Search for New Fermions (Quirks'') at the Fermilab Tevatron Collider

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)

Limits on anomalous couplings from higgs boson production at the fermilab tevatron collider

We estimate the attainable limits on the coefficients of dimension-6 operators from the analysis of Higgs boson phenomenology, in the framework of a SU L(2) × U y(1) gauge-invariant effective Lagrangian. Our results, based on the data sample already collected by the collaborations at Fermilab Tevatron, show that the coefficients of Higgs-vector boson couplings can be determined with unprecedented accuracy. Assuming that the coefficients of all blind operators are of the same magnitude, we are also able to impose mere restrictive bounds on the anomalous vector-boson triple couplings than the present limit from double gauge boson production at the Tevatron collider.

Measurement of the oscillation frequency of B s mesons in the hadronic decay mode B s pi D s (phi pi) X with the D0 detector at the Fermilab Tevatron collider

The standard model (SM) of particle physics is a theory, describing three out of four fundamental forces. In this model the Cabibbo-Kobayashi-Maskawa (CKM) matrix describes the transformation between the mass and weak eigenstates of quarks. The matrix properties can be visualized as triangles in the complex plane. A precise measurement of all triangle parameters can be used to verify the validity of the SM. The least precisely measured parameter of the triangle is related to the CKM element |Vtd|, accessible through the mixing frequency (oscillation) of neutral B mesons, where mixing is the transition of a neutral meson into its anti-particle and vice versa. It is possible to calculate the CKM element |Vtd| and a related element |Vts| by measuring the mass differences Dmd (Dms ) between neutral Bd and bar{Bd} (Bs and bar{Bs}) meson mass eigenstates. This measurement is accomplished by tagging the initial and final state of decaying B mesons and determining their lifetime. Currently the Fermilab Tevatron Collider (providing pbar{p} collisions at sqrt{s}=1.96 TeV) is the only place, where Bs oscillations can be studied. The first selection of the "golden", fully hadronic decay mode Bs->Ds pi(phi pi)X at DØ is presented in this thesis. All data, taken between April 2002 and August 2007 with the DØ detector, corresponding to an integrated luminosity of int{L}dt=2.8/fb is used. The oscillation frequency Dms and the ratio |Vtd|/|Vts| are determined as Dms = (16.6 +0.5-0.4(stat) +0.4-0.3(sys)) 1/ps, |Vtd|/|Vts| = 0.213 +0.004-0.003(exp)pm 0.008(theor). These results are consistent with the standard model expectations and no evidence for new physics is observable.

Search for resonant second generation slepton production at the Fermilab Tevatron

We present a search for supersymmetry in the R-parity violating resonant production and decay of smuons and muon sneutrinos in the channels mu ->chi(0)(1)mu, mu ->chi(0)(2,3,4)mu, and nu(mu)->chi(+/-)(1,2)mu. We analyzed 0.38 fb(-1) of integrated luminosity collected between April 2002 and August 2004 with the D0 detector at the Fermilab Tevatron Collider. The observed number of events is in agreement with the standard model expectation, and we calculate 95% C.L. limits on the slepton production cross section times branching fraction to gaugino plus muon, as a function of slepton and gaugino masses. In the framework of minimal supergravity, we set limits on the coupling parameter lambda(')(211), extending significantly previous results obtained in Run I of the Tevatron and at the CERN LEP collider.

Effects of SO(10) D terms on SUSY signals at the Fermilab Tevatron

We study signals for the production of superparticles at the Fermilab Tevatron in supergravity scenarios based on the grand unified group SO(10). The breaking of this group introduces extra contributions to the masses of all scalars, described by a single new parameter. We find that varying this parameter can considerably change the size of various expected signals studied in the literature, with different numbers of jets and/or charged leptons in the final state. The ratios of these signals can thus serve as a diagnostic to detect or constrain deviations from the much-studied scenario where all scalar masses are universal at the GUT scale. Moreover, under favorable circumstances some of these signals, and/or new signals involving hard b jets, should be observable at the next run of the Fermilab Tevatron collider even if the average scalar mass lies well above the gluino mass. ©2000 The American Physical Society.

First Observation of Vector Boson Pairs in a Hadronic Final State at the Tevatron Collider

We present the first observation in hadronic collisions of the electroweak production of vector boson pairs (VV, V=W, Z) where one boson decays to a dijet final state. The data correspond to 3.5  fb-1 of integrated luminosity of pp̅ collisions at √s=1.96  TeV collected by the CDF II detector at the Fermilab Tevatron. We observe 1516±239(stat)±144(syst) diboson candidate events and measure a cross section σ(pp̅ →VV+X) of 18.0±2.8(stat)±2.4(syst)±1.1(lumi)  pb, in agreement with the expectations of the standard model.

First Observation of Vector Boson Pairs in a Hadronic Final State at the Tevatron Collider

We present the first observation in hadronic collisions of the electroweak production of vector boson pairs (VV, V=W,Z) where one boson decays to a dijet final state . The data correspond to 3.5 inverse femtobarns of integrated luminosity of ppbar collisions at sqrt(s)=1.96 TeV collected by the CDFII detector at the Fermilab Tevatron. We observe 1516+/-239(stat)+/-144(syst) diboson candidate events and measure a cross section sigma(ppbar->VV+X) of 18.0+/-2.8(stat)+/-2.4(syst)+/-1.1(lumi) pb, in agreement with the expectations of the standard model.

Testing anomalous Higgs couplings in triple photon production at the Tevatron collider

We derive bounds on Higgs and gauge-boson anomalous interactions using the CDF data for the process pp̄ → γγγ + X. We use a linearly realized SU L(2) X U Y(1) invariant effective Lagrangian to describe the bosonic sector of the Standard Model, keeping the fermionic couplings unchanged. All dimension-six operators that lead to anomalous Higgs interactions involving γ and Z are considered. We also show the sensitivity that can be achieved for these couplings at Fermilab Tevatron upgrades. © 1998 Published by Elsevier Science B.V. All rights reserved.

Combination of Tevatron Searches for the Standard Model Higgs Boson in the W+W- Decay Mode

We combine searches by the CDF and D0 collaborations for a Higgs boson decaying to W+W-. The data correspond to an integrated total luminosity of 4.8 (CDF) and 5.4 (D0) fb-1 of p-pbar collisions at sqrt{s}=1.96 TeV at the Fermilab Tevatron collider. No excess is observed above background expectation, and resulting limits on Higgs boson production exclude a standard-model Higgs boson in the mass range 162-166 GeV at the 95% C.L.

Combined Tevatron upper limit on gg->H->W+W- and constraints on the Higgs boson mass in fourth-generation fermion models

We combine results from searches by the CDF and D0 collaborations for a standard model Higgs boson (H) in the process gg->H->W+W- in p=pbar collisions at the Fermilab Tevatron Collider at sqrt{s}=1.96 TeV. With 4.8 fb-1 of integrated luminosity analyzed at CDF and 5.4 fb-1 at D0, the 95% Confidence Level upper limit on \sigma(gg->H) x B(H->W+W-) is 1.75 pb at m_H=120 GeV, 0.38 pb at m_H=165 GeV, and 0.83 pb at m_H=200 GeV. Assuming the presence of a fourth sequential generation of fermions with large masses, we exclude at the 95% Confidence Level a standard-model-like Higgs boson with a mass between 131 and 204 GeV.

Diffractive W and Z production at the Fermilab Tevatron

We report on a measurement of the fraction of events with a W or Z boson produced diffractively in antiproton-proton collisions at a center of mass energy of 1.96 TeV, using data from 0.6 inverse femtobarns of integrated luminosity collected with the CDF-II detector equipped with a Roman-pot spectrometer that detects the antiproton (pbar) from pbar+p --> pbar+[X+W/Z]. We find that (0.97 +/- 0.11)% of Ws and (0.85 +/- 0.22)% of Zs are produced diffractively in a region of (anti)proton fractional momentum loss (\xi) of 0.03-1t p+[X+W/Z]+pbar, and on exclusive Z production, pbar+p-->pbar+Z+p. No signal is seen above background for these processes, and comparisons are made with expectations.

Probing neutrino mass with displaced vertices at the Fermilab Tevatron

Supersymmetric extensions of the standard model exhibiting bilinear R-parity violation can generate naturally the observed neutrino mass spectrum as well as mixings. One interesting feature of these scenarios is that the lightest supersymmetric particle (LSP) is unstable, with several of its decay properties predicted in terms of neutrino mixing angles. A smoking gun of this model in colliders is the presence of displaced vertices due to LSP decays in large parts of the parameter space. In this work we focus on the simplest model of this type that comes from minimal supergravity with universal R-parity conserving soft breaking of supersymmetry augmented with bilinear R-parity breaking terms at the electroweak scale (RmSUGRA). We evaluate the potential of the Fermilab Tevatron to probe the RmSUGRA parameters through the analysis of events possessing two displaced vertices stemming from LSP decays. We show that requiring two displaced vertices in the events leads to a reach in m(1/2) twice the one in the usual multilepton signals in a large fraction of the parameter space.

Trilepton signal for supersymmetry at the Fermilab Tevatron reexamined

Within a wide class of models, the CERN LEP2 lower limit of 95 GeV on the chargino mass implies gluinos are heavier than similar to 300 GeV. In this case electroweak (W) over tilde(1)(W) over tilde(1) production and (W) over tilde(1)(Z) over tilde(2) production are the dominant supersymmerry (SUSY) processes at the Fermilab Tevatron, and the extensively examined isolated trilepton signal From (W) over tilde(1)(Z) over tilde(2) production assumes an even greater importance. We update our previous calculations of the SUSY reach of luminosity upgrades of the Fermilab Tevatron in this channel incorporating (i) decay matrix elements in the computation of the momenta of leptons from chargino and neutralino decays, (ii) the trilepton background from W*Z* and W*gamma* production which, though neglected in previous analyses, turns out to be the dominant background, and finally, (iii) modified sets of cuts designed to reduce these new backgrounds and increase the range of model parameters for which the signal is observable. We show our improved projections for the reach for SUSY of both the Fermilab Main Injector and the proposed TeV33 upgrade. We also present opposite sign same flavor dilepton invariant mass distributions as well as the p(T) distributions of leptons in SUSY trilepton events, and comment upon how the inclusion of decay matrix elements impacts upon the Tevatron reach, as well as upon the extraction of neutralino masses.

gg -> h -> tau(+)tau(-) at the upgraded Fermilab Tevatron

We study the neutral Higgs boson production via the gluon fusion process with the tau(+)tau(-) final state at the upgraded Fermilab Tevatron, including a complete simulation of signal channels and leading background processes. For the SM Higgs boson, this h --> tau(+)tau(-) channel may provide important addition for the Higgs boson discovery in the mass range 120 - 140 GeV. In minimal supersymmetric models, natural enhancement for the signal rate over the SM expectation makes the h, H, A --> tau(+)tau(-) signal observable for large tan beta and low MA, which may lead to full coverage for SUSY Higgs parameters at the Tevatron with a moderate integrated luminosity.

Search for W H associated production in 5.3 fb(-1) of p(p)over-bar collisions at the Fermilab Tevatron

Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)