The processes e+e−→bbˉW±H∓e^+e^-\to b\bar bW^\pm H^\mp and e+e−→bbˉH+H−e^+e^-\to b\bar bH^+H^- at the Next Linear Collider in the Minimal Supersymmetric Standard Model

The complete matrix elements for $e^+e^-\to b\bar bW^\pm H^\mp$ and $e^+e^-\to b\bar bH^+H^-$ are computed at tree-level within the Minimal Supersymmetric Standard Model. Rates of interest to phenomenological analyses at the Next Linear Collider are given. In particular, we analyse: (i) $t\bar t$ pair production followed by the top decays into $b\bar bW^\pm H^\mp$ and $b\bar b H^+H^-$ final states; (ii) $H^+H^-$ signals in which $H^\pm\to h W^\pm \to b\bar b W^\pm$ and $H^\pm\to tb\to b\bar b W^\pm$. Top and Higgs finite width effects are included as well as all those due to the irreducible backgrounds.Comment: 34 pages, latex, epsfig, 15 postscript figures bitmapped at 1000dpi, complete paper including high resolution plots available at ftp://axpa.hep.phy.cam.ac.uk/moretti/cavendish_9701 and at http://www.hep.phy.cam.ac.uk/theory/papers

Heavy charged Higgs boson production at next generation e±γe^\pm\gamma colliders

We assess the potential of future electron-positron linear colliders operating in the $e^\pm\gamma$ mode in detecting charged Higgs bosons with mass around and larger than the top quark mass, using Compton back-scattered photons from laser light. We compare the pair production mode, $e^-\gamma\to e^- H^+H^-$, to a variety of channels involving only one charged Higgs scalar in the final state, such as the tree-level processes $e^-\gamma\to \nu_e H^- \Phi^0$ ($\Phi^0=h^0,H^0$ and $A^0$) and $e^-\gamma\to \nu_e f\bar f H^-$ ($f=b,\tau$ and $\nu_\tau$) as well as the loop-induced channel $e^-\gamma\to \nu_e H^-$. We show that, when the charged Higgs boson mass is smaller than or comparable to half the collider energy, \sqrt s_{ee}\gsim 2M_{H^\pm}, single production cross sections are of the same size as the pair production rate, whereas, for charged Higgs boson masses larger than $\sqrt s_{ee}/2$, all processes are heavily suppressed. In general, production cross sections of charged Higgs bosons via $e^\pm\gamma$ scatterings are smaller than those induced at an $e^+e^-$ collider and the latter represents a better option to produce and analyse such particles.Comment: 18 pages, latex, 9 figure

Gluino signals in 4jet events and vertex tagging at LEP1

Heavy flavour tagging provides a broad range of possibilities in testing QCD features at LEP. We present here a study of 4jets events at LEP I where the so-called light gluinos could be directly produced. We show that microvertex techniques offer a unique chance to exploit simple kinematical distributions in order to optimise the signal coming from gluino production with respect to the background of ordinary QCD events. Our results indicate that experimental analyses along the lines suggested here can exclude or reveal the presence of a gluino for masses up to 10 GeV and lifetimes below 10$^{-9}$ sec. We also point out that a large fraction of gluino events could decay in configurations carrying large missing energy, so to escape the usual selection criteria of 4jet samples. In our study, mass effects of quarks and gluinos have been taken into account exactly. Our results are independent from both the jet algorithm and its resolution parameter.}Comment: 28 pages, 9 figures embedded with epsfig, Latex. Major revision taking into account latest bounds. Version to appear in Zeit. f. Phys.

Regge-cascade hadronization

We argue that the evolution of coloured partons into colour-singlet hadrons has approximate factorization into an extended parton-shower phase and a colour-singlet resonance--pole phase. The amplitude for the conversion of colour connected partons into hadrons necessarily resembles Regge-pole amplitudes since qq-bar resonance amplitudes and Regge-pole amplitudes are related by duality. A `Regge-cascade' factorization property of the N-point Veneziano amplitude provides further justification of this protocol. This latter factorization property, in turn, allows the construction of general multi-hadron amplitudes in amplitude-squared factorized form from (1->2) link amplitudes. We suggest an algorithm with cascade-decay configuration, ordered in the transverse momentum, suitable for Monte-Carlo simulation. We make a simple implementation of this procedure in Herwig++, obtaining some improvement to the description of the event-shape distributions at LEP.Comment: 10 pages, 9 figure

The underlying event and fragmentation

A good fit to the CDF underlying event is obtained in the multiple parton scattering picture using HERWIG, after modifying the cluster hadronization algorithm as suggested by our previous study and adopting a larger maximum cluster size. The number of scatters per event is generated simply as a Poisson distribution. If our picture is correct, the baryon yield should be enhanced in the underlying event. This effect may be studied by measuring the proton-to-pion ratio.Comment: 23 pages, 8 figure