Recent Studies of Little Higgs Models in ATLAS

The ATLAS Collaboration at the LHC continues investigating the possibility to detect particles predicted by Little Higgs models. In this talk, the latest results on the Z/W h decays and on the hadronic decays of the new gauge bosons Z_H/W_H are reviewed.Comment: to appear in Proceedings of SUSY06, the 14th International Conference on Supersymmetry and the Unification of Fundamental Interactions, UC Irvine, California, 12-17 June 200

Performances du calorimètre électromagnétique et recherche de nouveaux bosons de jauge dans le canal diélectron auprès du détecteur ATLAS

Le XXe siècle a marqué le succès de la construction du modèle standard de la physique des particules. Elaborée entre les années 1930 et 1970, cette théorie des particules élémentaires et des interactions électromagnétique, faible et forte a depuis été abondamment vérifiée auprès des collisionneurs tels que le LEP et le Tevatron. Malgré ce succès, certaines questions laissées en supsens ont nécessité l'élaboration de nouvelles théories permettant de dépasser le cadre du modèle standard. Parmi ces théories nombreuses sont celles prédisant l'existance d'un nouveau boson Z' à l'échelle du TeV. Les données du LHC, recueillies depuis son démarrage à l'automne 2008, offrent une nouvelle fois l'opportunité de confronter le modèle standard à ses prédictions et de rechercher les signatures de l'existence de nouvelle physique jusqu'à des énergies inégalées. Le travail mené au sein de l'expérience ATLAS au cours de ces quatres premières années s'est ainsi orienté autour de la compréhension du détecteur et de l'analyse des premières données. Cette thèse couvre ces deux aspects. La première partie du travail présenté revient ainsi sur la mise en évidence d'une pathologie de l'électronique de lecture du calorimètre à argon liquide d'ATLAS ainsi que sur l'étude de larges déviations cohérentes du bruit observées depuis sa mise en service. La mise en place d'une stratégie de préservation des données collectées y est détaillée. La seconde partie de ce manuscrit se concentre sur la recherche d'un nouveau boson Z'. Si tant est qu'une telle particule existe, sa décroissance en un électron et un positron devrait donner lieu à l'apparition d'une nouvelle résonance massive dans le spectre en masse invariante diélectron. Les performances de reconstruction et d'identification des électrons, particulièrement à haute impulsion transverse, sont étudiées. L'analyse des 4.9 fb-1 de données collectées en 2011 est décrite. En l'absence de déviation significative par rapport aux prédictions du modèle standard, le spectre en masse invariante diélectron est réinterprété afin de dériver les limites sur l'existence de nouveaux bosons issus de théories de grande unification (E6) et sur l'existence d'un boson de type SSM. Ces limites et celles obtenues par l'expérience CMS sont à l'heure actuelle les plus contraignantes quant à l'existence de ces nouveaux bosons.The Standard Model of particle physics has known a tremendous rise during the twentieth century. Built up, from the early 1930s to the 1970s, this theory describing elementary particles and their interactions (electromagnetic, weak, strong) has now been intensivly tested by LEP and Tevatron colliders. Besides its succes, some problems remain and have lead to new theories attempting to go beyond the standard model. Many of them are predicting the existence of a new gauge boson Z', which is supposed to be observed at the TeV scale. Data recorded by the LHC since automn 2008 are a new opportunity to check the consistency of the Standard Model and to search for new physics evidence. Work that has been done by the ATLAS collaboration during the last four years has focused on understanding detector's behaviour and analysing the very first collected collisions. This thesis is reflecting these two aspects. Therefore, the first part of this thesis describes the caracterisation of a pathology of ATLAS liquid argon calorimeter electronics and of coherent noise bursts that have both been observed since the beginning of ATLAS operation. The policy deployed to preserve data quality is also detailled. The second part is focusing on the search for new Z' gauge boson. In case this particle was to exist, its decay into an electron and a positron would lead to a new massive resonance in the dielectron invariant mass spectrum. Therefore electron reconstruction and identification performances are closely looked at, especially at high transverse momentum. Analysis made on the 4.9 fb-1 of collected data is reported. As no significant excess with respect to Standard Model predictions is observed, the dielectron invariant mass spectrum is interpreted to derive mass limits concerning the existence of new Z' gauge bosons appearing in grand unification theories (E6) and effective sequential standard model (SSM). These limits and those derived by the CMS collaboration are the best ever set on such new bosons.SAVOIE-SCD - Bib.électronique (730659901) / SudocGRENOBLE1/INP-Bib.électronique (384210012) / SudocGRENOBLE2/3-Bib.électronique (384219901) / SudocSudocFranceF

Probing RS scenarios of flavour at LHC via leptonic channels

We study a purely leptonic signature of the Randall-Sundrum scenario with Standard Model fields in the bulk at LHC: the contribution from the exchange of Kaluza-Klein (KK) excitations of gauge bosons to the clear Drell-Yan reaction. We show that this contribution is detectable (even with the low luminosities of the LHC initial regime) for KK masses around the TeV scale and for sufficiently large lepton couplings to KK gauge bosons. Such large couplings can be compatible with ElectroWeak precision data on the Zff coupling in the framework of the custodial O(3) symmetry recently proposed, for specific configurations of lepton localizations (along the extra dimension). These configurations can simultaneously reproduce the correct lepton masses, while generating acceptably small Flavour Changing Neutral Current (FCNC) effects. This LHC phenomenological analysis is realistic in the sense that it is based on fermion localizations which reproduce all the quark/lepton masses plus mixing angles and respect FCNC constraints in both the hadron and lepton sectors.Comment: 15 pages, 6 Figures, Latex fil

Precision predictions for Z'-production at the CERN LHC: QCD matrix elements, parton showers, and joint resummation

We improve the theoretical predictions for the production of extra neutral gauge bosons at hadron colliders by implementing the Z' bosons in the MC@NLO generator and by computing their differential and total cross sections in joint p_T and threshold resummation. The two improved predictions are found to be in excellent agreement with each other for mass spectra, p_T spectra, and total cross sections, while the PYTHIA parton and ``power'' shower predictions usually employed for experimental analyses show significant shortcomings both in normalization and shape. The theoretical uncertainties from scale and parton density variations and non-perturbative effects are found to be 9%, 8%, and less than 5%, respectively, and thus under good control. The implementation of our improved predictions in terms of the new MC@NLO generator or resummed K factors in the analysis chains of the Tevatron and LHC experiments should be straightforward and lead to more precise determinations or limits of the Z' boson masses and/or couplings.Comment: 14 pages, 7 figures, version to appear in Nucl. Phys.

Study of Tau-pair Production in Photon-Photon Collisions at LEP and Limits on the Anomalous Electromagnetic Moments of the Tau Lepton

Tau-pair production in the process e+e- -> e+e-tau+tau- was studied using data collected by the DELPHI experiment at LEP2 during the years 1997 - 2000. The corresponding integrated luminosity is 650 pb^{-1}. The values of the cross-section obtained are found to be in agreement with QED predictions. Limits on the anomalous magnetic and electric dipole moments of the tau lepton are deduced.Comment: 20 pages, 9 figures, Accepted by Eur. Phys. J.

Energy dependence of Cronin momentum in saturation model for p+Ap+A and A+AA+A collisions

We calculate $\sqrt{s}$ dependence of Cronin momentum for $p+A$ and $A+A$ collisions in saturation model. We show that this dependence is consistent with expectation from formula which was obtained using simple dimentional consideration. This can be used to test validity of saturation model (and distinguish among its variants) and measure $x$ dependence of saturation momentum from experimental data.Comment: LaTeX2e, 12 pages, 8 figure

CP asymmetry in B→ϕKSB \to \phi K_S in a general two-Higgs-doublet model with fourth-generation quarks

We discuss the time-dependent CP asymmetry of decay $B \to \phi K_S$ in an extension of the Standard Model with both two Higgs doublets and additional fourth-generation quarks. We show that although the Standard Model with two-Higgs-doublet and the Standard model with fourth generation quarks alone are not likely to largely change the effective $\sin 2 \beta$ from the decay of $B \to \phi K_S$, the model with both additional Higgs doublet and fourth-generation quarks can easily account for the possible large negative value of $\sin 2 \beta$ without conflicting with other experimental constraints. In this model, additional large CP violating effects may arise from the flavor changing Yukawa interactions between neutral Higgs bosons and the heavy fourth generation down type quark, which can modify the QCD penguin contributions. With the constraints obtained from $b \to s \bar{s} s$ processes such as $B \to X_s \gamma$ and $\Delta m_{B_s^0}$, this model can lead to the effective $\sin 2 \beta$ to be as large as $- 0.4$ in the CP asymmetry of $B \to \phi K_S$.Comment: 13 pages, 5 figures, references added, to appear in Eur.Phys.J.

A Precise Measurement of the Tau Lifetime

The tau lepton lifetime has been measured with the e+e- -> tau+tau- events collected by the DELPHI detector at LEP in the years 1991-1995. Three different methods have been exploited, using both one-prong and three-prong tau decay channels. Two measurements have been made using events in which both taus decay to a single charged particle. Combining these measurements gave tau_tau (1 prong) = 291.8 +/- 2.3 (stat) +/- 1.5 (sys) fs. A third measurement using taus which decayed to three charged particles yielded tau_tau (3 prong) = 288.6 +/- 2.4 (stat) +/- 1.3 (sys) fs. These were combined with previous DELPHI results to measure the tau lifetime, using the full LEP1 data sample, to be tau_tau = 290.9 +/- 1.4 (stat) +/- 1.0 (sys) fs.Comment: 27 pages, 7 figure

Search for composite and exotic fermions at LEP 2

A search for unstable heavy fermions with the DELPHI detector at LEP is reported. Sequential and non-canonical leptons, as well as excited leptons and quarks, are considered. The data analysed correspond to an integrated luminosity of about 48 pb^{-1} at an e^+e^- centre-of-mass energy of 183 GeV and about 20 pb^{-1} equally shared between the centre-of-mass energies of 172 GeV and 161 GeV. The search for pair-produced new leptons establishes 95% confidence level mass limits in the region between 70 GeV/c^2 and 90 GeV/c^2, depending on the channel. The search for singly produced excited leptons and quarks establishes upper limits on the ratio of the coupling of the excited fermio

Search for lightest neutralino and stau pair production in light gravitino scenarios with stau NLSP

Promptly decaying lightest neutralinos and long-lived staus are searched for in the context of light gravitino scenarios. It is assumed that the stau is the next to lightest supersymmetric particle (NLSP) and that the lightest neutralino is the next to NLSP (NNLSP). Data collected with the Delphi detector at centre-of-mass energies from 161 to 183 \GeV are analysed. No evidence of the production of these particles is found. Hence, lower mass limits for both kinds of particles are set at 95% C.L.. The mass of gaugino-like neutralinos is found to be greater than 71.5 GeV/c^2. In the search for long-lived stau, masses less than 70.0 to 77.5 \GeVcc are excluded for gravitino masses from 10 to 150 \eVcc . Combining this search with the searches for stable heavy leptons and Minimal Supersymmetric Standard Model staus a lower limit of 68.5 \GeVcc may be set for the stau mas