QCD (&) Event Generators

Recent developments in QCD phenomenology have spurred on several improved approaches to Monte Carlo event generation, relative to the post--LEP state of the art. In this brief review, the emphasis is placed on approaches for 1) consistently merging fixed--order matrix element calculations with parton showers, 2) improving the parton shower algorithms themselves, and 3) improving the description of the underlying event in hadron collisions.Comment: Submitted to proceedings of DIS05, 12 page

Progress on Multiple Interactions

We report on the development of a new model for the underlying event in hadron-hadron collisions. The model includes parton showers for all interactions, as well as non-trivial flavour, momentum, and colour correlations between interaction initiators and beam remnant partons.Comment: To appear in the proceedings of HEP 2003, 3p

String Formation Beyond Leading Colour

We present a new model for the hadronisation of multi-parton systems, in which colour correlations beyond leading $N_C$ are allowed to influence the formation of confining potentials (strings). The multiplet structure of $SU(3)$ is combined with a minimisation of the string potential energy, to decide between which partons strings should form, allowing also for "baryonic" configurations (e.g., two colours can combine coherently to form an anticolour). In $e^+e^-$collisions, modifications to the leading-colour picture are small, suppressed by both colour and kinematics factors. But in $pp$ collisions, multi-parton interactions increase the number of possible subleading connections, counteracting their naive $1/N_C^2$ suppression. Moreover, those that reduce the overall string lengths are kinematically favoured. The model, which we have implemented in the PYTHIA 8 generator, is capable of reaching agreement not only with the important \left(n_\mathrm{charged}) distribution but also with measured rates (and ratios) of kaons and hyperons, in both $ee$ and $pp$ collisions. Nonetheless, the shape of their $p_\perp$ spectra remains challenging to explain.Comment: 48 pages, 22 figure

Soft-QCD and UE spectra in pp collisions at very high CM energies (a Snowmass white paper)

We make some educated guesses for the extrapolations of typical soft-inclusive (minimum-bias, pileup, underlying-event) observables to proton-proton collisions at center-of-mass energies in the range 13 - 100 TeV. The numbers should be interpreted with (at least) a 10% uncertainty.Comment: 5 pages. Update (v2) adds numbers for 13 TeV and corrects bug in UE ET density estimate

A Brief Introduction to PYTHIA 8.1

The PYTHIA program is a standard tool for the generation of high-energy collisions, comprising a coherent set of physics models for the evolution from a few-body hard process to a complex multihadronic final state. It contains a library of hard processes and models for initial- and final-state parton showers, multiple parton-parton interactions, beam remnants, string fragmentation and particle decays. It also has a set of utilities and interfaces to external programs. While previous versions were written in Fortran, PYTHIA 8 represents a complete rewrite in C++. The current release is the first main one after this transition, and does not yet in every respect replace the old code. It does contain some new physics aspects, on the other hand, that should make it an attractive option especially for LHC physics studies.Comment: 27 page

Branching ratios for B_{d,s}->J/psi eta and B_{d,s}->eta l+l-, extracting gamma from B_{d,s}->J/psi eta, and possibilities for constraining C_{10A} in semileptonic B decays

Estimates of the branching ratios for B_{d,s}->J/psi eta and B_{d,s}->eta l+l- are obtained by SU(3) relation to B_d->J/psi K and B_d->K l+l-, respectively, as functions of the eta mixing parameter, theta_P. Based on these estimates, a discussion of the prospects for HERA-B, CDF-II, and ATLAS on these processes is given. The CP violation in B_{d,s}-> J/psi eta is analyzed in depth and a method to extract the angle gamma of the unitarity triangle is discussed. Finally, a possible method to constrain the Wilson coefficient C_{10A} from measurements on semileptonic B decays such as B_d->J/psi K and B_s->eta l+l- is proposed along with a discussion of the prospects for future experiments and form factor calculations to reach the precision required for this method to be interesting.Comment: 14 pages, 2 figures, 4 tables. Fixed incorrect reference and replaced U-spin by SU(3) in several places. Corrected inconsistency in definition of a_s. Added various comment

Searching for L-Violating Supersymmetry at the LHC

The possibility to simulate lepton number violating supersymmetric models has been introduced into the recently updated PYTHIA event generator, now containing 1278 decay channels of SUSY particles into SM particles via lepton number violating interactions. This generator has been used in combination with the ATLFAST detector simulation to study the impact of lepton number violation (LV) on event topologies in the ATLAS detector, and trigger menus designed for LV-SUSY are proposed based on very general considerations. In addition, a rather preliminary analysis is presented on the possibility for ATLAS to observe a signal above the background in several mSUGRA scenarios, using a combination of primitive cuts and neural networks to optimize the discriminating power between signal and background events over regions of parameter space rather than at individual points. It is found that a 5 sigma discovery is possible roughly for m_{1/2} < 1TeV and m_0 < 2TeV with an integrated luminosity of 30fb^{-1}, corresponding to one year of data taking with the LHC running at ``mid-luminosity'', L = 3*10^{33}s^{-1}cm^{-2}.Comment: 12 page

Revisiting Radiation Patterns in e+e−e^+e^- Collisions

We propose four simple event-shape variables for semi-inclusive $e^+e^- \to 4$-jet events. The observables and cuts are designed to be especially sensitive to subleading aspects of the event structure, and allow to test the reliability of phenomenological QCD models in greater detail. Three of them, $\theta_{14}$, $\theta^*$, and $C_2^{(1/5)}$, focus on soft emissions off three-jet topologies with a small opening angle, for which coherence effects beyond the leading QCD dipole pattern are expected to be enhanced. A complementary variable, $M_L^2/M_H^2$, measures the ratio of the hemisphere masses in 4-jet events with a compressed scale hierarchy (Durham $y_{23}\sim y_{34}$), for which subleading $1\to 3$ splitting effects are expected to be enhanced. We consider several different parton-shower models, spanning both conventional and dipole/antenna ones, all tuned to the same $e^+e^-$ reference data, and show that a measurement of the proposed observables would allow for additional significant discriminating power between the models.Comment: 27 pages, 10 figure