Hard bremsstrahlung photons in thet\bar t production anddecay: A background for the intermediate Higgs search

We discuss the radiation of hard photons in the $t\bar t$ production anddecay as a background for the intermediate Higgs search at LHC/SSC experiments. For the purpose of experimental simulation of this process we propose to use the approximate Monte Carlo algorithm together with any physics Monte Carlo not including QED corrections. We verify, by comparison with the exact matrix element Monte Carlo TOPKI, that this method can be used to generate single- and double-photon bremsstrahlung in the $t\bar t$ production process with a precision of 10% for experimental cuts. The predictions for the ψψl background from photons radiated in theproduction of $t\bar t$ pair and their cascadedecays, within simplified experimental cuts, are presented for the first time. We show that, even after additional cuts on the minimal lepton-photon mass, photons radiated in the productionand decay of a $t\bar t$ pair increase this background by a factor of 2 with respect to previous estimates from radiation in the $t\bar t$ production alone.     

Effects of the mixing of scalar and pseudoscalar Higgs bosons in the process e^ + e^ - \to e^ + e^ - b\bar b at a future linear electron-positron collider

The sensitivity of data on the process $e^ + e^ - \to e^ + e^ - b\bar b$ at a future linear electron-positron collider to the deviation of the coupling of the Higgs boson to b quarks from the Standard Model predictions owing to the presence of a pseudoscalar Higgs boson state is analyzed for a collision energy of $\sqrt s = 500 Gev$ . The admixture of a new hypothetical pseudoscalar Higgs boson state in the $Hb\bar b$ vertex is parametrized in the form m _b /v(a + iγ ₅ b). On the basis of an analysis of data on the process $e^ + e^ - \to e^ + e^ - b\bar b$ , it is shown that experiments at the future linear collider TESLA will make it possible to constrain the parameters Δa = a ? 1 and b as ?0.056 ≤ Δa ≤ 0.055 and ?0.32 ≤ b ≤ 0.32, respectively.     

Effects of scalar and pseudoscalar Higgs bosons in the process e^ + e^ - \to b\bar bv\bar v at the LEP II collider

The possibility of setting constraints on the couplings of the scalar or pseudoscalar Higgs boson to b quarks on the basis of data on the process $e^ + e^ - \to b\bar bv\bar v$ at the LEP II collider is investigated. The mixing of the scalar and the hypothetical pseudoscalar Higgs boson in the $Hb\bar b$ vertex is parametrized in the form $\frac{{m_b }}{v}(a + i\gamma _5 b)$ . An analysis of the differential distributions for the process $e^ + e^ - \to b\bar bv\bar v$ reveals that the contribution of the fusion subprocess WW → H in the channel involving an electron neutrino enhances the sensitivity of data to the parameters under analysis. It is shown that the LEP II data at $\sqrt s = 200$ GeV and $\smallint \mathcal{L}dt = 600 pb^{ - 1} $ per experiment would make it possible to constrain the parameters Δa = a ? 1 and b as follows: ?0.75 ≤ Δa ≤ 1.4 at b = 0 and free Δa and ?0.97 ≤ b ≤ 0.97 at Δa = 0 and free b.     

Quark-diagram estimates ofCP violation in two-body baryonicB_d^0 - \bar B_d^0 decays

CP violation in partial-decay-rate asymmetries are examined for some two-body baryonic decays of \(B_d^0 - \bar B_d^0 \) system. We discuss two feasible experimental circumstances: the symmetrice ⁺ e ^? collisions (i) on theZ ⁰ resonance to produce incoherent \(B_d^0 \bar B_d^0 \) states, and (ii) just above the ?(4S) resonance to produceC=even \(B_d^0 \bar B_d^0 \) states. Using the quark-diagram scheme, we estimate the branching ratios of those decays, and the numbers ofb \(\bar b\) pairs needed for testing theCP-violating effects for 3σ signature. We find that the promising channels may beB _d ⁰ , \(\bar B_d^0 \to p\bar p\) , \(\Delta ^ + \bar \Delta ^ - \) , \(p\bar \Delta ^ - \) , \(\Delta ^ + \bar p\) , \(n\bar n\) , \(\Delta ^0 \bar \Delta ^0 \) , \(n\bar \Delta ^0 \) , \(\Delta ^0 \bar n\) , \(\Sigma _c^ + \bar \Sigma _c^ - \) , \(\Lambda _c^ + \bar \Lambda _c^ - \) , \(\Sigma _c^ + \bar \Lambda _c^ - \) , \(\Lambda _c^ + \bar \Sigma _c^ - \) , \(\Sigma _c^0 \bar \Sigma _c^0 \) , \(\Xi _c^0 \bar \Xi _c^0 \) , which should be interesting for experimental observation.     

Conserved-vector-current hypothesis and the\bar v_e e^ - \to \pi ^ - \pi ^0 process

Based on the conserved-vector-current (CVC) hypothesis and a four-ρ-resonance unitary and analytic VMD model of the pion electromagnetic form factor, theσ _tot(E _v ^lab ) and dσdE _π ^lab of the weak \(\bar v_e e^ - \to \pi ^ - \pi ^0\) process are predicted theoretically for the first time. Their experimental approval could verify the CVC hypothesis for all energies above the two-pion threshold. Since, unlike the electromagnetic e⁺e^?→π⁺π^? process, there is no isoscalar vector-meson contribution to the weak \(\bar v_e e^ - \to \pi ^ - \pi ^0\) reaction, accurate measurements of theσ _tot(E _v ^lab ) that moreover is strengthened with energyE _v ^lab linearly could solve now a widely discussed problem of the mass specification of the first excited state of theρ(770) meson. As a by-product, an equality \(\sigma _{tot} (\bar v_e e^ - \to \pi ^ - \pi ^0 ) = \sigma _{tot} (e^ + e^ - \to \pi ^ - \pi ^0 )\) is predicted for \(\sqrt s \approx 70 GeV\) .     

Properties ofe^ + e^ - \to h\bar h in the\Upsilon (b\bar b)-continuum

The unflavored hadrons frome ⁺ e ^? annihilation in the \(\Upsilon (b\bar b)\) -continuum are analyzed in terms of the covariant Boltzmann distribution with an equilibrium temperature 151 MeV according to the Stefan's law. It is found that the effective fugacities of quark contents are approximately equal to the reciprocals of those from othere ⁺ e ^? annihilations outside the γ-continuum, whereas the fireballs are practically stationary, reflecting the decay at rest of \(B\bar B\) mesons produced by thee ⁺ e ^? annihilation.     

Factorizable electroweak \mathcal{O}(\alpha) corrections for top quark pair production and decay at a linear e^+e^- collider

We calculate the standard model predictions for top quark pair production and decay into six fermions at a linear e⁺e^- collider. We include the factorizable electroweak $\mathcal{O}(\alpha)$ corrections in the pole approximation and QED corrections due to the initial state radiation in the structure function approach. The effects of the radiative corrections on the predictions are illustrated by showing numerical results for two selected six-fermion reactions $e^+e^-\rightarrow b\nu_{\mu}\mu^+\bar{b}\mu^-\bar{\nu}_\mu$ and $e^+e^-\rightarrow b\nu_{\mu}\mu^+\bar{b}d\bar{u}$ .     

Charged Higgs boson discovery potential at a 500 GeV e+e− linear collider

The discovery potential for charged Higgs bosons has been studied with full-statistics background simulations for $\sqrt s$ =500 GeV and ?=10fb^?1. For the hadronic decay channels $H^ + H^ - \to \operatorname{c} \bar s\bar cs$ , a microvertex detector is crucial for establishing a signal over the $e^ + e^ - \to t\bar t$ background. A combination with a search in the channels $H^ + H^ - \to c\bar s\tau ^ - \nu ,\tau ^ + \nu \tau ^ - \bar \nu$ allows detection sensitivity for charged Higgs bosons up to a mass of about 210GeV, independent of the charged Higgs decay modes. Sensitivity regions in them _A-tanβ parameter space of the Minimal Supersymmetric extention of the Standard Model (MSSM) are given.     

Probing Higgs boson — and supersymmetry-induced CP violation in top quark production by (un-) polarized electron-positron collisions

We consider, both for unpolarized and longitudinally polarized electron beams, the reaction $e^ + e^ - \to t\bar t$ with subsequent semileptonict and nonleptonic $\bar t$ decay and vice versa and investigate optimized angular correlations which are sensitive to CP non-conservation in the $t\bar t$ production vertex. We calculate these correlations for two-Higgs-doublet extensions and the minimal super-symmetric extension of the Standard Model (SM) with CP violation beyond the Kobayashi-Maskawa phase. While the sensitivity of the optimal correlation for tracing dispersive CP effects is enhanced with longitudinally polarized electron beams, we find that the sensitivity of the best correlation for probing absorptive CP effects is almost independent of the polarization degree.     

Higgs and background production in the reactione^ + e^ - \to b\bar b + 2 jets at LEPII and NLC energies

A complete tree-level Standard Model calculation of the reaction $e^ + e^ - \to b\bar b + 2$ jets in the energy range of LEPII and the Nexte ⁺e^? Linear Collider is presented. The 2-jet system represents either aq $\bar q$ pair or two gluons. The matrix elements were calculated by means of the package CompHEP, and phase space integration and event generation were carried out with the computer programs BASES/SPRING. Cross sections of the Higgs boson, the total background and important subsets of background are studied as functions of the c.m.s. energy and the Higgs mass, taking into account possible interferences. Various event distributions calculated for realistic integrated luminosities indicate encouraging prospects regarding the extraction of the Higgs signal.     

Mixings and CP violations of theB^0 - \bar B^0 system in the four-generation model

We study the mixings and CP violations in the \(B^0 - \bar B^0 \) system together with the \(K^0 - \bar K^0 \) one in the four-generation model, taking into account the ARGUS data for the \(B_d^0 - \bar B_d^0 \) mixing. We take typical six cases of the mixing hierarchy and search for the allowed regions of three phases in the 4×4KM matrix. The CP violating parameter ? _K in the \(K^0 - \bar K^0 \) system gives a severe constraint for the phases, and the smaller mixing than that in the three-generation model is predicted for the \(B_s^0 - \bar B_s^0 \) system. The asymmetry parameters for the two-body non-leptonic decays of theB ⁰ and \(\bar B^0 \) mesons are also predicted.     

Suggestion for Einstein-Podolsky-Rosen experiments using reactions likee^ + e^ - \to \Lambda \bar \Lambda \to \pi ^ - p\pi ^ + \bar p

Since weakly decaying particles are their own polarimeters, reactions like \(\eta _c \to \Lambda \bar \Lambda , \psi \to \Lambda \bar \Lambda ,e^ + e^ - \to \mu ^ + \mu ^ -\) , etc. are interesting for testing the non-locality of quantum mechanical predictions. Although such reactions, in principle, do not exclude all classes of hidden variable theories, they can be used to complement current experiments with external polarimeters. The reaction \(\eta _c \to \Lambda \bar \Lambda \to \pi ^ - p\pi ^{ + - } \bar p\) is conceptually the simplest and most useful as agedanken experiment, although it has not yet been seen experimentally. The reaction \(e^ + e^ - \to \Lambda \bar \Lambda \to \pi ^ - p\pi ^ + \bar p\) near threshold or at the φ resonance can be used for essentially the same test. This is feasible with presently available data and would be the first EPR experiment involving weak interactions.     

Spectroscopy ofQ\bar Qg hybrid mesons

Hybrid mesons composed of a quark, an antiquark, and a gluon are studied in the case of heavy quarks. Their masses are calculated with the potential model which can interpret heavy quarkonium spectroscopy. The ground state of the hybrid mesons \(c\bar cg\) and \(b\bar bg\) is found to be almost spherically symmetric, whereas that of \(t\bar tg\) is two-centered as anH ₂ ⁺ molecule. The \(b\bar bg\left[ {t\bar tg} \right]\) ground state turns out to have a mass below the \(B\bar B\left[ {T\bar T} \right]\) threshold. The excited states contain 0^??, 1^?+ exotic states and 1^?? states which may be examined bye ⁺ e ^? colliders.     

Thep - \bar p decays ofP-wave quarkonium and the helicity characteristics of massless QCD

We consider the exclusive \(p - \bar p\) decays of the quarkoniumP-states. Due to the helicity conservation of massless QCD the \(p - \bar p\) mode is forbidden in this limit for the¹ P ₁ and the³ P ₀ states. The angular distributions for the decays of the remaining states in the cascade \(^3 S\prime _1 \to \gamma ^3 P_J \to \gamma p\bar p\) are specific to QCD and can serve as a test of the theory. The same is true of the formation process \(p\bar p \to ^3 P_J \to ^3 S_1 \gamma \) . In lowest order QCD we obtain overall branching ratios for charmonium of the order of 10^?4.     

On the threshold behaviour of heavy top production

If the top is heavy, as now seems likely, the \(t\bar t\) threshold behaviour is given by perturbative QCD. The QCD threshold interaction can be formulated in terms of a potential, attractive or repulsive depending on whether the \(t\bar t\) is in a colour singlet or octet state. This gives a suppression factor for octet production. Singlet production is enhanced, both above threshold and, by resonance formation, below it. Whilee ⁺ e ^? annihilation only proceeds in the singlet \(t\bar t\) channel, hadronhadron collisions contain a non-trivial mixture of the two. In this paper we review the relevant threshold factor formulae, and present phenomenological consequences for hadron colliders, current and future.     

Continuum mixing and coupled channel effects inc\bar c andb\bar b quarkonium

We apply the unitarized quark model to heavy \(c\bar c\) and \(b\bar b\) quarkonium and study mass shifts and mixings induced by hadronic coupled channel effects. In particular, we study the two-meson continuum mixing in the quarkonium wave functions. It is found that the continuum component is 2–20% in the \(c\bar c\) and \(b\bar b\) states, measured as the squared sum of two-meson amplitudes \((D\bar D,D\bar D^ * + cc,D^ * \bar D^ * ,F\bar F,F\bar F^ * + cc,F^ * \bar F^ * for c\bar c)\) . It is the largest for states near or above the first threshold. These continuum mixings reduce the predicted radiative widths by 5–30%. The mass shifts of theP andF wave \(q\bar q\) states are similar to those ofS andD wave states previously studied. The computed resonance mixing matrices are explicitly given, enabling future calculations of relativistic effects, QCD effects, etc., to include also the coupled channel effects.     

Multi-quark structure ofU(3.1)

We interpret the recently observedU(3.1) mesons with the \(\Lambda \bar p\) + pions decays as the bound state of \(\Lambda ,\bar p\) andX ₀(1480). TheX ₀(1480) is a mesonium with \(Q^2 \bar Q^2 \) structures observed in γγ reactions and \(\bar pn\) annihilations. With this interpretation, we can understand its decay modes. Furthermore, we predict the ratio of \(\sigma (\Lambda \bar p\pi ^ + \pi ^ - )/\sigma (\Lambda \bar p\pi ^ + \pi ^ + )\) to be ?3.1 for centrally produced events and that the width of \(U^ - (\Lambda \bar p\pi ^ + \pi ^ - )\) to be greater than that of \(U^ + (\Lambda \bar p\pi ^ + \pi ^ + )\) . Both predictions seem to be in reasonable accord with the available data. We call for the detection of the \(\Lambda \bar p\pi ^ - \pi ^ - \) mode to verify the present interpretation.     

Rigorous QCD-potential for thet\bar t-system at threshold

Recent evidence for the top mass in the region of 160 GeV for the first time provides an opportunity to use the full power of relativistic quantum field theoretical methods, available also for weakly bound systems. Because of the large decay width Γ of the top quark individual energy-levels in “toponium” will be unobservable. However, the potential for the $t\bar t$ system, based on a systematic expansion in powers of the strong coupling constant α _s can be rigorously derived from QCD and plays a central role in the threshold region. It is essential that the neglect of nonperturbative (confining) effects is fully justified here for the first time to a large accuracy, also justbecause of the large Γ. The different contributions to that potential are computed from real level corrections near the bound state poles of the $t\bar t$ -system which for Γ≠0 move into the unphysical sheet of the complex energy plane. Thus, in order to obtain the different contributions to that potential we may use the level corrections at that (complex) pole. Within the relevant level shifts we especially emphasize the corrections of orderO(α _s ⁴ m _t ) and numerically comparable ones to that order also from electroweak interactions which may become important as well.