Radiative QCD backgrounds to exclusive $H\to b\bar{b}$ production: radiation from the screening gluon

Central exclusive Higgs boson production, pp→p ⊕ H ⊕ p, at the LHC can provide an important complementary contribution to the comprehensive study of the Higgs sector in a remarkably clean topology. The \(b\bar{b}\) Higgs decay mode is especially attractive, and for certain BSM scenarios may even become the discovery channel. Obvious requirements for the success of such exclusive measurements are strongly suppressed and controllable backgrounds. One potential source of background comes from additional gluon radiation which leads to a three-jet \(b\bar{b}g\) final state. We perform an explicit calculation of the subprocesses \(gg\to q\bar{q}g\), gg→ggg in the case of ‘internal’ gluon radiation from the spectator, t-channel screening gluon, when the two incoming active t-channel gluons form a color octet. We find that the overall contribution of this source of background is orders of magnitude lower than that caused by the main irreducible background resulting from the \(gg^{\mathrm{PP}}\to b\bar{b}\) subprocess. Therefore, this background contribution can be safely neglected.     

Measurement of the $b\overline{b}$ production cross section in 920 GeV fixed-target proton-nucleus collisions

Using the HERA - B detector, the production cross section has been measured in 920 GeV proton collisions on carbon and titanium targets. The production was tagged via inclusive bottom quark decays into by exploiting the longitudinal separation of decay vertices from the primary proton-nucleus interaction. Both e ⁺ e ^- and channels have been reconstructed and the combined analysis yields the cross section . Received: 6 June 2002 / Revised version: 2 September 2002 / Published online: 8 November 2002     

Prospects for observing an invisibly decaying Higgs boson in the ${t \bar t H}$ production at the LHC

The prospects for observing an invisibly decaying Higgs boson in the production at LHC are discussed. An isolated lepton, reconstructed hadronic top-quark decay, two identified b-jets and a large missing transverse energy are proposed as the final state signature for event selection. Only the standard model backgrounds are taken into account. It is shown that the , , and backgrounds can individually be suppressed below the signal expectation. The dominant source of background remains the production. The key for the observability will be an experimental selection which allows further suppression to be achieved of the contributions from the events with one of the top quarks decaying into a tau lepton. Depending on the details of the final analysis, an excess of the signal events above the standard model background of about 10% to 100% can be achieved in the mass range m_H= 100-200 GeV.Received: 1 July 2002, Revised: 16 May 2003, Published online: 3 July 2003E. Richter-Was: Supported in part by Polish Government grant KBN 2P03B11819     

Dilepton-tagged $Q\overline{Q}+\mathrm{jet}$ events at the LHC

We propose a new method for identifying and isolating events through semileptonic decays of the pair. Employing these decay dileptons to tag the jet in a specific kinematic region provides a clean signature of jets associated with heavy-quark production. The measurement, in both pp and heavy-ion collisions, is essential for addressing heavy-quark fragmentation in vacuum and in a dense medium. We present next-to-leading order calculations of production (leading order in production) in TeV pp collisions at the LHC and discuss the feasibility of the measurement in heavy-ion collisions at TeV.     

Predictions for ${\bar \nu} \nu \gamma$ production at LEP

We study predictions for the reaction . The complete one-loop corrections are taken into account and higher order contributions, in particular those for the observed real photons, are added whenever necessary. The event generator MC, a general-purpose Monte Carlo generator for the process based on the method of exclusive exponentiation, is used as the environment. We extend its applicability to the process ,, where the observation of at least a single is required. The exponentiation is implemented in much the same way as for the s-channel process alone. In particular, all photonic effects present in the case of W exchange, which cannot be included in the s-channel exponentiation scheme, are calculated to a finite order only. The real hard photon matrix element is calculated up to . Leading logarithmic contributions of the two-loop corrections and one-loop photonic corrections accompanying real single-photon emission are included. The electroweak corrections are calculated with the DIZET library of the ZFITTER package. Numerical tests and predictions for typical observables are presented. Received: 30 October 2001 / Revised version: 5 March 2002 / Published online: 22 May 2002     

Determination of the forward slope in pp and $\bar{p}p$ elastic scattering up to LHC energy

In the analysis of experimental data on pp (or [`(p)] p\bar{p} p) elastic differential cross section it is customary to define an average forward slope b in the form exp (−b|t|), where t is the momentum transfer. Taking as working example the results of experiments at Tevatron and SPS, we will show with the help of the impact picture approach that this simplifying assumption hides interesting information on the complex non-flip scattering amplitude, and that the slope b is not a constant. We investigate the variation of this slope parameter, including a model-independent way to extract this information from an accurate measurement of the elastic differential cross section. An extension of our results to the LHC energy domain is presented in view of future experiments.     

The Semileptonic Decay Modes {\bar{B} \rightarrow D\ell \bar{\nu}} and {\bar{B}_{s} \rightarrow D_{s} \ell \bar{\nu}}: A New Analysis in Potential Model

We consider the Schrödinger equation with a combination of Deng–Fan-type and harmonic terms. To solve the corresponding differential equation, we split the equation to two parts: the parent and the perturbation terms. We use the Nikiforov–Uvarov technique to solve the parent part. For the perturbation part, we apply the series expansion method. Next, using the calculated wave function, we investigate some bottom and charm mesons within the Isgur–Wise function formalism. We present especially semileptonic \({\bar{B} \rightarrow D\ell \bar{\nu}}\) and \({\bar{B}_{s} \rightarrow D_s \ell \bar{\nu }}\) decay widths, branching ratios and \({|V_{cb}|}\) (element of the CKM matrix). Masses of some pseudoscalar mesons are also indicated. Comparisons of our results with experimental values and other approaches are included.     

The rare annihilation decays $\bar{B}^0_{s,d} \to J/\psi\gamma$

We investigate the physics potential of the annihilation decays in the standard model and beyond. In a naive factorization approach, the branching ratios are estimated to be and . In the framework of QCD factorization, we compute the non-factorizable corrections and get , . Future measurements of these decays would be useful for testing the factorization frameworks. The smallness of these decays in the SM makes them sensitive probes of new physics. As an example, we will consider the possible admixture of the (V + A) charge current to the standard (V-A) current. This admixture will give a significant contribution to the decays.Received: 29 August 2003, Revised: 17 January 2004, Published online: 19 March 2004Corresponding author: Y.D. Yang     

The 2-site Hubbard and $\mathsf{t}$-$\mathsf{J}$ models

The fermionic and bosonic sectors of the 2-site Hubbard model have been exactly solved by means of the equation of motion and Greens function formalism. The exact solution of the t-J model has been also reported to investigate the low-energy dynamics. We have successfully searched for the exact eigenoperators, and the corresponding eigenenergies, having in mind the possibility to use them as an operatorial basis on the lattice. Many local, single-particle, thermodynamical and response properties have been studied as functions of the external parameters and compared between the two models and with some numerical and exact results. It has been shown that the 2-site Hubbard model already contains the most relevant energy scales of the Hubbard model: the local Coulomb interaction U and the spin-exchange one . As a consequence of this, for some relevant properties (kinetic energy, double occupancy, energy, specific heat and entropy) and as regards the metal-insulator transition issue, it has resulted possible to almost exactly mime the behavior of larger systems, sometimes using a higher temperature to get a comparable level spacing. The 2-site models have been also used as toy models to test the efficiency of the Greens function formalism for composite operators. The capability to reproduce the exact solutions, obtained by the exact diagonalization technique, gives a firm ground to the approximate treatments based on this formalism.Received: 16 July 2003, Published online: 30 January 2004PACS: 71.10.-w Theories and models of many-electron systems - 71.10.Fd Lattice fermion models (Hubbard model, etc.)     

Search for resonant $\tilde\nu$ production at $\sqrt{s} = $ 183 to 208 GeV

Searches for resonant production in e ⁺ e ^- collisions under the assumption that R-parity is not conserved and that the dominant R-parity violating coupling is or used data recorded by DELPHI in 1997 to 2000 at centre-of-mass energies of 183 to 208 GeV. No deviation from the Standard Model was observed. Upper limits are given for the and couplings as a function of the sneutrino mass and total width. The limits are especially stringent for sneutrino masses equal to the centre-of-mass energies with the highest integrated luminosities recorded. Received: 14 January 2003 / Published online: 14 March 2003     

Isospin mixing \bar{K}N\hbox{-}{\pi}{\Sigma} interaction and \bar{K}NN\hbox{-}{\pi} {\Sigma}N quasi-bound state

A phenomenological isospin-dependent $\bar{K}N\hbox{-}\pi\Sigma$ potential reproducing a medium KEK value of 1s kaonic hydrogen level shift instead of a K ^? p scattering length is constructed. The corresponding three-body $\bar{K}NN\hbox{-}\pi\Sigma N$ calculation using the obtained potential is performed.