The Pythia program can be used to generate high-energy-physics ''events'', i.e. sets of outgoing particles produced in the interactions between two incoming particles. The objective is to provide as accurate as possible a representation of event properties in a wide range of reactions, within and beyond the Standard Model, with emphasis on those where strong interactions play a role, directly or indirectly, and therefore multihadronic final states are produced. The physics is then not understood well enough to give an exact description; instead the program has to be based on a combination of analytical results and various QCD-based models. This physics input is summarized here, for areas such as hard subprocesses, initial- and final-state parton showers, underlying events and beam remnants, fragmentation and decays, and much more. Furthermore, extensive information is provided on all program elements: subroutines and functions, switches and parameters, and particle and process data. This should allow the user to tailor the generation task to the topics of interest.

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PYTHIA 6.4 Physics and Manual

Theory and phenomenology of two-Higgs-doublet models

We review the present status of QCD corrections to weak decays beyond the leading-logarithmic approximation, including particle-antiparticle mixing and rare and $\mathrm{CP}$-violating decays. After presenting the basic formalism for these calculations we discuss in detail the effective Hamiltonians of all decays for which the next-to-leading-order corrections are known. Subsequently, we present the phenomenological implications of these calculations. The values of various parameters are updated, in particular the mass of the newly discovered top quark. One of the central issues in this review are the theoretical uncertainties related to renormalization-scale ambiguities, which are substantially reduced by including next-to-leading-order corrections. The impact of this theoretical improvement on the determination of the Cabibbo-Kobayashi-Maskawa matrix is then illustrated. [S0034-6861(96)00304-2]

Weak decays beyond leading logarithms

There is almost universal agreement among astronomers that most of the mass in the Universe and most of the mass in the Galactic halo is dark. Many lines of reasoning suggest that the dark matter consists of some new, as yet undiscovered, weakly-interacting massive particle (WIMP). There is now a vast experimental effort being surmounted to detect WIMPS in the halo. The most promising techniques involve direct detection in low-background laboratory detectors and indirect detection through observation of energetic neutrinos from annihilation of WIMPs that have accumulated in the Sun and/or the Earth. Of the many WIMP candidates, perhaps the best motivated and certainly the most theoretically developed is the neutralino, the lightest superpartner in many supersymmetric theories. We review the minimal supersymmetric extension of the Standard Model and discuss prospects for detection of neutralino dark matter. We review in detail how to calculate the cosmological abundance of the neutralino and the event rates for both direct- and indirect-detection schemes, and we discuss astrophysical and laboratory constraints on supersymmetric models. We isolate and clarify the uncertainties from particle physics, nuclear physics, and astrophysics that enter at each step in the calculation. We briefly review other related dark-matter candidates and detection techniques.

Supersymmetric Dark Matter

Two-Higgs-doublet models predict nonstandard physical effects through the interactions of charged Higgs scalars. We analyze the experimental measurements that bound these effects and quantify the resulting constraints on the charged-Higgs-boson mass and coupling parameter $tan\ensuremath{\beta}$. We consider low-energy data relating to ${B}_{d}^{0}\ensuremath{-}{\overline{B}}_{d}^{0}$, ${D}^{0}\ensuremath{-}{\overline{D}}^{0}$, and ${K}^{0}\ensuremath{-}{\overline{K}}^{0}$ mixing, and high-energy measurements at $p\overline{p}$ colliders relating to searches for the top quark and events with large missing ${E}_{T}$. Our analysis systematically takes account of theoretical uncertainties in bag factors, decay constants, and quark mixing matrix elements by Monte Carlo sampling. We combine our results to determine the regions in the charged-Higgs-boson parameter space that are consistent with present data. An important result is that the Collider Detector at Fermilab top-quark mass bound ${m}_{t}g77$ GeV can probably not be evaded by appealing to $t\ensuremath{\rightarrow}b{H}^{+}$ decays, since only a very small sliver of parameter space is consistent with ${m}_{t}l77$ GeV in this scenario. Consistency with either the CERN NA31 or Fermilab E731 $\frac{{\ensuremath{\epsilon}}^{\ensuremath{'}}}{\ensuremath{\epsilon}}$ measurements is possible. The rare decays $B\ensuremath{\rightarrow}{K}^{*}\ensuremath{\gamma}$, ${K}^{+}\ensuremath{\rightarrow}{\ensuremath{\pi}}^{+}\ensuremath{\nu}\overline{\ensuremath{\nu}}$, and ${K}_{L}\ensuremath{\rightarrow}\ensuremath{\mu}\ensuremath{\mu}$ are also examined in these models within the context of our combined analysis.

New constraints on the charged Higgs sector in two-Higgs-doublet models.

We evaluate the influence of coherent forward scattering in matter upon neutrino oscillations in the three-neutrino picture. We write down the exact solution and also approximate first-order solutions that exhibit general features more transparently. Oscillation characteristics in matter that could be observed in deep-mine experiments are discussed and illustrated using an oscillation solution suggested by solar and reactor data.

Matter effects on three-neutrino oscillations

MESON REGGE EXCHANGES FROM SIMULTANEOUS ANALYSIS OF $pi$N SCATTERING DATA AND DISPERSION SUM RULES.

Remarks on the KARMEN anomaly

Quarks whose left- and right-handed chiral components are both singlets with respect to the SU(2) weak-isospin gauge group, offer interesting physics possibilities beyond the standard model (SM) already studied in many contexts. We address here some further aspects. We first collect and update the constraints from present data on their masses and mixings with conventional quarks. We discuss possible effects on b\ensuremath{\rightarrow}s\ensuremath{\gamma} and Z\ensuremath{\rightarrow}bb\ifmmode\bar\else\textasciimacron\fi{} decays and give fresh illustrations of CP asymmetries in ${\mathit{B}}^{0}$ decays differing dramatically from SM expectations. We analyze singlet effects in grand unification scenarios: d-type singlets are most economically introduced in 5+${5}^{\mathrm{*}}$ multiplets of SU(5), with up to three generations, preserving gauge coupling unification with perturbative values up to the GUT scale; u-type singlets can arise in 10+1${0}^{\mathrm{*}}$ multiplets of SU(5) with at most one light generation. With extra matter multiplets the gauge couplings are bigger; we give the two-loop evolution equations including exotic multiplets and a possible extra U(1) symmetry. Two-loop effects can become important, threatening unification (modulo threshold effects), perturbativity, and asymptotic freedom of ${\mathrm{\ensuremath{\alpha}}}_{3}$. In the Yukawa sector, the top-quark fixed-point behavior is preserved and singlet-quark couplings have infrared fixed points too, but the unification of b and \ensuremath{\tau} couplings is not possible in a three-generation ${\mathit{E}}_{6}$ model.

R.J.N. Phillips

Papers

New constraints on the charged Higgs sector in two-Higgs-doublet models.

Matter effects on three-neutrino oscillations

MESON REGGE EXCHANGES FROM SIMULTANEOUS ANALYSIS OF $pi$N SCATTERING DATA AND DISPERSION SUM RULES.

Remarks on the KARMEN anomaly

Quark singlets: Implications and constraints.