Combined search for the Higgs boson with the D0 experiment

We perform a combination of searches for standard model Higgs boson production in $p\bar{p}$ collisions recorded by the D0 detector at the Fermilab Tevatron Collider at a center of mass energy of $\sqrt{s}=1.96$ TeV. The different production and decay channels have been analyzed separately, with integrated luminosities of up to 9.7 fb$^{-1}$ and for Higgs boson masses $90\leq M_H \leq 200$ GeV. We combine these final states to achieve optimal sensitivity to the production of the Higgs boson. We also interpret the combination in terms of models with a fourth generation of fermions, and models with suppressed Higgs boson couplings to fermions. The result excludes a standard model Higgs boson at 95% C.L. in the ranges $90 M_HM_

Search for ZH to llbb production in 4.2 fb^-1 of ppbar collisions at sqrt(s) = 1.96 TeV

We present a search for the standard model Higgs boson produced in association with a Z boson in 4.2 fb^-1 of p-pbar collisions, collected with the D0 detector at the Fermilab Tevatron at sqrt(s) = 1.96 TeV. Selected events contain one reconstructed Z to llbb candidate and at least two jets, including at least one b-tagged jet. In the absence of an excess over the background expected from other standard model processes, limits on the ZH cross section multiplied by the branching ratios are set. The limit at M_H = 115 GeV is a factor of 5.9 larger than the standard model prediction

Rare Decays and CP Violation in the BsB_s System

CP violating phenomena and rare decays of $B_s$ mesons offer interesting probes to test the quark-flavor sector of the Standard Model. In view of plenty of data reported in particular from the Large Hadron Collider, this topic has received a lot of attention in 2012. We give an overview of the the most recent experimental results, new theoretical developments, and discuss the prospects for the future exploration of the $B_s$-meson system.Comment: 40 pages, 10 figures, Invited contribution to Annual Review of Nuclear and Particle Science, Vol. 63 (2013

Search for a fourth generation t' quark in ppbar collisions at sqrt{s}=1.96 TeV

We present a search for pair production of a fourth generation t' quark and its antiparticle, followed by their decays to a W boson and a jet, based on an integrated luminosity of 5.3/fb of proton-antiproton collisions at sqrt{s}=1.96 TeV collected by the D0 Collaboration at the Fermilab Tevatron Collider. We set upper limits on the t't'bar production cross section that exclude at the 95% C.L. a t' quark that decays exclusively to W+jet with a mass below 285 GeV. We observe a small excess in the muon+jets channel which reduces the mass range excluded compared to the expected limit of 320 GeV in the absence of a signal

Observation of a narrow mass state decaying into Upsilon(1S) + gamma in ppbar collisions at sqrt(s) = 1.96 TeV

Using data corresponding to an integrated luminosity of 1.3 fb^-1, we observe a narrow mass state decaying into Upsilon(1S)+gamma, where the Upsilon(1S) meson is detected by its decay into a pair of oppositely charged muons, and the photon is identified through its conversion into an electron-positron pair. The significance of this observation is 5.6 standard deviations. The mass of the state is centered at 10.551 \pm 0.014 (stat.) \pm 0.017 (syst.) GeV/c^2, which is consistent with that of the state recently observed by the ATLAS Collaboration

Combined search for the standard model Higgs boson decaying to b bbar using the D0 Run II data set

We present the results of the combination of searches for the standard model Higgs boson produced in association with a W or Z boson and decaying into b bbar using the data sample collected with the D0 detector in p pbar collisions at sqrt{s}=1.96 TeV at the Fermilab Tevatron Collider. We derive 95% CL upper limits on the Higgs boson cross section relative to the standard model prediction in the mass range 100 Ge

Search for the standard model Higgs boson in ZH -> l+ l- bbbar production with the D0 detector in 9.7 fb-1 of ppbar collisions at sqrt(s)=1.96 TeV

We present a search for the standard model (SM) Higgs boson produced in association with a Z boson in 9.7 fb-1 of ppbar collisions collected with the D0 detector at the Fermilab Tevatron Collider at sqrt(s) = 1.96 TeV. Selected events contain one reconstructed Z-> e+e- or Z-> mu+mu- candidate and at least two jets, including at least one jet identified as likely to contain a b quark. To validate the search procedure, we also measure the cross section for ZZ production in the same final state. It is found to be consistent with its SM prediction. We set upper limits on the ZH production cross section times branching ratio for H->bbbar at the 95% C.L. for Higgs boson masses 9

A Test of Lepton Flavour Universality in the Decays of the Top Quark and Data driven validation of the Hard Scatter Vertex Selection Efficiency at ATLAS

The first part of the thesis covers the work done for the ‘qualification task’ at ATLAS, contributing to the maintenance of the experiment, measuring the efficiency of Hard Scatter identification in isolated lepton events: Z 0 → ee¯, Z 0 → µµ¯ and tt¯ → eµ . Code was developed to extract the HS efficiency, the efficiency at which the ‘true’ first interaction of an event is identified by ATLAS software , with an ‘estimation’ method and a ‘fitting’ method in Release 21 of AnalysisBase. It was found that the Z 0 decay processes have a large discrepancy in the percentage of misidentified Hard Scatter vertices between Monte Carlo simulation and data unlike in the tt¯ channel where Monte Carlo and data agreed. The percentage of misidentified Hard Scatter vertices was also found to be far greater in Z 0 decays than tt¯. The fitting method was then developed for Release 22, and found that the HS efficiency was much better than Release 21, with the HS efficiency for Z 0 → ℓℓ events comparable to the efficiency of tt¯ events in Release 21. The second part is the main physics analysis, aiming to test a fundamental axiom of the Standard Model, Lepton Flavour Universality, the hypothesis that the couplings of each lepton flavour to the electroweak gauge bosons are the same regardless of flavour. The aim of the analysis presented is to test lepton flavour universality through the measurement of the of the rate of decay of W bosons to τ-leptons and electrons, R(τ/e) = B(W → τντ )/B(W → eνe). Di-leptonic tt¯ events are based on 139 f b−1 of data recorded with the ATLAS detector in proton–proton collisions at √s = 13TeV. The analysis follows a ‘tag and probe’ method; one lepton is the tag to select events, the other a probe to determine if the lepton comes from a ‘prompt’ decay, W → eνe, or an intermediate τ lepton, W → τντ → eνeντντ . The result of the analysis is still blinded as it is still in the approval process by the ATLAS collaboration. The expected precision of the measurement of R(τ/e) is 0.0234

Evidence for an anomalous like-sign dimuon charge asymmetry

We measure the charge asymmetry $A\equiv(N^{++} - N^{--})/(N^{++} + N^{--})$ of like-sign dimuon events in 6.1~fb$^{-1}$ of $p\bar{p}$ collisions recorded with the D0 detector at a center-of-mass energy $\sqrt{s}=1.96$~TeV at the Fermilab Tevatron collider. From $A$ we extract the like-sign dimuon charge asymmetry in semileptonic $b$-hadron decays: \aslb=-0.00957\pm0.00251\thinspace({\rm stat})\pm0.00146\thinspace({\rm sys}). It differs by 3.2~standard deviations from the standard model prediction \aslb({\rm SM})=(-2.3^{+0.5}_{-0.6})\times10^{-4}, and provides first evidence of anomalous $CP$ violation in the mixing of neutral $B$ mesons