W. Kilian

SUSY Les Houches accord: interfacing SUSY spectrum calculators, decay packages, and event generators

An accord specifying a unique set of conventions for supersymmetric extensions of the Standard Model together with generic file structures for 1) supersymmetric model specifications and input parameters, 2) electroweak scale supersymmetric mass and coupling spectra, and 3) decay tables is presented, to provide a universal interface between spectrum calculation programs, decay packages, and high energy physics event generators.

WHIZARD—simulating multi-particle processes at LHC and ILC

We describe the universal Monte-Carlo (parton-level) event generator WHIZARD (http://whizard.event-generator.org), version 2. The program automatically computes complete tree-level matrix elements, integrates them over phase space, evaluates distributions of observables, and generates unweighted partonic event samples. These are showered and hadronized by calling external codes, either automatically from within the program or via standard interfaces. There is no conceptual limit on the process complexity; using current hardware, the program has successfully been applied to hard scattering processes with up to eight particles in the final state. Matrix elements are computed as helicity amplitudes, so spin and color correlations are retained. For event generation, processes can be concatenated with full spin correlation, so factorized approximations to cascade decays are possible when complete matrix elements are not desired. The Standard Model, the MSSM, and many alternative models such as Little Higgs, anomalous couplings, or effects of extra dimensions or noncommutative SM extensions have been implemented. Using standard interfaces to parton shower and hadronization programs, WHIZARD covers physics at hadron, lepton, and photon colliders.

Resonances and Unitarity in Weak Boson Scattering at the LHC

A crucial test of the Standard Model is the measurement of electroweak gauge-boson scattering. In this paper, we describe a generic parameterization aimed at a realistic simulation of weak-boson scattering at the LHC. The parameterization implements resonances of all possible spin and isospin combinations, properly matched to the low-energy effective (chiral) Lagrangian, includes leading higher-order effects and contains a minimal unitarization scheme. We implement the parameterization in the Monte-Carlo event generator WHIZARD and present results for complete partonic cross-section integration and event generation. We provide a comparison with the effective W approximation that previously has been used for most WW scattering studies at hadron colliders.

Pseudo-axions in little Higgs models

A motivation for physics beyond the Standard Model (SM) in the electroweak (EW) sector lies in the vast difference between the Planck (or unification) and the EW scale (hierarchy problem), that requires some stabilization of light scalar masses (fine tuning problem). In contrast to the supersymmetric solution to that problem where the quadratic sensitivity of the scalar masses to the cut-off is cancelled above the SUSY breaking scale between partners of opposite statistics, models have been constructed that contain a spontaneously broken global symmetry with the Higgs being light because it is one of the Goldstone bosons appearing in this breaking. In the simplest variant this construction fails because the scale for new strong interactions is too close to the electroweak scale, leaving traces in the low-energy effective action which would have shown up at LEP and Tevatron. There are two ways to evade these complications, either to use non-simple global groups (deconstruction models) or to entangle the global with the local symmetry breaking to forbid one-loop contributions to the Higgs mass parameters [1]. This shifts the scale where new strong dynamics naturally appears by one order of magnitude upwards. If the Higgs is among the Goldstone bosons of a broken global symmetry group, one always has a reduction of the rank of the global group. The Higgs thereby corresponds to a broken non-diagonal generator like the kaon in chiral symmetry breaking.

Supersymmetry simulations with off-shell effects for the CERN LHC and an ILC

At the LHC and at an ILC, serious studies of new physics benefit from a proper simulation of signals and backgrounds. Using supersymmetric sbottom pair production as an example, we show how multiparticle final states are necessary to properly describe off-shell effects induced by QCD, photon radiation, or by intermediate on-shell states. To ensure the correctness of our findings we compare in detail the implementation of the supersymmetric Lagrangian in madgraph, sherpa and whizard. As a future reference we give the numerical results for several hundred cross sections for the production of supersymmetric particles, checked with all three codes.

NLO event generation for chargino production at the ILC

We present a Monte Carlo event generator for simulating chargino pair production at the International Linear Collider (ILC) at next-to-leading order in the electroweak couplings. By properly resumming photons in the soft and collinear regions, we avoid negative event weights, so the program can simulate physical (unweighted) event samples. Photons are explicitly generated throughout the range where they can be experimentally resolved. Inspecting the dependence on the cutoffs separating the soft and collinear regions, we evaluate the systematic errors due to soft and collinear approximations. In the resummation approach, the residual uncertainty can be brought down to the per-mil level, coinciding with the expected statistical uncertainty at the ILC.

Determination of new electroweak parameters at the ILC : sensitivity to new physics

We present a study of the sensitivity of the International Linear Collider (ILC) to electroweak parameters in the absence of a light Higgs boson. In particular, we consider those parameters that have been inaccessible at previous colliders, quartic gauge couplings. Within a generic effective-field theory context we analyze all processes that contain quasi-elastic weak-boson scattering, using complete six-fermion matrix elements in unweighted event samples, fast simulation of the ILC detector, and a multi-dimensional parameter fit of the set of anomalous couplings. The analysis does not rely on simplifying assumptions such as custodial symmetry or approximations such as the equivalence theorem. We supplement this by a similar new study of triple weak-boson production, which is sensitive to the same set of anomalous couplings. Including the known results on triple gauge couplings and oblique corrections, we thus quantitatively determine the indirect sensitivity of the ILC to new physics in the electroweak symmetry-breaking sector, conveniently parameterized by real or fictitious resonances in each accessible spin/isospin channel.

Distinguishing Little Higgs product and simple group models at the CERN LHC and ILC

We propose a means to discriminate between the two basic variants of Little Higgs models, the Product Group and Simple Group models, at the next generation of colliders. It relies on a special coupling of light pseudoscalar particles present in Little Higgs models, the pseudo-axions, to the Z and the Higgs boson, which is present only in Simple Group models. We discuss the collider phenomenology of the pseudo-axion in the presence of such a coupling at the LHC, where resonant production and decay of either the Higgs or the pseudo-axion induced by that coupling can be observed for much of parameter space. The full allowed range of parameters, including regions where the observability is limited at the LHC, is covered by a future ILC, where double scalar production would be a golden channel to look for.

Unification without doublet-triplet splitting

Abstract Matter–Higgs unification in string-inspired supersymmetric grand unified theories predicts the existence of colored states in the Higgs multiplets and calls for two extra generations of Higgs-like fields (‘unhiggses’). If these states are present near the TeV scale, gauge-coupling unification points to the existence of two distinct scales, 10 15 GeV where right-handed neutrinos and a Pati–Salam symmetry appear, and 10 18 GeV where complete unification is achieved. Baryon-number conservation, while not guaranteed, can naturally emerge from an underlying flavor symmetry. Collider signatures and dark-matter physics may be drastically different from the conventional MSSM.

Renormalized soft-higgs theorems

The Higgs couplings to matter fields are proportional to their masses. Thus Higgs amplitudes can be obtained by differentiating amplitudes without Higgs with respect to masses. We show how this well-known statement can be extended to higher order when renormalization effects are taken into account. We establish the connection with the Callan-Symanzik and renormalization group equations and consider also pseudoscalar Higgs couplings to fermions. Furthermore, we address the case where the Higgs couples to a heavy particle that is integrated out from the low-energy effective Lagrangian. We derive effective interactions where mass logarithms are resummed by renormalization-group methods, and give expansions of the results up to next-to-leading order.