Search for neutral supersymmetric Higgs bosons in multijet events at root s=1.96 TeV

We have performed a search for neutral Higgs bosons produced in association with bottom quarks in p (p) over bar collisions, using 260 pb(-1) of data collected with the D0 detector in Run II of the Fermilab Tevatron Collider. The cross sections for these processes are enhanced in many extensions of the standard model (SM), such as in its minimal supersymmetric extension at large tan beta. The results of our analysis agree with expectations from the SM, and we use our measurements to set upper limits on the production of neutral Higgs bosons in the mass range of 90 to 150 GeV.

Search for neutral Higgs bosons decaying to tau pairs in p(p)over-bar collisions at root s=1.96 TeV

A search for the production of neutral Higgs bosons Phi decaying into tau(+)tau(-) final states in p (p) over bar collisions at a center-of-mass energy of 1.96 TeV is presented. The data, corresponding to an integrated luminosity of approximately 325 pb(-1), were collected by the D0 experiment at the Fermilab Tevatron Collider. Since no excess compared to the expectation from standard model processes is found, limits on the production cross section times branching ratio are set. The results are combined with those obtained from the D0 search for Phi b((b) over bar)-> b (b) over barb((b) over bar) and are interpreted in the minimal supersymmetric standard model.

Flavor changing interaction models and strictly massless neutrinos

It is well known that experimental data, coming from solar and atmospheric neutrino detectors and also from experiments which look for neutrino oscillations. strongly suggest that neutrinos must have a mass different from zero. However at least the solar and/or the atmospheric neutrino data can be related to new flavor changing interactions beyond the standard model instead to the finite mass of neutrinos. This new physics may induce i) extra effects in neutrino-matter interactions, ii) CP violation in pion and lepton decays and, iii) muonium to antimuonium transition. We give two examples of models in which all those effects arise even with strictly massless neutrinos: the 331 model and multi-Higgs doublet extension of the standard model (mHDM) with flavor changing neutral currents in the charged lepton sector. It means that in this kind of models if neutrino masses were eventually needed, they will be independent of the parameters of the new interactions.

Electric charge quantization and the muon anomalous magnetic moment

We investigate some proposals to solve the electric charge quantization puzzle that simultaneously explain the recent measured deviation on the muon anomalous magnetic moment. For this we assess extensions of the electro-weak standard model spanning modifications on the scalar sector only. It is interesting to verify that one can have modest extensions which easily account for the solution for both problems.

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.

Constraints on a scalar-pseudoscalar Higgs mixing at future e(+)e(-) colliders: An update

We perform an update of our previous analysis of the constraints on possible deviations of Hb (b) over bar coupling parametrized as (m(b)/v)(a+igamma(5)b), arising from a scalar-pseudoscalar mixing, where the process e(+)e(-)-->b (b) over bar nu(ν) over bar was used. In this paper we include a complete simulation of the process e(+)e(-)-->b (b) over bare(+)e(-) and combine these results to obtain tighter bounds on the deviations of the parameters a and b from their standard model values that could be measured at the Next Linear Collider.

Closing the SU(3)(L)circle times U(1)(X) symmetry at the electroweak scale

We show that some models with SU(3)(C)circle times SU(3)(L)circle times U(1)(X) gauge symmetry can be realized at the electroweak scale and that this is a consequence of an approximate global SU(2)(L+R) symmetry. This symmetry implies a condition among the vacuum expectation value of one of the neutral Higgs scalars, the U(1)(X)'s coupling constant, g(X), the sine of the weak mixing angle sin theta(W), and the mass of the W boson, M-W. In the limit in which this symmetry is valid it avoids the tree level mixing of the Z boson of the standard model with the extra Z(') boson. We have verified that the oblique T parameter is within the allowed range indicating that the radiative corrections that induce such a mixing at the 1-loop level are small. We also show that a SU(3)(L+R) custodial symmetry implies that in some of the models we have to include sterile (singlets of the 3-3-1 symmetry) right-handed neutrinos with Majorana masses, since the seesaw mechanism is mandatory to obtain light active neutrinos. Moreover, the approximate SU(2)(L+R)subset of SU(3)(L+R) symmetry implies that the extra nonstandard particles of these 3-3-1 models can be considerably lighter than it had been thought before so that new physics can be really just around the corner.

Search for stopped gluinos from p(p)over-bar collisions at root s=1.96 TeV

Long-lived, heavy particles are predicted in a number of models beyond the standard model of particle physics. We present the first direct search for such particles' decays, occurring up to 100 h after their production and not synchronized with an accelerator bunch crossing. We apply the analysis to the gluino (g), predicted in split supersymmetry, which after hadronization can become charged and lose enough momentum through ionization to come to rest in dense particle detectors. Approximately 410 pb(-1) of p (p) over bar collisions at root s = 1.96 TeV collected with the D0 detector during Run II of the Fermilab Tevatron collider are analyzed in search of such stopped gluinos decaying into a gluon and a neutralino ((chi) over tilde (0)(1)). Limits are placed on the (gluino cross section) x (probability to stop) x [BR((g) over tilde -> g (chi) over tilde (0)(1))] as a function of the gluino and (chi) over tilde (0)(1) masses, for gluino lifetimes from 30 mu s-100 h.

Analysis of the Hb(b)over-bar coupling constant and CP-violation effects at the future e(+)e(-) collider

The e(+)e(-)-->b (B) over bar nu(ν) over bar process, where nu is an electron, muon, or tau-lepton neutrino, is analyzed in detail for the general form of the coupling constant of a Higgs boson with b quarks, with the (m(b)/v)(a + igamma(5)b) parameterization of the Hb (b) over bar interaction. This process is shown to be highly sensitive to this coupling constant. Experiments at the future with roots = 500-GeV linear collider will provide limits of 2 and 20% for deviations of the parameters a and b, respectively, from their Standard Model values. Results concerning the e(+)e(-)-->b (b) over bar nu(ν) over bar process in combination with the independent measurements of the partial width Gamma(H --> b (b) over bar) can testify to the CP origin of the Higgs sector of the theory. (C) 2003 MAIK Nauka/Interperiodica.

Search for anomalous heavy-flavor quark production in association with W bosons

We search for anomalous production of heavy-flavor quark jets in association with W bosons at the Fermilab Tevatron p(p) over bar Collider in final states in which the heavy-flavor quark content is enhanced by requiring at least one tagged jet in an event. Jets are tagged using one algorithm based on semileptonic decays of b/c hadrons, and another on their lifetimes. We compare e+jets (164 pb(-1)) and mu+jets (145 pb(-1)) channels collected with the D0 detector at root s = 1.96 TeV to expectations from the standard model and set upper limits on anomalous production of such events.

Measurement of the W boson helicity in top quark decays at D0

We present a measurement of the fraction f(+) of right-handed W bosons produced in top quark decays, based on a candidate sample of tt events in the l + jets and dilepton decay channels corresponding to an integrated luminosity of 370 pb(-1) collected by the D0 detector at the Fermilab Tevatron pp Collider at root s = 1.96 TeV. We reconstruct the decay angle theta* for each lepton. By comparing the cos theta* distribution from the data with that for the expected background and signal for various values of f(+) (where we assume that the fraction of longitudinally-polarized W bosons has the standard model value of 0.70), we find f(+) = 0.056 +/- 0.080 (stat) +/- 0.057 (syst) (f(+) < 0.23 at 95% C. L.), consistent with the standard model prediction of f(+) = 3.6 X 10(-4).

Evidence for production of single top quarks and first direct measurement of vertical bar V-tb vertical bar

The D0 Collaboration presents first evidence for the production of single top quarks at the Fermilab Tevatron < p(p)over bar > collider. Using a 0.9 fb(-1) dataset, we apply a multivariate analysis to separate signal from background and measure sigma(< p(p)over bar >-> tb+X,tqb+X)=4.9 +/- 1.4 pb. The probability to measure a cross section at this value or higher in the absence of a signal is 0.035%, corresponding to a 3.4 standard deviation significance. We use the cross section measurement to directly determine the Cabibbo-Kobayashi-Maskawa matrix element that describes the Wtb coupling and find 0.68 standard model.

Search for pair production of scalar bottom quarks in p(p)over-bar collisions at root s = 1.96 TeV

A search for direct production of scalar bottom quarks ((b) over bar) is performed with 310 pb(-1) of data collected by the D0 experiment in p (p) over bar collisions at root s = 1.96 TeV at the Fermilab Tevatron Collider. The topology analyzed consists of two b jets and an imbalance in transverse momentum due to undetected neutralinos (chi(0)(1)), with chi(0)(1) assumed to be the lightest supersymmetric particle. We find the data consistent with standard model expectations, and set a 95% C.L. exclusion domain in the (m(b), m(chi 1)(0)) mass plane, improving significantly upon the results from run I of the Tevatron.

Search for a Higgs boson produced in association with a Z boson in p(p)over-bar collisions

We describe a search for the Standard Model Higgs boson with a mass of 105 GeV/c(2) to 145 GeV/c(2) in data corresponding to an integrated luminosity of approximately 450 pb(-1) collected with the D phi detector at the Fermilab Tevatron p (p) over bar collider at a center-of-mass energy of 1.96 TeV. The Higgs boson is required to be produced in association with a Z boson, and the Z boson is required to decay to either electrons or muons with the Higgs boson decaying to a b (b) over bar pair. The data are well described by the expected background, leading to 95% confidence level cross section upper limits sigma (p (p) over bar -> ZH) x B(H -> b (b) over bar) in the range of 3.1 pb to 4.4 pb. (C) 2007 Elsevier B.V. All rights reserved.

Lepton transverse polarization in the B -> Dl nu(l) decay due to the electromagnetic final state interaction

The effect of lepton transverse polarization in B-0-->D(-)l(+)nu(l), B+-->(D) over bar (0)l(+)nu(l) decays (l=tau,mu) is analyzed within the framework of the standard model in the leading order of heavy quark effective theory. It is shown that a nonzero transverse polarization appears due to the electromagnetic final state interaction. The diagrams with intermediate D,D* mesons contributing to the nonvanishing P-T are considered. Regarding only the contribution of these mesons, the values of the tau-lepton transverse polarization averaged over the physical region in the B-0-->D(-)tau(+)nu(l) and B+-->(D) over bar (0)tau(+)nu(l) decays are equal to 2.60x10(-3) and -1.59x10(-3), respectively. In the case of muon decay modes the values of [P-T] are equal to 2.97x10(-4) and -6.79x10(-4).