Simulating New Physics at the LHC Peter Richardson IPPP, Durham University Imperial 28th May

Summary Introduction Simulation of BSM Physics Herwig++ Conclusions Imperial 28th May

A Monte Carlo Event Hard Perturbative scattering: Usually calculated at leading order in QCD, electroweak theory or some BSM model. Modelling of the soft underlying event Multiple perturbative scattering. Perturbative Decays calculated in QCD, EW or some BSM theory. Initial and Final State parton showers resum the large QCD logs. Finally the unstable hadrons are decayed. Non-perturbative modelling of the hadronization process. Imperial 28th May

Monte Carlo Event Generators For BSM physics the main pieces of the event generators are Hard Process New intermediate particles New particles produced Changes to SM distributions Decays Decays of new particles produced in the hard process or previous decays. Imperial 28th May

Historically Traditionally models of new physics are built into the event generator. This will often include hard processes and decays. Relatively few models have been implemented and the sophistication of the simulation varies. Each one was hard-coded by an author of the general purpose generator which was very time consuming. Imperial 28th May

Progress In the last few years things have moved on. Less new models are being implemented inside the event generators. Relying more on both. Matrix element generators for specific processes, interfaced via the Les Houches matrix element accord. Matrix element generators which automatically calculate the processes from the Feynman rules and allow the Feynman rules for new models to be implemented. Imperial 28th May

Progress The four main matrix element generators for BSM physics are: COMPHEP/CALCHEP; MadGraph; Omega/Whizard; SHERPA. All of these have the Feynman rules for a range of models included. Can also implement new models relatively easily from either the Feynman rules or Lagrangian. Recently progress with FeynRules to automated further automated this. Imperial 28th May

BSM Simulation In general there are two different classes of models to be simulated. Models which only have either new hard scattering processes, or modifications to the Standard Model ones. Models in which new heavy particles are produced and subsequently decay. The first type are relatively simple to simulate. The second class, e.g. SUSY, UED, Little Higgs with T-parity are more complicated. Imperial 28th May

Cascade Decays These models were implemented as follows: implement the production of the new particles in 2g2 scatterings; recursively decay the new particles using either phase space or matrix elements. This neglects both: spin correlation effects, which will be important in determining what a signal is; some off-shell effects, which may be important for specific models or values of parameters. Imperial 28th May

Cascade Decays There are two ways round these limitations. Calculate the matrix element for the hard scattering as a 2gn scattering process. Ensures that both the spin correlations and off-shell effects are correctly treated. Can be inefficient for long decay chains or many decay modes. Still factorize the process into production and decay but include correlations. Efficient for long decay chains and large numbers of decay modes. Only gets the spin correlations right, although some off-shell effects can be included. Imperial 28th May

Herwig++ M. Baehr, S. Gieseke, M. Gigg, D. Grellscheid, K. Hamilton, S. Latunde-Dada, S. Plaetzer, PR, M.H. Seymour, A. Sherstnev, J. Tully , B.R. Webber A. Siodmok Imperial 28th May

Introduction Herwig++ is an ongoing project to provide a replacement for the FORTRAN HERWIG program. Based on the same physics philosophy but with improved physics simulation based on the theoretical developments of the last 10 years, not just a rewrite. There are many improvements to the simulation for both Standard Model and BSM physics. In this talk I will concentrate on BSM physics. Work in hep-ph/0703199 and arXiv:0805.3037 Gigg and Richardson Imperial 28th May

Herwig++ Herwig++ uses a improved angular ordered parton shower. An improved cluster model including excited baryon multiplets for the hadronization. A multiple scattering model for the underlying event. A sophisticated treatment of BSM physics and hadron decays including spin correlations. Some processes at NLO in the POWHEG scheme. Imperial 28th May

Underlying Event Major new feature is a multiple scattering model of the underlying event. In good agreement with CDF data on the underlying event. Imperial 28th May

POWHEG In the last two releases we have included a number of processes accurate to next-to-leading order in the POsitive Weight Hardest Emission Generator scheme of Nason. This allows the simulation of processes with NLO accuracy and only positive weights, unlike MC@NLO. Imperial 28th May

POWHEG method for Drell-Yan Herwig++ POWHEG MC@NLO CDF Run I Z pT D0 Run II Z pT Imperial 28th May

POWHEG Method for gggH Tevatron LHC Associate Higgs W/Z production also available and work in progress on VBF. Imperial 28th May

BSM Physics in Herwig++ Herwig++ we wanted the following good features of the FORTAN: retain the good features of FORTRAN HERWIG, e.g. the spin correlations; make adding new models much easier; simulate perturbative and non-perturbative decays in the same way so all correlations can be generated for taus; generate QCD radiation from coloured BSM particles. Imperial 28th May

BSM Physics in Herwig++ The simulation of both Standard Model and Beyond the Standard Model hard processes and decays in Herwig++ is: based on a reimplementation of the HELAS library in C++.; all interactions are coded as vertex classes; the Standard Model hard processes and decays are implemented using hard-coded matrix elements using these classes; for BSM models the matrix elements for 2g2 scattering processes and 1g2 decays are automatically generated based on the spin structure of the process. Imperial 28th May

BSM Physics in Herwig++ Implementing a new model in Herwig++ is then simply a matter of: implementing a new model class which inherits from the Herwig++ Standard Model class and stores or calculates any parameters needed in the model; implementing the Vertex classes, specifying the interactions in the model; specifying the particle content of the model. Still requires some coding for each model. Imperial 28th May

Correlations in e+e- Unpolarised + Hw++ HERWIG Imperial 28th May

UED Look at the decay e- near e- far q e- near q*L Z* e+ far e+ near e*R g* Herwig++ compared to hep-ph/0507170 Smillie and Webber Imperial 28th May

Correlations in Tau Decays Left Handed stau Right Handed stau + Hw++ HERWIG+TAUOLA Fraction of visible energy carried by the charged pion Imperial 28th May

Correlations in Tau Decays Based on hep-ph/0612237 Choi et al. Imperial 28th May

Different Spin Structures With the exception of the code implementing the vertex the rest of the simulation doesn’t care what the Lorentz structure of the vertex is, e.g. for a fermion-fermion-vector coupling only the code calculating G needs to know what it is. Introduce a new level of abstract classes to allow any Lorentz structure to be implemented. Imperial 28th May

Example: VBF Higgs Production To explore the CP structure of the Higgs can implement a new vertex class with the CP-even and CP-odd operators. Then from the input files replace the Standard Model HWW vertex and get the result using the Standard Model matrix elements. Based on hep-ph/0105325 Plehn et. al Imperial 28th May

Example: Z’ with Anomalous gZZ’ coupling Another example is the model of with a Z’ coupling to both the SM fermions and gZ via an anomalous coupling hep-ph/0501154 Kozlov . The vertex for the Z’ coupling to the Standard Model fermions can b essentially copied from the Z vertex. Need a new Lorentz structure for the anomalous vertex but then the generation of the hard matrix elements and decays proceeds using the general code. Imperial 28th May

Example: Z’ with Anomalous gZZ’ coupling Imperial 28th May

Conclusions There’s been a lot of work done in the simulation of BSM physics. To get everything right, including interferences, need to use matrix elements but not currently practical. In practice for models like SUSY need to use different approaches which treat spin correlations correctly but treat the processes as a series of cascade decays. This approach is full implemented in Herwig++ for both SUSY and UED. Imperial 28th May