Observing b → s μ+μ− decays at hadron colliders

We examine the prospects for observing weak flavour-changing neutral current (FCNC) decays ofB mesons at hadron colliders, including effects of anomalousWWZ vertices. Since it is very difficult to measure the inclusive rate B X_s ^+– one should consider exclusive modes such as BK^{* + –} and B K^{+ –}. Even though this requires one to compute hadronic matrix elements, we show that experimentally observable quantities (ratios of decay rates) are not strongly parametrisation dependent. Some possibilities for reducing the theoretical uncertainties from other experimental data are discussed.     

Higgs boson physics in the s-channel at μ+μ− colliders

Techniques and strategies for discovering and measuring the properties of Higgs bosons via s-channel production at a μ⁺μ⁻ collider, and the associated requirements for the machine and detector, are discussed in detail. The unique feature of s-channel production is that, with good energy resolution, the mass, total width and partial widths of a Higgs boson can be directly measured with remarkable accuracy in most cases. For the expected machine parameters and luminosity the standard model (SM) Higgs boson h_SM, with mass ≲ 2m_W, the light h⁰ of the minimal supersymmetric standard model (MSSM), and the heavier MSSM Higgs bosons (the CP-odd A⁰ and the CP-even H⁰) can all be studied in the s-channel, with the heavier states accessible up to the maximal √s over a large fraction of the MSSM parameter space. In addition, it may be possible to discover the A⁰ and H⁰ by running the collider at full energy and observing excess events in the bremsstrahlung tail at lower energy. The integrated luminosity, beam resolution and machine/detector features required to distinguish between the h_SM and h⁰ are delineated.     

Predictions for anomalous τ+τ−γ production at LEP 1

We calculate distributions for τ⁺τ^?γ production at LEP 1 taking into account a potentially existing anomalous magnetic moment a _τ of the τ lepton. The existing upper limits for ¦a _τ¦ are known from the dependence of the decay Z ⁰ → τ⁺τ^?γ on a _τ ² and are of the order of (1–5)%. We show that such limits are also sensitive to linear terms in a _τ, which are of equal importance at ¦a _τ¦ ～ (1–2)% and dominate below this value. Contributions from an electric dipole moment dτ do not interfere with the electromagnetic vertex or with the anomalous magnetic moment. Appropriate formulae are derived.     

A possible signature of new physics at BES-Ⅲ

The recent observations of the purely leptonic decay Ds+→μ+νμ and τ+ντ at CLEO-c and B factory may allow a possible contribution from a charged Higgs bosom One such measurement of the decay constant fDs differs from the most precise unquenched lattice QCD calculation by a level of 4σ.Meanwhile,the measured ratio,BR(D+s+→μ.+νμ)/BR(D+→μ+νμ),is larger than the standard model prediction at a 2.0σ level.We discuss that the precise measurement of the ratio BR(Ds+→μ+νμ)/BR(D+μ+νμ) at BES-Ⅲ will shed light on the presence of new intermediate particles by comparing the data with the theoretical predictions,especially,the predictions of high precise unquenched lattice QCD calculations.     

A possible signature of new physics at BES-Ⅲ

The recent observations of the purely leptonic decay Ds＋→μ＋νμ and τ＋ντ at CLEO-c and B factory may allow a possible contribution from a charged Higgs boson. One such measurement of the decay constant fD. differs from the most precise unquenched lattice QCD calculation by a level of 4 σ. Meanwhile, the measured ratio, BR（Ds＋→μ＋νμ）/BR（D＋→μ＋νμ）, is larger than the standard model prediction at a 2.0σ level. We discuss that the precise measurement of the ratio BR（Ds＋→μ＋νμ）/BR（D＋→μ＋νμ） at BES-Ⅲ will shed light on the presence of new intermediate particles by comparing the data with the theoretical predictions, especially, the predictions of high precise unquenched lattice QCD calculations.     

Signatures of MSSM charged Higgs bosons via chargino–neutralino decay channels at the LHC

We assess the potential of detecting a charged Higgs boson of the MSSM at the LHC via its decays into a chargino and a neutralino. We focus our attention on the region of parameter space with and , where identification of the via other decay modes has proven to be ineffective. Searching for means to plug this hole, we simulate the decays and – the former can yield a single hard lepton (from the chargino decay) while the latter can yield three leptons (from the chargino and neutralino decays). Coupled with the dominant top quark + charged Higgs boson production mode, the resulting signature is one or three hard, isolated leptons, substantial missing transverse momentum and a reconstructed (via a 3-jet invariant mass) top quark. The single lepton channel is swamped by background processes; however, with suitable cuts, a trilepton signal emerges. While such a signal suffers from a low number of surviving events (after cuts) and is dependent on several MSSM input parameters (notably , and slepton masses), it does fill at least some of the void left by previous investigations. Received: 21 October 2000 / Published online: 23 February 2001     

Aspects of two-photon physics at lineare + e − colliders

We discuss various reactions at futuree ⁺ e ^– and colliders involving real (beamstrahlung or backscattered laser) or quasi-real (bremsstrahlung) photons in the initial state and hadrons in the final state. The production of two central jets with large transverse momentump _T is described in some detail; we give distributions for the rapidity andp _T of the jets as well as the di-jet invariant mass, and discuss the relative importance of various initial state configurations and the uncertainties that arise from the at present rather poor knowledge of the parton content of the photon. We also present results for mono-jet production where one jet goes down a beam pipe, for the production of charm, bottom and top quarks, and for single production ofW andZ bosons. Where appropriate, the two-photon processes are compared with annihilation reactions leading to similar final states. We also argue that the behaviour of the total inelastic cross section at high energies will probably have little impact on the severity of background problems caused by soft and semi-hard (minijet) two-photon reactions. We find very large differences in cross sections for all two-photon processes between existing desings for futuree ⁺ e ^– colliders, due to the different beamstrahlung spectra; in particular, both designs with 1 and 1 events per bunch crossing exist. The number of hadronic two-photon events is expected to rise quickly with the beam energy. Hadronic backgrounds will be even worse if thee ⁺ e ^– collider is converted into a collider.     

Search for charged Higgs bosons in e+e− collisions at s=189 GeV

A search for pair-produced charged Higgs bosons is performed with the L3 detector at LEP using data collected at a centre-of-mass energy of 188.6 GeV, corresponding to an integrated luminosity of 176.4 pb⁻¹. Higgs decays into a charm and a strange quark or into a tau lepton and its associated neutrino are considered. The observed events are consistent with the expectations from Standard Model background processes. A lower limit of 65.5 GeV on the charged Higgs mass is derived at 95% confidence level, independent of the decay branching ratio Br(H^±→τν).     

Quark-antiquark pair production via γW fusion at high energye + e − colliders

We calculate numerically the cross section for the process \(e^ + e^ - \to ve^ + Q_d \bar Q_u \) using the Weizsäcker-Williams approximation for the virtual photon. This process is a possible background for the detection of an “intermediate” Higgs via its decay into \(Q\bar Q\) pairs. The comparison with the signal allows us to conclude that actually it is not a problem for the Higgs search at high energye ⁺ e ^? colliders.     

Angular distribution in reactions involving the formation of three low energy particles,with application to τ+ meson decay

The effect of interaction in the final state on the angular distribution in reactions involving the formation of three low energy particles is considered. In the case of non-resonance interaction the angular distribution can be expressed in terms of scattering amplitudes of pairs of particles. It is shown how the meson-meson scattering amplitudes can be deduced from the angular distribution of the decay products of the τ⁺ → 2π⁺+π⁻, τ⁺ → 2π⁰+π⁺ reactions. Experimental data presently available are insufficient for determining these amplitudes.     

A Monte-Carlo generator for statistical hadronization in high energy e+e− collisions

We extend some previous attempts to explain the origin and evolution of primordial magnetic fields during inflation induced from a 5D vacuum. We show that the usual quantum fluctuations of a generalized 5D electromagnetic field cannot provide us with the desired magnetic seeds. We show that special fields without propagation on the extra non-compact dimension are needed to arrive at appreciable magnetic strengths. We also identify a new magnetic tensor field B _ij in this kind of extra dimensional theory. Our results are in very good agreement with observational requirements, in particular from TeV blazars and CMB radiation limits we see that primordial cosmological magnetic fields should be close to scale invariance.     

On the Coulomb corrections to the total cross section of the interaction of the π+π− atom with ordinary atoms at high energy

The size of π⁺π⁻ atom in the low lying states is considerably smaller than the radius of atomic screening. Due to that we can neglect this screening calculating the contribution of multi-photon exchanges. We obtain the analytic formula for Coulomb corrections which works with a very good accuracy for the ground state of π⁺π⁻ atom. Received: 30 November 1998     

Experimental momentum spectra of identified hadrons ate + e − colliders compared to QCD calculations

Experimental data on the shape of hadronic momentum spectra are compared to theoretical predictions in the context of calculations in the Modified Leading Log Approximation (MLLA), under the assumption of Local Parton Hadron Duality (LPHD). Considered are experimental measurements ate ⁺ e ^?-colliders of ξ _p ^* the position of the maximum in the distribution of ξ _p =log(1/x _p ), wherex _p =p/p _beam. The parameter ξ _p ^* is determined for various hadrons at various centre of mass energies. The dependence on the hadron type poses some interesting questions about the process of hadron-formation. The dependence of ξ _p ^* on the centre of mass energy is seen to be described adequately by perturbation theory. A quantitative check of LPHD + MLLA is possible by extracting a value of α _s from an overall fit to the scaling behaviour of ξ _p ^* .