed to probe them individually. With these combinations it is possible to probe both these CP-and T˜-even couplings cleanly, using linearly polarized beams. We find that longitudinal beam polarization can improve the sensitivity to CP-odd ZZH couplings viz. ℜ(˜bZ), ℑ(˜bZ), by a factor of about 6 −7. We also construct observables for final state τ’s with definite helicity. We make a plausible assumption that it should be possible to isolate events with τ’s in definite helicity state with an efficiency of 40%. With this assumption we demonstrate that the use of final state τ polarization can improve the sensitivity to the CP-even and T˜-odd ZZH coupling (ℑ(bZ)) by a factor of about 3. Moreover use of final state τ-polarization measurement along with linearly polarized beams can improve the sensitivity for one of the CP-odd ZZH couplings (ℜ(˜bZ))bya factor of about 2.Use of longitudinally polarized beams can also help to reduce the contamination in the measurement of the WWH couplings coming from the ZZH vertex contribution. We also perform χ2-analysis using the observables for different polarizations. The cross section of the t–channel diagram increases with increasing center of mass energy. Therefore, off hand it may look like that going to higher energy can increase the sensitivity to WWH couplings. Hence in this chapter we further investigate possible gain in sensitivity going to higher center of mass energies[3, 4]. We use the same observables constructed with unpolarized beams and consider various center of mass energies ranging from 300 GeV to 3 TeV. We find that it is possible to increase the bZ)byabouta factor 2 1 TeVas compared to the case of 500 GeV. In this analysis we include the effects of initial state radiation (ISR) and beamstrahlung. Both the ISR and beamstrahlung =500 GeV, the ISR can affect cross sections for s–channel processes by 10−15%.However, we observe that the effects of ISR and beamstrahlung change both the SM and anomalous contributions more beneficial for the study of anomalous V V H couplings. In the last chapter we investigate the role of transversely polarized beams to constrain the anomalous V V H couplings[5]. Using transverse spin direction of e±it is possible to devise observables which are nonzero only for transversely polarized beams. Use of transverse beam polarization allows construction of completely independent probes of both the CP-and T˜-even anomalous ZZH couplings (ΔaZ, ℜ(bZ)), leading to independent determination of all the anomalous parts of the ZZH vertex. In addition the use of transverse beam polarization can also add to the sensitivity for one of the CP-odd ZZH couplings viz. ℜ(˜bZ). Measurement of final state τ-polarization with transversely polarized beams can in fact also offer improvement on the sensitivity for ℑ(bZ) which is even under CP-and odd under T˜-transformation. Use of transverse beam polarization cannot improve the bounds on the anomalous WWH couplings as the squared matrix element of the t– channel WW–fusion diagram does not have any transverse beam polarization dependent term. A summary of the results obtained in this thesis is follows. We have developed a general procedure to construct observables with specific CP and T˜transformation properties to probe various anomalous couplings of the Higgs boson to a pair of vector bosons (V = W/Z) at an e+/e−Linear Collider. We investigate probes of these couplings in the process e+e−ffH. This process gives access to those anomalous couplings which cannot be probed using angular distribution of the Z boson in the process e+eZH. We showed that it would be possible to obtain stringent bounds on some of the parts of the anomalous ZZH vertex even without using polarized beams and/or information on polarization of final state particles. Use of longitudinal beam polarization and/or final state τ polarization can significantly enhance the sensitivity in probing most of the anomalous parts of a general ZZH vertex. Use of longitudinal beam polarization also reduces the contamination from the ZZH couplings in the determination of the ˜T-even anomalous WWH couplings (ℜ(bW), ℑ(˜bW)). However, two missing neutrinos in the final state do not allow any direct probe of the T˜-odd WWH couplings (ℑ(bW), ℜ(˜bW)).We find that use of transverse polarization of the beams is essential to construct independent probes of the two anomalous ZZH couplings, which are even under CP and T˜transformations, viz.ΔaZand ℜ(bZ).We observed that there will be no significant gain 500 GeV), but with polarized beams is preferable from the point of view of studying anomalous V V H coupling. (For mathematical equations pl see the pdf file.)

Le thème des analyses présentées dans ce document est la mesure des propriétés du boson de Higgs se désintégrant dans le mode H→ZZ→4l dans l'expérience ATLAS au CERN. Le document commence par un résumé détaillé concernant la procédure d'étalonnage des électrons: l'algorithme de combinaison trace-cluster améliore la résolution en énergie (surtout pour les électrons ayant une faible énergie transverse) en exploitant les informations du cluster et de la trace dans un ajustement par maximum de vraisemblance. L'amélioration en résolution est approximativement de 18-20% pour les désintégrations du J/Ψ en di-électrons, et 3% pour Z→ee. Par la suite, la combinaison E-p est appliquée au canal H→ZZ→4l avec électrons dans l'état final permettant d’obtenir un gain modéré sur la distribution de la masse invariante (4-5%). En deuxième lieu, la masse du boson de Higgs et sa largeur sont estimées, en particulier afin de comprendre les effets apportés par l'utilisation de l'algorithme de combinaison trace-cluster. La masse a été calculée en se servant d'un ajustement à deux dimensions appliqué sur la masse invariante m4l et un score de discrimination du signal contre le bruit de fond ZZ*. Cette discrimination est obtenue en exploitant les corrélations angulaires dont les distributions sont sensibles au spin et à la parité du boson de Higgs. L’étude sur la largeur du boson est ensuite détaillée : les résultats sont basés sur une approche qui vise à contraindre cette largeur en analysant la région de haute masse m4l où le boson de Higgs se comporte comme un propagateur. La section efficace au pic de la résonance (« on-shell ») dépend de la largeur totale du boson de Higgs, ce qui n’est pas le cas pour la production dans la région de haute masse (« off-shell »). Par conséquent, des limites indirectes sur la largeur peuvent être déterminées en combinant les régions « on-shell » et « off-shell ». Une limite à 6.7 fois la largeur Higgs ΓSMH est obtenue via le canal 4l. En combinant la mesure « on-shell » avec tous les canaux de désintégration étudiés (notamment ZZ→4l, ZZ→2l2ν and WW→lνlν), les résultats aboutissent à une limite observée (attendue) sur la largeur totale de 22.7 (33.0) MeV. La dernière partie de ce travail de thèse est consacrée à l'analyse sur la largeur du boson de Higgs en quatre leptons à haute (High-Luminosity LHC) luminosité intégrée (respectivement 300 fb⁻¹ et 3000 fb⁻¹) : il s’agit d’une étude extrapolant à √s =14 TeV les techniques utilisées pour l’analyse à 8 TeV (Run 1). / The theme of the analyses presented in this Thesis is the measurement of the Higgs boson properties in the H→ZZ→4l decay channel with the ATLAS experiment at the LHC. A detailed overview on the electron calibration process is first presented. In this regard, the track-cluster combination algorithm is found to improve the energy resolution of low ET electrons by exploiting both track and cluster information into a maximum likelihood fit. The improvement in resolution is approximately 18-20% for J/Ψ dielectron decays, and of the order of 3% for Z→ee events. In addition, the E-p combination algorithm has also been applied to the H→ZZ→4l channel with electrons in the final state resulting in a non-negligible gain on the invariant mass distribution (4-5%). Secondly, the Higgs mass and its total width are evaluated in the H→ZZ→4l channel. The Higgs mass is measured in the 4l decay channel with particular interest on the beneficial effects brought by the improved electron calibration and the track-cluster combination. The mass on the full 2011 and 2012 datasets is worked out with a 2-dimensional fit on the invariant mass of the 4 lepton final state, m4l, and on a boosted decision tree (BDT)-based output conceived against the main ZZ irreducible background and constructed on variables that are sensitive to the Higgs boson spin-parity state. Regarding the Higgs width, results are based on a relatively recent approach aimed at indirectly constraining the Higgs boson width by exploiting the m4l high-mass region where the Higgs boson acts as a propagator. The Higgs production cross section in the on-shell m4l region, where the Higgs boson is a resonance, depends on the total Higgs width, whereas this is not the case for the high mass m4l (off-shell). Limits on the Higgs width can be therefore set when merging the off-shell results with the on-shell ones. A limit of ∼ 6.7 times ΓSMH is obtained in the four lepton channel. Secondly, by combining with the on-shell measurement and using all the decay channels in the analysis, i.e. ZZ→4l, ZZ→2l2ν and WW→lνlν, the results lead to an observed (expected) 95% C.L. upper limit on the Higgs boson total width of 22.7 (33.0) MeV (4.2 MeV is the Standard Model predicted Higgs width at mH=125 GeV).The last section of the thesis is devoted to the evaluation of the Higgs width at √s=14 TeV in the high luminosity scenario (High Luminosity LHC), 300 fb⁻¹ and 3000 fb⁻¹, by employing the same techniques exploited in the previous Run 1 analysis at √s=8 TeV.

A first measurement of Electroweak W + 2 jets production at high dijet mass is performed using sqrt(s) = 7 TeV pp collision data from the Atlas experiment corresponding to 4.6 fb−1 of integrated luminosity. The background only hypothesis is excluded with a significance of 4.89-σ. A cross section of σ = 325 ± 6 (lumi) ±32 (stat) +63−70(syst) ±86 (theo) fb is extracted in the fiducial region.