Herwig++ Monte Carlo Event Generator Update

1. Herwig++ S. Gieseke, D. Grellscheid, K. Hamilton, A. Ribon, PR, P. Stephens, M.H. Seymour, B.R. Webber Peter Richardson IPPP, Durham University M. Baehr, M. Gigg, S. Latunde-Dada, S. Plaetzer, A. Sherstnev, J. Tully Moriond 20th March

2. Introduction • Monte Carlo event generators are an essential part of most experimental analyses. • The HERWIG program was highly successful during at LEP/HERA and the Tevatron. • However our understanding of the physics involved has improved over the last 20 years. • It has been possible to improve and extend the program a great deal but it has reached the end of its life. Moriond 20th March

3. Introduction • In order to include all the new theoretical ideas from the last 5-10 years for the LHC major changes were needed. • The Herwig++ project was to write a new generator, using the same physics philosophy as HERWIG, but including new developments wherever possible. • The initial plan was • Recode the cluster hadronization model making minor improvements to fix problems related to the number of excited mesons included. • Write a new angular-ordered parton shower with better: • treatment of mass effects; • Lorentz invariance properties. Moriond 20th March

4. Shower Improvements • Most of the recent progress in Monte Carlo simulations has been in better simulation of hard radiation. • While there are a number of different ideas all of them rely on being able to understand what the shower does. • To make these improvements we often need an analytic understanding on what the shower is doing. • Also some of the approximations which we made in the past were shown to be poor. Moriond 20th March

5. Dead-Cone • For massive particles radiation with angle less than m/E is suppressed, the “dead-cone”. • However in order to implement this we had to make an extreme approximation which leads to problems in physical distributions. Soft radiation pattern from a top quark with 1 TeV energy. Moriond 20th March

6. Shower Improvements • In the FORTRAN program the shower implemented angular ordering using and the DGLAP splitting functions. • The major change with the new algorithm were to generalise the evolution variable and use the quasi-collinear splitting functions of S. Catani et. al. Phys.Lett.B500:149-160,2001 • Also changed the definition of z to give invariance under boosts along the jet direction. Moriond 20th March

7. Shower Improvements • The main aim was to allow evolution down to zero pT for radiation from massive particles and to avoid the ‘dead-cone’ approximation used in the FORTRAN program. Moriond 20th March

8. Status • After the first release which only did e+e- collisions we decided to make a number of further improvements: • The extension to hadron collisions; • Developments to make implementing Standard Model scattering processes and Beyond the Standard Model physics easier; • Improvements to the simulation of QED radiation; • Improvements to the hadron decays. Moriond 20th March

9. pT of the Z compared with CDF data Moriond 20th March

10. Top Shower • In the FORTRAN algorithm the simulation of QCD radiation in the decay of heavy particles was performed in the rest frame of the decaying particle. • Within the formalism the decaying particle did not radiate in this process. • However if we consider tgbW+ the radiation from the bottom quark did not fill all the soft region. Moriond 20th March

11. Top Shower and Matrix Element Correction in tgbW+g • In the new formalism there is radiation from the top quark in the decay ensuring that the soft region is filled. • However the soft matrix element is required to give smooth coverage in the soft region Keith Hamilton, Peter Richardson, hep-ph/0612236, JHEP0702:069,2007 Moriond 20th March

12. Simulation of QED Radiation • In the FORTRAN there was no simulation of QED radiation. This is important for leptonic W and Z decay. • Also in some hadronic decay processes. • In the C++ we have a new simulation of QED radiation based on the YFS formalism. • This is a formalism for simulating soft electromagnetic radiation which can be systematically improved by including higher-order corrections and collinear emission. Moriond 20th March

13. Simulation of QED Radiation K. Hamilton and PR hep-ph/0603034, JHEP 0607:010, 2006. Moriond 20th March

14. Status • The current release 2.0, S. Gieseke et. al. hep-ph/0609306 includes: • Initial-State showers; • Top Decay Shower; • UA5 Soft Underlying event model; • QED Radiation; • Many important hadron-hadron matrix elements. • This version can be used for hadron collider physics. Moriond 20th March

15. Status • After this version there are a number of features which we still need to include: • JIMMY multiple scattering model for the underlying event; • different kinematic reconstruction procedures for the shower; • BSM Physics; • new hadron decay model; • spin correlations throughout the simulation. In order for the simulation to be as good as, or better than, the FORTRAN for everything. Moriond 20th March

16. Hard Processes and New Physics • In the FORTRAN each hard process and decay matrix element was typed in by hand. • Isn’t a good use of time. • Meant that models of new physics were very hard to include. • In the C++ we have used an entirely different philosophy. • A C++ helicity library based on the HELAS formalism is used for all matrix element and decay calculations. • Code the hard 2g2 matrix elements based on the spin structures. • Code the 1g2 decays in the same way and use phase space for the 1g3 decays to start with. • Easy to include spin correlations as we have access to the spin unaveraged matrix elements. M. Gigg and PR hep-ph/0703199. Moriond 20th March

17. New Physics M. Gigg and PR hep-ph/0703199. Moriond 20th March

18. HERWIG + Hw++ New Physics Unpolarised M. Gigg and PR hep-ph/0703199. Moriond 20th March

19. HERWIG+TAUOLA + Hw++ Tau Decays Right Handed stau Left Handed stau Fraction of visible energy carried by the charged pion M. Gigg and PR hep-ph/0703199. Moriond 20th March

20. Tau Decays H,Agt+t-gp+np-n Moriond 20th March

21. UED • The idea was that with the new approach to simulating BSM models other than SUSY should be easier. • The first test of that is the inclusion of the Universal Extra Dimensions (UED) model. • This is a model where all the particle propagate in an extra dimension. • Gives a similar spectrum of new particles to SUSY but the new particles have the same spin as their Standard Model partners. • It is a useful “straw-man” model to decide if the spins of new particles can be measured Moriond 20th March

22. q e- near Z* e*R g* UED Look at the decay e- near e- far e+ far q*L e+ near e+ far J. Smillie, B. Webber JHEP 0510:069,2005, hep-ph/0507170 Moriond 20th March

23. Hadron Decays • The simulation includes detailed modelling of many decays. Moriond 20th March

24. Future Shower Improvements • In addition to the other features one of the main reasons for going to C++ was to allow improvements to the shower algorithm. • CKKW matrix element matching • The multi-scale shower • MC@NLO • The Nason approach to MC@NLO Moriond 20th March

25. Nason Approach to MC@NLO hep-ph/0612281 Oluseyi Latunde-Dada, Stefan Gieseke, Bryan Webber Moriond 20th March

26. Summary • Herwig++ is now ready for hadron collisions. • Many further improvements are planned. • There will be a new release in the summer which will include the new simulation of BSM physics, hadron decays and some shower improvements. • In the near future the C++ simulation will replace the FORTRAN. Moriond 20th March