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Standard Model Higgs Boson Searches in the Weak Boson Decay Channels with the ATLAS Detector

German David Carrillo Montoya

The search of the Standard Model Higgs boson decaying into a pair of weak bosons with the subsequent leptonic decay of the W or Z bosons is presented. The contributions achieved by this work range from the réévaluation of Higgs boson normalisation cross-sections, to the development of the analysis strategies using detailed Monte Carlo simulations and the search results for the H -► ZZ -► l+l-ννbar decay mode based on 4.7 fb-1 of proton-proton collision data at sqrt(s) = 7 TeV taken during 2011 by the ATLAS detector. The computation of cross-sections values for Higgs boson production at centre-of-mass energies varying from 6 to 14 TeV, for the main production mechanisms at the LHC were documented for posterior usage among the Collaboration. Preliminary detector resolution studies on Z -► μμ events using early data collected by ATLAS are also described. Simulation studies for the H -► ZZ(*) -► l+l-l+l- and H — W+W- — l+νl-ν channels included feasibility checks used to quantify the expected sensitivity of the analyses for various scenarios with different centre-of-mass energy, luminosity and detector calibration conditions. These results were part of the input that the Collaboration used to feed the schedule and plans of the LHC. On the H -► ZZ -► l+l-ννbar channel, upper limits on the Higgs boson production cross-section in a mass range between 200 and 900 GeV are set. With this channel by itself, the Standard Model Higgs boson particle is excluded at 95% confidence level in the mass range between 319 and 558 GeV To complete the current picture of the Higgs boson searches in the ATLAS detector, a compilation of the different Higgs boson analyses performed by the ATLAS Collaboration are introduced. The Higgs boson existence is constrained to be in a narrow window between 122.5 GeV and 129 GeV with 95% confidence level; where an excess of 2.6 a local significance is observed.

EPFL2012

Interacting spin-1 bosons in a two-dimensional optical lattice

We study, using quantum Monte Carlo (QMC) simulations, the ground state properties of spin-1 bosons trapped in a square optical lattice. The phase diagram is characterized by the mobility of the particles (Mott insulating or superfluid phase) and by their magnetic properties. For ferromagnetic on-site interactions, the whole phase diagram is ferromagnetic and the Mott insulators-superfluid phase transitions are second order. For antiferromagnetic on-site interactions, spin nematic order is found in the odd Mott lobes and in the superfluid phase. Furthermore, the superfluid-insulator phase transition is first or second order depending on whether the density in the Mott is even or odd. Inside the even Mott lobes we observe a singlet-to-nematic transition for certain values of the interactions. This transition appears to be first order.

Amer Physical Soc2013

Quantum Monte Carlo Study of a Resonant Bose-Fermi Mixture

Gianluca Bertaina, Elisa Fratini

We study a resonant Bose-Fermi mixture at zero temperature by using the fixed-node diffusion Monte Carlo method. We explore the system from weak to strong boson-fermion interaction, for different concentrations of the bosons relative to the fermion component. We focus on the case where the boson density nB is smaller than the fermion density nF, for which a first-order quantum phase transition is found from a state with condensed bosons immersed in a Fermi sea, to a Fermi-Fermi mixture of composite fermions and unpaired fermions. We obtain the equation of state and the phase diagram, and we find that the region of phase separation shrinks to zero for vanishing nB

2013

Standard Model Higgs Boson Searches in the Weak Boson Decay Channels with the ATLAS Detector

German David Carrillo Montoya

EPFL2012